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C H A P T E R 3 Module 7 Neural and Hormonal Systems Module 8 The Brain M The Biological Bases of Behavior Perhaps you’re wondering what this chapter is doing here. After all, you signed up for a course in psychology, not biology! In the next two modules, we’ll be covering material that looks suspiciously as though it belongs in a biology textbook. What’s going on? Think of it this way. If your biological being suddenly disappeared, there would be nothing left. Without a body, there could be no behavior, and without a brain, there could be no mental processes. You couldn’t play a sport or a musical instrument. You couldn’t enjoy the taste of a ripe melon or a freshly baked chocolate chip cookie. You couldn’t solve a problem or fantasize about the upcoming weekend. You could neither laugh at a joke (a behavior) nor understand the humor behind it (a mental process). You couldn’t feel anxiety about an upcoming test or fall in love. In a nutshell, if biology disappeared, so would the stuff of psychology. It’s possible to study behavior and mental processes from a number of perspectives, including the cognitive perspective, the behavioral perspective, and the socialcultural perspective, and we will do that in other chapters of this book. But now it’s biology’s turn for the spotlight, and you may be surprised at the insight it provides. O D U L E 7 Neural and Hormonal Systems Neurons: The Building Blocks of the Nervous System Neural Communication The Structure of the Nervous System The Endocrine System Your body is an incredible organization of functioning systems. Your skeletal system supports your body, your digestive system extracts nutrients from food, your immune system wards off disease, your respiratory system allows you to take in oxygen and rid your cells of carbon dioxide, and so on. But the systems that psychologists focus on are the nervous and endocrine (hormonal) systems, which enable communication and information processing within our bodies. Neurons: The Building Blocks of the Nervous System What’s the point? chapter open photo to come 1. What are the primary parts of a typical neuron, and what functions do those parts perform? The nervous system is your body’s electrochemical communication system. Through it, your brain tells your body parts to move, your face to express emotion, and your internal organs to go about their business. Your nervous system, in partnership with your sensory systems, also gathers information so your brain can respond appropriately to stubbed toes, fire alarms, and the smell of popcorn. Like every other system in your body, your nervous system is built of cells, and taking a look at those cells is a good starting point for understanding the system as a whole. Module 7 ■ Neural and Hormonal Systems 121 Your brain, spinal cord, and nerves are formed from neurons, the highly specialized and unique cells of the nervous system. A neuron exists only to perform three tasks: • To receive information (in the form of electrochemical impulses) from the neurons that feed into it • To carry this information down its length • To pass the information on to the next neurons in line Figure caption will be 3 to 5 lines here. Figure caption will be 3 to 5 lines here. Figure caption will be 3 to 5 lines here. Figure title. neuron A nerve cell; the basic building block of the nervous system. dendrite The bushy, branching extensions of a neuron that receive messages and conduct impulses toward the cell body (soma). soma The cell body of a neuron, which contains the nucleus and other parts that keep the cell healthy. axon The extension of a neuron through which neural impulses are sent. Every behavior, thought, and emotion you’ve ever experienced depends on the neuron’s remarkable ability to move and process information. The wonder of it all is that neurons are so limited in function— their main capability is transmitting an impulse, or “firing.” In some ways, the guts of modern powerful computers operate in a similar way. Computers are binary—each electronic switch (or bit) in a central processor can be either on or off, set to represent either a 1 or a 0. All of a computer’s extraordinary capabilities—its communication functions, elaborate games, “number crunching,” mind-dazzling graphics, and sound—are ultimately accomplished by setting switches in the proper onor-off pattern. Neurons work in a similar way: They can “fire” (that is, send an impulse down their length) or not “fire.” That’s it. The beautiful colors you see in a sunset, the intense emotions you experienced during your first crush, the memory of your first day of kindergarten, the taste of pepperoni pizza, the thrill you feel when riding a roller coaster, and the devastating depression so many thousands suffer from—all emerge from a certain sequence of neurons