250wrd

Images.com/Corbis

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 brain or spinal cord, where they transmit information about the outside world to the central nervous system.

Sensory neurons situated anywhere in the body below the neck normally have axons that termi- nate in the spinal cord. In Figure 2.7 you can see that the axons of sensory neurons enter the spinal cord on its dorsal side. Located next to the dorsal aspect of the spinal cord are the dorsal root gan- glia, where the soma of many sensory neurons are found. Axons leave the dorsal root ganglion and enter the dorsal root of the spinal cord, along with all the other axons that enter the spinal cord.

Take a moment to think about the organization of the spinal cord. The ventral portion contains motor neurons, and its dorsal aspect processes information from sensory neurons. According to the Bell-Magendie law, motor nerves exit from the ventral areas of the spinal cord, and sensory

Gray matter Sensory neurons Dorsal

Ventral

White matter Motor neurons

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CHAPTER 2Section 2.2 The Structure of Neurons

information enters the dorsal areas. What can you predict about damage to the spinal cord? Dam- age to the ventral part of the spinal cord will produce motor dysfunction, whereas damage to the dorsal part will impair sensation. Obviously, damage to the entire spinal cord, as when the spinal cord is totally transected or torn apart, will affect both sensory and motor functions.

Sensory neurons in the head and neck send their axons directly to the brain, via cranial nerves, which we will examine in Chapter 4. Therefore, damage to the spinal cord does not affect sensory or motor functions of the head. Damage to a cranial nerve will impair the specific sense associ- ated with that nerve. For example, damage to the olfactory nerve, or cranial nerve I, will impair the sense of smell, and injury to the optic nerve, also known as cranial nerve II, will impair vision.

In the spinal cord an interneuron is a neuron that receives information from one neuron and passes it on to another neuron. That is, as its name implies [inter means “between”], an inter- neuron is situated between two other neurons. The axons and dendrites of interneurons do not extend beyond their cell clusters in the gray matter of the spinal cord. See Figure 2.7 for their loca- tion in the spinal cord.

Special Features of Neurons

As you’ve just learned, neurons are a unique type of cell. They have many special features that set them apart from other cells in the body. Before we launch into a discussion of the function of neurons, let’s consider a number of their distinctive characteristics, because these will help you understand better the peculiar nature of neurons.

You’ve already learned about two special features of neurons. Neurons have unusual shapes, with dendrites and long processes called axons. In addition, neurons produce unique substances called neurotransmitters, which they use to transmit signals.

Another special feature of neurons is their enormous and constant need for oxygen, more than any other cell in the body. Neurons require large amounts of oxygen, even when they are inactive or “resting.” Remember that neurons have large numbers of mitochondria throughout their cell bodies, dendrites, and axons. These mitochondria are miniature factories that use huge quanti- ties of oxygen to produce ATP, the principal fuel of the cell. Neurons that don’t get enough oxygen cannot make ATP, which supports the cell’s vital functions. Think about it for a moment. Neurons keep us alive: They keep us awake and conscious, they allow us to see and hear, they make us breathe. If your brain does not get the oxygen it needs, if it is deprived of oxygen for more than 10 to 15 seconds, then you lose consciousness. If oxygen deprivation continues for longer than that, coma and, finally, death ensue. The “Case Study” in this chapter illustrates the effects of oxygen deprivation on the brain.

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CHAPTER 2Section 2.2 The Structure of Neurons

Another unusual characteristic of neurons is that they are picky eaters. The other cells in the body can metabolize almost any nutrient. For example, muscles prefer fat but can also metabolize protein and carbohydrates. Neurons, however, can only metabolize glucose, a simple sugar. This means that any shortage in glucose supply will affect neuron function and that any disorder that affects glucose availability will affect the brain and, ultimately, behavior. Perhaps the best-known glucose-related disorder is diabetes, which afflicts more than 25 million North American men, women, and children. People with diabetes lack adequate amounts of insulin, which is needed to transport glucose out of the blood and into cells. Without insulin, diabetic individuals cannot get glucose into their neurons, and hence, neurons cannot function properly. Diabetic coma, a state of unconsciousness in which the individual does not respond to any stimulation, occurs when people with diabetes do not take their prescribed insulin and their neurons become starved for glucose.

Still another unique feature of neurons is their inability to regenerate. Other cells in the body, except for fat cells, will regenerate to replace dead cells. For example, skin cells are replaced every few days. On the other hand, when neurons in the central nervous system are damaged or die,

Case Study: Baby Trapped in a Recliner

Baby Amanda was nearly 15 months old when she was admitted to a metropolitan hospital in a coma. Shortly before the ambulance brought her to the hospital, Amanda had been toddling happily around the living room of her home, where she lived with her mother and father. Unfortunately, someone had left a recliner in its laid-back position, with its back down and its footstool up. Amanda threw herself down on the footstool, giggling. The weight of her body pushed the footstool down, causing the chair to automatically assume its upright position.

Amanda’s neck was trapped in the space between the footstool and the chair, and when the chair folded upright, the footstool squeezed against her neck, obstructing the flow of blood to her brain. Amanda’s mother was in the living room at the time of this incident, and she raced over to the chair as soon as she realized what had happened. It took her nearly 20 seconds to get Amanda free from the chair. However, by that time, Amanda was purple in the face and unconscious. Unable to revive her baby, Amanda’s mother dialed 911 and summoned an ambulance.

When Amanda arrived at the hospital, she was breathing on her own but did not respond to any stimulation whatsoever. She was examined by a team of neurologists who determined that she was in a coma. The anoxia [an- means “without,” and -oxia means “oxygen”] caused by the compression of the chair on her neck, which stopped the flow of blood to her brain, damaged Amanda’s brain to such an extent that she was not expected to regain consciousness. Amanda remained in a vegetative state until her death. This sad (but true) story illustrates that the brain requires a constant supply of oxygen. Irreversible brain damage occurs when the brain is deprived of oxygen, even for a short time.

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CHAPTER 2Section 2.2 The Structure of Neurons

they generally are not replaced, although there is some evidence that cells in some parts of the brain do regenerate (Barinaga, 1998; Gould & McEwen, 1993). What does this mean for brain function? Obviously, if enough brain cells are destroyed, the affected person’s behavior could be severely disrupted, as happens in chronic alcoholism. A large percentage of chronic alcoholics show evidence of brain damage and develop symptoms of mental confusion and memory loss (Sugawara, Namura, & Hishikawa, 1997).

Finally, unlike other cells in the body, neurons have helper-companion cells called glia, or glial cells. In the next section we will examine the structure and function of glial cells. Without these special cells, neurons could not carry out their important functions.

The Role of Glia

Glial cells are the “glue” that holds the nervous system together. It is estimated that there are 10 times as many glial cells as neurons in the nervous system and that they make up more than 50% of the brain’s volume (Travis, 1994). Therefore, because there are more than 200 billion neurons in the human body, this means that our bodies contain at least 2 trillion glia!

In addition to being more plentiful than neurons, glia are much smaller. The soma of most glial cells ranges from 6 to 10 micrometers in diameter (Hammond, 1996). Like neurons, glia come in many shapes and sizes (Figure 2.8). Astrocytes, also called astroglia, are star-shaped and relatively large. Microglia are the tiniest of glial cells. Radial glia have long appendages that radiate out from the soma. Two other common types of glia, the Schwann cells and oligodendrocytes, have flattened shapes.

Figure 2.8: Two common types of glia

How do glia differ from the neurons shown in Figures 2.5 and 2.6?

