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Learning Objectives

After completing this chapter, you should be able to:

• Identify the major functions of the central nervous system. • Explain how the meninges protect the central nervous system. • Compare the functions of the five levels of the spinal cord: cervical, thoracic, lumbar, sacral, and coccygeal. • Draw a rough map of the brain and indicate the location of the hindbrain, midbrain, and forebrain on the map. • Identify the structures in the hindbrain and relate their functions to human behavior. • Discuss the organization of the midbrain. • Describe the differences between the thalamus and the hypothalamus. • Explain the function of the various structures in the limbic system. • Differentiate between the functions of the cerebellum and the basal ganglia. • Name the lobes of the cerebrum and locate each on a drawing of the cerebrum. • Describe the organization of the peripheral nervous system, referring to spinal nerves and cranial nerves.

The Organization of the Nervous System

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CHAPTER 4The Organization of the Nervous System

On a Saturday night shortly before midnight, 15-year-old Tyler was carried by his worried parents into the emergency room of a large medical center. Tyler was conscious but unable to speak. In addi- tion, he appeared to be paralyzed on the right side of his body. The doctors who examined him were puzzled: Here was a teenager who appeared to have suffered a stroke, a disorder typically associated with older adults. Tyler’s symptoms, inability to speak and right-sided paralysis, suggested damage to the left side of his brain.

Although Tyler couldn’t speak, he could nod “yes” or shake his head “no” when questioned by the emergency room doctors. At first Tyler refused to answer the doctors’ questions. However, he began to cooperate when he was told that the doctors couldn’t help him unless he told them what he’d been doing earlier that evening.

The doctors surmised that Tyler had been trying to get high on some type of inhalant. Their ques- tions focused on determining the chemical he had used. It turned out that Tyler and his friends had been sniffing butane, the propellant used in disposable lighters. Butane is known to cause spasms in the muscular walls of blood vessels. In Tyler’s case the butane he inhaled caused sudden, extreme constriction of the arteries in his brain, causing a major artery in his left hemisphere to develop a blockage. Neurons in that side of his brain were deprived of oxygen as a result of the blockage and were damaged, producing a loss of function associated with the left hemisphere in the brain.

The World Congress of Neuroscience has hailed the 21st century as the Century of the Brain. The Century of the Brain! How much do you know about the brain? Do you understand the mecha- nisms by which it controls behavior? If you do, you should be able to explain how brain damage caused by sniffing butane affects behavior. If not, you will be able to explain the effects of brain damage on behavior when you finish this chapter.

This chapter will introduce you to the brain and its organization. As you read this chapter, you will learn about the important structures in the brain and how each participates in the control of bodily functions and behavior. Later in this book, we will look at vital human behaviors such as speech, reasoning, and emotions. We will examine how the brain is involved in mental illness. And we will explore the role of the brain in behaviors that we have in common with other animals, such as eating, sleeping, drinking, and sexual behavior. We will also probe the brain’s regulation of movement and sensory processes like vision, taste, smell, touch, and hearing.

Remember from Chapter 2 that the nervous system has two divisions: the central nervous system and the peripheral nervous system. The central nervous system is composed of the brain and the spinal cord. All of the neurons located outside the brain and spinal cord make up the peripheral nervous system. In this chapter we will focus on the organization of the nervous system and, in particular, on the structure of the principal organ of the nervous system, which is the brain. Let’s consider the organization of the central nervous system first.

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CHAPTER 4Section 4.1 An Overview of the Central Nervous System

4.1 An Overview of the Central Nervous System

The primary function of the central nervous system is to process information that it receives from the peripheral nervous system and then organize a response to that information. For example, imagine that a spider climbs onto your ankle and begins to walk up your leg. The tickling sensation travels through your peripheral nervous system to your central nervous system. This information travels up your spinal cord to your brain, which then processes the information and sends messages back to your peripheral nervous system, directing your eyes to look at your leg for the source of the tickling and directing your hand to brush the offending creature off your leg.

The brain can also initiate behavior on its own, with- out prompting from the peripheral nervous system. You might, for example, suddenly think of your sister in a faraway city and decide to telephone her. Your central nervous system in this case directs the move- ments needed to walk to the telephone, pick up the receiver, and dial your sister’s phone number.

Thus, the central nervous has two major functions: (1) to receive and act upon information received from the peripheral nervous system, and (2) to ini- tiate thoughts and behaviors independently without prompting from the peripheral nervous system. As you learned in Chapter 2, information coming from neurons in the peripheral nervous system located below your neck must travel through the spinal cord to be relayed to the brain. Information from neu- rons in your head and neck is transmitted though the peripheral nervous system directly to the brain.

Protecting the Central Nervous System

The central nervous system is so important to your survival that it is encased in bone for pro- tection. The skull surrounds the brain, and the column of vertebral bones that you know as the backbone protects the spinal cord. Underneath these bones, three layers of tissues, called the meninges, cover the brain and spinal cord, providing further protection (Figure 4.1).

Flirt/SuperStock

Photo 4.1 Your brain can initiate behavior on its own, prompting the central nervous system to act. An example of this would be thinking of someone and then calling that person.

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CHAPTER 4Section 4.1 An Overview of the Central Nervous System

Figure 4.1: The meninges, the protective covering of the central nervous system

The meninges consist of three layers: the dura mater (outer layer), arachnoid (middle layer), and pia mater (innermost layer).

Scalp

Skull

Dura mater

Arachnoid

Pia mater

Dura mater Spinal nerve

Spinal cord

Arachnoid

Pia mater

Bone of vertebral column (backbone)

Scalp

Skull

Meninges -Arachnoid -Dura mater -Pia mater

Cerebral cortex

Meninges in detail

The outermost layer of the meninges is known as the dura mater. The dura mater is a tough, fibrous protective coating that helps maintain the integrity of the brain and spinal cord. Beneath the dura mater is the arachnoid layer, which gets its name from its spider web–like appearance

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CHAPTER 4Section 4.1 An Overview of the Central Nervous System

(arachne means “spider” in Greek). The innermost layer of the meninges is called the pia mater. This delicate tissue is in direct contact with the surface of the brain and spinal cord.

The arachnoid layer’s web is composed of blood vessels and other vessels containing a special fluid found only in the central nervous system, called cerebrospinal fluid. The word cerebrospinal comes from two words, cerebrum, which is the Latin word for “brain,” and spinal, which refers to the spinal cord. Cerebrospinal fluid is manufactured in a structure located near the top of the brain, and it flows down through the brain and spinal cord through a system of canals, called ventricles. When the cerebrospinal fluid reaches the base of the spinal cord, it travels back up the spinal cord through the arachnoid layer of the meninges to the brain.