either firing or not firing. Neurons, like trees and dogs, come in a tremendous variety of shapes and sizes, but all neurons have similar important structures. Take a minute now to look at Figure 7.1, which show, these structures in a motor neuron, a nerve cell that carries messages to muscles and glands. In this discussion, we will examine neuron parts following the order in which information travels. A neuron has endings known as dendrites, which receive information. Dendrites look like branches, and in fact the word dendrite comes from the Greek word for “tree.” The neuron’s thickest part is the soma, or cell body. The soma is not responsible for transmitting information; rather, it contains the cell nucleus and other parts that keep the cell healthy and functioning properly. Perhaps the most interesting part of the neuron is the axon, an extension that adds length to the cell. The 122 Part Title ■ Chapter 3 The Biological Bases of Behavior Figure 7.1 A Typical Motor neuron’s purpose is to move information from point A to point B, and the axon creates distance between these points. Axons of neurons in the brain may be very short, because information doesn’t have to travel very far between the cells. But in some neurons in the leg, axons extend more than a meter, making these giant redwoods of the nervous system the longest cells in your body! Finally, the neuron ends with the axon terminals, which, as you will see, are the points of departure for information making its way to the dendrites of the next neurons in the sequence. Let’s look more closely at what happens when a neuron “fires.” Neuron Neural Communication The Neural Impulse 2. What roles do the action potential, refractory period, and resting potential play in generating a neural impulse? When a neuron “fires,” a tiny electrical charge, called an action potential, works its way from the dendrites to the axon terminals, much as a bite of swallowed food makes its way from your mouth to your stomach. This action potential represents the “on” condition of the neuron. Each action potential is followed by a brief recharging phase known as the refractory period (think of a camera flash that has to recharge before it can be used again). After the refractory period, the neuron is capable of another action potential when it is stimulated. When the cell is recharged and ready to fire again, a resting potential exists. Table 7.1 illustrates these steps. axon terminal The endpoint of a neuron, where neurotransmitters are stored. action potential A neural impulse; a brief electrical charge that travels down the axon of a neuron. refractory period The “recharging phase” when a neuron, after firing, cannot generate another action potential. Module 7 ■ Neural and Hormonal Systems 123 Communication Between Neurons TABLE 7.1 THREE PHASES OF COMMUNICATION WITHIN A NEURON Action potential The neural impulse created when a neuron “fires.” The impulse travels from thedendrites down the axon to the terminal branches. Refractory period The brief instant when a new action potential cannot be generated because the neuron is“recharging” after the previous action potential. Resting potential The state of a neuron when it is “charged” but waiting for the next action potential tobe generated. resting potential The state of a neuron when it is at rest and capable of generating an action potential. all-or-none principle The principle stating that if a neuron fires, it always fires at the same intensity; all action potentials are the same strength. synapse Tiny, fluid-filled gap between the axon terminal of one neuron and the dendrite of another. neurotransmitter A chemical messenger that travels across the synapse from one neuron to the next and influences whether a neuron will generate an action potential (impulse). 3. What role do neurotransmitters play in neural communication? So far, we have been discussing how information passes down the length of a single neuron. But how do messages travel from one neuron to the next? Amazingly, despite their large numbers, this happens without any two neurons actually coming in contact with one another! At every place where an axon terminal of one neuron and the dendrite of an adjacent neuron meet (and there may be thousands of such places on any single neuron), a very small, fluid-filled gap called a synapse exists that action potentials cannot jump. At this point, chemical messengers known as neurotransmitters continue the job and carry the information across the gap. When an action potential works its way to the end of a neuron, it causes the release of neurotransmitters from the axon terminals. The neurotransmitter molecules, which have a distinctive chemical shape, rapidly cross the synapse and fit into receptor sites on the dendrite of the next neuron (Figure 7.2). Figure 7.2 Communication Between Neurons An interesting