2 pages due in 16 hours

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Learning Objectives

After completing this chapter, you should be able to:

• Discuss the differences between the central and peripheral nervous systems, the somatic and autonomic nervous systems, and the sympathetic and parasympathetic nervous systems.

• Give examples of body changes associated with activation of the sympathetic nervous system. • Identify the major organelles in a neuron. • Describe how neurons differ from other cells in the body. • Explain the differences between unipolar, bipolar, and multipolar neurons and between motor neurons,

sensory neurons, and interneurons. • List the functions of astroglia, microglia, radial glia, oligodendrocytes, and Schwann cells. • Draw a picture of an action potential and describe the actions of sodium and potassium during an action

potential. • Define summation and explain its role in the production of an action potential. • Compare excitation and inhibition of neurons.

2

Introduction to the Nervous System

PASIEKA/Science Photo Library/Corbis

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CHAPTER 2Section 2.1 The Organization of the Nervous System

Camille, a psychology major, was a junior in college when she began to experience some troubling symptoms. Sometimes she had trouble lifting her legs when climbing stairs, and sometimes her hands and arms stiffened when she was typing on the computer keyboard. Most troubling was the double vision that Camille experienced when she tried to read for long periods. The words on the pages of her textbook would swim around when she studied, making it difficult for her to focus on her reading.

During winter break, Camille made an appointment to see her doctor in her hometown. She told her physician about her symptoms, including the intermittent weakness in her arms and legs and her double vision. Camille’s physician ordered a number of tests for her. Before she returned to spring semester classes, Camille learned that she had developed multiple sclerosis, a disorder in which the covering on her nerves progressively deteriorates. When the nerves lose their protective covering, information cannot be transmitted effectively from the brain to muscles. Thus, Camille was slowly losing control of the muscles in her arms, legs, and head.

In this chapter we will examine the nervous system and the important cells, called neurons and glial cells, that make up the nervous system. We will look at the function of neurons and glial cells, and we will discuss how information is transmitted within a neuron. Later in the chapter, we will come back to the topic of multiple sclerosis and examine the cause of this devastating disorder. First, let’s focus on the organization of the nervous system.

2.1 The Organization of the Nervous System

My son, Tony, came home from school one day and shared with me a tidbit that he had learned in his fourth-grade science class: “Systems are made of organs, organs are made of tissues, and tissues are made of cells.” A bit simplistic perhaps, but it’s a good place to begin our study of the nervous system. As you probably learned in fourth grade, or sometime in elementary school, our bodies are composed of systems that have particular functions that serve to keep us alive: for example, the digestive system, the respiratory system, the immune system, the urinary system, the skeletomuscular system, and the cardiovascular system. The nervous system, which is the focus of this textbook, is just another of the body’s many systems.

Some systems are confined to particular regions of our bodies. For example, the respiratory sys- tem is located in the chest (the lungs), the neck (the trachea), and the head (throat, mouth, and nasal passages). The nervous system, however, is more similar to the cardiovascular system, which is spread from head to toe, fingertip to fingertip, in our bodies. Like the cardiovascular system, the nervous system courses throughout the entire body, sending messages to all parts of the body except for the epidermis (the dead layers of skin), the fingernails and toenails, and hair.

The brain is the major organ of the nervous system, but many other structures make up the ner- vous system as well. The nervous system consists of the brain, the spinal cord, and the extensive pathways of nervous tissue located throughout the body. It is divided into the central nervous system and the peripheral nervous system. The brain and the spinal cord compose the central nervous system, whereas all the nervous tissue located outside the brain and spinal cord com- pose what is known as the peripheral nervous system (Figure 2.1). In the next section of this chapter, we’ll examine the organization of the peripheral nervous system. We will examine the organization of the central nervous system in Chapter 4.

