Outline of Chapter 2

chapter 2 chapter outline module 5 Neurons: The Basic Elements of Behavior The Structure of the Neuron How Neurons Fire Where Neurons Connect to One Another: Bridging the Gap Neurotransmitters: Multitalented Chemical Couriers module 6 module 7 The Brain The Nervous System and the Endocrine System: Communicating within the Body The Nervous System The Endocrine System: Of Chemicals and Glands

Studying the Brain’s Structure and Functions: Spying on the Brain The Central Core: Our “Old Brain” The Limbic System: Beyond the Central Core The Cerebral Cortex: Our “New Brain” Neuroplasticity and the Brain The Specialization of the Hemispheres: Two Brains or One? Exploring Diversity: Human Diversity and the Brain Try It! Assessing Brain Lateralization The Split Brain: Exploring the Two Hemispheres Becoming an Informed Consumer of Psychology: Learning to Control Your Heart—and Mind—through Biofeedback Psychology on the Web The Case of .

. . The Fallen Athlete Full Circle: Neuroscience and Behavior 46

The Deepest Cut Wendy Nissley carried her two-year-old daughter, Lacy, into O. R. 12 at Johns Hopkins Hospital to have half of her brain removed. Lacy suffers from a rare malformation of the brain, known as hemimegalencephaly, in which one hemisphere grows larger than the other. The condition causes seizures, and Lacy was having so many—up to forty in a day—that at an age when other toddlers were trying out sentences, she could produce only a few language-like sounds. As long as Lacy’s malformed right hemisphere was attached to the rest of her brain, it would prevent her left hemisphere from functioning normally.

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So Lacy’s parents had brought her to Johns Hopkins for a hemispherectomy, which is probably the most radical procedure in neurosurgery. (Kenneally, 2006, p. 36) neuroscience and behavior It took nearly a day, but the surgery to remove half of Lacy’s brain was a success. Within a few months, Lacy was crawling and beginning to speak. Although the long-term effects of the radical operation are still unclear, it brought substantial improvement to Lacy’s life. The ability of surgeons to identify and remove damaged portions of the brain is little short of miraculous. The greater miracle, though, is the brain itself.

An organ roughly half the size of a loaf of bread, the brain controls our behavior through every waking and sleeping moment. Our movements, thoughts, hopes, aspirations, dreams—our very awareness that we are human—all depend on the brain and the nerves that extend throughout the body, constituting the nervous system. Because of the importance of the nervous system in controlling behavior, and because humans at their most basic level are biological beings, many researchers in psychology and other fields as diverse as computer science, zoology, and medicine have made the biological underpinnings of behavior their specialty.

These experts collectively are called neuroscientists (Beatty, 2000; Posner & DiGirolamo, 2000; Gazzaniga, Ivry, & Mangun, 2002; Cartwright, 2006). Psychologists who specialize in considering the ways in which the biological structures and functions of the body affect behavior are known as Behavioral neuroscientists Psychologists who specialize in behavioral neuroscientists (or biopsychologists). They seek to answer sevconsidering the ways in which the eral key questions: How does the brain control the voluntary and involunbiological structures and functions tary functioning of the body?

How does the brain communicate with other of the body affect behavior. parts of the body? What is the physical structure of the brain, and how does this structure affect behavior? Are psychological disorders caused by biological factors, and how can such disorders be treated? As you consider the biological processes that we’ll discuss in this chapter, it is important to keep in mind why behavioral neuroscience is an essential part of psychology: our understanding of human behavior requires knowledge of the brain and other parts of the nervous system.

Biological factors are central to our sensory experiences, states of consciousness, motivation and emotion, development throughout the life span, and physical and psychological health. Furthermore, advances in behavioral neuroscience have led to the creation of drugs and other treatments for psychological and physical disorders. In short, we cannot understand behavior without understanding our biological makeup (Plomin, 2003a; Compagni & Manderscheid, 2006; Plomin et al. , 2008). 47 looking ahe ad module 5 Neurons The Basic Elements of Behavior learning outcomes 5. 1 Explain the structure of a neuron.

The nervous system is the pathway for the instructions that permit our bodies to carry out everyday activities such as scratching an itch as well as more remarkable skills like climbing to the top of Mount Everest. Here we will look at the structure and function of neurons, the cells that make up the nervous system, including the brain. 5. 2 Describe how neurons fire. 5. 3 Summarize how messages travel from one neuron to another. 5. 4 Identify neurotransmitters. The Structure of the Neuron LO 1 Playing the piano, driving a car, or hitting a tennis ball depend, at one level, on exact muscle coordination.