fact about how a neuron fires is called the all-or-none principle, which states that a neuron always fires with the same intensity. It doesn’t matter if there is strong stimulation or weak stimulation at the cell’s dendrites. As long as there is enough energy to trigger the cell, it will fire with the same intensity. One of the best analogies to a neuron and how it fires is, perhaps unfortunately, a toilet. Stop for a moment and think of how a toilet is similar to a neuron. Here are some similarities (perhaps you will be able to think of more!): • Like a neuron, a toilet has an action potential. When you flush, an “impulse” is sent down the sewer pipe. • Like a neuron, a toilet has a refractory period. There is a short delay after flushing when the toilet cannot be flushed again because the tank is being refilled. • Like a neuron, a toilet has a resting potential. The toilet is “charged” when there is water in the tank and it is capable of being flushed again. • Like a neuron, a toilet operates on the all-or-none principle—it always flushes with the same intensity, no matter how much force you apply to the handle (providing, of course, you provide enough force to trigger the mechanism). 124 Part Title ■ Chapter 3 The Biological Bases of Behavior Module 7 ■ Neural and Hormonal Systems 125 The neurotransmitter molecules can only come to rest in receptor sites designed to fit their shape, just as a key can only open locks with a certain configuration. Once in the receptor site, neurotransmitters can serve two broad functions. Under some circumstances, they have an excitatory effect. This means their arrival makes the receiving neuron more likely to fire. Other times, neurotransmitters have an inhibitory effect, which means their arrival makes a neuron less likely to generate an action potential. The excitatory role is like a green light. It shouts, “Just do it!” The inhibitory role is like a red light. Its message is “Just say no!” There are dozens of neurotransmitters, though so far researchers have learned the specific functions of only a few (Table 7.2). Neurotransmitters serve different functions, depending not only on the type of receptor site each locks into, but also on the place where they are released in the brain. (See Psychology Is a Science: Neurotransmitters and Drugs). The Neural Chain 4. What are the steps of the neural chain? excitatory effect A neurotransmitter effect that makes it more likely that the receiving neuron will generate an action potential (impulse). inhibitory effect A neurotransmitter effect that makes it less likely that a receiving neuron will generate an action potential (impulse). receptor cells Specialized cells in the sensory systems of the body that can turn other kinds of energy into action potentials (impulses) that the nervous system can process. The neural chain describes the path information follows as it is processed by the nervous system. To understand it, consider the example of playing your favorite radio station through your stereo system. What is necessary for this task? First, information must be available from the station. Whenever the station is broadcasting, its radio waves are present in the room, but people are not equipped to intercept and interpret these waves directly. That’s why we need the stereo system! The stereo’s antenna picks up the radio waves and sends them as an electronic message along a wire to the radio receiver. The receiver must process this information by amplifying and filtering it. Then the information is sent to the speakers, again via a wire. Finally, the speakers vibrate to create the sound of your favorite new hit. The stereo goes through this process of receiving, processing, and outputting information continuously. Your nervous system also specializes in receiving and processing information, and it contains functional components similar to those that make up your stereo system. First, you need to gather information from your environment. Your “antennae” are the receptor cells of your various sensory systems. These amazing cells have the ability to take other kinds of energy and put them in the form of neural impulses your brain can understand. Your eyes, for example, have receptor cells that take light energy and turn it into nerve impulses. Your ears have similar cells 126 Part Title ■ Chapter 3 The Biological Bases of Behavior that process sound energy, and elsewhere in your body other such cells process smells, tastes, and touch into nerve impulses. Without these receptor cells, your brain would be helpless. By itself, your brain cannot detect light, or sound, or smell. Just as you need your stereo to turn radio waves into something meaningful, your brain needs your senses and their receptor cells to gather and transform information into a form your brain can understand. The sense organs are not actually located