Ryan B., a college freshman, had suffered from occasional epileptic seizures since childhood. He had been taking drugs for his seizures for many years, but lately the medication wasn’t helping—his seizures were becoming more frequent. His neurologist increased the dose of the medication, but the seizures persisted, and the drug made it difficult for Ryan to concentrate on his studies. He was afraid that he would have to drop out of school.

He made an appointment with his neurologist and asked whether another drug was available that might work better and not affect his ability to concentrate. “No,” said the neurologist, “you’re taking the best medication we have right now. But I want to send you to Dr. L., a neurosurgeon at the medical school. I think you might be a good candidate for seizure surgery.”

Ryan had a focal-seizure disorder. His problems were caused by a localized region of the brain that contained some scar tissue. Periodically, this region would irritate the surrounding areas, triggering epileptic seizures—wild, sustained firing of cerebral neurons that result in cognitive disruption and, sometimes, uncontrolled movements. Ryan’s focus was probably a result of brain damage that occurred when he was born. Dr. L. ordered some tests that indicated that the seizure focus was located in the left side of his brain, in a region known as the medial temporal lobe.

Ryan was surprised to learn that he would remain awake during his surgery. In fact, he would be called on to provide information that the surgeon would need to remove a region of his brain that included the seizure focus. As you might expect, he was nervous when he was wheeled into the surgery, but after the anesthesiologist injected something through the tube in one of his veins, Ryan relaxed and thought to himself, “This won’t be too bad.”

Dr. L. marked something on his scalp, which had previously been shaved, and then made several injections of a local anesthetic. Then he cut the scalp and injected some more anesthetic. Finally, he used a drill and a saw to remove a piece of skull. He then cut and folded back the thick membrane that covers the brain, exposing the surface of the brain.

When removing a seizure focus, the surgeon wants to cut away all the abnormal tissue while sparing brain tissue that performs important functions, such as the comprehension and production of speech. For this reason, Dr. L. began stimulating parts of the brain to determine which regions he could safely remove. To do so, he placed a metal probe against the surface of Ryan’s brain and pressed a pedal that delivered a weak electrical current. The stimulation disrupts the firing patterns of the neurons located near the probe, preventing them from carrying out their normal functions. Dr. L. found that stimulation of parts of the temporal lobe disrupted Ryan’s ability to understand what he and his associates were saying. When he removed the part of the brain containing the seizure focus, he was careful not to damage these regions.

The operation was successful. Ryan continued to take his medication but at a much lower dose. His seizures disappeared, and he found it easier to concentrate in class. I met Ryan during his junior year, when he took a course I was teaching. I described seizure surgery to the class one day, and after the lecture he approached me and told me about his experience. He received the third highest grade in the class.

The goal of neuroscience research is to understand how the brain works. To understand the results of this research, you must be acquainted with the basic structure of the nervous system. The number of terms introduced in this chapter is kept to a minimum (but as you will see, the minimum is still a rather large number). With the framework you will receive from this chapter and from the features on MyPsychLab, you should have no trouble learning the material presented in subsequent chapters.

Basic Features of the Nervous System

FIGURE 3.1 Views of Alligator and Human

These side and frontal views show the terms used to denote anatomical directions.

Before beginning a description of the nervous system, I want to discuss the terms that are used to describe it. The gross anatomy of the brain was described long ago, and everything that could be seen without the aid of a microscope was given a name. Early anatomists named most brain structures according to their similarity to commonplace objects: amygdala, or “almond-shaped object”; hippocampus, or “sea horse”; genu, or “knee”; cortex, or “bark”; pons, or “bridge”; uncus, or “hook,” to give a few examples. Throughout this book I will translate the names of anatomical terms as I introduce them, because the translation makes the terms more memorable. For example, knowing that cortex means “bark” (like the bark of a tree) will help you to remember that the cortex is the outer layer of the brain.

When describing features of a structure as complex as the brain, we need to use terms denoting directions. Directions in the nervous system are normally described relative to the neuraxis , an imaginary line drawn through the length of the central nervous system, from the lower end of the spinal cord up to the front of the brain. For simplicity’s sake, let us consider an animal with a straight neuraxis. Figure 3.1 shows an alligator and two humans. This alligator is certainly laid out in a linear fashion; we can draw a straight line that starts between its eyes and continues down the center of its spinal cord. (See Figure 3.1 . ) The front end is anterior , and the tail is posterior . The terms rostral (toward the beak) and caudal (toward the tail) are also employed, especially when referring specifically to the brain. The top of the head and the back are part of the dorsal surface, while the ventral (front) surface faces the ground. (Dorsum means “back,” and ventrum means “belly.”) These directions are somewhat more complicated in the human; because we stand upright, our neuraxis bends, so the top of the head is perpendicular to the back. (You will also encounter the terms superior and inferior. In referring to the brain, superior means “above,” and inferior means “below.” For example, the superior colliculi are located above the inferior colliculi.) The frontal views of both the alligator and the human illustrate the terms lateral and medial : toward the side and toward the middle, respectively. (Look again at Figure 3.1 . )

lateral Toward the side of the body, away from the middle.

medial Toward the middle of the body, away from the side.

neuraxis An imaginary line drawn through the center of the length of the central nervous system, from the bottom of the spinal cord to the front of the forebrain.

anterior With respect to the central nervous system, located near or toward the head.

posterior With respect to the central nervous system, located near or toward the tail.

rostral “Toward the beak”; with respect to the central nervous system, in a direction along the neuraxis toward the front of the face.

caudal “Toward the tail”; with respect to the central nervous system, in a direction along the neuraxis away from the front of the face.

dorsal “Toward the back”; with respect to the central nervous system, in a direction perpendicular to the neuraxis toward the top of the head or the back.

ventral “Toward the belly”; with respect to the central nervous system, in a direction perpendicular to the neuraxis toward the bottom of the skull or the front surface of the body.

Two other useful terms are ipsilateral and contralateral. Ipsilateral refers to structures on the same side of the body. If we say that the olfactory bulb sends axons to the ipsilateral hemisphere, we mean that the left olfactory bulb sends axons to the left hemisphere and the right olfactory bulb sends axons to the right hemisphere. Contralateral refers to structures on opposite sides of the body. If we say that a particular region of the left cerebral cortex controls movements of the contralateral hand, we mean that the region controls movements of the right hand.

ipsilateral Located on the same side of the body.

contralateral Located on the opposite side of the body.

To see what is in the nervous system, we have to cut it open; to be able to convey information about what we find, we slice it in a standard way. Figure 3.2 shows a human nervous system. We can slice the nervous system in three ways:

· 1. Transversely, like a salami, giving us cross sections (also known as frontal sections when referring to the brain)

frontal section A slice through the brain parallel to the forehead.

cross section With respect to the central nervous system, a slice taken at right angles to the neuraxis.

· 2. Parallel to the ground, giving us horizontal sections

horizontal section A slice through the brain parallel to the ground.

· 3. Perpendicular to the ground and parallel to the neuraxis, giving us sagittal sections . The midsagittal plane divides the brain into two symmetrical halves. The sagittal section in Figure 3.2 lies in the midsagittal plane.

sagittal section (sadj i tul ) A slice through the brain parallel to the neuraxis and perpendicular to the ground.

midsagittal plane The plane through the neuraxis perpendicular to the ground; divides the brain into two symmetrical halves.

Note that because of our upright posture, cross sections of the spinal cord are parallel to the ground. (See Figure 3.2 . )

FIGURE 3.2 Brain Slices and Planes

This figure shows planes of section as they pertain to the human central nervous system.

An Overview

The nervous system consists of the brain and spinal cord, which make up the central nervous system (CNS), and the cranial nerves, spinal nerves, and peripheral ganglia, which constitute the peripheral nervous system (PNS). The CNS is encased in bone: The brain is covered by the skull, and the spinal cord is encased by the vertebral column. (See Table 3.1 . )

Figure 3.3 illustrates the relationship of the brain and spinal cord to the rest of the body. Do not be concerned with unfamiliar labels on this figure; these structures will be described later. (See Figure 3.3 . ) The brain is a large mass of neurons, glia, and other supporting cells. It is the most protected organ of the body, encased in a tough, bony skull and floating in a pool of cerebrospinal fluid. The brain receives a copious supply of blood and is chemically guarded by the blood–brain barrier.

The brain receives approximately 20 percent of the blood flow from the heart, and it receives it continuously. Other parts of the body, such as the skeletal muscles or digestive system, receive varying quantities of blood, depending on their needs, relative to those of other regions. But the brain always receives its share. The brain can store only a small amount of its fuel (primarily glucose), and it cannot temporarily extract energy without oxygen, as the muscles can; therefore, a consistent blood supply is essential. A 1-second interruption of the blood flow to the brain uses up much of the dissolved oxygen; a 6-second interruption produces unconsciousness. Permanent damage begins within a few minutes.

Meninges

The entire nervous system—brain, spinal cord, cranial and spinal nerves, and peripheral ganglia—is covered by tough connective tissue. The protective sheaths around the brain and spinal cord are referred to as the meninges (singular: meninx, the Greek word for “membrane”). The meninges consist of three layers, which are shown in Figure 3.3 . The outer layer is thick, tough, and flexible but unstretchable; its name, dura mater , means “hard mother.” The middle layer of the meninges, the arachnoid membrane , gets its name from the weblike appearance of the arachnoid trabeculae that protrude from it (from the Greek arachne, meaning “spider”; trabecula means “track”). The arachnoid membrane, soft and spongy, lies beneath the dura mater. Closely attached to the brain and spinal cord, and following every surface convolution, is the pia mater (“pious mother”). The smaller surface blood vessels of the brain and spinal cord are contained within this layer. Between the pia mater and arachnoid membrane is a gap called the subarachnoid space . This space is filled with a liquid called cerebrospinal fluid (CSF) . (Look again at Figure 3.3 . )

meninges (singular: meninx) ( men in jees ) The three layers of tissue that encase the central nervous system: the dura mater, arachnoid membrane, and pia mater.

dura mater The outermost of the meninges; tough and flexible.

arachnoid membrane ( a rak noyd ) The middle layer of the meninges, located between the outer dura mater and inner pia mater.

pia mater The layer of the meninges that clings to the surface of the brain; thin and delicate.

subarachnoid space The fluid-filled space that cushions the brain; located between the arachnoid membrane and the pia mater.

cerebrospinal fluid (CSF) A clear fluid, similar to blood plasma, that fills the ventricular system of the brain and the subarachnoid space surrounding the brain and spinal cord.