The ventricles produce nearly a pint of cerebrospinal fluid each day. Blockage of the ventricles causes cerebrospinal fluid to build up in the brain, producing a disorder known as hydrocephalus. You can imagine that, as fluid builds up in the brain due to a blockage of the ventricle system, pressure on neurons in the brain increases, and neurons are damaged or destroyed. Therefore, hydrocephalus is a serious condition that can produce death or permanent damage to the nervous system. We will discuss hydrocephalus in greater detail in Chapter 13.

Scientists have identified several functions of cerebrospinal fluid. First of all, it acts as a watery cush- ion to protect the central nervous system from smacking against the skull or backbone following a sud- den jarring (Johanson et al., 2008; Sakka, Coll, & Chazal, 2011). Cere- brospinal fluid produces a pressure inside the brain that prevents the brain from collapsing like a deflated balloon, and it also plays an impor- tant role in brain development (Sakka et al., 2011). Chemicals pro- duced by the brain are transported in the cerebrospinal fluid to the spinal cord. These chemicals found in the cerebrospinal fluid can tell us a lot about the brain (Simonsen, 2012). For example, they can indi- cate the presence of an infection in the brain or a chemical imbalance.

Cerebrospinal fluid, then, is a win- dow into the brain—it can tell us

what is happening in the brain. For example, a number of investigators have linked abnormal lev- els of serotonin byproducts and GABA found in the cerebrospinal fluid to a greater risk of depres- sion and suicidal behavior (Asberg, 1997; Lee, Petty, & Coccaro, 2009; Loefberg, Agren, Harro, & Oreland, 1998; Raedler, 2011; Singareddy & Balon, 2001). A spinal tap is a medical procedure that is used to extract cerebrospinal fluid from the arachnoid layer of the meninges. This procedure can inform physicians about the state of the brain without their actually going into the brain.

Southern Illinois University/Science Source

Photo 4.2 In hydrocephalus, a portion of the ventricles is blocked, causing accumulation of cerebrospinal fluid in the brain. Because the bones of the skull are not fixed in very young children, the head of the infant with hydrocephalus will swell as pressure from accumulating cerebrospinal fluid builds. In the newborn infant shown above, light placed behind the head shines through the skull because cerebrospinal fluid has compressed and replaced brain tissue.

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CHAPTER 4Section 4.1 An Overview of the Central Nervous System

Sometimes bacteria or other microorganisms can invade the meninges, producing a disorder called meningitis. Meningitis is a life-threatening illness that can produce long-term disabilities in those who survive. In meningitis the meninges swell in response to the infection by the microorganism, putting pressure on the spinal cord or brain, which can kill neurons or produce permanent brain damage. The onset of the illness is very rapid. A healthy individual will develop a fever and flu-like symptoms, which rapidly progresses to rigidity of the neck and severe headache. If the infection is left untreated, the individual will lapse into a coma and eventually die. Even with antibiotic treat- ment, 10% to 14% of people who contract meningitis will die (Centers for Disease Control and Prevention, 2005). In the United States meningitis is quite rare, occurring in fewer than 3 out of every 100,000 people each year.

The Spinal Cord

You have already been introduced to the spinal cord in Chapter 2. Recall that, in cross section, the spinal cord resembles a gray butterfly surrounded by a white background (Figure 2.7). The gray “butterfly” is actually composed of neuronal cell bodies, and the white matter is composed of bundles of axons. We see this pattern repeated throughout the nervous system: groups of neuro- nal cell bodies clustered together forming the gray matter and bundles of axons forming the white matter. In the central nervous system, a cluster of gray matter is called a nucleus (plural is nuclei), and a bundle of axons is called a tract. In the peripheral nervous system, a cluster of gray matter is called a ganglion (plural is ganglia), whereas a bundle of axons is called a nerve.

The spinal cord extends from the base of the skull to the tailbone. It is divided into five sections based on its location within the body: cervical, thoracic, lumbar, sacral, and coccygeal (Figure 4.2). The cervical portion of the spinal cord is located in the neck. As its name implies, the thoracic portion of the spinal cord is the section that runs through the chest area or thorax. The lumbar section is located within the vertebrae that make up the small of the back. The position of “lumbar support” in the driver’s seat of your car corresponds with the location of the lumbar area of the spinal cord. The sacral portion of the spinal cord is found within the backbones that are attached to the pelvic girdle, the circle of bones that make up your hips and pelvis. The coccygeal portion, which is located at the very base of the spinal cord, is very small and virtually useless in humans.

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CHAPTER 4Section 4.1 An Overview of the Central Nervous System

Figure 4.2: The five segments of the spinal cord

Each of the five segments receives messages from and controls different parts of the body. Which parts of the body does the sacral section command?

Cervical region

Thoracic region

Lumbar region

Sacral region

Coccygeal region

Spinal cord

Each of the five segments of the spinal cord receives information from and controls the muscles and organs in a particular part of the body. The cervical portion of the spinal cord innervates the shoulders, arms, and hands. The thoracic portion of the spinal cord regulates the functioning of muscles and organs in the chest and upper abdomen. The lower abdominal muscles and organs, as well as muscles in the legs and feet, are controlled by the lumbar portion of the spinal cord. Finally, the sacral portion of the spinal cord directs the workings of the organs within the pelvis,

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CHAPTER 4Section 4.2 The Brain

Scott Camazine/Science Source

Photo 4.3 This MRI scan of a normal brain highlights the different areas of the brain and its structures.

including the reproductive organs, the bladder, and rectum. In an animal with a tail, the coccygeal region of the spinal cord controls tail movements and receives sensory information from the tail. In humans, the coccygeal spinal cord controls only a small group of muscles above the anus.

Spina bifida is a disorder in which the bony spinal column fails to close properly during devel- opment, leaving a part of the spinal cord exposed. The amount of exposed spinal cord varies, depending on the extent of the opening in the backbone. Approximately 1 out of every 1,000 chil- dren born has spina bifida, although the disorder can be detected early in pregnancy. Because the spinal cord protrudes out of the backbone in spina bifida, it is vulnerable to infection and damage, which can leave the afflicted child severely physically and sometimes mentally challenged. Surgi- cal treatment, including closure of the bony defect, can sometimes help children born with this disorder. This surgical treatment can be performed prenatally (while the fetus is still in the uterus) or postnatally (after birth). Recently, a team of investigators demonstrated that prenatal surgery is more effective than postnatal surgery in reducing motor and mental impairment in children with spina bifida (Adzick et al., 2011).

4.2 The Brain

I remember the first time I saw an MRI scan of my own brain. I was fascinated and, at the same time, disappointed to find that my brain looked just like any other healthy human brain I’d ever seen. See the following image for an example of a typical brain. I’m sure my brain looks just like your brain. Most of us have virtually identical structures situated in the same locations within the skull.