But if we consider how the muscles can be activated so precisely, we see that there are more fundamental processes involved. For the muscles to produce the complex movements that make up any meaningful physical activity, the brain has to provide the right messages to them and coordinate those messages. Such messages—as well as those which enable us to think, remember, and experience emotion—are passed through specialized cells called neurons. Neurons Nerve cells, the basic Neurons, or nerve cells, are the basic elements of the nervous system. Their elements of the nervous system. uantity is staggering—perhaps as many as 1 trillion neurons throughout Dendrites A cluster of fibers at the body are involved in the control of behavior (Boahen, 2005). one end of the neuron that receives messages from other neurons. Although there are several types of neurons, they all have a similar strucAxon The part of the neuron that ture, as illustrated in Figure 1. In contrast to most other cells, however, carries messages destined for other neurons have a distinctive feature: the ability to communicate with other neurons. cells and transmit information across relatively long distances.

Many of the body’s neurons receive signals from the environment or relay the nervous system’s messages to muscles and other target cells, but the vast majority of neurons communicate only with other neurons in the elaborate information system that regulates behavior. As you can see in Figure 1, a neuron has a cell body with a cluster of fibers called dendrites at one end. Those fibers, which look like the twisted Remember that Dendrites branches of a tree, receive messages from other neurons. On the opposite Detect messages from other of the cell body is a long, slim, tubelike extension called an axon.

The axon neurons; Axons carry signals carries messages received by the dendrites to other neurons. The axon is conAway from the cell body. siderably longer than the rest of the neuron. Although most axons are several s tudy aler t 48 Chapter 2 neuroscience and behavior Dendrites Terminal buttons Cell body M o ec vem tric ent of al i mpu lse el Myelin sheath Axon (inside myelin sheath) Figure 1 The primary components of the specialized cell called the neuron, the basic element of the nervous system (Van De Graaff, 2000).

A neuron, like most types of cells in the body, has a cell body and a nucleus, but it also contains structures that carry messages: the dendrites, which receive messages from other neurons, and the axon, which carries messages to other neurons or body cells. In this neuron, as in most neurons, the axon is protected by the sausagelike myelin sheath. What advantages does the treelike structure of the neuron provide? millimeters in length, some are as long as three feet. A xons end in small bulges called terminal buttons, which send messages to other neurons.

The messages that travel through a neuron are electrical in nature. Although there are exceptions, those electrical messages, or impulses, generally move across neurons in one direction only, as if they were traveling on a one-way street. Impulses follow a route that begins with the dendrites, continues into the cell body, and leads ultimately along the tubelike extension, the axon, to adjacent neurons. To prevent messages from short-circuiting one another, axons must be insulated in some fashion (just as electrical wires must be insulated).

Most axons are insulated by a myelin sheath, a protective coating of fat and protein that wraps around the axon like links of sausage. Terminal buttons Small bulges at the end of the axons that send messages to other neurons. Myelin sheath A protective coat of fat and protein that wraps around the axon. All-or-none law The rule that neurons are either on or off. Resting state The state in which there is a negative electrical charge of about 70 millivolts within a neuron. s tudy aler t Think of a neuron as a sausage, and the myelin sheath as the case around it.

LO 2 How Neurons Fire Like a gun, neurons either fire—that is, transmit an electrical impulse along the axon—or don’t fire. There is no in-between stage, just as pulling harder on a gun trigger doesn’t make the bullet travel faster. Similarly, neurons follow an all-or-none law: they are either on or off, with nothing in between the on state and the off state. Once there is enough force to pull the trigger, a neuron fires. Before a neuron is triggered—that is, when it is in a resting state—it has a negative electrical charge of about 70 millivolts.

When a message arrives at a neuron, gates along the cell membrane open briefly to allow positively charged ions to rush in at rates as high as 100 million ions per second. The sudden arrival of these positive ions causes the charge within the nearby part of the cell to change momentarily from negative to positive. When the positive charge reaches a critical level, the “trigger” is pulled, and an electrical impulse, known as an action potential, travels along the axon of the neuron (see Figure 2). psych 2. 0 www. mhhe. com/psychlife Neurons 49 Module 5 neurons: the basic elements of behavior

Figure 2 Movement of the action potential across the axon. Just before Time 1, positively charged ions enter the cell membrane, changing the charge in the nearby part of the neuron from negative to positive and triggering an action potential. The action potential travels along the axon, as illustrated in the changes occurring from Time 1 to Time 3 (from top to bottom in this drawing). Immediately after the action potential has passed through a section of the axon, positive ions are pumped out, restoring the charge in that section to negative.