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CHAPTER 2

Brain

Cerebellum

Skull

Spinal cord

Central nervous system

1st lumbar vertebra

Sacrum

Nerves of peripheral nervous system

1st cervical vertebra

1st thoracic vertebra

Section 2.1 The Organization of the Nervous System

Organization of the Peripheral Nervous System

The peripheral nervous system has two divisions, the somatic nervous system and the autonomic nervous system. The somatic nervous system controls striated muscles, which get their name from their striated, or striped, appearance under the microscope. Striated muscles are attached to the bones of the skeleton and are sometimes referred to as skeletal muscles. In addition, sensory information arising from the skeletal muscles and the skin is relayed to the brain and spinal cord by the somatic nervous system.

Think about how you move about: Skeletal muscles, which are attached to your bones, contract, pulling the bones in one direction or another. These muscles are under your voluntary control. If you decide to wiggle your toes, you can—thanks to your somatic nervous system. The same is true with raising your hand in class, jotting down notes, or asking a question. The somatic nervous system allows you to move your skeleton about voluntarily.

Figure 2.1: The nervous system

The nervous system has two divisions: the central nervous system (composed of the brain and spinal cord) and the peripheral nervous system (all of the nervous tissue located outside the brain and spinal cord).

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CHAPTER 2Section 2.1 The Organization of the Nervous System

In contrast, the autonomic nervous system is not ordinarily under your conscious control. It acts automatically, in response to signals from the central nervous system. Smooth muscles are con- trolled by the autonomic nervous system. They have a characteristically smooth appearance under the microscope and are found throughout your body. For example, smooth muscles control the dilation and constriction of the pupils in your eyes. They also regulate the dilation and constriction of blood vessels. One of the reasons that blushing is so embarrassing for people who blush very noticeably is that they can’t control it. That is, blushers turn quite red in the face due to activation of the autonomic nervous system, and there is nothing they can do to stop it.

Another example of smooth muscles is the muscles that open and close the ducts in certain glands in the body, like the sweat glands or salivary glands. You cannot will yourself to stop sweating. It is under the control of the autonomic nervous system. Mammary glands, too, are regulated by the autonomic nervous system, which makes it impossible to stop the flow of milk from the nipples if you are a nursing mother.

Many organs, too, are lined with smooth muscles (for example, the stomach, the small and large intestines, the uterus, and the bladder). This means that you cannot consciously make your stom- ach digest your dinner faster, and you cannot will the uterus to stop contracting if you are giving birth. The heart is composed of a special type of muscle, called cardiac muscle, that closely resem- bles smooth muscle. Cardiac muscle is also under the control of the autonomic nervous system.

With special training, people can gain some control over the autonomic nervous system. Yogi masters, who are trained in meditation and body exercises, can control autonomic functions such as heart rate, brain waves, and body metabolism. However, you don’t have to be a yogi master to control the autonomic nervous system. Anyone can learn control over smooth muscles through biofeedback training. Biofeedback involves giving the trainee information, or feedback, about the state of a particular autonomic function, in an operant conditioning paradigm. This feedback acts as a reward that reinforces changes in autonomic function. For example, individuals can learn to lower their blood pressure through biofeedback (Di Cara & Miller, 1968; Nakao, Nomura, Shimo- sawa, Fujita, & Kuboki, 2000; Norris, Lee, Burshteyn, & Lea-Aravena, 2001). Information about a decrease in blood pressure is given to the trainee in the form of an auditory stimulus such as a tone. That is, a tone is heard whenever the trainee has a decrease in blood pressure. The trainee then tries to keep the tone on for as long as possible, thereby lowering blood pressure.

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CHAPTER 2Section 2.1 The Organization of the Nervous System

Organization of the Autonomic 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. It functions to prepare the body for an emergency, channeling resources so that a person can react quickly and effectively. Activa- tion of the sympathetic nervous system causes the heart to beat faster and breathing to speed up. It also shunts blood from the organs in the gut to the skeletal muscles, readying an individual to respond to a stressful situation. In 1927 Walter Cannon, an eminent American physiologist, referred to the responses of the sympathetic nervous system as fight-or-flight reactions. In the face of a stressful stimulus such as a verbal threat, we typically make one of two responses: We attack, or we retreat. The sympathetic nervous system organizes the body’s reaction to fight or flee.