Glia play a number of important roles in the smooth running of the nervous system: (1) they provide nourishment to neurons, (2) they remove waste products and dead neurons, (3) they form scar tissue in the nervous system, (4) they direct the development of the nervous system in the embryo, (5) they provide insulation for axons, (6) they contribute to the blood-brain barrier, (7) they communicate information to each other and to neurons, and (8) they function as the brain’s immune system (Hammond, 1996; Laming et al., 2000; Pfrieger & Barres, 1997; Streit & Kincaid-Colton, 1995). Let’s examine each of these functions separately.

A. Astrocyte B. Oligodendrocyte

Blood vessel

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CHAPTER 2Section 2.2 The Structure of Neurons

Providing Nourishment for Neurons Glial cells, particularly astrocytes, are responsible for making sure that the neurons get the nutri- ents that they need, such as glucose, water, amino acids, and oxygen. Figure 2.9 is an illustration from Ramon y Cajal (1933). In this drawing you can see exactly what Ramon y Cajal found when he studied certain astroglial cells. These astrocytes surround blood vessels, as in Figure 2.9, with several of their appendages actually penetrating the outer lining of the blood vessel. Ramon y Cajal called these appendages “sucking apparatuses” because he surmised that astrocytes remove nutrients from the blood. The glia then transfer these nutrients to neurons.

Figure 2.9: Astrocytes surrounding a blood vessel

The foot processes of astrocytes completely surround the walls of capillaries in the brain. Using a micro- scope, Ramon y Cajal observed that some of these foot processes actually penetrate the outer lining of the capillaries, and he called them “sucking apparatuses.”

Source: Adapted from “Neurological Cells” by Legado Cajal, 1933.

Astrocyte foot process

Astrocyte foot process

Blood vessel

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CHAPTER 2Section 2.2 The Structure of Neurons

Removing Waste Products and Dead Neurons You can imagine that a cell that uses as much glucose and oxygen as a neuron does will have a lot of waste products, such as carbon dioxide and urea, to dispose of. It is the job of glial cells, especially microglia and astrocytes, to gather up waste and package it properly for transport in blood vessels back to the lungs and kidneys. Likewise, when there are dead neurons lying about, as might happen after an injury to the nervous system or, to a lesser extent, after a night of heavy alcohol consump- tion, glia package up these dead cells for transport in the blood. Microglia also function as macro- phages, gobbling up dead neurons and foreign substances (Graeberg, Kreutzberg, & Streit, 1993).

Forming Scar Tissue in the Nervous System When neurons die, new neurons are not generated to replace them. Glial cells are there to pick up the debris, as you just learned. But what about the empty space that the dead neurons used to occupy? Is it left empty? No, glial cells, typically astrocytes, migrate into the empty space, forming scar tissue. Gliosis, or the accumulation of glia in brain tissue, is a prominent feature in a number of neurodegenerative illnesses that occur in old age (reviewed in Chapter 13). In these illnesses brain cells die in large numbers in particular regions of the brain, producing mental confu- sion, memory loss, seizures, speech impairment, and motor dysfunction in the afflicted individual (Andreasen et al., 2001; Jellinger, 2001; Lescaudron, Fulop, Sutton, Geller, & Stein, 2001). These patches of dead neurons are replaced by glial cells, which multiply and take over large areas in the brain, disrupting brain function.

Directing the Development of the Nervous System in the Embryo Early in embryonic development, the cells of the embryo fold in on themselves, creating a tube-like structure called the neural tube. The walls of the neural tube contain germinal cells, which give rise to all the neurons in the body. That is, all of the neurons in an individual’s body are formed along the neural tube and then migrate out to their proper destination in the body. Each neuron is created as a particular type of neuron with a particular address. It is the responsibility of the radial glial cells to make sure that neurons arrive at their correct destination. Newly created neurons move along the hair-like processes of the radial glia to their proper position in the body (Cowan, 1979). Think about what a complicated process this must be: More than 200 billion neurons must migrate to their rightful place in the nervous system.

During the development of the human embryo, neurons are created at a rate of 250,000 per minute, which means that radioglia must orchestrate the movements of a large number of neurons very quickly over relatively long distances. Astrocytes also play an important role linking neurons together during maturation of the nervous system to promote communication between neurons (Ullian, Sap- perstein, Christopherson, & Barres, 2001). In Chapter 13 we will examine in greater detail the devel- opment of the nervous system and the problems that can arise from faulty brain development.

Providing Insulation for Axons Most axons in the nervous system are surrounded by a special type of insulation composed of specialized glial cells, which are wrapped around selected axons dozens of times and form a fatty substance called myelin (Figure 2.10). In places where one glial cell ends and another begins, there

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CHAPTER 2Section 2.2 The Structure of Neurons

are breaks in the insulation, about 1 μm in length, known as nodes of Ranvier. In the central ner- vous system, the glial cells forming the myelin sheaths are called oligodendrocytes; whereas in the peripheral nervous system, they are called Schwann cells.

Figure 2.10: A myelinated axon

Most axons are covered with a myelin sheath, composed of glial cells called oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.

An oligodendrocyte consists of a cell body with up to 70 long processes. Each process of the oli- godendrocyte forms a myelin segment and wraps itself around a short distance (about 1 mm) of an axon (Hammond, 1996). Therefore, one oligodendrocyte can provide myelin segments for as many as 70 different axons. This means that damage to one oligodendrocyte can have a devastat- ing effect on the functioning of large numbers of neurons. In contrast, the entire Schwann cell is wrapped around one axon, each Schwann cell forming only one myelin segment. Damage to one Schwann cell will only affect the function of one axon.

Another important difference between oligodendrocytes and Schwann cells accounts for why axonal regeneration and repair are possible in the peripheral nervous system but not in the cen- tral nervous system. When an axon is severed in the peripheral nervous system, the axon grows back, and its function is often restored. This regrowth does not happen in the central nervous

Dendrite

Nucleus

Axon hillock

Soma

Node of Ranvier

Myelin sheath

Presynaptic terminals

Axon

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CHAPTER 2Section 2.2 The Structure of Neurons

system. For example, when the spinal cord is damaged, the axons do not regenerate, leaving the injured person permanently disabled. In the central nervous system, when an axon is damaged, the myelin segments provided by oligodendrocytes collapse, and astrocytes fill in the empty space left by the degenerated axon, making it impossible for the regenerating axon to find its way back to the neurons with which it formerly communicated.

However, Schwann cells do not collapse when an axon is damaged. Instead, they maintain their shape and provide an open tunnel through which the regenerating axon can grow. They also excrete a growth factor that stimulates the regrowth of the degenerated axon. Investigators are working to unlock the secret of the Schwann cell. If oligodendrocytes can be made to act like Schwann cells, spinal cord injuries and other damage to the central nervous system may be reversed. One promising treatment of injuries to the central nervous system involves implanting a peripheral nerve graft at the site of injury. The Schwann cells in the graft promote the regenera- tion and regrowth of central nervous system axons over relatively long distances (Berry, Carlisle, & Hunter, 1996; Brecknell & Fawcett, 1996).

Multiple sclerosis, which was introduced in the opening box in this chapter, is a disorder caused by degeneration of the myelin covering of axons. Although there is disagreement as to what causes this degeneration, multiple sclerosis appears to be an autoimmune illness in which the body’s immune system targets myelin as a foreign substance and attempts to destroy it. When the myelin insulation is stripped from the axon, communication of information down the axon becomes dis- rupted, and the neuron cannot function properly. This is especially devastating for motor neurons, which send their information via relatively long axons to muscles. Therefore, multiple sclerosis begins as a motor disorder in which movement becomes slow, weak, and uncontrolled. Symptoms typically begin in early adulthood. Most people with multiple sclerosis experience periods of poor motor function interspersed with periods of remission, in which symptoms disappear or diminish.