Time 1 Voltage Time 2 ++ +++ – – – – – – Time 3 Voltage Voltage Positive charge Negative charge Direction of impulse Action potential An electric nerve impulse that travels through a neuron when it is set off by a “trigger,” changing the neuron’s charge from negative to positive. Mirror neurons Neurons that fire when a person enacts a particular behavior and also when a person views others’ behavior. The action potential moves from one end of the axon to the other like a flame moving along a fuse.

Just after an action potential has occurred, a neuron cannot fire again immediately no matter how much stimulation it receives. It is as if the gun has to be reloaded after each shot. Eventually, though, the neuron is ready to fire once again. Neurons differ not only in terms of how quickly an impulse moves along the axon but also in their potential rate of firing. Some neurons are capable of firing as many as a thousand times per second; others fire at much slower rates. The intensity of a stimulus determines how much of a neuron’s potential firing rate is reached.

A strong stimulus, such as a bright light or a loud sound, leads to a higher rate of firing than a less intense stimulus does. Thus, even though all impulses move at the same strength or speed through a particular axon—because of the all-or-none law—there is variation in the frequency of impulses, providing a mechanism by which we can distinguish the tickle of a feather from the weight of someone standing on our toes. Although all neurons operate through the firing of action potentials, there is significant specialization among different types of neurons.

For example, in the last decade, neuroscientists have discovered the existence of mirror neurons, neurons that fire not only when a person enacts a particular behavior, but also when a person simply observes another individual carrying out the same behavior (Lepage & Theoret, 2007; Schulte-Ruther et al. , 2007). 50 Chapter 2 neuroscience and behavior Mirror neurons may help explain how (and why) humans have the capacity to understand others’ intentions. Specifically, mirror neurons may fire when we view others’ behavior, helping us to predict what their goals are and what hey may do next (Oberman, Pineda, & Ramachandran, 2007; Triesch, Jasso, & Deak, 2007). Mirror neurons may help explain how (and why) humans have the capacity to understand others’ intentions. LO 3 Where Neurons Connect to One Another: Bridging the Gap Synapse The space between two If you have looked inside a computer, you’ve seen that each part is physically connected to another part. In contrast, evolution has produced a neural transmission system that at some points has no need for a structural connection between its components.

Instead, a chemical connection bridges the gap, known as a synapse, between two neurons (see Figure 3). The synapse is the space between two neurons where the axon of a sending neuron 1 Neurotransmitters are produced and stored in the axon. neurons where the axon of a sending neuron communicates with the dendrites of a receiving neuron by using chemical messages. 2 If an action potential arrives, the axon releases neurotransmitters. 3 Neurotransmitters travel across the synapse to receptor sites on another neuron’s dendrite. Axon Axon Synapse Dendrite Synapse Neurotransmitter Neurotransmitter Synapse Receptor site

Receptor site 4 When a neurotransmitter fits into a receptor site, it delivers an excitatory or inhibitory message. If enough excitatory messages are delivered, the neuron will fire. A Neurotransmitter Dendrite B Figure 3 (A) A synapse is the junction between an axon and a dendrite. The gap between the axon and the dendrite is bridged by chemicals called neurotransmitters (Mader, 2000). (B) Just as the pieces of a jigsaw puzzle can fit in only one specific location in a puzzle, each kind of neurotransmitter has a distinctive configuration that allows it to fit into a specific type of receptor cell (Johnson, 2000).

Why is it advantageous for axons and dendrites to be linked by temporary chemical bridges rather than by the hard wiring typical of a radio connection or telephone hookup? Module 5 neurons: the basic elements of behavior 51 communicates with the dendrites of a receiving neuron by using chemical messages (Fanselow & Poulos, 2005; Dean & Dresbach, 2006). carry messages across the synapse to When a nerve impulse comes to the end of the axon and reaches a terminal the dendrite (and sometimes the cell button, the terminal button releases a chemical courier called a neurotransbody) of a receiver neuron. mitter.