Furthermore, our human brains bear a close resem- blance to the brains of other mammals (Figure 4.3). In fact, if you are really observant, you will notice many similarities between your brain and the brains of other vertebrates, like fish, cats, and birds. For all vertebrates, the brain can be divided into three parts: the forebrain, the midbrain, and the hindbrain. These terms make a lot of sense if you consider the brain of a typical animal, one that is oriented horizontally with respect to the ground. The hindbrain is located in the back of the brain toward the animal’s hindquarters. The forebrain is situated in the front of the brain, and the midbrain is situated in the middle.

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CHAPTER 4Section 4.2 The Brain

Figure 4.3: Comparison of the human brain with the brains of other creatures

Homo sapiens may not see, hear, or smell as well as many other species, but greater relative brain size and the greater relative size of the “thinking” part—the association cortex—of the human brain—have given us an evolutionary advantage.

Cortex

Vertical brain orientation

Horizontal brain orientation

Source: Copyright Hungry Coyote Limited. Used by permission.

In humans and other animals that have a vertical orientation, the hindbrain is located at the bot- tom, the midbrain is located on top of the hindbrain, and the forebrain, which is enormous in humans, sits on top, covering not only the tiny midbrain but also the larger hindbrain. Figure 4.4 shows the positions of the forebrain, midbrain, and hindbrain in the human brain.

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CHAPTER 4Section 4.2 The Brain

Figure 4.4: The forebrain, midbrain, and hindbrain

This figure shows the development of the forebrain, midbrain, and hindbrain as one ages. What changes do you see?

Forebrain

Midbrain

MidbrainHindbrain

Hindbrain

Neural tube

A. 5 weeks (in utero) B. 8 weeks (in utero)

C. 7 months (in utero) D. Adult

Spinal cord

Forebrain

Midbrain

Hindbrain

Spinal cord

Armbud

The Hindbrain

Three structures make up the hindbrain: the medulla, the pons, and the cerebellum (Figure 4.5). The medulla is located directly above, or superior to, the spinal cord. This means that all informa- tion coming from and going to the spinal cord must pass through the medulla. Indeed, the medulla contains a great deal of white matter containing tracts that relay information between the spinal cord and higher brain areas. Gray matter is also found in the medulla, however. Clusters of neu- rons, known as nuclei, are scattered throughout the medulla. Each nucleus has a specific function, as you will learn in later chapters. Some nuclei regulate life-support functions, such as breathing, coughing, or vomiting. Damage to these nuclei can result in death. Other nuclei in the medulla receive sensory information or send motor commands to muscles in the head, neck, and trunk.

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CHAPTER 4Section 4.2 The Brain

Figure 4.5: Major structures in the brain

Although the major structures of the brain function in different ways, they all allow neural messages to come through to the higher brain.

Cerebrum

Corpus callosum

Pituitary gland

Thalamus

Hypothalamus Midbrain

Pons

Cerebellum

Medulla

Located directly above the medulla is the pons. The word pons means “bridge” in Latin. The pons is a bridge-like structure that is composed almost entirely of white matter, or tracts, conveying information between the higher brain regions and the medulla and spinal cord. Information leav- ing your forebrain travels down tracts through your midbrain to your hindbrain and spinal cord, passing through the pons. Neural messages coming from the medulla and spinal cord pass through the pons before traveling to higher brain areas, as you will learn in later chapters.

The cerebellum lies dorsal to both the medulla and the pons. We will discuss its structure in greater detail in Chapter 5. For now, you should know that it is divided into two hemispheres: right and left. Each hemisphere contains a cerebellar cortex (gray matter) and underlying white matter. This architecture (gray matter situated over tracts of white matter) gives the cerebellum a characteristic tree-like appearance when it is viewed in cross section.

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CHAPTER 4Section 4.2 The Brain

For Further Thought: Sobriety Tests—Evaluating Cerebellar Impairment

Alex was in Atlanta for a professional conference, where he had the opportunity to meet with col- leagues and former college friends. The first night of the conference, he went out to a bar with a group of friends. After a fun evening of swapping industry stories, Alex left the bar around midnight with four friends. He had drunk six beers but felt that he could drive safely. Within minutes, he was stopped at a roadblock by police who were looking for drunk drivers.

Alcohol affects the functioning of neurons throughout the brain, including the cerebel- lum. Because the cerebellum controls coor- dinated movements, alcohol interferes with coordination. A sobriety test allows police officers to check for signs of cerebellar dys- function that are caused by alcohol intoxica- tion. A police officer approached Alex’s car and asked for his driver’s license. Nervously, Alex fumbled in his wallet for his license. The officer scanned it briefly, and then asked Alex to say the alphabet as quickly as he could. Alex did well until he got to “L, M, N, O, P.”

Because he was intoxicated, his tongue stumbled over those letters.

The police officer asked Alex to get out of the car and walk along a straight line that was painted on the pavement. Alex had a difficult time walking the line. He staggered, weaving from side to side, and nearly fell over at one point. Finally, the officer told Alex to close his eyes, extend his right arm, and then touch his nose with his right index finger. Alex closed his eyes and tried to touch his nose, but he slammed his hand into his face very hard. Alex failed all three phases of the sobriety test. He demon- strated fine motor impairment when he slipped up saying the letters of the alphabet. Impairment of gross movement was evident when Alex could not walk along a straight line. In addition, Alex failed the finger-to-nose test, a test that assesses the ability of the cerebellum to locate the position of a particular limb in space and to direct the movement of that limb based on its location.

After Alex failed the sobriety test, he was subjected to a Breathalyzer test, which measures alcohol content of the expired breath. This test indicated that the level of alcohol in Alex’s body was above the legal limit. Alex was given a ticket for DUI and taken to jail. His car was towed away, and his intoxicated friends had to take a taxi home.

Charlie Neuman/ZUMA Press/Corbis

Photo 4.4 Alcohol’s influence on the cerebellum causes a loss of coordination.

The neurons in the cerebellum are responsible for coordinating muscular activities, especially those involved in rapid and repetitive movements. Alcohol is known to especially impair the func- tioning of the cerebellum, causing intoxicated persons to lose their muscle coordination. Sobriety tests performed by police officers are designed to detect impairment of the cerebellum (see the “For Further Thought” box in this chapter).

The Midbrain

The midbrain is the smallest of the three major divisions of the brain. In the human brain, it is roughly the size of a large acorn. The midbrain can be divided into three parts: the dorsal portion, the ventral portion, and the tegmentum, which lies between the dorsal and ventral areas. The

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CHAPTER 4Section 4.2 The Brain

dorsal area is also known as the tectum, and it contains four structures that look like little hills, the superior colliculi and the inferior colliculi. (The term colliculi means “little hills” in Latin.) You have two superior colliculi and two inferior colliculi in your midbrain: one of each on the left side of your brain and one of each on the right side. Located above the inferior colliculi, the superior colliculi are important in processing visual information, as you will learn in Chapter 6. The inferior colliculi are involved in relaying auditory information to the cerebellum and forebrain.