TABLE 3.1 The Major Divisions of the Nervous System

Central Nervous System (CNS)	Peripheral Nervous System (PNS)
Brain	Nerves
Spinal cord	Peripheral ganglia

The peripheral nervous system (PNS) is covered with two layers of meninges. The middle layer (arachnoid membrane), with its associated pool of CSF, covers only the brain and spinal cord. Outside the central nervous system, the outer and inner layers (dura mater and pia mater) fuse and form a sheath that covers the spinal and cranial nerves and the peripheral ganglia.

In the first edition of this book I said that I did not know why the outer and inner layers of the meninges were referred to as “mothers.” I received a letter from medical historians at the Department of Anatomy at UCLA that explained the name. (Sometimes, it pays to proclaim one’s ignorance.) A tenth-century Persian physician, Ali ibn Abbas, used the Arabic term al umm to refer to the meninges. The term literally means “mother” but was used to designate any swaddling material, because Arabic lacked a specific term for the word membrane. The tough outer membrane was called al umm al djafiya, and the soft inner one was called al umm al rigiga. When the writings of Ali ibn Abbas were translated into Latin during the eleventh century, the translator, who was probably not familiar with the structure of the meninges, made a literal translation of al umm. He referred to the membranes as the “hard mother” and the “pious mother” (pious in the sense of “delicate”) rather than using a more appropriate Latin word.

FIGURE 3.3 The Nervous System

The figures show (a) the relation of the nervous system to the rest of the body, (b) detail of the meninges that cover the central nervous system, and (c) a closer view of the lower spinal cord and cauda equina.

The Ventricular System and Production of CSF

The brain is very soft and jellylike. The considerable weight of a human brain (approximately 1400 g), along with its delicate construction, necessitates that it be protected from shock. A human brain cannot even support its own weight well; it is difficult to remove and handle a fresh brain from a recently deceased human without damaging it.

Fortunately, the intact brain within a living human is well protected. It floats in a bath of CSF contained within the subarachnoid space. Because the brain is completely immersed in liquid, its net weight is reduced to approximately 80 g; thus, pressure on the base of the brain is considerably diminished. The CSF surrounding the brain and spinal cord also reduces the shock to the central nervous system that would be caused by sudden head movement.

FIGURE 3.4 The Ventricular System of the Brain

The figure shows (a) a lateral view of the left side of the brain, (b) a frontal view, (c) a dorsal view, and (d) the production, circulation, and reabsorption of cerebrospinal fluid.

The brain contains a series of hollow, interconnected chambers called ventricles (“little bellies”), which are filled with CSF. (See Figure 3.4 . ) The largest chambers are the lateral ventricles , which are connected to the third ventricle . The third ventricle is located at the midline of the brain; its walls divide the surrounding part of the brain into symmetrical halves. A bridge of neural tissue called the massa intermedia crosses through the middle of the third ventricle and serves as a convenient reference point. The cerebral aqueduct , a long tube, connects the third ventricle to the fourth ventricle . The lateral ventricles constitute the first and second ventricles, but they are never referred to as such. (Look again at Figure 3.4 . )

ventricle (ven trik ul ) One of the hollow spaces within the brain, filled with cerebrospinal fluid.

lateral ventricle One of the two ventricles located in the center of the telencephalon.

third ventricle The ventricle located in the center of the diencephalon.

cerebral aqueduct A narrow tube interconnecting the third and fourth ventricles of the brain, located in the center of the mesencephalon.

fourth ventricle The ventricle located between the cerebellum and the dorsal pons, in the center of the metencephalon.

FIGURE 3.5 A Scanning Electron Micrograph of the Choroid Plexus

BV = blood vessel, CE = choroid plexus, V = ventricle.

(From Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, by Richard G. Kessel and Randy H. Kardon. Copyright © 1979 by W. H. Freeman and Co. Reprinted by permission of Barbara Kessel and Randy Kardon.)

Cerebrospinal fluid is extracted from the blood and resembles blood plasma in its composition. CSF is manufactured by special tissue with an especially rich blood supply called the choroid plexus , which protrudes into all four of the ventricles. CSF is produced continuously; the total volume of CSF is approximately 125 ml, and the half-life (the time it takes for half of the CSF present in the ventricular system to be replaced by fresh fluid) is about 3 hours. Therefore, several times this amount is produced by the choroid plexus each day. The continuous production of CSF means that there must be a mechanism for its removal. The production, circulation, and reabsorption of CSF are illustrated in Figure 3.4d . A scanning electron micrograph of the choroid plexus is shown in Figure 3.5 .

choroid plexus The highly vascular tissue that protrudes into the ventricles and produces cerebrospinal fluid.

Figure 3.4(d) shows a slightly rotated midsagittal view of the central nervous system, which shows only the right lateral ventricle (because the left hemisphere has been removed). Cerebrospinal fluid is produced by the choroid plexus of the lateral ventricles, and it flows into the third ventricle. More CSF is produced in this ventricle, which then flows through the cerebral aqueduct to the fourth ventricle, where still more CSF is produced. The CSF leaves the fourth ventricle through small openings that connect with the subarachnoid space surrounding the brain. The CSF then flows through the subarachnoid space around the central nervous system, where it is reabsorbed into the blood supply through the arachnoid granulations . These pouch-shaped structures protrude into the superior sagittal sinus , a blood vessel that drains into the veins serving the brain. (Look again at Figure 3.4d and Simulate meninges and CSF on MyPsychLab.)

arachnoid granulation Small projections of the arachnoid membrane through the dura mater into the superior sagittal sinus; CSF flows through them to be reabsorbed into the blood supply.

superior sagittal sinus A venous sinus located in the midline just dorsal to the corpus callosum, between the two cerebral hemispheres.

Occasionally, the flow of CSF is interrupted at some point in its route of passage. For example, a brain tumor growing in the midbrain may push against the cerebral aqueduct, blocking its flow, or an infant may be born with a cerebral aqueduct that is too small to accommodate a normal flow of CSF. This occlusion results in greatly increased pressure within the ventricles, because the choroid plexus continues to produce CSF. The walls of the ventricles then expand and produce a condition known as obstructive hydrocephalus (hydrocephalus literally means “water-head”). If the obstruction remains and if nothing is done to reverse the increased intracerebral pressure, blood vessels will be occluded, and permanent—perhaps fatal—brain damage will occur. Fortunately, a surgeon can usually operate on the person, drilling a hole through the skull and inserting a shunt tube into one of the ventricles. The tube is then placed beneath the skin and connected to a pressure relief valve that is implanted in the abdominal cavity. When the pressure in the ventricles becomes excessive, the valve permits the CSF to escape into the abdomen, where it is eventually reabsorbed into the blood supply. (See Figure 3.6 . )

obstructive hydrocephalus A condition in which all or some of the brain’s ventricles are enlarged; caused by an obstruction that impedes the normal flow of CSF.

SECTION SUMMARY: Basic Features of the Nervous System

Anatomists have adopted a set of terms to describe the locations of parts of the body. Anterior is toward the head, posterior is toward the tail, lateral is toward the side, medial is toward the middle, dorsal is toward the back, and ventral is toward the front surface of the body. In the special case of the nervous system, rostral means toward the beak (or nose), and caudal means toward the tail. Ipsilateral means “same side,” and contralateral means “other side.” A cross section (or, in the case of the brain, a frontal section) slices the nervous system at right angles to the neuraxis, a horizontal section slices the brain parallel to the ground, and a sagittal section slices it perpendicular to the ground, parallel to the neuraxis.

The central nervous system consists of the brain and spinal cord, and the peripheral nervous system consists of the spinal and cranial nerves and peripheral ganglia. The CNS is covered with the meninges: dura mater, arachnoid membrane, and pia mater. The space under the arachnoid membrane is filled with cerebrospinal fluid, in which the brain floats. The PNS is covered with only the dura mater and pia mater. Cerebrospinal fluid is produced in the choroid plexus of the lateral, third, and fourth ventricles. It flows from the two lateral ventricles into the third ventricle, through the cerebral aqueduct into the fourth ventricle, then into the subarachnoid space, and finally back into the blood supply through the arachnoid granulations. If the flow of CSF is blocked by a tumor or other obstruction, the result is hydrocephalus: enlargement of the ventricles and subsequent brain damage.

FIGURE 3.6 Hydrocephalus in an Infant

A surgeon places a shunt tube in a lateral ventricle, which permits cerebrospinal fluid to escape to the abdominal cavity, where it is absorbed into the blood supply. A pressure valve regulates the flow of CSF through the shunt.

The Central Nervous System

Although the brain is exceedingly complicated, an understanding of the basic features of brain development makes it easier to learn and remember the location of the most important structures. With that end in mind, I introduce these features here in the context of development of the central nervous system.

Development of the Central Nervous System

The central nervous system begins early in embryonic life as a hollow tube and maintains this basic shape even after it is fully developed. During development, parts of the tube elongate, pockets and folds form, and the tissue around the tube thickens until the brain reaches its final form.

AN OVERVIEW OF BRAIN DEVELOPMENT

Development of the human nervous system begins around the eighteenth day after conception. Part of the ectoderm (outer layer) of the back of the embryo thickens and forms a plate. The edges of this plate form ridges that curl toward each other along a longitudinal line, running in a rostral–caudal direction. By the twenty-first day these ridges touch each other and fuse together, forming a tube—the neural tube —which gives rise to the brain and spinal cord. The top part of the ridges break away from the neural tube and become the ganglia of the autonomic nervous system, described later in this chapter. (See Figure 3.7 . )

neural tube A hollow tube, closed at the rostral end, that forms from ectodermal tissue early in embryonic development; serves as the origin of the central nervous system.

FIGURE 3.7 Neural Plate Development

The figure shows development of the neural plate into the neural tube, which gives rise to the brain and spinal cord. Left: Dorsal views. Right: Cross section at levels indicated by dashed lines.

By the twenty-eighth day of development the neural tube is closed, and its rostral end has developed three interconnected chambers. These chambers become ventricles, and the tissue that surrounds them becomes the three major parts of the brain: the fore-brain, the midbrain, and the hindbrain. (See Figures 3.8a and 3.8c . ) As development progresses, the rostral chamber (the forebrain) divides into three separate parts, which become the two lateral ventricles and the third ventricle. The region around the lateral ventricles becomes the telencephalon (“end brain”), and the region around the third ventricle becomes the diencephalon (“interbrain”). (See Figures 3.8b and 3.8d . ) In its final form, the chamber inside the midbrain (mesencephalon) becomes narrow, forming the cerebral aqueduct, and two structures develop in the hindbrain: the metencephalon (“afterbrain”) and the myelencephalon (“marrowbrain”). (See Figure 3.8e . )

Table 3.2 summarizes the terms I have introduced here and mentions some of the major structures found in each part of the brain. The colors in the table match those in Figure 3.8 . These structures will be described in the remainder of the chapter. (See Table 3.2 . )

PRENATAL BRAIN DEVELOPMENT

Brain development begins with a thin tube and ends with a structure weighing approximately 1400 g (about 3 lb) and consisting of several hundreds of billions of cells. Where do these cells come from, and what controls their growth?