Think about what happens when a person tries to eat when upset. Imagine that you have pre- pared your lunch and are just sitting down to eat when you receive a phone call from a friend who tells you that another close friend had just been seriously injured in an automobile accident. Immediately, your heart begins to pound, you break into a sweat, and your breathing becomes more rapid—all sympathetic responses. You sit down and try to eat, but the food is tasteless, hard to swallow, and sits like a rock in your stomach after you choke it down. This is because your saliva becomes thick and scant when the sympathetic nervous system is activated, making it difficult to taste and swallow food. The food in your stomach doesn’t digest readily because the stomach has been turned off and blood has been diverted from your stomach to your skeletal muscles.

In contrast, the parasympathetic nervous system plays an energy-conserving role. Picture your- self lounging on the sofa after a quiet, filling meal. You are in a parasympathetic state, totally relaxed, almost falling asleep. Your breathing is slow and regular. Your heart rate is decreased, too. Your stomach and intestines are engorged with blood, and these organs contract rhythmically in a process called peristalsis as your dinner is digested and absorbed. The functions of the autonomic nervous system are summarized in Figure 2.2.

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CHAPTER 2Section 2.1 The Organization of the Nervous System

Figure 2.2: Functions of the sympathetic and parasympathetic nervous systems

The two divisions of the autonomic nervous system, the sympathetic and parasympathetic nervous systems, produce opposite effects throughout the body.

As you will learn, the nervous system is quite complex. Information from the peripheral nervous system is processed in the central nervous system. In response to this information, the central nervous system sends out orders to the peripheral nervous system, directing the action of muscles and glands. We can identify the nervous system anywhere in the body because of the presence of nervous tissue, which consists of two different types of cells, neurons and glia.

Dilates pupil

Relaxes bronchi

Accelerates, strengthens heartbeat

Inhibits activity

Constricts vessels

Constricts pupil

Sympathetic

1. Eyes

2. Lungs

3. Heart

4. Stomach, intestines

5. Blood vessels of internal organs

Parasympathetic

Constricts bronchi

Slows heartbeat

Stimulates activity

Dilates vessels

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CHAPTER 2Section 2.2 The Structure of Neurons

Originally, scientists believed that nervous tissue, unlike other tissue, was not made of cells. When examined under the microscope, neurons are typically bunched together in tight clumps, mak- ing it difficult to discern where one cell ends and another begins, especially given their irregular shapes. It wasn’t until the very end of the 19th century when Santiago Ramon y Cajal, using a new staining technique developed by Camillo Golgi, was able to demonstrate that the brain is com- posed of a large number of cells, which he called “neurones” (Ramon y Cajal, 1933). Ramon y Cajal and Golgi were jointly awarded the Nobel Prize for their discovery of neurons in 1906. The glia, or glial cells, were identified shortly after neurons were discovered.

In this chapter we will examine in great detail the structure and function of the neuron and the glial cell. These two types of cells are considered to be the building blocks of the central and peripheral nervous systems. In the next section of this chapter, we’ll consider neurons.

2.2 The Structure of Neurons

Neurons are very much like other cells in the body. Each has a cell body, or soma (plural is somata), that is filled with a watery liquid called cytoplasm and is bounded by a cell membrane (Figure 2.3). Inside the soma, various tiny structures, called organelles, are found: the nucleus, nucleolus, endoplasmic reticulum, Golgi complex, microsomes, mitochondria, and ribo- somes. You probably learned about organelles some time ago in a science class, but let’s review the functions of the most important organelles in the neuron now.