The tegmentum contains a number of important structures, such as the red nucleus, the periaq- ueductal gray, and the substantia nigra, that we will cover later in this book, structures that play a vital role in movement, attention, pain control, emotions, and sensory processing. For the most part, the ventral area of the midbrain consists of tracts relaying neural information between the hindbrain and the forebrain.

The reticular formation, which runs from the hindbrain to the forebrain, takes up a large portion of the midbrain. The reticular formation plays an important role in keeping you awake and alert. When your alarm goes off, it is your reticular formation that rouses you to consciousness. The reticular formation also arouses you when someone calls your name, when some creepy-crawlie slithers up your arm, or when someone yells “Help!” Damage to the reticular formation can pro- duce a state of unconsciousness called a coma, in which a person is unable to respond to external stimuli.

The Forebrain

The largest part of the human brain is the forebrain (Figure 4.4). This area of the brain produces the most interesting human behaviors, such as thinking, creating, eating, speaking, and emoting. The forebrain contains a number of important structures, including the cerebrum, basal ganglia, limbic system, thalamus, and hypothalamus. Two forebrain structures, the thalamus and hypothalamus, make up the diencephalon, which gets its name from the Greek words for “two” (dio) and “brain” (encephalon). Let’s examine each of these forebrain structures, beginning with the diencephalon.

The Diencephalon The diencephalon is located directly above the midbrain. Information from the midbrain must pass through the diencephalon in order to reach the higher parts of the forebrain. The prefix hypo- means “under” in Greek. As its prefix hypo- implies, the hypothalamus lies under the thalamus (Figure 4.5).

The shape of the thalamus resembles an egg that has been flattened on one side. It is composed of a large number of nuclei, or clusters of neurons, that relay information to and from structures in the forebrain, especially the largest structure, which is called the cerebrum. The cerebrum is con- sidered the seat of consciousness by most behavioral neuroscientists. That is, you do not become consciously aware of a stimulus unless neural information about the stimulus makes its way to the cerebrum. It is the function of the nuclei in the thalamus to process incoming information and pass it on to the cerebrum. Thus, the thalamus acts like a switchboard operator relaying informa- tion between the cerebrum and other parts of the brain. There is evidence that the thalamus also receives information from the cerebrum and plays an important role in attention (Haber & Calzavara, 2009; McAlonan, Cavanaugh, & Wurtz, 2006; Sillito, Jones, Gerstein, & West, 1994; Zikopoulos & Barbas, 2006).

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CHAPTER 4Section 4.2 The Brain

The hypothalamus is located on the ventral surface of the brain superior to the midbrain (Figure 4.5). Below or ventral to the hypothalamus lies the pituitary gland, which is attached to the hypothalamus by a stalk. The hypothalamus directly controls the activity of the pituitary gland by releasing hormones that are sent to the pituitary gland. Hormones are specialized chemi- cals that are released by one structure in the body, typically called a gland, and affect another structure in the body. The hypothalamus directs the activity of the pituitary gland with hormones, and the pituitary gland uses hormones to control the activity of other glands and organs in the body, as you will learn in Chapters 8 through 11.

In addition, the hypothalamus is involved in the regulation of many motivated behaviors, such as eating, drinking, sleeping, temperature control, sexual behavior, and emotions. Like the thala- mus, the hypothalamus contains many tiny clusters of neurons called nuclei. Each nucleus in the hypothalamus plays a role in the regulation of a specific motivated behavior. We will examine the functions of the hypothalamus in Chapters 8 through 11.

The diencephalon, together with the midbrain and hindbrain, makes up the brain stem. In terms of evolutionary development, the brain stem is considered to be the oldest part of the brain. If you examine the brain of a reptile, like that of a snake or a turtle, you would find that the reptilian brain is very similar to the human brain stem.

The Limbic System The hippocampus, amygdala, and septum, together with a handful of regions in the midbrain, dien- cephalon, and cerebrum, make up the limbic system. The Canadian psychologist Paul MacLean coined the term limbic system. Limbic comes from the Latin word limbus, meaning “boundary.” To MacLean’s mind, the limbic system formed a boundary between the brain stem and the higher centers of the brain. The limbic system functions in the production and experience of emotion.

The hippocampus gets its name from its seahorse-like shape (hippo means “horse” and kampus means “sea monster” in Greek). In addi- tion to the role it plays in emotions, the hippocampus is responsible for some types of learning and for the creation of permanent, or long- term, memories. Damage to the hippocampus can interfere with and produce memory loss.

The amygdala is located at the tail end of the hippocampus’s seahorse shape. It is oval in appearance and has two distinct regions: one that produces fear and escape behav- iors when stimulated and another that elicits rage and aggressive attack behavior when activated.

Denkou Images/SuperStock

Photo 4.5 The amygdala has two separate regions: one that produces fear and escape behaviors and one that produces aggressive behavior when attacked.

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CHAPTER 4Section 4.2 The Brain

Rabies, which is caused by a virus that attacks the forebrain, especially in the region of the amyg- dala, produces a lack of natural fear, as well as vicious attack behavior, in rabid animals.

The septum is a complex forebrain structure that contains a number of different nuclei with diverse functions. Until 1954 the function of the septum was unknown. Studies by James Olds and Peter Milner in the early 1950s demonstrated that rats will eagerly press a bar to receive brain stimulation of the septum (Olds & Milner, 1954). Thus, the septum is known as the “reward center” of the brain and is involved in the development of addictions (Luo, Tahsili-Fahadan, Wise, Lupica, & Aston-Jones, 2011). In addition, recent research has also implicated the septum in the development of intense romantic love (Acevedo, Aron, Fisher, & Brown, 2011; Fisher, Aron, & Brown, 2005).

The Basal Ganglia The basal ganglia consist of several clusters of neurons found in the base of the forebrain. Together, these structures are responsible for the production of movement. For example, you notice that your hands are dirty and decide to wash your hands. The basal ganglia initiate this washing behav- ior. Hand-washing compulsions, in which people feel compelled to wash their hands hundreds of times a day, are a common manifestation of obsessive-compulsive disorder. Research has dem- onstrated that faulty circuits between the basal ganglia and other brain structures may cause obsessive-compulsive disorders (Rapoport, 1990). The basal ganglia have also been implicated in a number of movement disorders, such as tremors associated with Parkinson’s disease and Hun- tington’s chorea (Lieu & Subramanian, 2012), as you will learn in Chapter 5.