Let’s consider the development of the cerebral cortex, about which most is known. The principles described here are similar to the ones that apply to development of other regions of the brain. (For details of this process, see Cooper, 2008 , and Rakic, 2009 .) Cortex means “bark,” and the cerebral cortex , approximately 3 mm thick, surrounds the cerebral hemispheres like the bark of a tree. Corrected for body size, the cerebral cortex is larger in humans than in any other species. As we will see later in this book, circuits of neurons in the cerebral cortex play a vital role in perception, cognition, and control of movement.

cerebral cortex The outermost layer of gray matter of the cerebral hemispheres.

Stem cells that line the inside of the neural tube give rise to the cells of the central nervous system. The cerebral cortex develops from the inside out. That is, the first cells to be produced migrate a short distance and establish the first—and deepest—layer. The next wave of newborn cells passes through the first layer and forms the second one—and so on, until all six layers of the cerebral cortex are laid down. The last cells to be produced must pass through all the ones born before them.

FIGURE 3.8 Brain Development

This schematic outline of brain development shows its relation to the ventricles. Views (a) and (c) show early development. Views (b) and (d) show later development. View (e) shows a lateral view of the left side of a semitransparent human brain with the brain stem “ghosted in.” The colors of all figures denote corresponding regions.

TABLE 3.2 Anatomical Subdivisions of the Brain

Major division	Ventricle	Subdivision	Principal structures
Forebrain	Lateral	Telencephalon	Cerebral cortex
			Basal ganglia
			Limbic system
	Third	Diencephalon	Thalamus
			Hypothalamus
Midbrain	Cerebral aqueduct	Mesencephalon	Tectum Tegmentum
Hindbrain	Fourth	Metencephalon	Cerebellum
			Pons
		Myelencephalon	Medulla oblongata

The stem cells that give rise to the cells of the brain are known as progenitor cells . (A progenitor is a direct ancestor of a line of descendants.) During the first phase of development, progenitor cells in the ventricular zone (VZ) , located just outside the wall of the neural tube, divide, making new progenitor cells and increasing the size of the ventricular zone. Some progenitor cells migrate a short distance away from the ventricular zone, where they continue to divide into more progenitor cells and establish the subventricular zone (SVZ) . This phase is referred to as symmetrical division , because the division of each progenitor cell produces two new progenitor cells. This form of division increases the size of the ventricular and subventricular zones. Then, seven weeks after conception, progenitor cells receive a signal to begin a period of asymmetrical division . During this phase, progenitor cells form two different kinds of cells as they divide: another progenitor cell and a brain cell.

progenitor cells Cells of the ventricular zone that divide and give rise to cells of the central nervous system.

ventricular zone (VZ) A layer of cells that line the inside of the neural tube; contains progenitor cells that divide and give rise to cells of the central nervous system.

subventricular zone (SVZ) A layer of progenitor cells located just inside the ventricular zone; thicker in mammals with large brains.

symmetrical division Division of a progenitor cell that gives rise to two identical progenitor cells; increases the size of the ventricular zone and hence the brain that develops from it.

asymmetrical division Division of a progenitor cell that gives rise to another progenitor cell and a neuron, which migrates away from the ventricular zone toward its final resting place in the brain.

The first brain cells produced through asymmetrical division are radial glia . The cell bodies of radial glia remain close to the wall of the neural tube, in the VZ and SVZ, but they extend fibers radially outward from the ventricular zone, like spokes in a wheel. These fibers end in cuplike feet that attach to the pia mater, located at the outer surface of what becomes the cerebral cortex. As the cortex becomes thicker, the fibers of the radial glia grow longer and maintain their connections with the pia mater. (See Figure 3.9 . )

radial glia Special glia with fibers that grow radially outward from the ventricular zone to the surface of the cortex; provide guidance for neurons migrating outward during brain development.

FIGURE 3.9 Cortical Development

This cross section through the cerebral cortex shows it early in its development. The radially oriented fibers of glial cells help to guide the migration of newly formed neurons from the ventricular zone to their final resting place in the cerebral cortex. Each successive wave of neurons passes neurons that migrated earlier, so the most recently formed neurons occupy layers closer to the cortical surface.

(Adapted from Rakic, P. Trends in Neuroscience, 1995, 18, 383–388.)

The period of asymmetrical division lasts about three months. Because the human cerebral cortex contains about 100 billion neurons, there are about one billion neurons migrating along radial glial fibers on a given day. The migration path of the earliest neurons is the shortest and takes about one day. The neurons that produce the last, outermost layer have to pass through five layers of neurons, and their migration takes about two weeks. The end of cortical development occurs when the progenitor cells receive a chemical signal that causes them to die—a phenomenon known as apoptosis (literally, a “falling away”). Molecules of the chemical that conveys this signal bind with receptors that activate killer genes within the cells. (All cells have these genes, but only certain cells possess the receptors that respond to the chemical signals that turn them on.) At this time, radial glia are transformed into astrocytes.

apoptosis (ay po toe sis) Death of a cell caused by a chemical signal that activates a genetic mechanism inside the cell.

The brains of the earliest vertebrates were smaller than those of later animals and were simpler as well. The evolutionary process brought about genetic changes that were responsible for the development of more complex brains, with more parts and more interconnections. An important factor in the evolution of more complex brains is genetic duplication (Allman, 1999 ). As Lewis ( 1992 ) noted, most of the genes that a species possesses perform important functions. If a mutation causes one of these genes to do something new, the previous function would be lost, and the animal might not survive. However, geneticists have discovered that genes can sometimes duplicate themselves, and if these duplications occur in cells that give rise to ova or sperms, the duplication can be passed on to the organism’s offspring. This means that the offspring will have one gene to perform the important functions and another one to “experiment” with. If a mutation of the extra gene occurs, the old gene is still present and its important function is still performed.

As we saw in Chapter 1 , the human brain is larger than that of any other large animal when corrected for body size—more than three times larger than that of a chimpanzee, our closest relative. What types of genetic changes are required to produce a large brain?

Rakic ( 1988 , 2009 ) suggests that the size differences between these two brains could be caused by a very simple process. We just saw that the size of the ventricular zone increases during symmetrical division of the progenitor cells located there. The ultimate size of the brain is determined by the size of the ventricular zone. As Rakic notes, each symmetrical division doubles the number of progenitor cells and thus doubles the size of the brain. The human brain is ten times larger than that of a rhesus macaque monkey. Thus, between three and four additional symmetrical divisions of progenitor cells would account for the difference in the size of these two brains. In fact, the stage of symmetrical division lasts about two days longer in humans, which provides enough time for three more divisions. The period of asymmetrical division is longer, too, which accounts for the fact that the human cortex is 15 percent thicker. Thus, delays in the termination of the symmetrical and asymmetrical periods of development could be responsible for the increased size of the human brain. A few simple mutations of the genes that control the timing of brain development could be responsible for these delays.

The process I have just described explains the development of the brains of small mammals such as rodents. These brains have a smooth outer surface, which limits the size of the cerebral cortex that cover them. Larger brains, especially those of the larger primates, have convoluted brains—brains with a surface covered by grooves and bulges. Convolutions greatly increase the surface area of the cerebral cortex, which means that the cortex of a convoluted brain contains many more neurons than that of a smooth brain. The increased number of neurons in the convoluted human cerebral cortex makes possible the complex circuitry found in our brains.

Two studies appear to have discovered an important aspect of the process responsible for the development of convoluted brains. The subventricular zone of convoluted brains is much thicker than that of smooth brains. In fact, this zone can be divided into two parts, the inner SVZ and the outer SVZ. (The inner SVZ is located closer to the wall of the neural tube, and the outer SVZ is located closer to the surface of the brain.) In smooth-brained animals such as rodents, all of the cells of the brain derive from progenitor cells located in the ventricular and subventricular zones. Because the cell bodies of the radial glia that develop from the progenitor cells are locked in place, the surface of the developing cortex remains more or less parallel to the wall of the neural tube, which means that it will remain smooth. Fietz et al. ( 2010 ) and Hansen et al. ( 2010 ) found that, during development of the human brain, some newborn progenitor cells migrated into the inner SVZ, positioning themselves between the fibers of the radial glia whose cell bodies were anchored in place. These unattached progenitor cells undergo asymmetrical division, sending neurons into the upper layer of the developing cortex. This source of neurons increases the numbers of cells in the cerebral cortex, which forces it to bend and fold into convolutions. The genes that control this process have not yet been discovered.

Once neurons have migrated to their final locations, they begin forming connections with other neurons. They grow dendrites, which receive the terminal buttons from the axons of other neurons, and they grow axons of their own. Some neurons extend their dendrites and axons laterally, connecting adjacent columns of neurons or even establishing connections with other neurons in distant regions of the brain. The growth of axons is guided by physical and chemical factors. Once the growing ends of the axons (the growth cones) reach their targets, they form numerous branches. Each of these branches finds a vacant place on the membrane of the appropriate type of postsynaptic cell, grows a terminal button, and establishes a synaptic connection. Apparently, different types of cells—or even different parts of a single cell—secrete different chemicals, which attract different types of axons (Benson, Colman, and Huntley, 2001 ). Of course, the establishment of a synaptic connection also requires efforts on the part of the postsynaptic cell; this cell must contribute its parts of the synapse, including the postsynaptic receptors. The chemical signals that the cells exchange to tell one another to establish these connections are just now being discovered.

The ventricular zone gives rise to more neurons than are needed. In fact, these neurons must compete to survive. The axons of approximately 50 percent of these neurons do not find vacant postsynaptic cells of the right type with which to form synaptic connections, so they die by apoptosis. This phenomenon, too, involves a chemical signal; when a presynaptic neuron establishes synaptic connections, it receives a signal from the postsynaptic cell that permits it to survive. The neurons that come too late do not find any available space and therefore do not receive this life-sustaining signal. This scheme might seem wasteful, but apparently the evolutionary process found that the safest strategy was to produce too many neurons and let them fight to establish synaptic connections rather than trying to produce exactly the right number of each type of neuron.

POSTNATAL BRAIN DEVELOPMENT

Brain development continues after an animal is born. In fact, the human brain continues to develop for at least two decades, and subtle changes—for example those produced by learning experiences—continue to occur throughout life.

As we will see later in this chapter, different regions of the cerebral cortex perform specialized functions. Some receive and analyze visual information, some receive and analyze auditory information, some control movement of the muscles, and so on. Thus, different regions receive different inputs, contain different types of circuits of neurons, and have different outputs. What factors control this pattern of development?