in the brain, so your neural system must literally move the information your receptor cells pull in. This movement occurs as billions of neurotransmitter molecules pass messages among millions of neurons—from your fingertips, your eyeballs, your ears, your nose, and your mouth to the proper area of the brain for processing. As a stereo uses metal wires, your body uses living wires known as nerves, constructed of individual neurons. Those that connect the sense organs to the brain and spinal cord are sensory nerves. Without them, your brain would be no more effective than your stereo receiver would be if somebody cut the wire bringing information from the antenna. The brain, like a stereo receiver, is the real powerhouse of the system, processing the constant, massive barrage of sensory data flowing in from the sensory nerves. Your brain must process information about what you see, hear, taste, smell, and feel throughout your body, if only to ignore much of the information as probably insignificant. It is the brain’s responsibility to deal with it all and make appropriate decisions, just as your stereo receiver properly filters and amplifies an incoming radio signal. The billions of neurons that do this processing in your brain and spinal cord are called interneurons. Many times, your brain determines that some action is necessary to deal with incoming information. If your brain detects a ball moving toward your head, you need to either catch the ball or duck to avoid getting hit. If your brain detects a question asked by your teacher, you need to decide on an appropriate answer and say it. If your brain detects that you’re overheating, you need to begin sweating. The point is that while the brain can determine a course of action on its own (such as speaking or sweating), it cannot actually do these things. To trigger actions the brain must get word to the body’s muscles and glands, just as your stereo system has to convey the processed sound signal from the receiver to the speakers. Your stereo uses more wires for this purpose. Similarly, your nervous system uses motor nerves to carry information away from your brain to the parts of the body that can take action. Without motor nerves and the muscles and glands they attach to, your brain could not accomplish anything. (Your stereo wouldn’t be much good without speakers, would it?) sensory nerves Nerves that carry information to the central nervous system. interneurons Nerve cells in the brain and spinal cord responsible for processing information related to sensory input and motor output. motor nerves Nerves that carry information from the central nervous system. Module 7 ■ Neural and Hormonal Systems 127 Neurotransmitters and Drugs The synapse is where it’s at when it comes to the effects of many types of drugs. Let’s take a quick look at the role of a few neurotransmitters, and see what happens when outside chemicals are added to the mix. One neurotransmitter, acetylcholine (ACh), enables both memory and movement. ACh is present in every synapse of motor nerves and at the final connection between the last neuron and muscle Figure 7.3 Agonists and Antagonists fiber. Chemical substances can disrupt the normal effects of ACh, however. Some native tribes in South America use such a substance, a poison called curare, to coat the tips of the darts they use in their blowguns. When these darts strie an animal, the result is paralysis. Why? Because the curare molecules fill the receptor sites on dendrites that normally receive ACh, but the curare molecules do not stimulate an action potential in the receiving neu- ron the way ACh would. Thus, since ACh is effectively blocked from doing its job, movement ceases. Substances such as curare that block the effects of a neurotransmitter are called antagonists. Like curare, black widow spider venom also interacts with ACh, but not in the same way curare does. The venom fills the ACh receptor sites, but its chemical structure is so similar to ACh that it mimics ACh’s effect on the receiving neuron. So now two substances, ACh and spider venom, are doing the same thing. The result is excessive and uncontrollable movement, in the form of convulsions. The spider venom is called an agonist because it enhances the effect of a neurotransmitter. Figure 7.3 illustrates how antagonists and agonists interact with neurotransmitters. Another neurotransmitter with interesting effects is dopamine. Schizophrenia, a serious illness that disrupts a person’s sense of reality, is associated with high levels of dopamine. Drugs commonly prescribed for this illness alleviate some of the symptoms of schizophrenia by blocking the action of dopamine at the synapse. These drugs are dopamine antagonists. Another disorder, depression, is associated with low levels of the neurotransmitter serotonin. Some medications, the most famous of which