Neurotransmitters are chemicals that carry messages across the Excitatory messages Chemical synapse to a dendrite (and sometimes the cell body) of a receiving neuron. messages that make it more likely that a receiving neuron will fire and an The chemical mode of message transmission that occurs between neurons is action potential will travel down its axon. strikingly different from the means by which communication occurs inside Inhibitory messages Chemical neurons: although messages travel in electrical form within a neuron, they messages that prevent or decrease the move between neurons through a chemical transmission system. ikelihood that a receiving neuron will fire. There are several types of neurotransmitters, and not all neurons are Reuptake The reabsorption of capable of receiving the chemical message carried by a particular neuneurotransmitters by a terminal button. rotransmitter. In the same way that a jigsaw puzzle piece can fit in only one specific location in a puzzle, each kind of neurotransmitter has a distinctive configuration that allows it to fit into a specific type of receptor site on the receiving neuron (see Figure 3B). It is only when a neurotransmitter fits precisely into a receptor site that successful chemical communication is possible.

If a neurotransmitter does fit into a site on the receiving neuron, the chemical message it delivers is basically one of two types: excitatory or inhibitory. Excitatory messages make it more likely that a receiving neuron will fire and an action potential will travel down its axon. Inhibitory messages, in contrast, do just the opposite; they provide chemical information that prevents or decreases the likelihood that the receiving neuron will fire. Because the dendrites of a neuron receive both excitatory and inhibitory messages simultaneously, the neuron must integrate the messages by using a kind of chemical calculator.

Put simply, if the excitatory messages (“fire! ”) outnumber psych 2. 0 the inhibitory ones (“don’t fire! ”), the neuron fires. In contrast, if the inhibitory www. mhhe. com/psychlife messages outnumber the excitatory ones, nothing happens, and the neuron remains in its resting state (Mel, 2002; Flavell et al. , 2006). If neurotransmitters remained at the site of the synapse, receiving neurons would be awash in a continual chemical bath, producing constant stimulation or constant inhibition of the receiving neurons—and effective communication across the synapse would no longer be possible.

To solve this problem, neurotransmitters are either deactivated by enzymes or—more commonly— reabsorbed by the terminal button in an example of chemical recycling called reuptake. Like a vacuum cleaner sucking up dust, neurons reabsorb the neurotransmitters that are now clogging the synapse. All this activity Messages Traveling between Neurons occurs at lightning speed (Helmuth, 2000; Holt & Jahn, 2004). Neurotransmitters Chemicals that LO 4 Neurotransmitters: Multitalented Chemical Couriers Neurotransmitters are a particularly important link between the nervous system and behavior.

Not only are they important for maintaining vital brain and body functions, a deficiency or an excess of a neurotransmitter can produce severe behavior disorders. More than a hundred chemicals have been found to act as neurotransmitters, and neuroscientists believe that more may ultimately be identified (Penney, 2000; Schmidt, 2006). Neurotransmitters vary significantly in terms of how strong their concentration must be to trigger a neuron to fire. Furthermore, the effects of a particular neurotransmitter vary, depending on the area of the nervous system in 52 Chapter 2 neuroscience and behavior Dopamine Pathways Name Acetylcholine (ACh)

Location Brain, spinal cord, peripheral nervous system, especially some organs of the parasympathetic nervous system Brain, spinal cord Brain, spinal cord Effect Excitatory in brain and autonomic nervous system; inhibitory elsewhere Function Muscle movement, cognitive functioning Glutamate Gamma-amino butyric acid (GABA) Excitatory Main inhibitory neurotransmitter Memory Eating, aggression, sleeping Serotonin Pathways Dopamine (DA) Brain Inhibitory or excitatory Muscle disorders, mental disorders, Parkinson’s disease Sleeping, eating, mood, pain, depression Pain suppression, pleasurable feelings, appetities, placebos

Serotonin Brain, spinal cord Inhibitory Endorphins Brain, spinal cord Primarily inhibitory, except in hippocampus Figure 4 Some major neurotransmitters. which it is produced. The same neurotransmitter, then, can act as an excitatory message to a neuron located in one part of the brain and can inhibit firing in neurons located in another part. (The major neurotransmitters and their effects are described in Figure 4. ) One of the most common neurotransmitters is acetylcholine (or ACh, its chemical symbol), which is found throughout the nervous system. ACh is Michael J.