In addition to providing a covering for axons, oligodendrocytes and Schwann cells lend structural sup- port to the cell bodies of neurons (Hammond, 1996). In other organs of the body, connective tissue holds cells together and provides a structural framework for the organ. Glial cells perform this func- tion in the nervous system, surrounding and encapsulating the cell bodies in the gray matter of the nervous system. They also segregate groups of neurons from each other, thus forming a structural framework in the nervous system. Many authors refer to these encapsulating glia as satellite cells.

Contributing to the Blood-Brain Barrier Neurons get their nourishment from the blood, which carries oxygen, glucose, water, amino acids, and needed vitamins and minerals to the brain and all other parts of the nervous system. The blood, however, carries other chemicals and microorganisms, such as bacteria and viruses, that are harmful to the cells of the brain. Glial cells, especially astrocytes, help form a barrier between the blood and the brain by causing the cells in the walls of blood capillaries to form tight, overlap- ping junctions that keep out harmful substances that could interfere with the smooth function- ing of the brain. In other parts of the body, blood vessels are not connected to each other with tight junctions but instead have little gaps that allow substances to flow freely from the blood to body tissues (Figure 2.11). Astrocytes are suspected of promoting the development of these tight junctions because, when astrocytes are removed, brain capillaries do not develop tight junctions (Janzer & Raff, 1987).

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CHAPTER 2Section 2.2 The Structure of Neurons

Figure 2.11: Blood vessels in the brain

Most blood vessels in the body permit ready passage of molecules through the walls of the blood ves- sel. Blood vessels in the nervous system, which are found along the blood-brain barrier, have overlap- ping, tight junctions between cells of the capillary wall and do not permit passage of molecules across the capillary wall.

Bacteria are one example of a harmful agent that the blood-brain barrier tries to keep out of the brain. These microorganisms cause infection when they invade body tissues, producing swelling and inflammation. Obviously, swelling is not desirable in the brain because it is surrounded by a bony skull that does not expand. A bacterial infection in the brain would produce swelling, tis- sue damage, and neuronal death. Antibiotics are used to treat bacterial infections in the body. However, antibiotics cannot easily cross the blood-brain barrier, so attempts to stop the bacterial infection are quite limited.

Interstitial fluid Cerebrospinal fluid

Blood vessel

B L

O O

D F

L O

W

Neuron

Tight junction

Fenestrated (molecules pass easily through junction)

GLUCOSE

GLUCOSE

Glial cell

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CHAPTER 2Section 2.2 The Structure of Neurons

Viruses, on the other hand, are very tiny, about 1,000 times smaller than bacteria. Because of their small size, some viruses are capable of sneaking through the blood-brain barrier. These viruses can cause a brain infection called encephalitis. You’ve probably heard of encephalitis, because out- breaks of this illness are very disturbing and often publicized. For example, during the fall of 1997, an outbreak of encephalitis in Florida prompted Disney World and other amusement parks in Orlando to close before dusk to reduce human contact with mosquitoes, which carry the encepha- litis virus. When the virus enters the brain, it begins to reproduce, causing inflammation and swell- ing. Infected individuals at first feel like they have the flu, with a fever and achiness. But delirium and then a coma-like state soon follow. Because we have no drugs available to treat viruses (and even if we did, they probably couldn’t get through the blood-brain barrier), there is not much we can do to treat a person with encephalitis. About 50% of people who develop encephalitis die, and the other half who survive often show lingering signs of brain damage.

Communicating Information to Each Other and to Neurons Scientists have learned only recently that some glia have receptors that are capable of receiving messages from neurons and other glia (Gallo & Chittajallu, 2001; Pfrieger & Barres, 1997). Most astrocytes communicate by way of special electrical messages that are transmitted across pores in their cell membranes. In addition, there is evidence that some glial cells have receptors on their surfaces that respond to neurotransmitters released by neurons and hence are capable of responding to messages from neurons (Lino et al., 2001; Oliet, Pier, & Poulain, 2001; Travis, 1994). Other investigators have demonstrated that astrocytes even manufacture substances that regu- late brain activity (Shinoda, Marini, Cosi, & Schwartz, 1989; Stornetta, Hawelu-Johnson, Guyenet, & Lynch, 1988).

Functioning as the Brain’s Immune System The immune system’s major form of protection in the body is specialized white blood cells, which provide surveillance and defense against infection and cancer. However, the blood-brain barrier keeps these specialized white blood cells from passing into the brain because these cells secrete substances that can kill neurons as well as invading microorganisms. For many years scientists believed that the brain lacks immune protection. But recent research has confirmed that the cen- tral nervous system does indeed possess an immunological defense system: microglia. Microglia provide immune protection in a variety of ways. They devour foreign substances and microorgan- isms. They induce and organize attacks against foreign invaders in the brain. They also secrete chemicals, just like the ones released by white blood cells outside the central nervous system, that are capable of killing microbe invaders. Unfortunately, although these chemicals do a great job killing or disabling foreign invaders, they also damage cell membranes, proteins, or DNA in neu- rons. Microglia are suspected of participating in the development of a number of nervous system diseases, including Alzheimer’s disease, multiple sclerosis, Parkinson’s disease, and amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease (Streit & Kincaid-Colton, 1995).

By now you should have a good understanding of the many important functions of glial cells. They feed and clean up after neurons, they protect neurons and assist them in the communication of information, and they direct their migration in the body during embryonic development. The role of glia is to maintain the integrity and support the function of the nervous system. Let’s go back now and look at how neurons do the work of the nervous system.

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CHAPTER 2Section 2.3 The Function of Neurons

2.3 The Function of Neurons

Neurons have important work to do. They provide a communication network throughout the body, linking the central nervous system and the peripheral nervous system: (1) to enable rapid responses, (2) to allow the central nervous system to regulate activity in the peripheral ner- vous system, (3) to permit cells within the brain and spinal cord to influence each other, and (4) to communicate with the outside environment. Neurons communicate with each other in two ways: through chemical messengers released from the terminal buttons of their axons and through elec- trical signals passed through gap junctions. Let’s examine how this happens.

Synapses

Neurons communicate with each other across a junction called a synapse. Typically, in a chemi- cal synapse an axon from one neuron communicates with the dendrites of other neurons (Figure 2.12). The narrow space between the two neurons is referred to as the synaptic cleft. The neu- ron whose axon makes up half of the synapse is called the presynaptic neuron. It comes before the synapse. The postsynaptic neuron comes after the synapse; its dendrite forms the other half of the synapse. Thus, a synapse is composed of a presynaptic neuron, a synaptic cleft, and a postsynaptic neuron.

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CHAPTER 2Section 2.3 The Function of Neurons

Figure 2.12: Synapse between an axon and dendrite

The neurotransmitter is released from the terminal button of the presynaptic axon, crosses the synaptic cleft, and binds with receptor sites on the postsynaptic dendrite.