Some of the specialization is undoubtedly programmed genetically. The neurons produced by the asymmetrical division of a particular progenitor cell all follow a particular radial glial fiber, so they end up more or less above the progenitor cell. Thus, if the progenitor cells in different regions of the developing brain are themselves different, the neurons they produce will reflect these differences.

Experience also affects brain development. For example, one cue for depth perception arises from the fact that each eye gets a slightly different view of the world (Poggio and Poggio, 1984 ). This form of depth perception, stereopsis (“solid appearance”), is the kind obtained from a stereoscope or a three-dimensional movie. The particular neural circuits that are necessary for stereopsis, which are located in the cerebral cortex, will not develop unless an infant has experience viewing objects with both eyes during a critical period early in life. If an infant’s eyes do not move together properly—if they are not directed toward the same place in the environment (that is, if the eyes are “crossed”)—the infant never develops stereoscopic vision, even if the eye movements are later corrected by surgery on the eye muscles. This critical period occurs some time between one and three years of age (Banks, Aslin, and Letson, 1975 ). Similar phenomena have been studied in laboratory animals and have confirmed that sensory input affects the connections established between cortical neurons.

Evidence indicates that a certain amount of neural rewiring can be accomplished even in the adult brain. For example, after a person’s arm has been amputated, the region of the cerebral cortex that previously analyzed sensory information from the missing limb soon begins analyzing information from adjacent regions of the body, such as the stump of the arm, the trunk, or the face. In fact, the person becomes more sensitive to touch in these regions after the changes in the cortex take place (Elbert et al., 1994 ; Kew et al., 1994 ; Yang et al., 1994 ). In addition, musicians who play stringed instruments have a larger cortical region devoted to analysis of sensory information from the fingers of the left hand (which they use to press the strings), and when a blind person who can read Braille touches objects with his or her fingertips, an enlarged region of the cerebral cortex is activated (Elbert et al., 1995 ; Sadato et al., 1996 ).

For many years, researchers have believed that neurogenesis —production of new neurons—cannot take place in the fully developed brain. However, more recent studies have shown this belief to be incorrect—the adult brain contains some stem cells (similar to the progenitor cells that give rise to the cells of the developing brain) that can divide and produce neurons. Detection of newly produced cells is done by administering a small amount of a radioactive form of one of the nucleotide bases that cells use to produce the DNA that is needed for neurogenesis. The next day, the animals’ brains are removed and examined with methods described in Chapter 5 . Such studies have found evidence for neurogenesis in just two parts of the adult brain: the hippocampus, primarily involved in learning, and the olfactory bulb, involved in the sense of smell (Doetsch and Hen, 2005 ). Evidence indicates that exposure to new odors can increase the survival rate of new neurons in the olfactory bulbs, and training on a learning task can enhance neurogenesis in the hippocampus. (See Figure 3.10 . ) Chapter 13 has more to say about the role of neurogenesis in learning. In addition, as we will see in Chapter 16 , depression or exposure to stress can suppress neurogenesis in the hippocampus, and drugs that reduce stress and depression can reinstate neurogenesis. Unfortunately, there is no evidence that growth of new neurons can repair the effects of brain damage, such as that caused by head injury or strokes.

FIGURE 3.10 Effects of Learning on Neurogenesis

The figure shows sections through a part of the hippocampus of rats that received training on a learning task or were exposed to a control condition that did not lead to learning. Arrows indicate newly formed cells.

(From Leuner, B., Mendolia-Loffredo, S., Kozorovitskiy, Y., Samburg, D., Gould, E., and Shors, T.J. Journal of Neuroscience, 2004, 24, 7477–7481. Reprinted with permission.)

neurogenesis Production of new neurons through the division of neural stem cells; occurs in the hippocampus and olfactory bulb and appears to play a role in learning.

The Forebrain

As we saw, the forebrain surrounds the rostral end of the neural tube. Its two major components are the telencephalon and the diencephalon.

forebrain The most rostral of the three major divisions of the brain; includes the telencephalon and diencephalon.

TELENCEPHALON

The telencephalon includes most of the two symmetrical cerebral hemispheres that make up the cerebrum. The cerebral hemispheres are covered by the cerebral cortex and contain the limbic system and the basal ganglia. The latter two sets of structures are primarily in the subcortical regions of the brain—those located deep within it, beneath the cerebral cortex.

cerebral hemisphere ( sa ree brul ) One of the two major portions of the forebrain, covered by the cerebral cortex.

subcortical region The region located within the brain, beneath the cortical surface.

Cerebral Cortex.

As we saw in the previous section, the cerebral cortex surrounds the cerebral hemispheres like the bark of a tree. In humans the cerebral cortex is greatly convoluted; these convolutions, consisting of sulci (small grooves), fissures (large grooves), and gyri (bulges between adjacent sulci or fissures), greatly enlarge the surface area of the cortex, compared with a smooth brain of the same size. In fact, two-thirds of the surface of the cortex is hidden in the grooves; thus, the presence of these convolutions triples the area of the cerebral cortex. The total surface area is approximately 2360 cm2 (2.5 ft2), and the thickness is approximately 3 mm. The cerebral cortex consists mostly of glia and the cell bodies, dendrites, and interconnecting axons of neurons. Because cell bodies predominate, giving the cerebral cortex a grayish tan appearance, it is referred to as gray matter. (See Figure 3.11 . ) Beneath the cerebral cortex run millions of axons that connect the neurons of the cerebral cortex with those located elsewhere in the brain. The large concentration of myelin gives this tissue an opaque white appearance—hence the term white matter.

fissure A major groove in the surface of the brain, larger than a sulcus.

gyrus (plural: gyri) (jye russ, jye rye ) A convolution of the cortex of the cerebral hemispheres, separated by sulci or fissures.

Three areas of the cerebral cortex receive information from the sensory organs. The primary visual cortex , which receives visual information, is located at the back of the brain, on the inner surfaces of the cerebral hemispheres—primarily on the upper and lower banks of the calcarine fissure . (Calcarine means “spur-shaped.” See Figure 3.12 . ) The primary auditory cortex , which receives auditory information, is located on the lower surface of a deep fissure in the side of the brain—the lateral fissure . (See the inset in Figure 3.12 . ) The primary somatosensory cortex , a vertical strip of cortex just caudal to the central sulcus , receives information from the body senses. As Figure 3.12 shows, different regions of the primary somatosensory cortex receive information from different regions of the body. In addition, the base of the somatosensory cortex and a portion of the insular cortex , which is normally hidden from view by the frontal and temporal lobes, receives information concerning taste. (Look again at Figure 3.12 . )

primary visual cortex The region of the posterior occipital lobe whose primary input is from the visual system.

calcarine fissure (kal ka rine ) A fissure located in the occipital lobe on the medial surface of the brain; most of the primary visual cortex is located along its upper and lower banks.

primary auditory cortex The region of the superior temporal lobe whose primary input is from the auditory system.

lateral fissure The fissure that separates the temporal lobe from the overlying frontal and parietal lobes.

primary somatosensory cortex The region of the anterior parietal lobe whose primary input is from the somatosensory system.

central sulcus The sulcus that separates the frontal lobe from the parietal lobe.

insular cortex (in sue lur ) A sunken region of the cerebral cortex that is normally covered by the rostral superior temporal lobe and caudal inferior frontal lobe.

With the exception of olfaction and gustation (taste), sensory information from the body or the environment is sent to primary sensory cortex of the contralateral hemisphere. Thus, the primary somatosensory cortex of the left hemisphere learns what the right hand is holding, the left primary visual cortex learns what is happening toward the person’s right, and so on.

FIGURE 3.11 Cross Section of the Human Brain

This brain slice shows fissures and gyri and the layer of cerebral cortex that follows these convolutions.

The region of the cerebral cortex that is most directly involved in the control of movement is the primary motor cortex , located just in front of the primary somatosensory cortex. Neurons in different parts of the primary motor cortex are connected to muscles in different parts of the body. The connections, like those of the sensory regions of the cerebral cortex, are contralateral; the left primary motor cortex controls the right side of the body and vice versa. Thus, if a surgeon places an electrode on the surface of the primary motor cortex and stimulates the neurons there with a weak electrical current, the result will be movement of a particular part of the body. Moving the electrode to a different spot will cause a different part of the body to move. (Look again at Figure 3.12 . ) I like to think of the strip of primary motor cortex as the keyboard of a piano, with each key controlling a different movement. (We will see shortly who the “player” of this piano is.)

primary motor cortex The region of the posterior frontal lobe that contains neurons that control movements of skeletal muscles.

The regions of primary sensory and motor cortex occupy only a small part of the cerebral cortex. The rest of the cerebral cortex accomplishes what is done between sensation and action: perceiving, learning and remembering, planning, and acting. These processes take place in the association areas of the cerebral cortex. The central sulcus provides an important dividing line between the rostral and caudal regions of the cerebral cortex. (Look once more at Figure 3.12 . ) The rostral region is involved in movement-related activities, such as planning and executing behaviors. The caudal region is involved in perceiving and learning.

Discussing the various regions of the cerebral cortex is easier if we have names for them. In fact, the cerebral cortex is divided into four areas, or lobes, named for the bones of the skull that cover them: the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. Of course, the brain contains two of each lobe, one in each hemisphere. The frontal lobe (the “front”) includes everything in front of the central sulcus. The parietal lobe (the “wall”) is located on the side of the cerebral hemisphere, just behind the central sulcus, caudal to the frontal lobe. The temporal lobe (the “temple”) juts forward from the base of the brain, ventral to the frontal and parietal lobes. The occipital lobe (from the Latin ob, “in back of,” and caput, “head”) lies at the very back of the brain, caudal to the parietal and temporal lobes. Figure 3.13 shows these lobes in three views of the cerebral hemispheres: a ventral view (a view from the bottom), a midsagittal view (a view of the inner surface of the right hemisphere after the left hemisphere has been removed), and a lateral view. (See Figure 3.13 . )

frontal lobe The anterior portion of the cerebral cortex, rostral to the parietal lobe and dorsal to the temporal lobe.

parietal lobe ( pa rye i tul ) The region of the cerebral cortex caudal to the frontal lobe and dorsal to the temporal lobe.

temporal lobe (tem por ul ) The region of the cerebral cortex rostral to the occipital lobe and ventral to the parietal and frontal lobes.

occipital lobe ( ok sip i tul ) The region of the cerebral cortex caudal to the parietal and temporal lobes.

FIGURE 3.12 The Primary Sensory Regions of the Brain

The figure shows a lateral view of the left side of a human brain and part of the inner surface of the right side. The inset shows a cutaway of part of the frontal lobe of the left hemisphere, permitting us to see the primary auditory cortex on the dorsal surface of the temporal lobe, which forms the ventral bank of the lateral fissure.