Figure 2.3: Parts of the cell

The cell body, or soma, is bounded by a cell membrane and contains numerous structures, including the nucleus, ribosomes, and mitochondria.

Cell membrane

Golgi apparatus

Chromatin

Vacuole

Food storage particle

Granular endoplasmic reticulum

Smooth endoplasmic reticulum

Mitochondrion

Nuclear membrane

Nucleolus

Lysosome

Secretion granule

Ribosome

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CHAPTER 2Section 2.2 The Structure of Neurons

The nucleus of a cell is one of the most prominent structures in the cell. Under the microscope, even the most inexperienced eye can pick out this rather large, roundish structure. The nucleus is important because it contains the genetic information of the cell. This information is coded in the form of strings of nucleic acid that are located in chromosomes. The human cell typically contains 23 pairs of chromosomes, except for ova and sperm cells, which contain 23 unpaired chromosomes, as we will discuss in Chapter 10. Of these 23 pairs of chromosomes, 22 are called autosomes, and the final pair is referred to as the sex chromosomes, designated X and Y chromo- somes. Altogether, approximately 25,000 genes are encoded in 23 pairs of chromosomes in each human cell. The sum total of these genes is called the genome, and the same genome is found in every cell in an individual’s body.

Nucleic acids are specialized chemicals that are found in abundance in the nucleus of all cells. There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is found in chromosomes in the nucleus of the cell, whereas RNA is generally located in ribo- somes. In addition, DNA is composed of four nucleotide bases, known as adenine (A), guanine (G), cytosine (C), and thymine (T). The building blocks of RNA, on the other hand, are adenine (A), guanine (G), cytosine (C), and uracil (U).

Genetic variations are produced when the normal sequence of nucleotide bases in DNA is dis- rupted. One example of a genetic alteration occurs in fragile X syndrome, which produces intel- lectual disabilities in afflicted individuals. The problem has been associated with a repeat of three bases (CGG) on the X chromosome. Normally, the CGG triad is repeated 10 to 30 times on the X chromosome. In individuals with fragile X syndrome, however, this triad is repeated hundreds of times, producing a weakened, fragile arm of the affected X chromosome (Plomin, 1999). It is important to remember that fragile X is only one form of intellectual disability and that not all developmental disabilities are associated with the X chromosome. For example, intellectual dis- abilities that result from untreated phenylketonuria (PKU) have been linked to an altered gene on chromosome 12.

Genetic alterations are also produced when chromosomes are missing or when extra chromosomes are pres- ent in the cell nucleus. For example, people with Down syndrome (see Photo 2.1) have three copies of chro- mosome 21 instead of the usual pair of chromosomes (Photo 2.2). This chromosomal abnormality contrib- utes to faulty development of the brain, which leads to impairment of cognition. Thus, an individual with Down syndrome will present with intellectual disabilities, as well as a number of other skeletal and soft tis- sue abnormalities.Richard Hutchings/Science Source

Photo 2.1 Those with Down syndrome will present with intel- lectual deficits and a number of soft tissue abnormalities.

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CHAPTER 2Section 2.2 The Structure of Neurons

Ribosomes are tiny cellular structures responsible for the pro- duction of protein in the cell. RNA translation, which involves decoding strings of nucleotide bases into sequences of amino acids, occurs in the ribosomes (Tjian, 1995). Each sequence of amino acids is a particular protein that has a specific function in the cell. Some proteins are used by neurons to manufac- ture special substances called neurotransmitters, which are unique to neurons. Neurotransmitters permit the neuron to carry out its most important function, which is communicat- ing with other cells in the body. In Chapter 3 we will examine many different neurotransmitters and their functions.

The final organelle that I would like to bring to your attention is the mitochondrion (plural is mitochondria). Mitochondria have a very important function: to produce the fuel, or energy source, of the cell. For all cells, this energy source is adenosine triphosphate (ATP). Cells use ATP to fuel most metabolic reac- tions that keep us alive. Because of the special work that they do, neurons require a lot of ATP. Therefore, mitochondria are found in large numbers throughout the neuron.