Synaptic cleftNeurotransmitter

molecule

Postsynaptic membrane

Receptor site

Presynaptic neuron

Presynaptic neuron

Presynaptic membrane

Postsynaptic neuron

Postsynaptic neuron

Neural impulse

Neural impulse

Axon

Synaptic vesicles

Axon terminal

Neurotransmitter molecule

Postsynaptic membrane

Receptor site

Synaptic cleft

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CHAPTER 2Section 2.3 The Function of Neurons

At a chemical synapse, neurotransmitters are released from the terminal buttons of the presyn- aptic neurons, diffuse across the synaptic cleft, and bind to receptors that are located on the den- drites of the postsynaptic neurons. In a chemical synapse, information is transmitted in the form of chemical substances called neurotransmitters. Photo 2.3 is a photograph of a chemical synapse, produced by a scanning electron microscope. Note in Photo 2.3 that the terminal button of the axon contains a multitude of little round sacs, or vesicles. These vesicles contain a neurotransmit- ter substance that has been produced in the soma and then transported to the terminal button. You can see that many of the vesicles are lined up along the terminal wall of the axon next to the synapse. When the presynaptic neuron becomes activated, some of these vesicles release their contents into the synaptic cleft, discharging neurotransmitter into the extracellular fluid, where it can diffuse to the receptors on nearby neurons.

The type of synapse illustrated in Photo 2.3 is an axodendritic syn- apse, in which the axon terminates on a dendrite. Other types of syn- apses are shown in Figure 2.13. In an axoaxonic synapse, both the presynaptic and postsynaptic ele- ments are axons. That is, the ter- minal button of one axon abuts the axon of another neuron in an axoaxonic synapse. The axoso- matic synapse is composed of a presynaptic terminal button of an axon terminating on a cell body of a neuron. Recently, investiga- tors have identified another type of synapse, the dendrodendritic synapse, in which dendrites act as both presynaptic and postsynaptic elements. Some neurons terminate on muscles or glands, rather than another neuron, forming neuromuscular (neuron to muscle) and neuroglandular (neuron to gland) synapses.

Eye of Science/Science Source

Photo 2.3 The terminal buttons of the axon are located immedi- ately adjacent to the postsynaptic membrane of the dendrite.

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CHAPTER 2Section 2.3 The Function of Neurons

Figure 2.13: Types of synapses

How do these synapses differ from one another?

A. Axosomatic synapse

B. Axodendritic synapse

C. Ephapse

D. Hypothetical dendrodendritic synapse

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CHAPTER 2Section 2.3 The Function of Neurons

In an electrical synapse, neurons transmit information across a synapse by means of electrically charged molecules called ions. This type of synapse is often referred to as a gap junction (Figure 2.14). A gap junction is a place where the cell membranes of two cells come together and allow the transmission of ions (Jensen & Chiu, 1993). The gap junction, then, is nothing more than an opening in the cell membranes of two adjoining cells through which electrically charged particles enter and leave the cell, producing an electrical signal. When one cell is more negative than its adjacent cell, the current flows from the more positive cell through the gap junction to the more negative cell, but only while the gap junction is open (Burt & Spray, 1988).

Figure 2.14: The gap junction between two cells

A gap junction allows the transmission of ions from one cell to another. In a gap junction, the current flows from the more positive cell to the more negative cell. In A, cell 1 is more positive, so the current flows from cell 1 to cell 2. In B, cell 2 is more positive, so the current flows from cell 2 to cell 1.

Electrical synapses play an important role in the excitation of large groups of neurons (Levitan & Kaczmarek, 1997). When a large number of neurons are required to fire at the same time, they must be activated simultaneously. The instantaneous spread of electrical activation through gap junctions between neurons enables these neurons to release their important transmitter sub- stance at the same time. Gap junctions also appear to play an important role in the developing brain. Neurons in the brains of newborn rats communicate through gap junctions, but these same neurons do not use electrical synapses in the adult rat brain. Scientists theorize that gap junctions are important in the neurons of newborns because they allow a number of neurons to become electrically active at the same time. Research has demonstrated that developing neurons that are electrically active at the same time establish chemical synaptic connections among themselves.

−

−

− −

−

−

−

−

−

− −

−

−

−

+

+

+

+

+

+ +

+ + +

+

+

+

+ Cell 1 Cell 1

Cell 2

Flow of current Flow of current

A. B.

Cell 2 Cell nucleus Cell nucleus

Gap junction Gap junction

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CHAPTER 2Section 2.3 The Function of Neurons

Therefore, gap junctions in neurons in newborns direct the development of chemical synapses between these neurons (Lustig, 1994; Yuste, Peinado, & Katz, 1992).

Communication through gap junctions is the simplest form of intercellular communication and occurs in many types of cells, including heart cells and cells of the digestive tract, wherever activity must be synchronized. Electrical synapses are found between glia and between glia and neurons. The truth is that relatively little is known about the functioning of electrical synapses. For the most part, the synapses discussed in this textbook will be chemical synapses, not because they are more important than electrical synapses, but because we know more about chemical synapses.

Transmitting Information Within a Neuron

We have examined the two major ways in which neurons transmit information to other cells: (1) via chemical synapses, and (2) via electrical synapses. In this section we will look at how a neuron transmits information from its dendrites and soma to the terminal buttons of its axon. Intracellular communication (the communication of information within a cell) is an important process in neurons because neurons receive many different messages from numerous other neurons and must sort out these messages before sending information on to other cells.

Resting Potential First, it’s best to consider a neuron at rest (Figure 2.15). A neuron at rest is bathed, like all neurons, in an extracellular fluid that is an aqueous (or water-based) solution full of ions. For example, sodium chloride (NaCl), the common salt that we sprinkle on our food, is dissolved into sodium (Na1) and chloride (Cl2) ions in extracellular fluid.

Figure 2.15: A neuron at rest

A neuron at rest has a negative membrane potential, due to the uneven distribution of ions across the cell membrane.

CytoplasmCytoplasm

Glutamate Fumarate Aspartate

Bicarbonate

Glutamate Fumarate Aspartate

Bicarbonate

Extracellular fluid Na Cl

K ClCl

Na

Na

Na

Cl

Cl

Cl Cl

Cl

Cl Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

K

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CHAPTER 2Section 2.3 The Function of Neurons

The cytoplasm of the neuron is also an aqueous solution that contains charged molecules, includ- ing potassium (K1), chloride (Cl2), ammonium (NH41), and a host of negatively charged amino acids and bicarbonate ions. As you can see in Figure 2.15, the negatively charged molecules far outnumber the positively charged ions in the cytoplasm of a resting neuron. It is possible to mea- sure the voltage difference between the extracellular fluid and the cytoplasm by inserting a wire, connected to a voltmeter, into the extracellular fluid and another wire, connected to the same voltmeter, into the neuron. In a resting neuron the voltmeter will show a voltage difference of between 40 and 90 millivolts (mV) across the cell membrane, between the extracellular fluid and the inside of the neuron. By convention, negative potentials indicate that the inside of the cell is negative relative to the extracellular fluid. This means that the electric charge, or potential, inside the resting neuron is somewhere between –40 and –90 mV. For most neurons, it is typically around –70 mV. That is, most neurons have a negative resting potential of –70 mV.

Let’s look at the resting neuron more closely. When a neuron is at rest, it maintains a negative potential across its cell membrane, with the inside of the membrane being more negative than the outside. This negative resting potential is due to an unequal distribution of ions across the cell membrane. You might be wondering: Why don’t the negative ions just leave the inside of the neu- ron, or why don’t some of those positive sodium ions flow into the cell, just to even things up a bit? Certainly, that’s what you would expect, given that opposite charges attract and like charges repel.

But you must remember that the cell membrane of the neuron is semipermeable. This means that some substances can cross the cell membrane easily, whereas others cannot. In the case of neurons, uncharged molecules such as oxygen and carbon dioxide diffuse readily across the cell membrane. Charged substances such as ions cannot cross the membrane at all except through special pores known as ion channels.