The Cerebrum The cerebrum (cerebrum means “brain” in Latin) gets its name from the fact that it is by far the largest structure in the brain—so large, in fact, that when the skull is removed from the top of a person’s head, the only structure that can be seen is the cerebrum (Figure 4.6). The rest of the forebrain, the midbrain, and the hindbrain are tucked underneath the cerebrum in the skull. This means that damage to the skull, as in a skull fracture, will have its greatest impact on the cere- brum. In Chapter 13 we will look at different effects of damage to the cerebrum.

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CHAPTER 4Section 4.2 The Brain

Figure 4.6: The cerebrum in two views: in the skull and removed from the skull

Each lobe of the cerebrum has a specialized function in the brain, such as speech, hearing, and sight.

Prefrontal cortex

Frontal lobe

Temporal lobe

Central sulcus Frontal lobes

Central sulcus

Parietal lobes

Occipital lobes

Temporal lobe

Parietal lobe

Occipital lobe

Sylvian fissure

Occipital lobe

Anatomical areas (left lateral view)

Anatomical areas (left lateral view) Anatomical areas (top view)

Left hemisphereA.

Right hemisphere

Postcentral gyrus (somatosensory cortex)

Central sulcus

Primary visual cortex

Broca’s speech area

Olfactory bulb and tract

Sylvian fissure

Motor cortexPremotor area

The cerebrum is organized like the cerebellum, with a cortical layer called the cerebral cortex, which consists of a thin layer of gray matter (less than 2 millimeters in thickness) and white mat- ter beneath the cortex. The white matter is composed of axons leaving and entering the cerebral cortex. Some axons carry information from nuclei in the thalamus to particular regions of the cortex. Other axons carry information from the cerebral cortex to specific nuclei in the thalamus. Most axons in the white matter of the cerebrum, however, carry information from one area of the cortex to another.

The cerebrum has two halves, called cerebral hemispheres. That is, you have a left cerebral hemi- sphere and a right cerebral hemisphere. Four lobes make up each cerebral hemisphere: the fron- tal lobe, the parietal lobe, the temporal lobe, and the occipital lobe (Figure 4.6). Although each lobe has specialized functions, the four lobes in each hemisphere communicate with each other and with the lobes in the other hemisphere. Communication between the hemispheres is made

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CHAPTER 4Section 4.2 The Brain

possible by the corpus callosum (Figure 4.7). The corpus callosum is a large, thick tract that con- nects neurons in the left and right cerebral hemispheres.

Figure 4.7: The cerebrum in cross section

In this image the cerebrum is seen in two views in cross section: mid-sagittal cross section (left) and coronal cross section (right).

Corpus callosum

Frontal lobe

Motor cortex

Parietal lobe

Cingulate cortex

Mid-sagittal view Coronal view

Anterior commissure

Caudate nucleus

Gray matter (cortex)

Corpus callosum

Internal capsule

PutamenBasal ganglia

Caudate

Globus pallidus

Thalamus Thalamus

Occipital lobe Third ventricle

Corona radiata

Lateral ventricle

The Frontal Lobes Brain investigators divide the frontal lobe into three separate regions, each with its own special functions: the motor cortex, the premotor cortex, and the prefrontal cortex. The motor cortex directs fine motor coordination. The neurons in the motor communicate directly with the motor neurons that control muscle contractions. Located immediately anterior to (or in front of) the motor cortex, the premotor cortex processes information about intended movements and sends that information on to the motor cortex.

The prefrontal cortex contains a number of regions that have been demonstrated to control a number of executive functions, including short-term memory, working memory, decision making, and prioritizing behaviors. When you are trying to decide whether you should do your laundry, watch television, or call your mother, it is your prefrontal cortex that weighs the alternatives and empowers you to make a decision. Throughout this book, we will make many references to the frontal lobe because it plays an important role in who you are and what you do.

The Parietal Lobes Located directly posterior to the frontal lobes, the parietal lobes consist of the primary somato- sensory cortex and the secondary somatosensory cortex. The prefix somato- is derived from the Latin word soma, which means body. Somatosensation refers to sensations that arise from the body. The principal function of the parietal lobes, then, is to process sensory information coming in from the body, whether it be a stomachache or the sensation of someone stroking your arm or cold toes on a wintry day. You will learn more about the parietal lobe in Chapter 6.

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CHAPTER 4Section 4.3 The Peripheral Nervous System

The Occipital Lobes The occipital lobes are situated at the posterior end of the cerebrum, behind the parietal lobes. The principal function of the occipital lobes is to process visual information coming from the eyes. As you will learn in Chapter 6, neurons in the occipital lobe receive information about the images detected by the eyes, analyze that information by breaking each image into tiny components, and then recon- stitute the image after communicating with neurons in the frontal, parietal, and temporal lobes.

The Temporal Lobes The temporal lobes play a role in a number of important functions. The senses of taste, smell, and audition are processed in the temporal lobes. Language comprehension, too, is regulated in the temporal lobe, as is recognition of visual objects and faces. In addition, several structures, includ- ing the amygdala and the hippocampus, are located deep beneath the cortex of the temporal lobe. Therefore, damage to the temporal lobe can affect hearing, taste, smell, comprehension of language and facial expressions, mood, and memory.

Left or Right? Recall that the cerebrum comprises two cerebral hemispheres that are connected by a band of white matter called the corpus callosum. Psychologists such as Nobel Prize winner Roger Sperry have demonstrated that the left and right cerebral hemispheres have specialized functions, too. The new brain-scanning technologies have confirmed that the cerebrum divides its work asymmetrically across the two hemispheres. For most people the left hemisphere plays an important role in speech and language comprehension. Arithmetic and scientific reasoning also appear to be controlled by the left hemisphere. In contrast, neurons in the right hemisphere process information about emo- tional expression, face recognition, music, and other time-space relationships. However, it is impor- tant to remember that the differences between the left and right hemispheres are not clear-cut. Research has suggested that some people lack specialized hemispheric skills (Gazzaniga, 1989).

4.3 The Peripheral Nervous System

Recall from Chapter 2 that the peripheral nervous system has two divisions: the somatic ner-vous system and the autonomic nervous system. The somatic nervous system innervates striated muscles, which are under your voluntary control. In contrast, the autonomic nervous system innervates smooth muscles (and cardiac, or heart, muscles) and is not under your con- scious control. The autonomic nervous system acts automatically, in response to signals from the central nervous system.

The autonomic nervous system is composed of two divisions: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system becomes activated when a person is excited, aroused, or in another highly emotional state. On the other hand, the parasympathetic nervous system plays an energy-conserving role and is associated with a relaxed, vegetative state.