is Prozac, work to reduce depression by enhancing the availability of serotonin at the synapse. Prozac, therefore, is a serotonin agonist. Table 7.2 summarizes the effects of serotonin and some other neurotransmitters. Prescribed medications are not the only substances that exert their effects at the synapse. All mind-altering chemicals, ranging from caffeine to cocaine, operate by influencing neurotransmission. Many of these relationships between drugs and neurochemistry are very complicated. A single drug, such as alcohol, might influence several different neurotransmitters in different ways. Research on neurotransmitters is always in progress and brings fascinating and important results. TABLE 7.2 EXAMPLES OF NEUROTRANSMITTER FUNCTIONS Acetylcholine (ACh) 128 Part Title ■ Chapter 3 The Biological Bases of Behavior • Muscle action • Learning • Memory • ACh-producing neurons have deteriorated in people with Alzheimer’s disease. Dopamine • Learning • Attention • Emotion • Excess dopamine activity is associated with schizophrenia. Serotonin • Hunger • Sleep • Arousal • Mood • Low levels of dopamine are associated with depression. acetylcholine [ah-seat-elKO-leen] A neurotransmitter that triggers muscle contraction and affects learning and memory. antagonist A drug that blocks the effect of a neurotransmitter. agonist A drug that boosts the effect of a neurotransmitter. dopamine A neurotransmitter that affects learning, attention, and emotion; excess dopamine activity is associated with schizophrenia. serotonin [sare-oh-TON-in] A neurotransmitter that affects hunger, sleep, arousal, and mood; serotonin appears in lower than normal levels in depressed persons. Module 7 ■ Neural and Hormonal Systems 129 The central nervous system (CNS) includes the brain and the spinal cord, so important to the nervous system that they are both encased in bone for protection. The brain is the location where most information processing takes place, and the spinal cord is the main pathway information follows as it enters and leaves the brain. In shape, the spinal cord tapers from about the thickness of a broomstick where it joins the brain to the diameter of a pencil at the base of the back. The interneurons that make up the CNS are responsible for processing information. The peripheral nervous system (PNS) contains all the nerves that feed into and branch out from the brain, and more often, the spinal cord. The word peripheral means “outer region” (perhaps you’ve heard the phrase “peripheral vision,” which refers to your ability to see things that are on the outer regions of your visual field). The PNS divides into two subsystems: Figure 7.4 A Neural Chain • The somatic nervous system contains the motor nerves you use central nervous system (CNS) The brain and the spinal cord. peripheral nervous system (PNS) The sensory and motor nerves that connect the central nervous system to the rest of the body. somatic nervous system The division of the peripheral nervous system that controls the body’s skeletal muscles. autonomic [aw-tuh-NAHMik] nervous system The division of the peripheral nervous system that controls the glands and muscles of the internal organs. Its subdivisions are the sympathetic (arousing) division and the parasympathetic (calming) division. Figure 7.4 shows a neural chain so basic that the initial action is determined by the spinal cord without the involvement of the brain. In this case, the response to the heat from the flame is a simple reflex. To react quickly to a dangerous situation, an interneuron in the spinal cord sends the command to withdraw the finger even before other interneurons relay the information to your brain! to activate muscles voluntarily. You develop the idea to walk across a room using your central nervous system, but you rely on the motor and sensory nerves of your somatic nervous system to carry the CNS’s commands to the muscles of your legs and to get feedback about what your legs are actually doing. • The second component of the PNS is the autonomic nervous system, which monitors the automatic functions of your body. Your autonomic nervous system controls your breathing, blood pressure, and digestive processes. Did you notice this was the second time in this module we have a word built from the Greek root soma, which means “body”? Figure 7.5 Divisions of the Nervous System The Structure of the Nervous System 5. What are the various divisions of the nervous system, and what is the function of each of these subsystems? So far we’ve examined the nervous system by zooming in on its smaller pieces, sensory and motor nerves made up of tiny neurons sending their neurotransmitters to one another. Now it’s time to take a step back for a broader view of the whole communication system in which these pieces function. One good way to understand the nervous system is to study its major divisions, which you can see in Figure 7.5. The nervous system has two major components, the central nervous system and the peripheral nervous system. 