Mitochondria are also of extreme interest to behavioral geneticists, who study the role that genes play in the devel- opment of certain behaviors. This interest is due to the fact that mitochondria contain DNA. A number of disorders have been associated with mutations in mitochondrial DNA, including migraine headaches, movement disorders, mental depression, diabetes accompanied by blind- ness and deafness, and neurodegenerative brain disorders that produce seizures, blindness, deaf- ness, and severe headaches (Graf et al., 2000; Hanna & Bhatia, 1997; Hofmann et al., 1997; Kato, 2001; Katz, Newman, & Izenwasser, 1997; Kerrison, Howell, Miller, Hirst, & Green, 1995; Montine, Powers, Vogel, & Radtke, 1995; Onishi et al., 1997; Russell, Diamant, & Norby, 1997; Santorelli et al., 1997; Suomalainen, 1997; Uncini et al., 1995). Mitochondrial DNA is always inherited from the mother, whereas nuclear DNA is inherited from both parents. This is because ova contain mitochondria, whereas sperm do not. When an egg and sperm are united during fertilization, only the egg brings mitochondria to the newly created individual, and thus only the mother provides mitochondrial DNA.

The neuron is a cell and contains cytoplasm and the organelles found in other kinds of cells. But it is no ordinary cell. First, the neuron has a special function: gathering, processing, and send- ing information to other cells and communicating with the outside world. Second, as you’ve just learned, neurons manufacture neurotransmitters, which are used to signal other cells.

The neuron also differs from other types of cells in its appearance. Look closely at Figure 2.4. Can you spot any differences between neurons and other cells? You may notice that neurons have many projections, whereas most other cells have a smooth appearance. These projections found on neurons have names: dendrites and axons.

Leonard Lessin/Science Source

Photo 2.2 Down syndrome is associ- ated with the presence of three copies of chromosome 21.

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CHAPTER 2Section 2.2 The Structure of Neurons

Figure 2.4: Different types of cells

Neurons have a distinctly different appearance compared to other cells.

A neuron typically has many dendrites and one axon. Each dendrite is multibranched, and some are covered with dendritic spines, which are short spikes that increase the dendrites’ ability to receive messages. Most signals reaching the neuron are received by the dendrites, which have special proteins on their surface to process the signal. There is evidence that dendritic spines can change shape and thus alter the messages that are received (Koch, Zador, & Brown, 1992). The axon is a tube-shaped projection that arises from a thickened area on the soma known as the axon hillock. Unlike the dendrites, the axon is usually unbranched except at its end, where it branches into numerous button-shaped endings called terminal buttons in English or, more commonly, ter- minal boutons, as they were named by their French discoverers.

Figure 2.5 is an illustration of a typical neuron. You can see that dendrites are much shorter than the axon. In fact, in some neurons, they can be more than 1,000 times shorter than an axon, given that axons can be 1 meter (m) or more long and that most dendrites are less than 1 millimeter (mm, or 10–3 m) long. By contrast, the soma of a neuron is usually measured in micrometers (μm, or 10–6 m).

A. Stomach

D. Artery E. Neuron F. Blood cells

B. Skin C. Heart muscle

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CHAPTER 2Section 2.2 The Structure of Neurons

Figure 2.5: A typical neuron

Based on this neuron’s shape, how is its function different from the neurons in Figure 2.6?

The enormous length of the axon, relatively speaking, gives us a clue as to its function. Why would a neuron possess a long projection that is a million times longer than the diameter of its soma? The axon obviously stretches far away from the cell body, which means that it is capable of taking signals from the soma to cells in other parts of the body. For example, thousands of axons from neurons in your spinal cord extend down your leg to communicate with muscles in your foot.