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CHAPTER 4Section 4.3 The Peripheral Nervous System

Both the sympathetic and parasympathetic nervous systems innervate all organs and body struc- tures containing smooth muscles. However, the two divisions of the autonomic nervous system have opposite effects on the structures that they stimulate. For example, recall from Chapter 2 that the sympathetic nervous system speeds up the heart, and the parasympathetic system slows it down. Sympathetic activation causes the pupils in the eyes to dilate, and parasympathetic acti- vation causes the pupils to constrict. Likewise, when the sympathetic nervous system is activated, salivary glands produce a thick saliva in small quantities, whereas parasympathetic stimulation of the salivary glands causes copious secretion of a thin, watery saliva.

The word innervate is used several times in the preceding paragraphs. Notice that the root of innervate is nerve. The peripheral nervous system transmits information over relatively long dis- tances in the body by way of nerves. To innervate, then, means to send information by way of nerves. Usually this information serves to control or regulate the body structure that is innervated.

The peripheral nervous system transmits information to the entire body by way of 12 pairs of cranial nerves and 31 pairs of spinal nerves, or 43 pairs of nerves altogether. They are described as “pairs of nerves” because nerves exit from the right and left halves of the brain and spinal cord. That means that you have a left optic nerve that connects your left eye to your brain and a right optic nerve that connects your right eye. The same is true for all the nerves that we will discuss in this section. Each nerve is paired with a nerve on the opposite side of the body. Let’s consider the cranial nerves first.

Cranial Nerves

The cranial nerves permit direct communication between the brain and the peripheral nervous system. Cranial nerves allow for (1) sensory input from the head, neck, and upper abdomen to the brain; (2) motor output from the brain to the skeletal muscles in the head and neck; and (3) para- sympathetic output to smooth muscles in the head, neck, and upper abdomen. These nerves gain access to the brain via holes in the skull. Table 4.1 summarizes the functions of the twelve cranial nerves. Note that Roman numerals are used to denote each cranial nerve.

Table 4.1: Functions of the cranial nerves

Cranial nerve Function

Olfactory (I) Sensory: smell

Optic (II) Sensory: vision

Oculomotor (III) Motor: eye muscles

Trochlear (IV) Motor: eye muscles

Trigeminal (V) Sensory: skin of face, jaws, teeth Motor: muscles that close mouth

(continued)

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CHAPTER 4Section 4.3 The Peripheral Nervous System

Table 4.1: Functions of the cranial nerves

Cranial nerve Function

Abducens (VI) Sensory: eye muscles Motor: eye muscles

Facial (VII) Sensory: taste, facial muscles Motor: muscles of facial expression

Auditory (VIII) Sensory: hearing, vestibular senses

Glossopharyngeal (IX) Sensory: tongue, throat Motor: muscles of tongue and throat

Vagus (X) Sensory: taste, organs of chest and upper abdomen Motor: smooth muscles in neck, chest, upper abdomen

Accessory (XI) Motor: skeletal muscles of neck

Hypoglossal (XII) Motor: muscles in lower jaw, tongue

Spinal Nerves

In addition to the cranial nerves, the peripheral nervous system contains 31 pairs of spinal nerves. The spinal nerves are bundles of axons that exit and enter the spinal cord. Recall from our earlier discussion of the spinal cord that axons entering the dorsal aspect of the spinal cord are carrying sensory information and that axons leaving the ventral aspect of the spinal cord are carrying motor information to muscles. Each spinal nerve, then, is composed of a sensory branch and a motor branch (Figure 4.8). All spinal nerves are paired, with one nerve associated with the left side of the spinal cord and one nerve associated with the right side.

(continued)

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CHAPTER 4Section 4.3 The Peripheral Nervous System

Figure 4.8: The sensory and motor branches of spinal nerves

Sensory nerves enter the dorsal aspect of the spinal cord. Motor nerves exit from the ventral aspect of the spinal cord.

Motor branch Sensory branch

Sensory branch

Gray matter of spinal cord

Spinal cord (white matter)

Pia mater

Arachnoid

Dura mater

DorsalVentral

Vertebral column

(backbone)

You have already learned that the spinal cord is divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal. The spinal nerves are named after the region of the spinal cord from which they arise. Because the spinal cord is encased within the vertebral column, the spinal nerves can enter and exit the spinal cord only where there are breaks in the column of bones that make up your backbone (Figure 4.9). Therefore, the number of nerves that arise from each region of the spinal cord is determined by the number of vertebral bones that cover and protect that region.

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CHAPTER 4Section 4.3 The Peripheral Nervous System

Figure 4.9: Spinal nerves exiting between vertebrae

The number of nerves between your vertebrae is determined by the number of vertebral bones that shield that particular region.

Gray matter

Spinal nerve

Dura mater

Vertebra

Go back to Figure 4.2. Here you can see that 8 pairs of nerves arise from the cervical region of the spinal cord, 12 pairs of nerves arise from the thoracic region, 5 pairs of nerves from the lumbar region, 5 pairs of nerves from the sacral region, and 1 pair of nerves from the coccygeal region, for a grand total of 31 pairs of nerves.

Each spinal nerve is named after the region of the spinal cord from which it arises. A nerve from the cervical region is named C1 through C8, depending on where it arises, with C1 being the most superior cervical nerve and C8 the most inferior of the cervical nerves. Thoracic nerves are named T1 through T12, with T1 being most superior and T12 most inferior. Likewise, lumbar nerves are named L1, L2, L3, L4, and L5, and sacral nerves are named S1, S2, S3, S4, and S5. Each spinal nerve innervates a specific area of the body. The body area innervated by one spinal nerve is called a dermatome. Figure 4.10 illustrates the dermatomes associated with each spinal nerve.

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CHAPTER 4Section 4.3 The Peripheral Nervous System

Figure 4.10: Dermatomes

Spinal nerves innervate specific areas of the body: C1–C8 (cervical nerves), T1–T12 (thoracic nerves), L1–L5 (lumbar nerves), and S1–S5 (sacral nerves).

Anterior view Posterior view

S4 S5

T3 T2

T1T1

T5 T6 T7

T10

T11

T12

T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 L1

L3 L4

L2 L3

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CHAPTER 4Section 4.3 The Peripheral Nervous System

When a person suffers an injury to the spinal cord, we identify the injury by its location on the spinal cord. If a woman breaks her neck between the third and fourth cervical vertebrae, we say that she has a C4 injury. In general, the higher the lesion on the spinal cord, the more impairment of movement and sensation we expect to see. A person with a C4 injury, or an injury to the spinal cord above C4, is likely to develop quadriplegia, a disability in which the motor and sensory func- tions of all four limbs are impaired. If you look closely at Figure 4.10, you will see that the hands and forearms receive innervation from C6, C7, and C8. This means that some arm function may be spared with a C5 injury, and the person with a C4 injury will have weak or limited motor control of the forearms.

The Organization of the Peripheral Nervous System This final section will help you gain an understanding of how the peripheral nervous system is distributed across the cranial and spinal nerves. So far, you’ve learned that the peripheral nervous system comprises 12 cranial nerves and 31 spinal nerves. You’ve also learned that the periph- eral nervous system has two divisions: the somatic and autonomic nervous systems, and you’ve learned that the autonomic nervous system is further divided into the sympathetic and parasym- pathetic nervous systems.