130 Part Title ■ Chapter 3 The Biological Bases of Behavior Module 7 ■ Neural and Hormonal Systems 131 sympathetic division The part of the autonomic nervous system that arouses the body to deal with perceived threats. parasympathetic division The part of the autonomic nervous system that calms the body. A final split is within the autonomic nervous system, which has both a sympathetic division and a parasympathetic division (Figure 7.6). These two divisions work together in a masterful example of checks and balances—it’s not just our government that relies on this principle! The sympathetic division is in charge of arousal; it controls a number of responses, collectively referred to as the fight-or-flight response, that prepare you to deal with threats or challenges. If you hear footsteps closing in behind you late at night on a deserted sidewalk, if a teacher announces a pop quiz at the beginning of class, if you’re about to make a nervous call to a potential dating partner you’ve never phoned before, your sympathetic nervous system will kick in. The parasympathetic division of the autonomic nervous system opposes the sympathetic division and generates responses that calm you down. The sympathetic division may send your blood pressure higher when your parent catches you coming in after your curfew; your parasympathetic division brings your blood pressure back down to normal when your parent responds calmly to your explanation of car trouble. The Endocrine System endocrine [EN-duh-krin] system One of the body’s two 6. How does the way the endocrine system communicates differ from the way the nervous system communicates? Your body has another system for communicating information. This system, slower to awaken and slower to shut down than the nervous system, is the endocrine system. It is made up of the endocrine glands, which produce hormones, chemical substances that circulate throughout the body in the blood. Hormones and neurotransmitters are similar in function: Both carry messages, and both communicate by locking into receptor sites. Figure 7.7 illustrates the major endocrine glands. The most important is the pituitary gland, so crucial it is sometimes referred to as the “master gland.” The pea-sized pituitary is located at the base of the brain, and it actually connects to a brain part called the hypothalamus through tissue that is part glandular and part neural. This connection illustrates the close relationship between the nervous and communication systems; a set of glands that produce hormones, chemical messengers that circulate in the blood. hormone Chemical messengers produced by the endocrine glands and circulated in the blood. pituitary gland The endocrine system’s highly influential “master gland” that, in conjunction with the brain, controls the other endocrine glands. Figure 7.6 The Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System The sympa- thetic division arouses us and expends energy. The parasympathetic division calms us and conserves energy. 132 Part Title ■ Chapter 3 The Biological Bases of Behavior Figure 7.7 Major Glands of the Endocrine System Module 7 ■ Neural and Hormonal Systems 133 thyroid gland Endocrine gland that helps regulate energy level in the body. adrenal glands Endocrine glands that help to arouse the body in times of stress. endocrine systems. The brain may call on the pituitary to release hormones that stimulate or inhibit the release of other hormones from other endocrine glands. Can you see why the pituitary is called the master gland? It is also true that the brain monitors the levels of hormones circulating in the blood, and may be influenced by their levels. Hunger, for example, is a response to a complex interaction of the neural and endocrine systems. The hypothalamus and pituitary work together to monitor and control the levels of glucose (blood sugar that your cells use for fuel) and insulin (a hormone the pancreas gland secretes, which allows the cells to use the available glucose) and thus determine, after considering a host of other factors, how hungry you are at any given moment. The important pituitary also releases hormones related to physical growth and pregnancy. Other endocrine glands include the thyroid, the adrenals, and the sex glands (or gonads). The thyroid gland, located in the neck, helps to regulate energy level. The adrenal glands, which perch atop the kidneys, release epinephrine and norepinephrine (also called adrenaline and noradrenaline). These substances enhance strength and endurance in emergency situations. The sex glands—ovaries in females and testes in males—release hormones that influence emotion and physical development. The primary male hormone is testosterone