Not all neurons have extremely long axons, however. In fact, most neurons, more than 90%, are interneurons whose axons and dendrites are very short and do not extend beyond their cell clus- ter. These neurons are also called intrinsic or local neurons because they exchange messages with neighboring neurons and do not transmit information over long distances. Interneurons are found in both the peripheral and central nervous systems.

Classifying Neurons

Figure 2.6 illustrates the wide range of shapes and sizes of neurons. Ramon y Cajal, the inves- tigator who discovered neurons, classified all neurons into one of three groups, based on the number of processes possessed by the neuron: (1) unipolar neurons, (2) bipolar neurons, and (3) multipolar neurons (Ramon y Cajal, 1933). A unipolar neuron (Figure 2.6a) has only one process, which typically branches a short distance from the soma, whereas a bipolar neuron possesses two processes: one dendrite and one axon (Figure 2.6b). The third type of neuron, the multipolar neuron, has three or more processes, typically many dendrites and only one axon (Figure 2.6c). Multipolar neurons, which make up the majority of all neurons, are found through- out the central and peripheral nervous systems.

Nucleus

Dendrite Cell body

Axon hillock Axon Node of

Ranvier

Terminal button

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CHAPTER 2Section 2.2 The Structure of Neurons

Figure 2.6: Different shapes of neurons

Neurons differ in form and function. Typical forms include: (a) unipolar, (b) bipolar, and (c) multipolar.

Classification System for Neurons Based on Function The system that is widely used to classify neurons in the spinal cord is based on the function of the neuron. There are three types of neurons in this classification system: (1) motor neurons, (2) sensory neurons, and (3) interneurons.

Motor neurons carry information from the central nervous system to muscles and glands. Their soma are located in the central nervous system, and the terminal buttons of their axons are found interspersed among muscle fibers. When a motor neuron is excited, it produces contraction of muscle fibers.

A cross section of the spinal cord is illustrated in Figure 2.7. Examine this diagram closely. You will notice that the spinal cord, when cut in cross section, looks like a gray butterfly surrounded by a white border. The gray area, known as gray matter, contains the soma of neurons located in the spinal cord. The white area, or white matter, is comprised of axons. The axons appear white because most axons are covered with a white, fatty substance called myelin. Multiple sclerosis, the disorder described in the opening box, is caused by deterioration of myelin, which disrupts the flow of information down axons, interfering with smooth movement.

A. Unipolar neuron

B. Bipolar neuron

C. Multipolar neuron

Cell body

Axon

Cell body

Cell body

Axon

Axon

Dendrite

Dendrite

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CHAPTER 2Section 2.2 The Structure of Neurons

Figure 2.7: Cross section of the spinal cord

Sensory information enters the dorsal aspect of the spinal cord, and motor information exits via the ventral root.

In addition, the spinal cord can be divided into two portions: the ventral aspect and the dorsal aspect. The ventral aspect of the spinal cord is the part of the spinal cord that is closest to your belly, whereas the dorsal aspect is closest to your back. The important point to remember here is that we are all “wired” alike: Motor neurons are always found in the ventral portion of the spinal cord. Notice in Figure 2.7 that the soma of motor neurons are located in the gray matter in the ventral region of the spinal cord. Their axons leave the spinal cord and course through the periph- eral nervous system to reach the muscle fibers that each stimulates.

Motor neurons are also located in the brain. In the brain, the soma of motor neurons are grouped together in little nests of gray matter called nuclei. These motor nuclei are situated throughout the lower regions of the brain. Axons from the motor neurons leave the brain stem nuclei and course through the peripheral nervous system to stimulate muscles in the head and neck. Motor neurons located in the brain control muscles in the head and neck, whereas motor neurons in the spinal cord control muscles from the shoulders on down to the toes.

Sensory neurons are located in the peripheral nervous system and carry information to the central nervous system. The cell bodies of sensory neurons are found in the peripheral nervous system. Their axons enter the