Ion Channels The cell membrane is composed of a double layer of a fatty substance, known as phospholipid (Figure 2.16). The phospholipid molecule has two ends: a hydrophilic end and a hydrophobic end (hydro- means “water,” -philic means “loving,” and -phobic means “avoiding” in Greek). Because the cytoplasm and the extracellular fluid are water-based solutions, the two hydrophobic ends are tucked in the center of the double-walled membrane, whereas the hydrophilic ends are exposed to the watery environments of the extracellular fluid and the cytoplasm. This double-walled membrane will readily allow the passage of uncharged molecules. However, ions cannot pass the hydrophobic layers of the cell membrane.

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CHAPTER 2Section 2.3 The Function of Neurons

Figure 2.16: The bilipid cell membrane

The cell membrane of a neuron is composed of a double layer of a fatty substance called a phospho- lipid. Embedded in the bilipid cell membrane are ion channels that permit the passage of ions across the cell membrane.

Ion channels are specialized proteins in the cell membrane that permit the passage of ions into and out of the neuron (Figure 2.16). These proteins form pores in the membrane that can be opened and closed. When the pores are opened, ions pass freely; when the pores are closed, ions cannot move in or out of the cell. Most ion channels are specialized passageways that permit only one particular ion to pass through the cell membrane. For example, a sodium channel will only allow sodium to cross the cell membrane.

Ion Distribution and the Negative Resting Potential In the resting neuron the unequal distribution of ions, particularly potassium, across the cell mem- brane contributes to the negative resting potential. The concentration of potassium is 20 times greater on the inside of the neuron than it is on the outside. This means that potassium is approxi- mately 20 times more likely to leave the cell than to enter it. Because potassium channels are con- stantly open, potassium continually flows out of a neuron when that neuron is at rest. As potassium leaves the neuron, the cytoplasm loses some of its positive charge, becoming more negative than the extracellular fluid. Thus, there is a negative potential or charge across the neuronal cell mem- brane due to the movement of positively charged potassium ions out of a neuron at rest.

Outside of the cell, there is an abundance of sodium ions, compared to inside the neuron, where potassium ions and negatively charged molecules abound. In fact, the concentration of sodium outside the neuron is 10 times greater than that inside the cell (Fischbach, 1992). Sodium is attracted to the inside of the neuron because there are relatively few sodium ions inside the neu- ron, so positively charged sodium ions slip into the cell every chance they get.

Ion channels

Phospholipid molecules

Hydrophilic ends

Hydrophobic ends s

Io an

nn nelsschachacc a

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CHAPTER 2Section 2.3 The Function of Neurons

Depolarization When positively charged sodium ions enter a neuron, they bring their positive charge with them, which makes the inside of the cell more positive (Figure 2.17). Whenever a neuron becomes more positive, we say that the cell has become depolarized. Remember that, in its resting state, a neu- ron has a negative resting potential of approximately –70 mV and thus is polarized. (A polarized neuron is a neuron that carries a positive or negative electrical charge.) Any positive movement away from this polarized resting voltage toward 0 mV is called depolarization. Depolarization of the cell membrane causes sodium channels to open, which means more sodium ions can rush into the cell, further depolarizing the membrane and opening still more sodium channels.

However, the cell has a built-in mechanism, the sodium-potassium pump, to keep this process from getting out of control. The sodium-potassium pump has the duty of pumping sodium out of the neuron every time the ion slips in. This pump is constructed in such a way that, for every three sodium ions it pumps out, it pumps in two potassium ions. The sodium-potassium pump maintains the neuron’s negative resting potential at a great cost to the neuron. In order to actively pump sodium out of the cell against its concentration gradient, a lot of ATP is needed. Therefore, even when the neuron is “resting,” it is burning up huge quantities of fuel. To make this fuel requires large amounts of glucose, oxygen, and water.

Pioneering research on the squid’s giant axon by Hodgkin and Huxley (1952) demonstrated without a doubt that communication within a neuron is electrical. The giant motor neurons of the squid have axons that are relatively large, up to 500 times thicker in diameter than mammalian axons. Because the giant motor axon is so thick, tiny glass microelectrodes can be inserted safely into the axon without damaging its structure or interfering with its function. These microelectrodes allowed Hodgkin and Huxley to measure changes in the membrane potential as they manipulated the concentrations of potassium and sodium both inside and outside of the axon. Hodgkin and Huxley were able to show that depolarization causes the neuron to go from a resting state to a state of activation in which a wave of depolarization sweeps down the axon. For this research, Hodgkin and Huxley were awarded the Nobel Prize in 1963.

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CHAPTER 2Section 2.3 The Function of Neurons

Figure 2.17: Depolarization of a neuron

A neuron at rest A has a negative resting potential. Depolarization B occurs when positively charged sodium ions enter a neuron, which makes the inside of the cell more positively charged.

Action Potential

The resting potential is the negative voltage measured across the cell membrane of a neuron that is in an off state. When an axon turns on or fires, an action potential is recorded. In order for a neuron to turn on, a threshold must be reached. This threshold is usually between 10 and 20 mV above (more positive than) the resting potential. This means that, in a typical neuron with a rest- ing potential of –70 mV, the threshold is at about –55 mV. When the neuron’s internal voltage depolarizes to –55 mV, the neuron turns on, and an action potential is generated and travels all the way down the axon to its terminal buttons. The purpose of the action potential, remember, is to inform the terminal buttons that the neuron has been excited and to start the process of releas- ing neurotransmitter into the synapse.

K+

Na+Na+

Na+

Na+

Cl−

Cl− Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Na+

Na+ Na+

Na+

Na+ Na+

+ −

+ −−

+

−

+

+

−

− +

− +

−

+

− − −

+

−

+

−

−

− − − − −

Na+Na +

Na+ Cl−

Cl− Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Cl−

Na+

Na+ Na+

Na+

Na+

Na+

Na+ Na +

Na+

Na+

Na+

Na+

−−

− −−

−

−

−

−

− −

−

− −

− −

−

−

−

− − −

−

−

−

− −

−

− − − − −

A. At rest

B. Depolarization

Na+ Na+Na+

Na+Na+

Na+Na+

Na+Na+ Na+Na+ Na+Na+

Na+Na+ Na+Na+ K+

K+K+

K+K+

K+K+

K+K+

K+K+

K+K+

K+K+

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CHAPTER 2Section 2.3 The Function of Neurons

Unlike the resting potential, which is ordinarily maintained at a constant voltage, the action poten- tial fluctuates over time, with a rapid depolarization followed by a less rapid return to the resting potential (Figure 2.18). When the threshold is reached, a large number of sodium channels in the cell membrane open up, and sodium ions flood into the neuron, overwhelming the sodium- potassium pump. This is measured as a rapid depolarization to 0 mV and beyond to 130 mV or more, as sodium ions stream into the neuron unchecked. At the peak of the action poten- tial, sodium stops its movement into the cell, and potassium rushes out of the cell, which causes the voltage to fall from more than 130 mV to less than 270 mV. Because both potassium and sodium leave the neuron when the internal potential becomes positive, the internal charge drops to nearly 2100 mV. This state, when the internal charge of the cell is more negative than the resting potential, is called hyperpolarization. The sodium-potassium pump restores the resting potential by pumping out the remaining sodium ions and pumping in escaped potassium ions. Therefore, the action potential begins at threshold, becomes depolarized, is repolarized and then hyperpolarized, and finally is polarized to the negative resting potential. All of this takes place in a matter of 1 or 2 milliseconds.