Each primary sensory area of the cerebral cortex sends information to adjacent regions, called the sensory association cortex . Circuits of neurons in the sensory association cortex analyze the information received from the primary sensory cortex; perception takes place there, and memories are stored there. The regions of the sensory association cortex located closest to the primary sensory areas receive information from only one sensory system. For example, the region closest to the primary visual cortex analyzes visual information and stores visual memories. Regions of the sensory association cortex located far from the primary sensory areas receive information from more than one sensory system; thus, they are involved in several kinds of perceptions and memories. These regions make it possible to integrate information from more than one sensory system. For example, we can learn the connection between the sight of a particular face and the sound of a particular voice. (Look again at Figure 3.13 . )

sensory association cortex Those regions of the cerebral cortex that receive information from the regions of primary sensory cortex.

If people sustain damage to the somatosensory association cortex, their deficits are related to somatosensation and to the environment in general; for example, they may have difficulty perceiving the shapes of objects that they can touch but not see, they may be unable to name parts of their bodies (see the following case), or they may have trouble drawing maps or following them. Destruction of the primary visual cortex causes blindness. However, although people who sustain damage to the visual association cortex will not become blind, they may be unable to recognize objects by sight. People who sustain damage to the auditory association cortex may have difficulty perceiving speech or even producing meaningful speech of their own. People who sustain damage to regions of the association cortex at the junction of the three posterior lobes, where the somatosensory, visual, and auditory functions overlap, may have difficulty reading or writing.

Mr. M., a city bus driver, stopped to let a passenger climb board. The passenger asked him a question, and Mr. M. suddenly realized that he didn’t understand what she was saying. He could hear her, but her words made no sense. He opened his mouth to reply. He made some sounds, but the look on the woman’s face told him that she couldn’t understand what he was trying to say. He turned off the engine and looked around at the passengers and tried to tell them to get some help. Although he was unable to say anything, they understood that something was wrong, and one of them called an ambulance.

An MRI scan showed that Mr. M. had sustained an intracerebral hemorrhage—a kind of stroke caused by rupture of blood vessels in the brain. The stroke had damaged his left parietal lobe. Mr. M. gradually regained the ability to talk and understand the speech of others, but some deficits remained. A colleague, Dr. D., and I studied Mr. M. several weeks after his stroke. The dialogue went something like this:

“Show me your hand.”

“My hand…my hand.” Looks at his arms, then touches his left forearm.

“Show me your chin.”

“My chin.” Looks at his arms, looks down, puts his hand on his abdomen.

“Show me your right elbow.”

“My right…” (points to the right with his right thumb) “elbow.” Looks up and down his right arm, finally touches his right shoulder.

As you can see, Mr. M. could understand that we were asking him to point out parts of his body and could repeat the names of the body parts when we spoke them, but he could not identify which body parts these names referred to. This strange deficit, which sometimes follows damage to the left parietal lobe, is called auto-topagnosia, or “poor knowledge of one’s own topography.” (A better term would be autotopanomia, or “poor knowledge of the names of one’s own topography,” but, then, no one asked me to choose the term.) The parietal lobes are involved with space: the right primarily with external space and the left with one’s body and personal space. I will say more about disorders such as this one in Chapter 14 , which deals with brain mechanisms of language.

FIGURE 3.13 The Four Lobes of the Cerebral Cortex

This figure shows the location of the four lobes, the primary sensory and motor cortex, and the association cortex. (a) Ventral view, from the base of the brain. (b) Midsagittal view, with the cerebellum and brain stem removed. (c) Lateral view.

Just as regions of the sensory association cortex of the posterior part of the brain are involved in perceiving and remembering, the frontal association cortex is involved in the planning and execution of movements. The motor association cortex (also known as the premotor cortex) is located just rostral to the primary motor cortex. This region controls the primary motor cortex; thus, it directly controls behavior. If the primary motor cortex is the keyboard of the piano, then the motor association cortex is the piano player. The rest of the frontal lobe, rostral to the motor association cortex, is known as the prefrontal cortex . This region of the brain is less involved with the control of movement and more involved in formulating plans and strategies.

motor association cortex The region of the frontal lobe rostral to the primary motor cortex; also known as the premotor cortex.

prefrontal cortex The region of the frontal lobe rostral to the motor association cortex.

Although the two cerebral hemispheres cooperate with each other, they do not perform identical functions. Some functions are lateralized—located primarily on one side of the brain. In general, the left hemisphere participates in the analysis of information—the extraction of the elements that make up the whole of an experience. This ability makes the left hemisphere particularly good at recognizing serial events—events whose elements occur one after the other—and controlling sequences of behavior. (In a few people, the functions of the left and right hemispheres are reversed.) The serial functions that are performed by the left hemisphere include verbal activities, such as talking, understanding the speech of other people, reading, and writing. These abilities are disrupted by damage to the various regions of the left hemisphere. (I will say more about language and the brain in Chapter 14 .)

In contrast, the right hemisphere is specialized for synthesis; it is particularly good at putting isolated elements together to perceive things as a whole. For example, our ability to draw sketches (especially of three-dimensional objects), read maps, and construct complex objects out of smaller elements depends heavily on circuits of neurons that are located in the right hemisphere. Damage to the right hemisphere disrupts these abilities.

We are not aware of the fact that each hemisphere perceives the world differently. Although the two cerebral hemispheres perform somewhat different functions, our perceptions and our memories are unified. This unity is accomplished by the corpus callosum , a large band of axons that connects corresponding parts of the cerebral cortex of the left and right hemispheres: The left and right temporal lobes are connected, the left and right parietal lobes are connected, and so on. Because of the corpus callosum, each region of the association cortex knows what is happening in the corresponding region of the opposite side of the brain. The corpus callosum also makes a few asymmetrical connections that link different regions of the two hemispheres. Figure 3.14 shows the bundles of axons that constitute the corpus callosum, obtained by means of diffusion tensor imaging, a special scanning method described in Chapter 5 .

corpus callosum ( ka loh sum ) A large bundle of axons that interconnects corresponding regions of the association cortex on each side of the brain.

FIGURE 3.14 Bundles of Axons in the Corpus Callosum

This figure, obtained by means of diffusion tensor imaging, shows bundles of axons in the corpus callosum that serve different regions of the cerebral cortex.

(From Hofer, S., and Frahm, J. NeuroImage, 2006, 32, 989–994. Reprinted with permission.)

Figure 3.15 shows a midsagittal view of the brain. The brain (and part of the spinal cord) has been sliced down the middle, dividing it into its two symmetrical halves. The left half has been removed, so we see the inner surface of the right half. The cerebral cortex that covers most of the surface of the cerebral hemispheres (including the frontal, parietal, occipital, and temporal lobes) is called the neocortex (“new” cortex, because it is of relatively recent evolutionary origin). Another form of cerebral cortex, the limbic cortex , is located around the medial edge of the cerebral hemispheres (limbus means “border”). The cingulate gyrus , an important region of the limbic cortex, can be seen in this figure. (See Figure 3.15 . ) In addition, if you look back at the top two drawings of Figure 3.13 , you will see that the limbic cortex occupies the regions that have not been colored in. (Refer back to Figure 3.13 . )

neocortex The phylogenetically newest cortex, including the primary sensory cortex, primary motor cortex, and association cortex.

limbic cortex Phylogenetically old cortex, located at the medial edge (“limbus”) of the cerebral hemispheres; part of the limbic system.

cingulate gyrus (sing yew lett) A strip of limbic cortex lying along the lateral walls of the groove separating the cerebral hemispheres, just above the corpus callosum.

FIGURE 3.15 A Midsagittal View of the Brain and Part of the Spinal Cord

Figure 3.15 also shows the corpus callosum. To slice the brain into its two symmetrical halves, one must slice through the middle of the corpus callosum. (Recall that I described the split-brain operation, in which the corpus callosum is severed, in Chapter 1 .) (Look again at Figure 3.15 . )

Simulate the rotatable brain on MyPsychLab to view the brain from various angles and see the locations of the specialized regions of the cerebral cortex. Simulate brain slices on MyPsychLabto see the internal structures of slices of human brains.

Limbic System.

A neuroanatomist, Papez ( 1937 ), suggested that a set of interconnected brain structures formed a circuit whose primary function was motivation and emotion. This system included several regions of the limbic cortex (already described) and a set of interconnected structures surrounding the core of the forebrain. A physiologist, MacLean ( 1949 ), expanded the system to include other structures and coined the term limbic system . Besides the limbic cortex, the most important parts of the limbic system are the hippocampus (“sea horse”) and the amygdala (“almond”), located next to the lateral ventricle in the temporal lobe. The fornix (“arch”) is a bundle of axons that connects the hippocampus with other regions of the brain, including the mammillary (“breast-shaped”) bodies, protrusions on the base of the brain that contain parts of the hypothalamus. (See Figure 3.16 . )

limbic system A group of brain regions including the anterior thalamic nuclei, amygdala, hippocampus, limbic cortex, and parts of the hypothalamus, as well as their interconnecting fiber bundles.

hippocampus A forebrain structure of the temporal lobe, constituting an important part of the limbic system; includes the hippocampus proper (Ammon’s horn), dentate gyrus, and subiculum.

amygdala ( a mig da la ) A structure in the interior of the rostral temporal lobe, containing a set of nuclei; part of the limbic system.

fornix A fiber bundle that connects the hippocampus with other parts of the brain, including the mammillary bodies of the hypothalamus; part of the limbic system.

mammillary bodies (mam i lair ee) A protrusion of the bottom of the brain at the posterior end of the hypothalamus, containing some hypothalamic nuclei; part of the limbic system.

MacLean noted that the evolution of this system, which includes the first and simplest form of cerebral cortex, appears to have coincided with the development of emotional responses. As you will see in Chapter 13 , we now know that parts of the limbic system (notably, the hippocampal formation and the region of limbic cortex that surrounds it) are involved in learning and memory. The amygdala and some regions of limbic cortex are specifically involved in emotions: feelings and expressions of emotions, emotional memories, and recognition of the signs of emotions in other people.

FIGURE 3.16 The Major Components of the Limbic System

All of the left hemisphere except for the limbic system has been removed.

Basal Ganglia.

The basal ganglia are a collection of subcortical nuclei in the forebrain, which lie beneath the anterior portion of the lateral ventricles. Nuclei are groups of neurons of similar shape. (The word nucleus , from the Greek “nut,” can refer to the inner portion of an atom, to the structure of a cell that contains the chromosomes, and—as in this case—to a collection of neurons located within the brain.) The major parts of the basal ganglia are the caudate nucleus, the putamen, and the globus pallidus (the “nucleus with a tail,” the “shell,” and the “pale globe”). (See Figure 3.17 . ) The basal ganglia are involved in the control of movement. For example, Parkinson’s disease is caused by degeneration of certain neurons located in the midbrain that send axons to the caudate nucleus and the putamen. The symptoms of this disease are weakness, tremors, rigidity of the limbs, poor balance, and difficulty in initiating movements.

basal ganglia A group of subcortical nuclei in the telencephalon, the caudate nucleus, the globus pallidus, and the putamen; important parts of the motor system.

nucleus (plural: nuclei) An identifiable group of neural cell bodies in the central nervous system.

