Role of the Nerve and Endocrine Regulation for Human Behavior

 

Neural Communication

The body’s circuitry, the nervous system, consists of billions of individual cells called neurons. A neuron receives signals from other neurons through its branching dendrites and cell body, combines these signals in the cell body, and transmits an electrical impulse (the action potential) down its axon. When these signals reach the end of the axon, they stimulate the release of chemical messengers called neurotransmitters. These molecules pass on their excitatory or inhibitory messages as they traverse the synaptic gap between neurons and combine with receptor sites on neighboring neurons. Researchers are studying neurotransmitters to discern their role in behavior and emotion. Some drugs (agonists) excite by mimicking particular neurotransmitters or blocking their reuptake; others (antagonists) inhibit by blocking neurotransmitters.

 

Communication throughout the nervous system takes place via neurons—cells that are highly specialized to receive and transmit information from one part of the body to another. Most neurons, especially those in your brain, are extremely small. A bit of brain tissue no larger than a grain of rice contains about 10,000 neurons! Your entire brain contains an estimated 100 billion neurons. Special magnifying equipment, such as an electron microscope, is usually used to study neurons. Fortunately for neuroscientists, there are often striking similarities between the workings of the human nervous system and those of the nervous systems of many other creatures in the animal kingdom. Very simple creatures, such as sea snails and squid, tend to have larger neurons and simpler nervous systems than do humans. Neuroscientists have been able to closely observe the actions and reactions of a single neuron by studying the nervous systems of such simple animals.

Along with neurons, the human nervous system is made up of other types of specialized cells, called glial cells. Glial cells outnumber neurons by about 10 to 1, but are much smaller. Unlike neurons, glial cells do not send or receive information. Rather, they are cast in supporting roles to the major player, the neuron. Glia is Greek for “glue,” and at one time it was believed that glial cells were the glue that held the neurons of the brain together. Although they don’t actually glue neurons together, glial cells do provide structural support for neurons. Glial cells also provide nutrition, enhance the speed of communication between neurons, and remove waste products, including dead or damaged neurons. Neurons vary greatly in size and shape, reflecting their specialized functions. There are three basic types of neurons, each communicating different kinds of information.

Sensory neurons convey information about the environment, such as light or sound, from specialized receptor cells in the sense organs to the brain. Sensory neurons also carry information from the skin and internal organs to the brain. Motor neurons communicate information to the muscles and glands of the body. Simplyblinking your eyes activates thousands of motor neurons. Finally, interneurons communicate information between neurons. By far, most of the neurons in the human nervous system are interneurons, and many interneurons connect to other interneurons.

Characteristics of the Neuron

Most neurons have three basic components: a cell body, dendrites, and an axon. The cell body contains the nucleus, which provides energy for the neuron to carry out its functions. The cell body also contains genetic material and other structures that are found in virtually all the cells in the body.

Extending out from the cell body are many short, branching fibers, called dendrites. The term dendrite comes from a Greek word meaning “tree.” If you have a good imagination, the intricate branching of the dendrites do often resemble the branches of a tree. Dendrites receive messages from other neurons or specialized cells. Dendrites with many branches have a greater surface area, which increases the amount of information the neuron can receive. Some neurons have thousands of dendrites.

The axon is a single, elongated tube that extends from the cell body in most, though not all, neurons. (Some neurons do not have axons.) Axons carry information from the neuron to other cells in the body, including other neurons, glands, and muscles. In contrast to the potentially large number of dendrites, a neuron has no more than one axon exiting from the cell body. However, many axons have branches near their tips that allow the neuron to communicate information to more than one target.

Axons can vary enormously in length. Most axons are very small; some are no more than a few thousandths of an inch long. Other axons are quite long. For example, the longest axon in your body is that of the motor neuron that controls your big toe. This neuron extends from the base of your spine into your foot. If you happen to be a seven-foot-tall basketball player, this axon could be four feet long! For most of us, of course, this axon is closer to three feet long. The axons of many, though not all, neurons are surrounded by the myelin sheath. The myelin sheath is a white, fatty covering manufactured by special glial cells. In much the same way that you can bundle together electrical wires if they are insulated with plastic, myelin helps insulate one axon from the axons of other neurons. Rather than forming a continuous coating of the axon, the myelin sheath occurs in segments that are separated by small gaps where there is no myelin. The small gaps are called the nodes of Ranvier, or simply nodes. Neurons wrapped in myelin communicate their messages up to 20 times faster than do unmyelinated neurons.