Figure 2.18: The action potential

When an action potential is initiated, sodium ions flood into the neuron, causing a surge of depolariza- tion. At the peak of the action potential, sodium movement into the neuron is slowed, and potassium continues to exit the neuron. The sodium-potassium pump restores the resting potential by pumping sodium out of the cell and potassium back into the neuron.

Adapted from PhysiologyWeb.com. Copyright © 2000–2013 by PhysiologyWeb. Reprinted by permission.

How do we know that sodium ions and potassium ions are responsible for producing action poten- tials? Hodgkin and Huxley’s research on the giant axon of the squid demonstrated that the action potential consists of two components: inflowing sodium followed by outflowing potassium. Figure 2.18 shows the time course of the movement of sodium and potassium during an action poten- tial. When the diffusion of sodium or potassium in or out of the neuron is interrupted, the action potential is disrupted, as the “For Further Thought” box illustrates.

msec 0 1 2 3 4

M e m

b ra

n e p

o te

n ti

a l (m

V )

0

–80

–60

–40

–20

+20

+40

Hyperpolarization

Charge due to sodium flow

Action potential

Charge due to potassium flow

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CHAPTER 2Section 2.3 The Function of Neurons

Special Properties of an Action Potential

The action potential has a number of special properties that need to be discussed because we will be coming back to the action potential again and again over the course of this textbook. First of all, action potentials follow a rule known as the all-or-none law. According to the all-or-none law, once an action potential has begun, it always goes all the way to completion. If a neuron has a peak of 130 mV, its action potential will always be measured as peaking at 130 mV. This is true no matter how intense the stimulus is that triggers the action potential. A weak stimulus, as long as it is above threshold, will produce the same size action potential as a very strong stimulus.

If a neuron responds with the same-size action potential to all stimuli, no matter how weak or strong, then how does that neuron communicate information about the intensity of the stimulus? This is an important question because neurons, especially sensory neurons, must be able to con- vey information about intensity, as you will learn in Chapters 5 and 6. Research has demonstrated

For Further Thought: What Puffer Fish, Bees, Scorpions, and Snakes Can Tell Us About Ion Channels

Some animals, like puffer fish (see Photo 2.4), bees, scor- pions, and snakes, produce venoms that immobilize prey or repel predators. These venoms are called neurotoxins because they have their poisonous effect on neurons in the central and peripheral nervous systems. Research has demonstrated that these neurotoxins interfere with the flow of ions in and out of neurons by blocking sodium or potassium channels.

For example, the puffer fish is a poisonous, spiny fish that is capable of swallowing a great deal of air or water, which makes it swell up in size to scare away predators. For further protection, the puffer fish secretes a neuro- toxin, known as tetrodotoxin or TTX. Tetrodotoxin blocks sodium channels, thereby preventing action potentials. In large doses this neurotoxin leaves the victim feeling weak or unable to move, and it can be fatal. In very small doses, as when a person eats a properly prepared puffer fish,

tetrodotoxin will produce a tingling sensation in the mouth. However, fatalities can occur when people eat a puffer fish that contains too much tetrodotoxin.

Scorpions, snakes, and bees produce a number of different venoms that act as neurotoxins. Bees, for example, release several toxins from their stingers. Depending on their species, poisonous snakes secrete any of a number of neurotoxins. The neurotoxins produced by scorpions, snakes, and bees are quite similar in function. They block potassium channels and prevent potassium from leaving neurons. When potassium channels are blocked, potassium cannot leave the cell during an action potential, and hyperpolarization does not occur at the end of the action potential. Thus, these neurotoxins cause overexcitation of neurons, allowing them to fire rapidly, over and over. Seizures and immobilization can result from high doses of these toxins.

Hemera Technologies/Photos.com/Thinkstock

Photo 2.4 This puffer fish, if prepared correctly by a skilled chef, can be eaten safely. If it isn’t, however, the consumer can die by ingesting too much tetrodotoxin, a neurotoxin.

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CHAPTER 2Section 2.3 The Function of Neurons

that the intensity of stimulation is encoded by the neuron as frequency of the action potential. This means that the stronger the stimulus is, the more frequently the neuron fires. Likewise, it is also true that the weaker the stimulus is, the slower the rate of action potentials will be. Remem- ber that some stimuli are so weak that they evoke no action potentials whatsoever.

Axonal Propagation Axonal propagation is another property of the axon potential that you will encounter in later chapters of this textbook. The term axonal propagation refers to the movement of the action potential down the axon. In an unmyelinated axon the action potential begins at the axon hillock (Figure 2.19). Depolarization initially takes place in a very narrow region of the axon in the area of the axon hillock, opening sodium channels in the membrane at this site. As sodium ions rush into the axon, they depolarize the membrane a bit farther down, which causes sodium channels in this section of the axon to open. This step-by-step depolarization continues all the way to the terminal buttons of the axon.

Figure 2.19: Movement of the action potential down an unmyelinated axon

Depolarization moves in a stepwise fashion down the axon. When one section of the axon becomes depolarized, Na1 channels in the adjacent area open, allowing Na1 to flow into the axon at that point, which depolarizes that section of the axon. This depolarization leads to the entry of more sodium ions, which causes depolarization of the membrane farther down the axon. Bit by bit, the action potential moves down the axon as depolarization spreads from one site to the next, until the action potential reaches the terminal buttons, triggering the release of neurotransmitters.

+ + + + + + + + + + + +

+

Na+ channel closes; ion flow stops.

When depolarization in the cell body reaches the threshold level, adjacent Na+ channels open.

Successive Na+ channels open in response to depolarization in adjacent areas of axon.

Sodium channel Na+

Na+

Na+

Na+

Potassium channel

Cell body

– – – – – – – – – – – –

–

– – – – – – – – – – – –

+ + + + + + + + + + + +

+ + + + + + + + + +

+

– – – – – – – – – –

–

– – – – – – – – – –

+ + + + + + + + + +

Cell body

Depolarized area of axon

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CHAPTER 2Section 2.3 The Function of Neurons

Axonal Propagation in Myelinated Axons Propagation of the action potential in myelinated axons is quite different. First of all, the cell membrane under the myelin cannot be depolarized, which means that the action potential can- not move down the myelinated axon in a step-by-step fashion as it does in the unmyelinated neuron. The only places along the cell membrane that can be depolarized are at the nodes of Ran- vier. Therefore, the action potential in a myelinated axon can occur only at the nodes of Ranvier, where sodium ions can enter the axon. The action potential jumps from node of Ranvier to node of Ranvier as it moves down the axon (Figure 2.20). This jumping of the action potential is called saltatory conduction.

Figure 2.20: Movement of the action potential down a myelinated axon

1. In a myelinated axon, Na1 can only enter the axon through ion channels located at the nodes of Ranvier (the gaps in the myelin sheath). 2. Depolarization at one node of Ranvier causes Na1 channels to open in the adjacent node of Ranvier, allowing Na1 to flow into the axon at that point, which depo- larizes that section of the axon. 3. The action potential appears to leap from one node of Ranvier to the next as it moves down the myelinated axon.

You can imagine that an action potential that jumps from node of Ranvier to node of Ranvier moves a lot faster down an axon than does an action potential that spreads down the axon bit by bit. In fact, axonal propagation down the axon can be as much as 200 times faster in a myelinated axon than in an unmyelinated axon. Another factor that affects the speed with which an axon potential moves down an axon is the diameter of the axon. Diameters of axons can vary from 1 μm to 1 mm. The smaller the diameter of the axon, the slower the axon potential travels down its length. As a general rule, action potentials travel most rapidly down thick, myelinated axons, and they travel most slowly down thin, unmyelinated axons.