DIENCEPHALON

The second major division of the forebrain, the diencephalon , is situated between the telencephalon and the mesencephalon; it surrounds the third ventricle. Its two most important structures are the thalamus and the hypothalamus. (See Figure 3.17 . )

diencephalon (dy en seff a lahn) A region of the forebrain surrounding the third ventricle; includes the thalamus and the hypothalamus.

Thalamus.

The thalamus (from the Greek thalamos, “inner chamber”) makes up the dorsal part of the diencephalon. It is situated near the middle of the cerebral hemispheres, immediately medial and caudal to the basal ganglia. The thalamus has two lobes, connected by a bridge of gray matter called the massa intermedia, which pierces the middle of the third ventricle. (Look again at Figure 3.17 . ) The massa intermedia is probably not an important structure, because it is absent in the brains of many people. However, it serves as a useful reference point in looking at diagrams of the brain; it appears in Figures 3.4 , 3.15 , 3.16 , and 3.19 .

thalamus The largest portion of the diencephalon, located above the hypothalamus; contains nuclei that project information to specific regions of the cerebral cortex and receive information from it.

FIGURE 3.17 The Basal Ganglia and Diencephalon

The basal ganglia and diencephalon (thalamus and hypothalamus) are ghosted in to a semitransparent brain.

Most neural input to the cerebral cortex is received from the thalamus; indeed, much of the cortical surface can be divided into regions that receive projections from specific parts of the thalamus. Projection fibers are sets of axons that arise from cell bodies located in one region of the brain and synapse on neurons located within another region (that is, they project to these regions).

projection fiber An axon of a neuron in one region of the brain whose terminals form synapses with neurons in another region.

The thalamus is divided into several nuclei. Some thalamic nuclei receive sensory information from the sensory systems. The neurons in these nuclei then relay the sensory information to specific sensory projection areas of the cerebral cortex. For example, the lateral geniculate nucleus receives information from the eye and sends axons to the primary visual cortex, and the medial geniculate nucleus receives information from the inner ear and sends axons to the primary auditory cortex. Other thalamic nuclei project to specific regions of the cerebral cortex, but they do not relay sensory information. For example, the ventrolateral nucleus receives information from the cerebellum and projects it to the primary motor cortex. Still other nuclei receive information from one region of the cerebral cortex and relay it to another region. And as we will see in Chapter 9 , several nuclei are involved in controlling the general excitability of the cerebral cortex. To accomplish this task, these nuclei have widespread projections to all cortical regions.

lateral geniculate nucleus A group of cell bodies within the lateral geniculate body of the thalamus that receives fibers from the retina and projects fibers to the primary visual cortex.

medial geniculate nucleus A group of cell bodies within the medial geniculate body of the thalamus; receives fibers from the auditory system and projects fibers to the primary auditory cortex.

ventrolateral nucleus A nucleus of the thalamus that receives inputs from the cerebellum and sends axons to the primary motor cortex.

Hypothalamus.

As its name implies, the hypothalamus lies at the base of the brain, under the thalamus. Although the hypothalamus is a relatively small structure, it is an important one. It controls the autonomic nervous system and the endocrine system and organizes behaviors related to survival of the species—the so-called four F’s: fighting, feeding, fleeing, and mating.

hypothalamus The group of nuclei of the diencephalon situated beneath the thalamus; involved in regulation of the autonomic nervous system, control of the anterior and posterior pituitary glands, and integration of species-typical behaviors.

The hypothalamus is situated on both sides of the ventral portion of the third ventricle. The hypothalamus is a complex structure, containing many nuclei and fiber tracts. Figure 3.18 indicates its location and size. Note that the pituitary gland is attached to the base of the hypothalamus via the pituitary stalk. Just in front of the pituitary stalk is the optic chiasm , where half of the axons in the optic nerves (from the eyes) cross from one side of the brain to the other. (See Figure 3.18 . ) The role of the hypothalamus in the control of the four F’s (and other behaviors, such as drinking and sleeping) will be considered in several chapters later in this book.

optic chiasm (kye az’m ) An X-shaped connection between the optic nerves, located below the base of the brain, just anterior to the pituitary gland.

FIGURE 3.18 A Midsagittal View of Part of the Brain

This view shows some of the nuclei of the hypothalamus. The nuclei are situated on the far side of the wall of the third ventricle, inside the right hemisphere.

Much of the endocrine system is controlled by hormones produced by cells in the hypothalamus. A special system of blood vessels directly connects the hypothalamus with the anterior pituitary gland . (See Figure 3.19 . ) The hypothalamic hormones are secreted by specialized neurons called neurosecretory cells , located near the base of the pituitary stalk. These hormones stimulate the anterior pituitary gland to secrete its hormones. For example, gonadotropin-releasing hormone causes the anterior pituitary gland to secrete the gonadotropic hormones, which play a role in reproductive physiology and behavior.

anterior pituitary gland The anterior part of the pituitary gland; an endocrine gland whose secretions are controlled by the hypothalamic hormones.

neurosecretory cell A neuron that secretes a hormone or hormone-like substance.

FIGURE 3.19 The Pituitary Gland

Hormones released by the neurosecretory cells in the hypothalamus enter capillaries and are conveyed to the anterior pituitary gland, where they control its secretion of hormones. The hormones of the posterior pituitary gland are produced in the hypothalamus and carried there in vesicles by means of axoplasmic transport.

Most of the hormones secreted by the anterior pituitary gland control other endocrine glands. Because of this function, the anterior pituitary gland has been called the body’s “master gland.” For example, the gonadotropic hormones stimulate the gonads (ovaries and testes) to release male or female sex hormones. These hormones affect cells throughout the body, including some in the brain. Two other anterior pituitary hormones—prolactin and somatotropic hormone (growth hormone)—do not control other glands but act as the final messenger. The behavioral effects of many of the anterior pituitary hormones are discussed in later chapters.

The hypothalamus also produces the hormones of the posterior pituitary gland and controls their secretion. These hormones include oxytocin, which stimulates ejection of milk and uterine contractions at the time of childbirth, and vasopressin, which regulates urine output by the kidneys. They are produced by neurons in the hypothalamus whose axons travel down the pituitary stalk and terminate in the posterior pituitary gland. The hormones are carried in vesicles through the axoplasm of these neurons and collect in the terminal buttons in the posterior pituitary gland. When these axons fire, the hormone contained within their terminal buttons is liberated and enters the circulatory system.

posterior pituitary gland The posterior part of the pituitary gland; an endocrine gland that contains hormone-secreting terminal buttons of axons whose cell bodies lie within the hypothalamus.

The Midbrain

The midbrain (also called the mesencephalon ) surrounds the cerebral aqueduct and consists of two major parts: the tectum and the tegmentum.

midbrain The mesencephalon; the central of the three major divisions of the brain.

mesencephalon ( mezz en seff a lahn ) The midbrain; a region of the brain that surrounds the cerebral aqueduct; includes the tectum and the tegmentum.

TECTUM

The tectum (“roof”) is located in the dorsal portion of the mesencephalon. Its principal structures are the superior colliculi and the inferior colliculi , which appear as four bumps on the dorsal surface of the brain stem . The brain stem includes the midbrain and the hindbrain, and it is called the brain stem because it looks just like that: a stem. Figure 3.20 shows several views of the brain stem: lateral and posterior views of the brain stem inside a semitransparent brain, an enlarged view of the brain stem with part of the cerebellum cut away to reveal the inside of the fourth ventricle, and a cross section through the midbrain. (See Figure 3.20 . ) The inferior colliculi are a part of the auditory system. The superior colliculi are part of the visual system. In mammals they are primarily involved in visual reflexes and reactions to moving stimuli.

tectum The dorsal part of the midbrain; includes the superior and inferior colliculi.

superior colliculi ( ka lik yew lee ) Protrusions on top of the midbrain; part of the visual system.

inferior colliculi Protrusions on top of the midbrain; part of the auditory system.

brain stem The “stem” of the brain, from the medulla to the midbrain, excluding the cerebellum.

TEGMENTUM

The tegmentum (“covering”) consists of the portion of the mesencephalon beneath the tectum. It includes the rostral end of the reticular formation, several nuclei controlling eye movements, the periaqueductal gray matter, the red nucleus, the substantia nigra, and the ventral tegmental area. (See Figure 3.20d . )

tegmentum The ventral part of the midbrain; includes the periaqueductal gray matter, reticular formation, red nucleus, and substantia nigra.

The reticular formation is a large structure consisting of many nuclei (over ninety in all). It is also characterized by a diffuse, interconnected network of neurons with complex dendritic and axonal processes. (Indeed, reticulum means “little net”; early anatomists were struck by the netlike appearance of the reticular formation.) The reticular formation occupies the core of the brain stem, from the lower border of the medulla to the upper border of the midbrain. (Look again at Figure 3.20d . ) The reticular formation receives sensory information by means of various pathways and projects axons to the cerebral cortex, thalamus, and spinal cord. It plays a role in sleep and arousal, attention, muscle tonus, movement, and various vital reflexes. Its functions will be described more fully in later chapters.

reticular formation A large network of neural tissue located in the central region of the brain stem, from the medulla to the diencephalon.

The periaqueductal gray matter is so called because it consists mostly of cell bodies of neurons (“gray matter,” as contrasted with the “white matter” of axon bundles) that surround the cerebral aqueduct as it travels from the third to the fourth ventricle. The periaqueductal gray matter contains neural circuits that control sequences of movements that constitute species-typical behaviors, such as fighting and mating. As we will see in Chapter 7 , opiates such as morphine decrease an organism’s sensitivity to pain by stimulating receptors on neurons located in this region.

periaqueductal gray matter The region of the midbrain surrounding the cerebral aqueduct; contains neural circuits involved in species-typical behaviors.

The red nucleus and substantia nigra (“black substance”) are important components of the motor system. A bundle of axons that arises from the red nucleus constitutes one of the two major fiber systems that bring motor information from the cerebral cortex and cerebellum to the spinal cord. The substantia nigra contains neurons whose axons project to the caudate nucleus and putamen, parts of the basal ganglia. As we will see in Chapter 4 , degeneration of these neurons causes Parkinson’s disease.

red nucleus A large nucleus of the midbrain that receives inputs from the cerebellum and motor cortex and sends axons to motor neurons in the spinal cord.

substantia nigra A darkly stained region of the tegmentum that contains neurons that communicate with the caudate nucleus and putamen in the basal ganglia.