Fox, who suffers from Parkinson’s disease, like Muhammad Ali, has become a strong advocate for research into the disorder. The pair is seen here asking Congress for additional funds for Parkinson’s research. Module 5 neurons: the basic elements of behavior 53 involved in our every move, because—among other things—it transmits messages relating to our skeletal muscles. ACh is also involved in memory capabilities, and diminished production of ACh may be related to Alzheimer’s disease (Mohapel et al. , 2005). Another major neurotransmitter is dopamine (DA), which is involved in movement, attention, and learning.

The discovery that certain drugs can have a significant effect on dopamine release has led to the development of effective treatments for a wide variety of physical and mental ailments. For instance, Parkinson’s disease, from which actor Michael J. Fox suffers, is caused by a deficiency of dopamine in the brain. Techniques for increasing the production of dopamine in From the perspective of . . . A Health Care Provider How might your understanding of the nervous system help you explain the symptoms of Parkinson’s disease to a patient with the disorder?

Parkinson’s patients are proving effective (Kaasinen & Rinne, 2002; Willis, 2005; Iversen & Iversen, 2007). In other instances, over production of dopamine produces negative consequences. For example, researchers have hypothesized that schizophrenia and some other severe mental disturbances are affected or perhaps even caused by the presence of unusually high levels of dopamine. Drugs that block the reception of dopamine reduce the symptoms displayed by some people diagnosed with schizophrenia (Baumeister & Francis, 2002; Bolonna & Kerwin, 2005; Olijslagers, Werkman, & McCreary, 2006). recap

Explain the structure of a neuron. ¦ A neuron has a cell body (which contains a nucleus) with a cluster of fibers called dendrites, which receive messages from other neurons. On the opposite end of the cell body is a tubelike extension, an axon, which ends in a small bulge called a terminal button. Terminal buttons send messages to other neurons. (p. 48) message to fire, it releases an action potential, an electrical charge that travels through the axon. Neurons operate according to an all-ornone law: Either they are at rest, or an action potential is moving through them. There is no in-between state. p. 49) Summarize how messages travel from one neuron to another. ¦ Once a neuron fires, nerve impulses are carried to other neurons through the production of chemical substances, neurotransmitters, that actually bridge the gaps—known as synapses—between neurons. Neurotransmitters Describe how neurons fire. ¦ Most axons are insulated by a coating called the myelin sheath. When a neuron receives a 54 Chapter 2 neuroscience and behavior may be either excitatory, telling other neurons to fire, or inhibitory, preventing or decreasing the likelihood of other neurons firing. (p. 52) Identify neurotransmitters. Neurotransmitters are an important link between the nervous system and behavior. Common neurotransmitters include the following: acetylcholine, which transmits messages relating to our muscles and is involved in memory capabilities; glutamate, which plays a role in memory; gamma-amino butyric acid (GABA), which moderates behaviors from eating to aggression; dopamine, which is involved in movement, attention, and learning; serotonin, which is associated with the regulation of sleep, eating, mood, and pain; and endorphins, which seem to be involved in the brain’s effort to deal with pain and elevate mood. p. 53) evaluate 1. The is the fundamental element of the nervous system. and send messages through their 2. Neurons receive information through their . 3. Just as electrical wires have an outer coating, axons are insulated by a coating called the . 4. The gap between two neurons is bridged by a chemical connection called a 5. Endorphins are one kind of , the chemical “messengers” between neurons. . rethink How might psychologists use drugs that mimic the effects of neurotransmitters to treat psychological disorders? Answers to Evaluate Questions 1. neuron; 2. dendrites, axons; 3. yelin sheath; 4. synapse; 5. neurotransmitter key terms Behavioral neuroscientists (or biopsychologists) p. 47 Neurons p. 48 Dendrites p. 48 Axon p. 48 Terminal buttons p. 49 Myelin sheath p. 49 All-or-none law p. 49 Resting state p. 49 Module 5 neurons: the basic elements of behavior Action potential p. 50 Mirror neurons p. 50 Synapse p. 51 Neurotransmitters p. 52 Excitatory messages p. 52 Inhibitory messages p. 52 Reuptake p. 52 55 module 6 The Nervous System and the Endocrine System Communicating within the Body learning outcomes 6. 1 Explain how the structures f the nervous system are linked together. The complexity of the nervous system is astounding. Estimates of the number of connections between neurons within the brain fall in the neighborhood of 10 quadrillion—a 1 followed by 16 zeros. Furthermore, connections among neurons are not the only means of communication within the body; as we’ll see, the endocrine system, which secretes chemical messages that circulate through the blood, also communicates messages that influence behavior and many aspects of biological functioning (Kandel, Schwartz, & Jessell, 2000; Forlenza & Baum, 2004; Boahen, 2005). . 2 Describe the operation of the endocrine system and how it affects behavior. Central nervous system (CNS) The part of the nervous system that includes the brain and spinal cord. Spinal cord A bundle of neurons LO 1 The Nervous System that leaves the brain and runs down the length of the back and is the main means of transmitting messages between the brain and the body. The human nervous system has both logic and elegance. We turn now to a discussion of its basic structures. Central and Peripheral Nervous Systems