The importance of myelin becomes readily apparent when it is damaged. For example, multiple sclerosis is a disease that involves the degeneration of patches of the myelin sheath. This degeneration causes the transmission of neural messages to be slowed or interrupted, resulting in disturbances in sensation and movement. Muscular weakness, loss of coordination, and speech and visual disturbances are some of the symptoms that characterize multiple sclerosis.

 

Communication Within the Neuron

The All-or-None Action Potential

Essentially, the function of neurons is to transmit information throughout the nervous system. But exactly how do neurons transmit information? What form does this information take? In this section, we’ll consider the nature of communication within a neuron, and in the following section we’ll describe communication between neurons. As you’ll see, communication in and between neurons is an electrochemical process.

In general, messages are gathered by the dendrites and cell body and then transmitted along the axon in the form of a brief electrical impulse called an action potential. The action potential is produced by the movement of electrically charged particles, called ions, across the membrane of the axon. Some ions are negatively charged, others positively charged. Think of the axon membrane as a gatekeeper that carefully controls the balance of positive and negative ions on the interior and exterior of the axon. As the gatekeeper, the axon membrane opens and closes ion channels that allow ions to flow into and out of the axon.

Each neuron requires a minimum level of stimulation from other neurons or sensory receptors to activate it. This minimum level of stimulation is called the neuron’s stimulus threshold. While waiting for sufficient stimulation to activate it, the neuron is said to be polarized. In this state the axon’s interior is more negatively charged than is the exterior fluid surrounding the axon. Just in case you’re wondering, scientists have measured the negative electrical charge of the neuron’s interior, using giant squid neurons to do so. And how much electricity are we discussing? About  -70 millivolts (thousandths of a volt). The -70 millivolts is referred to as the neuron’s resting potential.

This polarized, negative-inside/positive-outside condition is primarily due to the different concentrations of two particular ions: sodium and potassium. While the neuron is in resting potential, the fluid surrounding the axon contains a larger concentration of sodium ions than does the fluid within the axon. The fluid within the axon contains a larger concentration of potassium ions than is found in the fluid outside the axon. When sufficiently stimulated by other neurons or sensory receptors, the neuron depolarizes, beginning the action potential. At each successive axon segment, sodium ion channels open for a mere thousandth of a second. The sodium ions rush to the axon interior from the surrounding fluid, and then the sodium ion channels close. Less than a thousandth of a second later, the potassium ion channels open, allowing potassium to rush out of the axon and into the fluid surrounding it. Then the potassium ion channels close. This sequence

of depolarization and ion movement continues in a self-sustaining fashion down the entire length of the axon.

As sodium ions penetrate the axon membrane and potassium ions exit, the electrical charge on the inside of the axon momentarily changes to a positive electrical charge of about +30 millivolts. The result is a brief positive electrical impulse that progressively occurs at each segment down the axon—the action potential. Although it’s tempting to think of the action potential as being conducted much as electricity is conducted through a wire, that’s not what takes place in the neuron. The axon is actually a poor conductor of electricity. At each successive segment of the axon, the action potential is regenerated in the same way in which it was generated in the previous segment—by depolarization and the movement of ions.

Once the action potential is started, it is self-sustaining and continues to the end of the axon. In other words, there is no such thing as a partial action potential. Either the neuron is sufficiently stimulated and an action potential occurs, or the neuron is not sufficiently stimulated and an action potential does not occur. This principle is referred to as the all-or-none law. Following the action potential, a refractory period occurs during which the neuron is unable to fire. This period lasts for a mere thousandth of a second or less. During the refractory period, the neuron repolarizes and reestablishes the negative-inside/positive-outside condition. Like depolarization, repolarization occurs progressively at each segment down the axon. This process of pumping sodium ions out and drawing potassium ions back in reestablishes the resting potential conditions so that the neuron is capable of firing again. The graph in Figure 2.3a depicts the complete sequence from resting potential to action potential and back to resting potential.