Except for cranial nerves I, II, and VIII, all cranial and spinal nerves innervate skeletal muscles and, thus, are part of the somatic nervous system. (Recall that cranial nerves I, II, and VIII carry sen- sory information only and have no motor component.) Not all cranial and spinal nerves innervate smooth muscles, however. Therefore, the autonomic nervous system is not distributed across all nerves. Of the cranial nerves, only cranial nerves III, V, VII, IX, and X innervate smooth muscles (Figure 4.11). Similarly, only the spinal nerves arising from the thoracic, lumbar, and sacral regions of the spinal cord relay information from the autonomic nervous system.

The two divisions of the autonomic nervous system, the sympathetic and the parasympathetic nervous systems, are also distributed unevenly across the spinal and cranial nerves. The sympa- thetic nervous system arises from the thoracic and lumbar regions of the spinal cord. Thus, only the thoracic and lumbar nerves carry sympathetic information. Messages from the sympathetic nervous system are sent to various organs and glands throughout the body, preparing the body to deal with arousing or stressful stimuli. The “Case Study” describes the case of a young boy with cancer of the sympathetic nervous system.

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CHAPTER 4Section 4.3 The Peripheral Nervous System

Figure 4.11: Distribution of the autonomic nervous system

The sympathetic nervous system arises from the thoracic and lumbar regions of the spinal cord. The parasympathetic nervous system arises from the sacral region of the spinal cord and from the brain (via cranial nerves).

C1 2 3 4 5 6 7 8

T1 2 3 4 5 6 7 8 9 10 11 12 L1 2 3 4 5

2 3 4 5

Kidney

T1 2 33 4 55 66 77 88 99 1010 11 1212 L1 2 3 4 5

S1S1S1 22 333 44 55

Eye

Lacrimal gland

Salivary gland

Hair follicle

Sweat gland

Vagus nerve

Blood vessel in periphery

Blood vessel in viscera

Heart

Larynx Trachea Bronchi Lungs

Stomach Adrenal gland

Pancreas

External genitalia

Large intestine

Bladder

Parasympathetic Cranial

Sympathetic Thoracic and lumbar

Parasympathetic Sacral

Adrenal glandgland

Pelvic nerve

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CHAPTER 4Section 4.3 The Peripheral Nervous System

The sympathetic nervous system is sometimes referred to as the thoracolumbar system because it arises from the thoracic and lumbar regions of the spinal cord. Similarly, the parasympathetic nervous system is called the craniosacral system because only certain cranial nerves and nerves from the sacral region of the spinal cord relay parasympathetic information to smooth muscles and organs. Cranial nerves III, V, VII, IX, and X conduct information in the parasympathetic nervous system to smooth muscles and glands in the head and neck. In addition, cranial nerve X carries parasympathetic stimulation to smooth muscles and organs in the chest and upper abdomen, such as the heart and the stomach. Sacral nerves relay parasympathetic stimulation to the smooth muscles and organs of the lower abdomen and pelvic area.

We will come back to a discussion of the autonomic nervous system many times during the course of this book, when we are examining eating behavior, sexual behavior, emotion, and the reaction to stress. Autonomic arousal, especially activation of the sympathetic nervous system, can also affect perception, attention, learning, and a number of cognitive processes. Therefore, it is impor- tant that you understand thoroughly the differences between the two divisions of the autonomic nervous system, the sympathetic and parasympathetic nervous systems.

Case Study: Neuroblastoma

Robyn was a happy, friendly baby who had big blue eyes and a ready smile. Soon after he first learned to walk, Robyn began to complain of pain in his back. He would take a few steps, grab his sides, and groan. When he started to talk, his first words included “Ow!” and “Hurt!”

Robyn’s mother took Robyn to his pediatri- cian, the first of many physicians who exam- ined Robyn. The pediatrician was doubtful that Robyn was really experiencing pain and suggested that he was just calling for more attention. However, Robyn continued to complain about pain in his back, especially

when he walked. His mother had him examined by two other pediatricians who could not find anything wrong.

When Robyn was 3 years old, his mother took him to a family practitioner who referred him to a phy- sician who specialized in the treatment of arthritis. The arthritis specialist examined Robyn carefully before he said to Robyn’s mother, “Your son does not have arthritis. He has neuroblastoma.”

Neuroblastoma is a form of cancer that strikes the neurons of the sympathetic nervous system. Tumors were found in the thoracic and lumbar areas of Robyn’s back, a common origin of neuroblastoma. That explained why Robyn experienced pain in the middle of his back. Over time, tumors spread along the entire distribution of Robyn’s sympathetic nervous system, to his lungs, liver, and other organs. Tumors developed even in Robyn’s eyes, blinding him. A cheerful little boy until the end, Robyn died shortly before his fifth birthday.

Nci-Tsokos/Phanie/SuperStock

Photo 4.6 Neuroblastoma is a form of cancer that strikes the neurons of the sympathetic nervous system. Shown is a microscopic view of neuroblastoma cells.

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CHAPTER 4Section 4.4 Chapter Summary

4.4 Chapter Summary An Overview of the Central Nervous System

• The central nervous system is composed of the brain and spinal cord. • The central nervous has two major functions: (1) to receive and act upon information

received from the peripheral nervous system and (2) to initiate thoughts and behaviors independently without prompting from the peripheral nervous system.

Protecting the Central Nervous System • The central nervous system is protected by bone and the meninges, which has three

layers. • The outermost layer of the meninges is the dura mater, the middle layer is the arachnoid

layer, and the innermost layer is the pia mater. • Vessels in the arachnoid layer contain a fluid found only in the central nervous system

called cerebrospinal fluid.

The Spinal Cord • The spinal cord is divided into five sections: cervical, thoracic, lumbar, sacral, and coccy-

geal, each of which controls the muscles and organs in a specific area of the body.

The Brain • The brain has three subdivisions: the hindbrain, the midbrain, and the forebrain.

The Hindbrain • The medulla, pons, and cerebellum are the major structures in the hindbrain. • Neurons in the medulla regulate life-support functions, receive sensory information, or

send motor commands to muscles in the head, neck, and trunk. • The pons permits passage of information between the spinal cord and higher regions of

the brain. • The cerebellum coordinates muscular activity and is especially vulnerable to alcohol,

which impairs its functioning.

The Midbrain • In the midbrain are located the superior and inferior colliculi, the tegmentum, and the

reticular formation. • The superior colliculus processes visual information, and the inferior colliculus processes

auditory information. • The reticular formation runs from the hindbrain to the forebrain via the midbrain and

plays an important role in keeping the individual awake and alert.