The Hindbrain

The hindbrain , which surrounds the fourth ventricle, consists of two major divisions: the metencephalon and the myelencephalon.

hindbrain The most caudal of the three major divisions of the brain; includes the metencephalon and myelencephalon.

METENCEPHALON

The metencephalon consists of the pons and the cerebellum.

Cerebellum.

The cerebellum (“little brain”), with its two hemispheres, resembles a miniature version of the cerebrum. It is covered by the cerebellar cortex and has a set of deep cerebellar nuclei . These nuclei receive projections from the cerebellar cortex and themselves send projections out of the cerebellum to other parts of the brain. Each hemisphere of the cerebellum is attached to the dorsal surface of the pons by bundles of axons: the superior, middle, and inferior cerebellar peduncles (“little feet”). (See Figure 3.20c . )

cerebellum ( sair a bell um ) A major part of the brain located dorsal to the pons, containing the two cerebellar hemispheres, covered with the cerebellar cortex; an important component of the motor system.

cerebellar cortex The cortex that covers the surface of the cerebellum.

deep cerebellar nuclei Nuclei located within the cerebellar hemispheres; receive projections from the cerebellar cortex and send projections out of the cerebellum to other parts of the brain.

cerebellar peduncle (pee dun kul ) One of three bundles of axons that attach each cerebellar hemisphere to the dorsal pons.

Damage to the cerebellum impairs standing, walking, or performance of coordinated movements. (A virtuoso pianist or other performing musician owes much to his or her cerebellum.) The cerebellum receives visual, auditory, vestibular, and somatosensory information, and it also receives information about individual muscle movements being directed by the brain. The cerebellum integrates this information and modifies the motor outflow, exerting a coordinating and smoothing effect on the movements. Cerebellar damage results in jerky, poorly coordinated, exaggerated movements; extensive cerebellar damage makes it impossible even to stand. Chapter 8 discusses the anatomy and functions of the cerebellum in more detail.

FIGURE 3.20 The Cerebellum and the Brain Stem

This figure shows (a) a lateral view of a semitransparent brain, showing the cerebellum and brain stem ghosted in, (b) a view from the back of the brain, and (c) a dorsal view of the brain stem. The left hemisphere of the cerebellum and part of the right hemisphere have been removed to show the inside of the fourth ventricle and the cerebellar peduncles. Part (d) shows a cross section of the midbrain.

Pons.

The pons , a large bulge in the brain stem, lies between the mesencephalon and medulla oblongata, immediately ventral to the cerebellum. Pons means “bridge,” but it does not really look like one. (Refer back to Figures 3.15 and 3.20a . ) The pons contains, in its core, a portion of the reticular formation, including some nuclei that appear to be important in sleep and arousal. It also contains a large nucleus that relays information from the cerebral cortex to the cerebellum.

pons The region of the metencephalon rostral to the medulla, caudal to the midbrain, and ventral to the cerebellum.

MYELENCEPHALON

The myelencephalon contains one major structure, the medulla oblongata (literally, “oblong marrow”), usually just called the medulla. This structure is the most caudal portion of the brain stem; its lower border is the rostral end of the spinal cord. (Refer again to Figures 3.15 and 3.20a . ) The medulla contains part of the reticular formation, including nuclei that control vital functions such as regulation of the cardiovascular system, respiration, and skeletal muscle tonus.

medulla oblongata (me doo la ) The most caudal portion of the brain; located in the myelencephalon, immediately rostral to the spinal cord.

The Spinal Cord

The spinal cord is a long, conical structure, approximately as thick as our little finger. The principal function of the spinal cord is to distribute motor fibers to the effector organs of the body (glands and muscles) and to collect somatosensory information to be passed on to the brain. The spinal cord also has a certain degree of autonomy from the brain; various reflexive control circuits (some of which are described in Chapter 8 ) are located there.

spinal cord The cord of nervous tissue that extends caudally from the medulla.

The spinal cord is protected by the vertebral column, which is composed of twenty-four individual vertebrae of the cervical (neck), thoracic (chest), and lumbar (lower back) regions and the fused vertebrae that make up the sacral and coccygeal portions of the column (located in the pelvic region). The spinal cord passes through a hole in each of the vertebrae (the spinal foramens). Figure 3.21 illustrates the divisions and structures of the spinal cord and vertebral column. (See Figure 3.21 . ) The spinal cord is only about two-thirds as long as the vertebral column; the rest of the space is filled by a mass of spinal roots composing the cauda equina (“horse’s tail”). (Refer back to Figure 3.3a . )

spinal root A bundle of axons surrounded by connective tissue that occurs in pairs, which fuse and form a spinal nerve.

cauda equina (ee kwye na ) A bundle of spinal roots located caudal to the end of the spinal cord.

Early in embryological development the vertebral column and spinal cord are the same length. As development progresses, the vertebral column grows faster than the spinal cord. This differential growth rate causes the spinal roots to be displaced downward; the most caudal roots travel the farthest before they emerge through openings between the vertebrae and thus compose the cauda equina. To produce the caudal block that is sometimes used in pelvic surgery or childbirth, a local anesthetic can be injected into the CSF contained within the sac of dura mater surrounding the cauda equina. The drug blocks conduction in the axons of the cauda equina.

caudal block The anesthesia and paralysis of the lower part of the body produced by injection of a local anesthetic into the cerebrospinal fluid surrounding the cauda equina.

Figure 3.22(a) shows a portion of the spinal cord, with the layers of the meninges that wrap it. Small bundles of fibers emerge from each side of the spinal cord in two straight lines along its dorsolateral and ventrolateral surfaces. Groups of these bundles fuse together and become the thirty-one paired sets of dorsal roots and ventral roots . The dorsal and ventral roots join together as they pass through the intervertebral foramens and become spinal nerves. (See Figure 3.22a . )

dorsal root The spinal root that contains incoming (afferent) sensory fibers.

ventral root The spinal root that contains outgoing (efferent) motor fibers.

FIGURE 3.21 Ventral View of the Spinal Column

Details show the anatomy of the bony vertebrae.

Figure 3.22(b) shows a cross section of the spinal cord. Like the brain, the spinal cord consists of white matter and gray matter. Unlike the brain’s, the spinal cord’s white matter (consisting of ascending and descending bundles of myelinated axons) is on the outside; the gray matter (mostly neural cell bodies and short, unmyelinated axons) is on the inside. In Figure 3.22(b) , ascending tracts are indicated in blue; descending tracts are indicated in red. (See Figure 3.22b . )

FIGURE 3.22 Ventral View of the Spinal Cord

The figure shows (a) a portion of the spinal cord, showing the layers of the meninges and the relationship of the spinal cord to the vertebral column; and (b) a cross section through the spinal cord. Ascending tracts are shown in blue; descending tracts are shown in red.

SECTION SUMMARY: The Central Nervous System

The brain consists of three major divisions, organized around the three chambers of the tube that develops early in embryonic life: the forebrain, the midbrain, and the hindbrain. The development of the neural tube into the mature central nervous system is illustrated in Figure 3.8 , and Table 3.2 outlines the major divisions and subdivisions of the brain.

During the first phase of brain development, symmetrical division of the progenitor cells of the ventricular and subventricular zones, which lines the neural tube, increases in size. During the second phase, asymmetrical division of these cells gives rise to neurons, which migrate up the fibers of radial glial cells to their final resting places. There, neurons develop dendrites and axons and establish synaptic connections with other neurons. Later, neurons that fail to develop a sufficient number of synaptic connections are killed through apoptosis. Although the basic development of the nervous system is genetically controlled, sensory stimulation plays a role in refining the details. In addition, the neural circuitry of even a fully mature brain can be modified through experience.

The duplication of genes—in particular, master genes that control groups of other genes—facilitated the increase in complexity of the brain during the process of evolution. When a gene is duplicated, one of the copies can continue to perform vital functions, leaving the other copy for “experimentation” through mutations. The large size of the human brain, relative to the brains of other primates, appears to be accomplished primarily by lengthening the first and second periods of brain development, and convolutions are produced by division of progenitor cells of the inner SVZ that are not anchored to the wall of the neural tube.

The forebrain, which surrounds the lateral and third ventricles, consists of the telencephalon and diencephalon. The telencephalon contains the cerebral cortex, the limbic system, and the basal ganglia. The cerebral cortex is organized into the frontal, parietal, temporal, and occipital lobes. The central sulcus divides the frontal lobe, which deals specifically with movement and the planning of movement, from the other three lobes, which deal primarily with perceiving and learning. The limbic system, which includes the limbic cortex, the hippocampus, and the amygdala, is involved in emotion, motivation, and learning. The basal ganglia participate in the control of movement. The diencephalon consists of the thalamus, which directs information to and from the cerebral cortex, and the hypothalamus, which controls the endocrine system and modulates species-typical behaviors.

The midbrain, which surrounds the cerebral aqueduct, consists of the tectum and the tegmentum. The tectum is involved in audition and the control of visual reflexes and reactions to moving stimuli. The tegmentum contains the reticular formation, which is important in sleep, arousal, and movement; the periaqueductal gray matter, which controls various species-typical behaviors; and the red nucleus and the substantia nigra, both parts of the motor system. The hindbrain, which surrounds the fourth ventricle, contains the cerebellum, the pons, and the medulla. The cerebellum plays an important role in integrating and coordinating movements. The pons contains some nuclei that are important in sleep and arousal. The medulla oblongata, too, is involved in sleep and arousal, but it also plays a role in control of movement and in control of vital functions such as heart rate, breathing, and blood pressure.

The outer part of the spinal cord consists of white matter: axons conveying information up or down. The central gray matter contains cell bodies.

The Peripheral Nervous System

The brain and spinal cord communicate with the rest of the body via the cranial nerves and spinal nerves. These nerves are part of the peripheral nervous system, which conveys sensory information to the central nervous system and conveys messages from the central nervous system to the body’s muscles and glands.

Spinal Nerves

The spinal nerves begin at the junction of the dorsal and ventral roots of the spinal cord. The nerves leave the vertebral column and travel to the muscles or sensory receptors they innervate, branching repeatedly as they go. Branches of spinal nerves often follow blood vessels, especially those branches that innervate skeletal muscles. (Refer back to Figure 3.3 . )

spinal nerve A peripheral nerve attached to the spinal cord.