Summation Before an action potential can begin, the membrane potential must reach threshold, as you’ve already learned. How does this occur? The threshold is nothing more than a depolarization of the

1. Sodium enters axon, depolarizing that segment.

2. Sodium channel on adjacent node of Ranvier opens.

Cell body Myelin sheath

Nodes of Ranvier

Na+

Na+

3. The action potential jumps from node of Ranvier to node of Ranvier.

Cell body Na+

Na+

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CHAPTER 2Section 2.3 The Function of Neurons

neuron 10 to 20 mV above the resting potential. The important point to remember here is that one neuron by itself is not typically capable of producing an action potential in another neuron. That is, one neuron cannot alter another neuron’s membrane potential by 10 to 20 mV all by itself. It takes many neurons firing over a brief period to raise a neuron’s membrane potential from 270 to 255 mV. A special mechanism called summation is needed. Summation involves the addition of depolarizations from many presynaptic neurons.

Figure 2.21 illustrates a simplified representation of input that one neuron receives from other neurons. The representation is “simplified” because, in Figure 2.20, the postsynaptic neuron is receiving input from only 6 other neurons, whereas a real neuron receives input from up to 10,000 other neurons. Neurons A, B, C, D, E, and F in Figure 2.21 cannot individually depolarize the post- synaptic neuron from 270 to 255 mV. In fact, most neurons can change a postsynaptic neuron’s membrane potential by only about 0.5 mV.

Figure 2.21: Summation

Presynaptic neurons A through F make a synaptic connection with a postsynaptic neuron. A postsynaptic neuron typically receives input from thousands of presynaptic neurons. The charges supplied by (A–F) sum in cell G (that is, the cell in the center of the drawing): only when G reaches threshold can it fire to activate H.

Postsynaptic neuron

A B

C G

D

E

F

H

+

+

++ +

+ +

+

+

+

+

+

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CHAPTER 2Section 2.3 The Function of Neurons

In order for the postsynaptic neuron to reach threshold, a number of different presynaptic neurons must fire at the same time. This requires that the postsynaptic neuron process input from many dif- ferent neurons and add them together in order to reach threshold and initiate an action potential. If neurons A through F were to stimulate the postsynaptic neuron in Figure 2.21 at the same time, the postsynaptic neuron would use summation to add the input from these presynaptic neurons.

Let’s complicate the picture just a bit. Not all presynaptic neurons cause depolarization in the postsynaptic neuron. Some presynaptic neurons actually produce hyperpolarization. As you will learn in Chapter 3, some neurotransmitters released by presynaptic neurons cause sodium chan- nels to open, and some cause potassium channels to open. Recall that when sodium channels open, sodium rushes into the cell, producing depolarization. When potassium channels open, potassium leaves the cell, making the inside of the neuron more negative and producing hyper- polarization. Therefore, the postsynaptic neuron is bombarded by a host of depolarizations and hyperpolarizations, which it must summate.

Excitation and Inhibition Some presynaptic neurons release neurotransmitters that depolarize the postsynaptic neurons. They are called excitatory neurons because they cause the postsynaptic neuron to become more positive. On the other hand, inhibitory neurons release neurotransmitters that hyperpolarize the postsynaptic neuron, making it more negative. A postsynaptic neuron receives stimulation from both excitatory and inhibitory neurons. The excitatory neuron causes an excitatory postsynaptic potential (EPSP) of approximately 10.5 mV in the postsynaptic neuron, whereas the inhibitory neuron causes an inhibi- tory postsynaptic potential (IPSP) of about 20.5 mV in the postsynaptic neuron.

Imagine in Figure 2.21 that neurons A, B, C, and D are excitatory neurons and that neurons E and F are inhibitory neurons. If all six presynaptic neurons fire once at the same time, they would have a net effect of raising the membrane potential of the postsynaptic neuron by 11.0 mV. That is, the potential of the resting postsynaptic neuron would increase from 270 to 269 mV. Do you under- stand how this would happen? Neurons A, B, C, and D would increase the postsynaptic potential by 4 (10.5 mV), or 12.0 mV, whereas neurons E and F would decrease the potential by 2 (20.5 mV), or 21.0 mV, for a net effect of (12.0 mV) 1 (21.0 mV), or 11.0 mV.

Remember that this example is a very simplistic representation of what actually occurs in the brain. Hundreds or thousands of neurons are activated in producing most behaviors. Another point to remember here is that inhibition is a very important process in the brain. Inhibition serves to control or check the activities of excitatory neurons. Think what would happen if you acted on every impulse that you experienced. Inhibitory neurons prevent postsynaptic neurons from responding to all impulses from excitatory neurons. They provide needed checks and balances to the nervous system.

Storage and Release of Neurotransmitters Neurotransmitters are manufactured inside the neuron and stored in little sacs called vesicles. These vesicles are then transported to the terminal buttons of the axon, where they remain until their contents are released into the synapse. For the release of neurotransmitter to occur, the membrane potential in the terminal button must be depolarized by an action potential.

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CHAPTER 2Section 2.4 Chapter Summary

In Chapter 3 we will examine neurotransmitters released by excitatory and inhibitory neurons and consider various drugs and chemicals that affect neurotransmitter function.

2.4 Chapter Summary The Organization of the Nervous System

• The nervous system is one of the body’s many systems, including the digestive system, the respiratory system, the cardiovascular system, and the reproductive system.

• The nervous system has two divisions: the central nervous system, which is composed of the brain and the spinal cord, and the peripheral nervous system, which is composed of all the other nervous tissue outside the brain and spinal cord.

• The peripheral nervous system is divided into the somatic system, which controls skeletal muscles, and the autonomic nervous system, which controls smooth and cardiac muscles.

• The autonomic nervous system is divided into the sympathetic nervous system, which is activated when a person becomes excited or aroused, and the parasympathetic nervous system, which is activated when a person relaxes.

Neurons and Glial Cells • Neurons and glia are the main cells that make up the nervous system. • Like other cells in the body, neurons possess a soma, cytoplasm, cell membrane, and

numerous organelles, including a nucleus (which contains the genetic information of the cell stored in chromosomes), ribosomes (which are involved in protein production), mito- chondria (which produce ATP), dendrites, and one axon.

• The function of neurons is to communicate with other neurons, muscles, and glands, and they possess special processes called dendrites and axons for receiving and sending information.

• Neurons can be classified as unipolar (containing only one axon), bipolar (containing one axon and one dendrite), or multipolar neurons (containing many dendrites and one axon), or they can be classified as motor neurons, sensory neurons, or interneurons.

• Glial cells function to support the activity of neurons, including providing nourishment to neurons (astrocytes), removing waste products (microglia and astrocytes), forming scar tissue in the nervous system (astrocytes), guiding the development of the nervous system (radial glia), providing insulation for neurons (oligodendrocytes and Schwann cells), con- tributing to the blood-brain barrier (astrocytes), communicating with neurons through gap junctions (astrocytes), and functioning as the brain’s immune system (microglia).

• Multiple sclerosis is a disorder caused by degeneration of the myelin covering of axons.

The Function of Neurons • Neurons provide a communication network linking the central nervous system and the

peripheral nervous system. They communicate with each other across chemical synapses, which involve neurotransmitters released by presynaptic terminals, and across electrical synapses, which involve ions passed through gap junctions.

• Electrical synapses allow for rapid communication between neurons, whereas chemical synapses involve the release of neurotransmitters, which carry a message from a presyn- aptic neuron to postsynaptic neurons.