and the primary female hormone is estrogen, but both males and females have each hormone present in their systems. I’m seated at my desk right now, working on a computer that will process e-mail, connect to the Internet, and fax with a click of the mouse. It does this through a cable modem, which is also the source of the television programming I can access with the remote control that sits next to the telephone I used to talk to my son, 90 miles away, a few minutes ago. Also on the desk is a stack of bills, delivered via the U.S. Postal Service. Later this afternoon I will pay them—electronically—by using the computer to send instructions to my credit union. While I depend on these methods of communication to function effectively, my body would be unable to function without the communication capabilities of the nervous and endocrine systems. They are our personal information highways. 134 Part Title ■ Chapter 3 The Biological Bases of Behavior R E V I E W Module 7: Neural and Hormonal Systems What’s the Point? Neurons: The Building Blocks of the Nervous System 1. What are the primary parts of a typical neuron, and what functions do those parts perform? Neurons are cells that are unique to the nervous system. Their primary parts are the dendrites, soma, axon, and axon terminals. Dendrites are bushy endings that receive information from other cells. The soma is the cell body, which contains the nucleus and other parts that maintain the cell’s health. The axon ends in a number of axon terminals, which are the points at which messages leave the neuron for transmission to dendrites on other cells. Neural Communication 2. What roles do the action potential, refractory period, and resting potential play in generating a neural impulse? Neurons, like guns, can either fire or not fire, and they always fire with the same intensity. When a neuron fires, the action potential—a tiny electrical charge— works its way down the axon to the axon terminals. Following this firing, the neuron requires a brief period of recharging—the refractory period. A neuron that has recharged but has not yet fired again is in a state known as the resting potential. 3. What role do neurotransmitters play in neural communication? The transfer of messages from one neuron to another is an electrochemical process. Within a neuron, most of the action is electrical. But when messages pass from one neuron to another, they take the form of chemical substances that travel across the synapse, the tiny gap between two neurons. These substances are called neurotransmitters, and they have unique shapes that fit like keys into sites on the dendrites of the cells receiving the messages. Once in place in the site, neurotransmitters either encourage the receiving cell to fire (excitatory effect), or they help to repress firing in the receiving cell (inhibitory effect). 4. What are the steps of the neural chain? A neural chain is the sequence of events that take place when your nervous system gathers information, translates it into a form your brain can process, moves it to the brain for processing, and enables your body to take any necessary actions. In the first step, receptor cells in your sensory systems gather information and turn it into nerve impulses. In the second step, neurons and neurotransmitters move the impulses along sensory nerves to appropriate areas of your brain. In the third step, interneurons in your brain and spinal cord process the information and determine whether an action is necessary. In the fourth and final step, the brain uses motor nerves to transmit information to parts of your body that can react appropriately. Module 7 ■ Neural and Hormonal Systems 135 The Structure of the Nervous System 5. What are the divisions of the nervous system, and what is the function of each of these subsystems? The two major divisions of the nervous system are the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain, where most information processing takes place, and the spinal cord, the main pathway information follows into and out of the brain. The PNS contains all nerves that connect the CNS to the rest of the body. The PNS is further divided into the somatic nervous system (which controls voluntary movements of the body’s skeletal muscles) and the autonomic nervous system (which controls the self-regulated action of internal organs and glands). The autonomic nervous system in turn has two more subdivisions: the sympathetic division, which arouses us and expends energy, and the parasympathetic division, which calms us and conserves energy. The Endocrine System 6. How does the way the endocrine system communicates differ from the way the nervous system communicates? The endocrine system comprises all the glands that produce hormones. The major endocrine glands are the pituitary, thyroid, adrenals, pancreas, and sex glands (ovaries