Remember, action potentials are generated in mere thousandths of a second. Thus, a single neuron can potentially generate hundreds of neural impulses per second. Given these minute increments of time, just how fast do neural impulses zip around the body? The fastest neurons in your body communicate at speeds of up to 270 miles per hour. In the slowest neurons, messages creep along at about 2 miles per hour. This variation in communication speed is due to two factors: the axon diameter and the myelin sheath. The greater the axon’s diameter, the faster the axon conducts action potentials. And, as we said earlier, myelinated neurons communicate faster than unmyelinated neurons. In myelinated neurons, the sodium ion channels are concentrated at each of the nodes of Ranvier where the myelin is missing. So, in myelinated neurons the action potential jumps from node to node rather than progressing down the entire length of the axon.

The primary function of a neuron is to communicate information to other cells, most notably other neurons. The point of communication between two neurons is called the synapse. At this communication junction, the message-sending neuron is referred to as the presynaptic neuron. The message-receiving neuron is called the postsynaptic neuron. For cells that are specialized to communicate information, neurons have a surprising characteristic: They don’t touch each other. The presynaptic and postsynaptic neurons are separated by a tiny, fluidfilled space, called the synaptic gap, which is only about five-millionths of an inch wide.

The transmission of information between two neurons occurs in one of two ways: electrically or chemically. When communication is electrical, the synaptic gap is extremely narrow, and special ion channels serve as a bridge between the neurons. Electrical communication between the two neurons is virtually instantaneous.

Although some neurons in the human nervous system communicate electrically, over 99 percent of the synapses in the brain use chemical transmission (Greengard, 2001). In general terms, chemical communication occurs when the presynaptic neuron creates a chemical substance that diffuses across the synaptic gap and is detected by the postsynaptic neuron. This one-way communication process between one neuron and another has many important implications for human behavior.

More specifically, here’s how chemical communication takes place between neurons. As we’ve seen, when the presynaptic neuron is activated, it generates an action potential that travels to the end of the axon. At the end of the axon are several small branches called axon terminals. Floating in the interior fluid of the axon terminals are tiny sacs called synaptic vesicles. The synaptic vesicles hold special chemical messengers manufactured by the neuron, called neurotransmitters.

When the action potential reaches the axon terminals, some of the synaptic vesicles “dock” on the axon terminal membrane, then release their neurotransmitters into the synaptic gap. These chemical messengers cross the synaptic gap and attach to receptor sites on the dendrites of the surrounding neurons. This journey across the synaptic gap is slower than electrical transmission, but is still extremely rapid; it takes less than ten-millionths of a second. The entire process of transmitting information at the synapse is called synaptic transmission. What happens to the neurotransmitter molecules after they’ve attached to the receptor sites of the postsynaptic neuron? Most often, they detach from the receptor and are reabsorbed by the presynaptic neuron so they can be recycled and used again. This process is called reuptake. Reuptake also occurs with many of the neurotransmitters that failed to attach to a receptor and are left floating in the synaptic gap. Neurotransmitter molecules that are not reabsorbed or that remain attached to the receptor site are broken down or destroyed by enzymes. As you’ll see in the next section, certain drugs can interfere with both of these processes, prolonging the presence of the neurotransmitter in the synaptic gap.

The number of neurotransmitters that a neuron can manufacture varies. Some neurons produce only one type of neurotransmitter, whereas others manufacture three or more. Although estimates vary, scientists have thus far identified more than 100 different compounds that function as neurotransmitters in the brain (Greengard, 2001).

Each type of neurotransmitter has a chemically distinct, different shape. When released by the presynaptic neuron, neurotransmitters search for the correctly shaped receptor sites on the dendrites of the postsynaptic neurons. Like a key in a lock, a neurotransmitter’s shape must precisely match that of a receptor site on the postsynaptic neuron’s dendrites for the neurotransmitter to affect that neuron. Keep in mind that the postsynaptic neuron can have many differently shaped receptor sites on its dendrites and thus may accommodate several different neurotransmitters.