• In an electrical synapse, information flows from the more positive neuron through the gap junction to the more negative neuron.

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• A neuron at rest has a voltage difference of approximately –70 mV across the cell mem- brane, with the inside of the neuron being more negative than the outside.

• The unequal distribution of ions across the cell membrane, with high concentrations of potassium inside the neuron and high concentrations of sodium outside, produces the negative resting potential.

• When a neuron becomes excited, sodium enters the neuron, producing depolarization. When a neuron becomes depolarized, its membrane potential becomes more positive.

• If the depolarization reaches 10 to 20 mV above the resting potential, an action potential is initiated.

• Axonal propagation is fastest in large, myelinated axons and slowest in small, unmyelin- ated axons.

• A postsynaptic neuron receives stimulation from both excitatory neurons, which produce an excitatory postsynaptic potential (EPSP), and inhibitory neurons, which produce an inhibitory postsynaptic potential (IPSP).

• Before an action potential can begin, summation of excitatory and inhibitory postsynap- tic potentials occur, thereby increasing the membrane potential from –70 mV to some threshold level.

• When an action potential reaches the terminal buttons of an axon, the membrane of the terminal buttons becomes depolarized, and neurotransmitters are released into the syn- aptic cleft.

Questions for Thought

1. Describe the physical symptoms you would expect in a person with an overactive sympa- thetic nervous system.

2. Predict what would occur if a person injured the ventral aspect of his or her spinal cord. 3. When are electrical synapses necessary? When are chemical synapses most useful? 4. What are the major differences between the somatic and autonomic nervous systems? 5. Identify the mitochondrion and the nucleus of a cell. How are they alike? How are they

different? 6. Discuss the functions of glial cells and neurons. 7. Describe how sodium ions and potassium ions contribute to the maintenance of a neu-

ron’s resting potential. How do they contribute to the initiation of an action potential?

Web Links

Search “The Life and Death of a Neuron” on the National Institute of Neurological Disorders and Stroke’s website to view an in-depth summary of the life cycle of a neuron. The summary includes the construction, performance, and death of a neuron. http://www.ninds.nih.gov/index.htm

Neural Development is a peer-reviewed, online journal that evaluates manuscripts that discuss aspects of research that use various methods to provide insights into the nervous system. This journal is a great resource for unique discoveries in the field. http://www.neuraldevelopment.com/

CHAPTER 2Web Links

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CHAPTER 2Key Terms

Key Terms

action potential The voltage change recorded in an excited neuron.

adenosine triphosphate (ATP) The principal fuel of the cell.

all-or-none law A rule that states that once an action potential has begun, it always goes all the way to completion.

astrocytes Also called astroglia; star-shaped glial cells.

autonomic nervous system The division of the peripheral nervous system that controls smooth muscles.

axon Single, long process that conducts infor- mation away from the cell body of the neuron.

axon hillock A thickened area on the cell body of the neuron.

axonal propagation The movement of the action potential down the axon.

Bell-Magendie law A rule that states that motor nerves exit from the ventral areas of the spinal cord, and sensory information enters the dorsal areas.

biofeedback The process of gaining greater awareness of and control over various physi- ological functions through information, or feedback, about the state of a particular auto- nomic function.

bipolar neuron A type of neuron that has two processes: one dendrite and one axon.

cell membrane The outer surface of any ani- mal cell.

central nervous system The division of the nervous system consisting of the brain and spinal cord.

chemical synapse A junction between two neurons that communicate using neurotransmitters.

chromosomes Structures found in the cell nucleus that contain strings of nucleic acids that code for various genes.

cytoplasm Fluid found within a cell.

dendrite A multibranched neuronal process that receives messages from other neurons.

dendritic spines Short spikes that increase the dendrites’ ability to receive messages.

deoxyribonucleic acid (DNA) The type of nucleic acid found in chromosomes.

depolarization A state in which a neuron loses its negative charge.

dorsal Toward the back.

electrical synapse A type of synapse in which neurons transmit information by means of electrically charged molecules called ions.

excitatory postsynaptic potential (EPSP) A small increase (10.5 mV) in the charge across a postsynaptic membrane.

gap junction A place where the cell mem- branes of two cells come together and allow the transmission of electrically charged mol- ecules called ions.

genome The complete collection of genes that is encoded in the cell’s chromosomes.

glial cells Supporting cells in the nervous system.

gliosis The accumulation of glial cells.

hyperpolarization A state in which a neu- ron becomes more negatively charged than normal.

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CHAPTER 2Key Terms

inhibitory postsynaptic potential (IPSP) A small decrease (–0.5 mV) in the charge across a postsynaptic membrane.

interneurons Intrinsic neurons that receive information from one neuron and pass it on to another.

ions Electrically charged molecules.

ion channels Special pores in the cell mem- brane that permit passage of ions into and out of the cell.

microglia Tiny glial cells.

mitochondrion An organelle that produces ATP, the energy source for the cell.

motor neurons Neurons in the central nervous system that stimulate muscle contractions.

multiple sclerosis A disorder caused by degeneration of the myelin covering of axons.

multipolar neuron A type of neuron that has three or more processes, typically many den- drites and only one axon.

myelin A fatty material that forms a sheath around axons.

negative resting potential The electrical charge across the cell membrane when a neu- ron is at rest.

nervous system One of the body’s major systems that consists of the brain, the spinal cord, and the extensive pathways of nervous tissue located throughout the body that per- mits communication between specialized cells called neurons.

neurotransmitter A chemical produced and used by neurons to communicate across synapses.

nodes of Ranvier Places where one glial cell ends and another begins, causing gaps in the insulation of the axon that are about 1 μm in length.

nuclei Areas in the brain where the soma of motor neurons are grouped together in little nests of gray matter.

nucleus A cluster of neuronal soma (cell bod- ies) in the central nervous system.

oligodendrocytes Glial cells located in the central nervous system that have a flattened shape that enables them to wrap around axons of neurons.

organelles Specialized structures within a cell.

parasympathetic nervous system A division of the autonomic nervous system that is con- cerned with energy conservation.

peripheral nervous system All of the neurons in the nervous system found outside of the brain and spinal cord.

postsynaptic neuron The neuron that receives communication across a synapse.

presynaptic neuron The neuron that sends a message across a synapse.

radial glia Glial cells that have long append- ages that radiate out from the soma and direct the development of the nervous system.

ribonucleic acid (RNA) The type of nucleic acid found in ribosomes.

ribosomes Special cellular structures that direct the production of protein in the cell.

saltatory conduction The jumping of an action potential from node of Ranvier to node of Ranvier along a myelinated axon.

Schwann cells Glial cells located in the periph- eral nervous system that have a flattened shape that enables them to wrap around axons of neurons.

sensory neurons Neurons that respond to stimuli in the periphery and send informa- tion about the stimuli to the central nervous system.

soma The cell body of a neuron.

somatic nervous system A division of the peripheral nervous system that controls skel- etal muscles.

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CHAPTER 2Key Terms

summation A mechanism that involves the addition of depolarizations from many presyn- aptic neurons.

sympathetic nervous system A division of the autonomic nervous system that produces fight-or-flight responses.

synapse The junction between two neurons.

synaptic cleft The narrow space between the two neurons.

terminal buttons The place where the end of the axon branches into numerous button- shaped endings.

unipolar neuron A type of neuron that has only one process, which typically branches a short distance from the cell body.

ventral Toward the belly, or front.

vesicles Tiny sacs found in the terminal but- tons of axons in which neurotransmitters are stored.

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