As you can see from the schematic representation in Figure 1, the nervous system is divided into two main parts: the central nervous system and the peripheral nervous system. The central nervous system (CNS) is composed of the brain and spinal cord. The spinal cord, which is about the thickness of a pencil, contains a bundle of neurons that leaves the brain and runs down the length of the back (see Figure 2). As you can see in Figure 1, the spinal cord is the primary means for transmitting messages between the brain and the rest of the body. 56 Chapter 2 euroscience and behavior The Nervous System Consists of the brain and the neurons extending throughout the body Peripheral Nervous System Made up of long axons and dendrites, it contains all parts of the nervous system other than the brain and spinal cord Central Nervous System Consists of the brain and spinal cord Somatic Division (voluntary) Specializes in the control of voluntary movements and the communication of information to and from the sense organs Autonomic Division (involuntary) Concerned with the parts of the body that function involuntarily without our awareness

Brain An organ roughly half the size of a loaf of bread that constantly controls behavior Spinal Cord A bundle of nerves that leaves the brain and runs down the length of the back; transmits messages between the brain and the body Sympathetic Division Acts to prepare the body in stressful emergency situations, engaging resources to respond to a threat Parasympathetic Division Acts to calm the body after an emergency situation has engaged the sympathetic division; provides a means for the body to maintain storage of energy sources Figure 1 A schematic diagram of the relationship of the parts of the nervous system.

However, the spinal cord is not just a communication channel. It also Reflex An automatic, involuntary controls some simple behaviors on its own, without any help from the response to an incoming stimulus. brain. An example is the way the knee jerks forward when it is tapped with a rubber hammer. This behavior is a type of reflex, an automatic, involuntary response to an incoming stimulus. A reflex is also at work when psych 2. 0 you touch a hot stove and immediately withdraw your hand. Although the www. mhhe. com/psychlife brain eventually analyzes and reacts to the situation (“Ouch—hot stove— pull away! ), the initial withdrawal is directed only by neurons in the spinal cord. Three kinds of neurons are involved in reflexes. Sensory (afferent) neurons transmit information from the perimeter of the body to the central nervous system. Motor (efferent) neurons communicate information from the nervous system to muscles and glands. Interneurons connect sensory and motor neurons, carrying messages between the two. Organization of the Nervous System Module 6 the nervous system and the endocrine system 57 Central Nervous System Brain Spinal cord Peripheral Nervous System Spinal nerves

Figure 2 The central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system. Sensory (afferent) neurons Neurons that transmit information from the perimeter of the body to the central nervous system. Motor (efferent) neurons Neurons that communicate information from the nervous system to muscles and glands. Interneurons Neurons that connect sensory and motor neurons, carrying messages between the two. Peripheral nervous system The part As suggested by its name, the peripheral nervous system branches out from the spinal cord and brain and reaches the extremities of the body.

Made up of neurons with long axons and dendrites, the peripheral nervous system encompasses all the parts of the nervous system other than the brain and spinal cord. There are two major divisions— the somatic division and the autonomic division— both of which connect the central nervous system with the sense organs, muscles, glands, and other organs. The somatic division specializes in the control of voluntary movements—such as the motion of the eyes to read this sentence or those of the hand to turn this page—and the communication of information to and from the sense organs.

On the other hand, the autonomic division controls the parts of the body that keep us alive—the heart, blood vessels, glands, lungs, and other organs that function involuntarily without our awareness. As you are reading at this moment, the autonomic division of the peripheral nervous system is pumping blood through your body, pushing your lungs in and out, and overseeing the digestion of your last meal. Activating the Divisions of the Autonomic Nervous System The autonomic division plays a particularly crucial role during emergencies. Suppose that as you are reading in bed you suddenly sense that someone is outside your bedroom window.

As you look up, you see the glint of an object that might