The Forebrain • The forebrain is the largest part of the human brain. • The forebrain contains the diencephalon, the basal ganglia, the limbic system, and the

cerebrum.

The Diencephalon • The diencephalon is composed of the thalamus and hypothalamus. • The thalamus contains a cluster of nuclei that relay information to and from the cerebrum. • The hypothalamus is involved in the regulation of the pituitary gland and motivates behav-

iors such as sleeping, eating, and drinking.

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CHAPTER 4Web Links

• Together, the midbrain, hindbrain, and diencephalon make up the brain stem, which regu- lates our most primitive behaviors, such as breathing, sleep, sexual behavior, drinking, and eating.

• The limbic system produces emotional behavior and contains the hippocampus, amyg- dala, and septum.

• The hippocampus is a seahorse-shaped structure that plays a role in emotion as well as the creation of long-term memories.

• The amygdala is involved in emotional behaviors such as fear, escape, rage, and aggression. • The basal ganglia are composed of clusters of neurons located in the base of the forebrain

and play a role in the production of movement. • The cerebrum is the largest structure in the brain and is organized like the cerebellum with

two hemispheres, a cortical layer (the cerebral cortex, which contains the cell bodies of hundreds of millions of neurons) and underlying white matter.

• Mental activities requiring consciousness are processed in the four lobes of the cerebrum. • The frontal lobes consist of three regions: the motor cortex, the premotor cortex, and the

prefrontal cortex. Processing of somatosensation is one of the principal functions of the parietal lobes, and processing of vision is the principal function of the occipital lobes. The temporal lobes are involved in a number of functions, including hearing, taste, smell, and emotions.

The Peripheral Nervous System • The peripheral nervous system is composed of 12 cranial and 31 spinal nerves.

The Cranial Nerves • Cranial nerves allow for direct communication between the brain and the peripheral ner-

vous system.

The Spinal Nerves • Spinal nerves are named after the five regions of the spinal cord: cervical, thoracic, lum-

bar, sacral, and coccygeal. • The body region innervated by one spinal nerve is called a dermatome.

Questions for Thought

1. Which parts of the brain are highly developed in alligators? In birds? How did you reach your conclusion?

2. How do you think the limbic system influences the functioning of the cerebrum? 3. How does the function of the cerebellum differ from that of the basal ganglia?

Web Links

To put brain development into context, search “Adolescent Brain Development” on the U.S. Department of Health & Human Services’ website. Here you will learn about the factors that contribute to brain development in adolescents and the effects on teenage behavior, including decision-making and risk-taking. http://www.hhs.gov/

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http://www.hhs.gov/

CHAPTER 4Key Terms

To learn more about disorders that affect the peripheral nervous system, visit the MedlinePlus website. Information provided includes symptoms, diagnosis, treatment, and the recent studies of these disorders. http://medlineplus.gov

Key Terms

amygdala An almond-shaped structure located in the medial temporal lobe that is implicated in the experience of negative emotions.

arachnoid layer The middle layer of the meninges.

basal ganglia A group of subcortical nuclei that sends and receives information about movement to and from the cerebrum.

brain stem Consisting of the midbrain, hind- brain, and diencephalon, the brain stem regulates our most primitive behaviors, such as breathing, sleep, sexual behavior, drinking, and eating.

cerebellum The hindbrain structure that controls motor coordination and coordinates movement in response to sensory stimuli.

cerebral cortex The outermost layers of the cerebrum that contain neurons.

cerebral hemispheres The two (left and right) halves of the cerebrum.

cerebrospinal fluid A watery fluid found in the ventricles of the brain and in the spinal cord.

cerebrum The largest structure in the brain, believed to be the seat of consciousness.

cervical The region of the spinal cord located in the neck.

coccygeal The region of the spinal cord located at the base of the spine.

corpus callosum The large tract that con- nects neurons in the left and right cerebral hemispheres.

cranial nerves Twelve pairs of nerves that enter and exit the brain through holes in the skull.

diencephalon The forebrain area that con- tains the thalamus and hypothalamus.

dura mater The tough outer layer of the meninges.

forebrain The largest part of the brain; it contains the cerebrum, basal ganglia, limbic system, thalamus, and hypothalamus.

frontal lobe The most anterior lobe of the cerebrum; it contains the motor cortex and other centers of executive function.

ganglion A cluster of neuronal soma in the peripheral nervous system.

hindbrain The part of the brain immediately superior to the spinal cord; it consists of the medulla, pons, and cerebellum.

hippocampus The limbic system structure that plays a role in emotions and memory.

hypothalamus A structure in the diencepha- lon that controls the pituitary gland and regu- lates motivated behaviors.

inferior colliculi Midbrain structures that receive and process auditory information.

limbic system The brain structures that regu- late emotions.

lumbar The region of the spinal cord located in the lower back.

medulla The hindbrain structure that controls life-support functions.

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http://medlineplus.gov

CHAPTER 4Key Terms

meninges The protective covering of the cen- tral nervous system.

meningitis An infection of the meninges pro- duced by bacteria.

midbrain The middle section of the brain; it consists of the tectum, the tegmentum, and a ventral region that contains the reticular formation.

motor cortex An area in the frontal lobe that directs fine motor coordination.

nerve A bundle of axons in the peripheral nervous system.

occipital lobe The most posterior lobe of the cerebrum; it processes visual information.

parietal lobe The region of the cerebrum located immediately posterior to the frontal lobe; it processes somatic information relayed from the body.

pia mater The innermost layer of the meninges.

pituitary gland The master gland located below the ventral surface of the brain; it is connected to the hypothalamus by a stalk and receives commands from the hypothalamus to regulate the activity of other major glands in the body.

pons The hindbrain structure that relays infor- mation from the medulla to the higher brain structures.

prefrontal cortex The most anterior region of the frontal lobe that controls executive decision- making functions.

premotor cortex An area of the frontal lobe involved in planning, organizing, and integrat- ing movements of the head, trunk, and limbs.

reticular formation A group of neurons located in the brain stem that alerts the fore- brain to important stimuli.

sacral The region of the spinal cord located in the pelvic girdle.

septum The structure in the limbic system associated with positive emotions and pleasur- able feelings.

spina bifida A disorder in which the spinal cord is left exposed due to failure of the back- bone to close during embryonic development.

spinal nerves Nerves that enter and exit the spinal cord between bones of the spinal column.

superior colliculi Midbrain structures that receive visual information and process the location of objects in the environment.

temporal lobe The most inferior lobe of the cerebrum; it processes auditory, smell, and taste information and contains the hippocam- pus and amygdala.

thalamus A structure in the diencephalon that relays information from the brain stem to the cerebrum.

thoracic The region of the spinal cord located in the chest.

ventricles Cavities in the brain that contain cerebrospinal fluid.

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