Now let us consider the pathways by which sensory information enters the spinal cord and motor information leaves it. The cell bodies of all axons that bring sensory information into the brain and spinal cord are located outside the CNS. (The sole exception is the visual system; the retina of the eye is actually a part of the brain.) These incoming axons are referred to as afferent axons because they “bear toward” the CNS. The cell bodies that give rise to the axons that bring somatosensory information to the spinal cord reside in the dorsal root ganglia , rounded swellings of the dorsal root. (See Figure 3.23 . ) These neurons are of the unipolar type (described in Chapter 2 ). The axonal stalk divides close to the cell body, sending one limb into the spinal cord and the other limb out to the sensory organ. Note that all of the axons in the dorsal root convey somatosensory information.

afferent axon An axon directed toward the central nervous system, conveying sensory information.

dorsal root ganglion A nodule on a dorsal root that contains cell bodies of afferent spinal nerve neurons.

Cell bodies that give rise to the ventral root are located within the gray matter of the spinal cord. The axons of these multipolar neurons leave the spinal cord via a ventral root, which joins a dorsal root to make a spinal nerve. The axons that leave the spinal cord through the ventral roots control muscles and glands. They are referred to as efferent axons because they “bear away from” the CNS. (Look again at Figure 3.23 . )

efferent axon (eff ur ent ) An axon directed away from the central nervous system, conveying motor commands to muscles and glands.

Cranial Nerves

Twelve pairs of cranial nerves are attached to the ventral surface of the brain. Most of these nerves serve sensory and motor functions of the head and neck region. One of them, the tenth, or vagus nerve , regulates the functions of organs in the thoracic and abdominal cavities. It is called the vagus(“wandering”) nerve because its branches wander throughout the thoracic and abdominal cavities. (The word vagabond has the same root.) Figure 3.24 presents a view of the base of the brain and illustrates the cranial nerves and the structures they serve. Note that efferent (motor) fibers are drawn in red and that afferent (sensory) fibers are drawn in blue. (See Figure 3.24 . )

cranial nerve A peripheral nerve attached directly to the brain.

vagus nerve ( vay guss) The largest of the cranial nerves, conveying efferent fibers of the parasympathetic division of the autonomic nervous system to organs of the thoracic and abdominal cavities.

FIGURE 3.23 A Cross Section of the Spinal Cord

The figure shows the routes taken by afferent and efferent axons through the dorsal and ventral roots.

As I mentioned in the previous section, cell bodies of sensory nerve fibers that enter the brain and spinal cord (except for the visual system) are located outside the central nervous system. Somatosensory information (and the sense of taste) is received, via the cranial nerves, from unipolar neurons. Auditory, vestibular, and visual information is received via fibers of bipolar neurons (described in Chapter 2 ). Olfactory information is received via the olfactory bulbs , which receive information from the olfactory receptors in the nose. The olfactory bulbs are complex structures that contain a considerable amount of neural circuitry; actually, they are part of the brain. Sensory mechanisms are described in more detail in Chapters 6 and 7 .

olfactory bulb The protrusion at the end of the olfactory nerve; receives input from the olfactory receptors.

The Autonomic Nervous System

The part of the peripheral nervous system that I have discussed so far—which receives sensory information from the sensory organs and that controls movements of the skeletal muscles—is called the somatic nervous system . The other branch of the peripheral nervous system—the autonomic nervous system (ANS) —is concerned with regulation of smooth muscle, cardiac muscle, and glands. (Autonomicmeans “self-governing.”) Smooth muscle is found in the skin (associated with hair follicles), in blood vessels, in the eyes (controlling pupil size and accommodation of the lens), and in the walls and sphincters of the gut, gallbladder, and urinary bladder. Merely describing the organs innervated by the autonomic nervous system suggests the function of this system: regulation of “vegetative processes” in the body.

somatic nervous system The part of the peripheral nervous system that controls the movement of skeletal muscles or transmits somatosensory information to the central nervous system.

autonomic nervous system (ANS) The portion of the peripheral nervous system that controls the body’s vegetative functions.

The ANS consists of two anatomically separate systems: the sympathetic division and the parasympathetic division. With few exceptions, organs of the body are innervated by both of these subdivisions, and each has a different effect. For example, the sympathetic division speeds the heart rate, whereas the parasympathetic division slows it.

FIGURE 3.24 The Cranial Nerves

The figure shows the twelve pairs of cranial nerves and the regions and functions they serve. Red lines denote axons that control muscles or glands; blue lines denote sensory axons.

SYMPATHETIC DIVISION OF THE ANS

The sympathetic division is most involved in activities associated with expenditure of energy from reserves that are stored in the body. For example, when an organism is excited, the sympathetic nervous system increases blood flow to skeletal muscles, stimulates the secretion of epinephrine (resulting in increased heart rate and a rise in blood sugar level), and causes piloerection (erection of fur in mammals that have it and production of “goose bumps” in humans).

sympathetic division The portion of the autonomic nervous system that controls functions that accompany arousal and expenditure of energy.

The cell bodies of sympathetic motor neurons are located in the gray matter of the thoracic and lumbar regions of the spinal cord (hence, the sympathetic nervous system is also known as the thoracolumbar system). The fibers of these neurons exit via the ventral roots. After joining the spinal nerves, the fibers branch off and pass into sympathetic ganglia (not to be confused with the dorsal root ganglia). Figure 3.25 shows the relationship of these ganglia to the spinal cord. Note that individual sympathetic ganglia are connected to the neighboring ganglia above and below, thus forming the sympathetic ganglion chain . (See Figure 3.25 . )

sympathetic ganglia Nodules that contain synapses between preganglionic and postganglionic neurons of the sympathetic nervous system.

sympathetic ganglion chain One of a pair of groups of sympathetic ganglia that lie ventrolateral to the vertebral column.

The axons that leave the spinal cord through the ventral root belong to the preganglionic neurons . Sympathetic preganglionic axons enter the ganglia of the sympathetic chain. Most of the axons form synapses there, but others pass through these ganglia and travel to one of the sympathetic ganglia located among the internal organs. With one exception (mentioned in the next paragraph), all sympathetic preganglionic axons form synapses with neurons located in one of the ganglia. The neurons with which they form synapses are called postganglionic neurons . The postganglionic neurons send axons to the target organs, such as the intestines, stomach, kidneys, or sweat glands. (See Figure 3.25 . )

preganglionic neuron The efferent neuron of the autonomic nervous system whose cell body is located in a cranial nerve nucleus or in the intermediate horn of the spinal gray matter and whose terminal buttons synapse upon postganglionic neurons in the autonomic ganglia.

postganglionic neuron Neurons of the autonomic nervous system that form synapses directly with their target organ.

The sympathetic nervous system controls the adrenal medulla , a set of cells located in the center of the adrenal gland. The adrenal medulla closely resembles a sympathetic ganglion. It is innervated by preganglionic axons, and its secretory cells are very similar to postganglionic sympathetic neurons. These cells secrete epinephrine and norepinephrine when they are stimulated. These hormones function chiefly as an adjunct to the direct neural effects of sympathetic activity; for example, they increase blood flow to the muscles and cause stored nutrients to be broken down into glucose within skeletal muscle cells, thus increasing the energy available to these cells.

adrenal medulla The inner portion of the adrenal gland, located atop the kidney, controlled by sympathetic nerve fibers; secretes epinephrine and norepinephrine.

The terminal buttons of sympathetic preganglionic axons secrete acetylcholine. The terminal buttons on the target organs, belonging to the postganglionic axons, secrete another neurotransmitter: norepinephrine. (An exception to this rule is provided by the sweat glands, which are innervated by acetylcholine-secreting terminal buttons.)

PARASYMPATHETIC DIVISION OF THE ANS

The parasympathetic division of the autonomic nervous system supports activities that are involved with increases in the body’s supply of stored energy. These activities include salivation, gastric and intestinal motility, secretion of digestive juices, and increased blood flow to the gastrointestinal system.

parasympathetic division The portion of the autonomic nervous system that controls functions that occur during a relaxed state.

Cell bodies that give rise to preganglionic axons in the parasympathetic nervous system are located in two regions: the nuclei of some of the cranial nerves (especially the vagus nerve) and the intermediate horn of the gray matter in the sacral region of the spinal cord. Thus, the parasympathetic division of the ANS has often been referred to as the craniosacral system. Parasympathetic ganglia are located in the immediate vicinity of the target organs; the postganglionic fibers are therefore relatively short. The terminal buttons of both preganglionic and postganglionic neurons in the parasympathetic nervous system secrete acetylcholine.

Table 3.3 summarizes the major divisions of the peripheral nervous system.

TABLE 3.3 The Major Divisions of the Peripheral Nervous System

Somatic Nervous System	Autonomic Nervous System (ANS)
Spinal Nerves	Sympathetic Branch
Afferents from sense organs	Spinal nerves (from thoracic and lumbar regions)
Efferents to muscles	Sympathetic ganglia
Cranial Nerves	Parasympathetic Branch
Afferents from sense organs	Cranial nerves (3rd, 7th, 9th, and 10th)
Efferents to muscles	Spinal nerves (from sacral region)
	Parasympathetic ganglia (adjacent to target organs)

FIGURE 3.25 The Autonomic Nervous System

The schematic figure shows the target organs and functions served by the sympathetic and parasympathetic branches of the autonomic nervous system.

SECTION SUMMARY: The Peripheral Nervous System

The spinal nerves and the cranial nerves convey sensory axons into the central nervous system and motor axons out from it. Spinal nerves are formed by the junctions of the dorsal roots, which contain incoming (afferent) axons, and the ventral roots, which contain outgoing (efferent) axons. The autonomic nervous system consists of two divisions: the sympathetic division, which controls activities that occur during excitement or exertion, such as increased heart rate, and the parasympathetic division, which controls activities that occur during relaxation, such as decreased heart rate and increased activity of the digestive system. The pathways of the autonomic nervous system contain preganglionic axons, from the brain or spinal cord to the sympathetic or parasympathetic ganglia, and postganglionic axons, from the ganglia to the target organ. The adrenal medulla, which secretes epinephrine and norepinephrine, is controlled by axons of the sympathetic nervous system.

Review Questions

Study and Review on MyPsychLab

Explain the origins of the names of brain structures and the terms used to indicate directions and planes of section.

Describe the blood supply to the brain, the meninges, the ventricular system, and flow of cerebrospinal fluid through the brain and its production.

Outline the development of the central nervous system and the evolution of the human brain.

Describe the telencephalon, one of the two major structures of the forebrain.

Describe the two major structures of the diencephalon.

Describe the two major structures of the midbrain, the two major structures of the hindbrain, and the spinal cord.

Describe the peripheral nervous system, including the two divisions of the autonomic nervous system.

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■ THE NERVOUS SYSTEM

In order to understand the function of the nervous system, one must first learn its structure and its development. The virtual brain includes two modules that will help you master the material. The Development of the Nervous System module shows the major division of the brain. The Nervous Systemmodule that will help you become familiar with the names and locations of more specific structures (e.g. the ventricles) and brain regions.