in females and testes in males). Although both the neural system and the endocrine system carry messages and communicate by locking chemicals into receptor sites, the endocrine system communicates more slowly than the nervous system. Another difference is that the neural system’s chemical messengers are neurotransmitters, which transmit information along nerves, whereas the endocrine system’s chemical messengers are hormones, which travel through the bloodstream. Key Terms neuron excitatory effect central nervous system (CNS) dendrite inhibitory effect peripheral nervous system (PNS) soma acetylcholine somatic nervous system axon antagonist autonomic nervous system axon terminal agonist sympathetic division action potential dopamine parasympathetic division refractory period serotonin endocrine system resting potential receptor cells hormone all-or-none principle sensory nerves pituitary gland synapse interneurons thyroid gland neurotransmitter motor nerves adrenal gland 136 Part Title ■ Chapter 3 The Biological Bases of Behavior Self-Test Multiple-Choice Questions: Choose the best answer for each of the following questions: 1. The nervous system is a. an electrical communication system. b. a chemical communication system. c. a hormonal communication system. d. an electrochemical communication system. 2. The all-or-none principle states that a. a neuron always fires with the same intensity; neurons either fire or they don’t fire. b. neurotransmitters are found in all neural chains but not in the hormonal system. c. all live humans have firing neurons, but no dead humans have firing neurons. d. all the brain can do is determine a course of action; none of our behaviors are performed directly by the brain. 3. The brief recharging period when a neuron cannot fire is known as the a. action potential. b. refractory period. c. resting potential. d. neural impulse. 4. The chemical messengers in the neural system are a. neurotransmitters. b. hormones. c. agonists. d. antagonists. 5. In the nervous system, __________ pick up information (about images, smells, tastes, and so on) from the world around us and transform that information into neural impulses that your brain can understand. a. dendrites c. receptor cells b. axon terminals d. interneurons 137 Part Title ■ Chapter 3 The Biological Bases of Behavior 6. The two major components of the nervous system are a. the somatic nervous system and the autonomic nervous system. b. the central nervous system and the peripheral nervous system. c. the sympathetic nervous system and the parasympathetic nervous system. d. the hormonal system and the endocrine system. 7. The __________ gland is called the “master gland” because it has a close connection with the brain and helps to monitor and control other glands. c. thyroid a. adrenal b. hypothalamus d. pituitary 8. Matching Terms and Definitions: For each definition, choose the best matching term from the list that follows. Definition a. The tiny, fluid-filled gap between the axon terminal of one neuron and the dendrite of another. b. The part of the autonomic nervous system that arouses the body to deal with perceived threats. c. Nerves that carry information TO the central nervous system. d. The division of the peripheral nervous system that controls the body’s skeletal muscles. e. Bushy, branching extensions of a neuron that receive messages and conduct impulses toward the cell body. f. The part of the autonomic nervous system that calms the body. g. The division of the peripheral nervous system that controls the glands and muscles of the internal organs. Module 7 ■ Neural and Hormonal Systems 137 h. Nerves that carry information FROM the central nervous system. i. The cell body of a neuron, which contains the nucleus and other parts that keep the cell healthy. j. The state of a neuron when it has recharged and is able to fire. Term (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) dendrite soma resting potential refractory period synapse sensory nerves motor nerves interneurons somatic nervous system autonomic nervous system sympathetic division parasympathetic division 138 Part Title ■ Chapter 3 The Biological Bases of Behavior Fill-in-the-Blank Questions 9. __________ are specialized cells found in the brain, spinal cord, and nerves. 10. The __________ __________ is a brief electrical charge that travels down the axon of a neuron. 11. A neurotransmitter that exerts an __________ effect makes the receiving neuron MORE likely to fire; a neurotransmitter that exerts an __________ effect makes the receiving neuron LESS likely to fire. Brief Essay Question 12. How exactly would a neural message travel from one neuron to the next in its path through the body? Name the parts of the neuron that the message would travel through, and be sure to follow the correct order. Then discuss what happens at the synapse.