 

Excitatory and Inhibitory Messages

A neurotransmitter communicates either an excitatory or an inhibitory message to a postsynaptic neuron. An excitatory message increases the likelihood that the postsynaptic neuron will activate and generate an action potential. Conversely, an inhibitory message decreases the likelihood that the postsynaptic neuron will activate. If a postsynaptic neuron receives an excitatory and an inhibitory message simultaneously, the two messages cancel each other out.

It’s important to note that the effect of any particular neurotransmitter depends on the particular receptor to which it binds. So, the same neurotransmitter can have an inhibitory effect on one neuron and an excitatory effect on another. Depending on the number and kind of neurotransmitter chemicals that are taken up by the dendrites of the adjoining neurons, the postsynaptic neurons are more or less likely to activate. If the net result is a sufficient number of excitatory messages, the postsynaptic neuron depolarizes, generates an action potential, and releases its own neurotransmitters. When released by a presynaptic neuron, neurotransmitter chemicals cross hundreds, even thousands, of synaptic gaps and affect the intertwined dendrites of adjacent neurons. Because the receiving neuron can have thousands of dendrites that intertwine with the axon terminals of many presynaptic neurons, the number of potential synaptic interconnections between neurons is mind-boggling. On the average, each neuron in the brain communicates directly with 1,000 other neurons (Greengard, 2001). Thus, in your brain alone, there are up to 100 trillion synaptic interconnections.

 

Neurotransmitters and Their Effects

Your ability to perceive, feel, think, move, act, and react depends on the delicate balance of neurotransmitters in your nervous system. Too much or too little of a given neurotransmitter can have devastating effects. Yet neurotransmitters are present in only minuscule amounts in the human body. If you imagine trying to detect a pinch of salt dissolved in an Olympic-sized swimming pool, you will have some idea of the infinitesimal amounts of neurotransmitters present in brain tissue. In this section, you’ll see that researchers have linked abnormal levels of specific neurotransmitters to various physical and behavioral problems. Nevertheless, it’s important to remember that any connection between a particular neurotransmitter and a particular effect is not a simple one-to-one relationship. Many behaviors are the result of the complex interaction of different neurotransmitters. Further, neurotransmitters sometimes have different effects in different areas of the brain.

 

Important Neurotransmitters

Acetylcholine -learning, memory, muscle contractions

Dopamine – movement, thought processes, rewarding sensations

Serotonin - emotional states, sleep

Norepinephrine - physical arousal, learning, memory

GABA - Inhibition of brain activity

Endorphins Pain perception Positive emotions

Neurotransmitter and Receptor Site Shapes Each neurotransmitter has a chemically distinct shape. Like a key in a lock, a neurotransmitter must perfectly fit the receptor site on the receiving neuron for its message to be communicated. In this figure, NE is the abbreviation for the neurotransmitter norepinephrine and ACh is the abbreviation for acetylcholine.

Acetylcholine, the first neurotransmitter discovered, is found in all motor neurons. It stimulates muscles to contract, including the heart and stomach muscles. Whether it is as simple as the flick of an eyelash or as complex as a back flip, all movement involves acetylcholine. Acetylcholine is also found in many neurons in the brain, and it is important in memory, learning, and general intellectual functioning. People with Alzheimer’s disease, which is characterized by progressive loss of memory and deterioration of intellectual functioning, have a severe depletion of several neurotransmitters in the brain, most notably acetylcholine. The neurotransmitter dopamine is involved in movement, attention, learning, and pleasurable or rewarding sensations (Nader & others, 1997; Schultz & others, 1997). Evidence suggests that the addictiveness of many drugs, including cocaine and nicotine, is related to their dopamine-increasing properties (Greengard, 2001; Volkow & others, 2001a). The degeneration of the neurons that produce dopamine in one brain area causes Parkinson’s disease, which is characterized by rigidity, muscle tremors, poor balance, and difficulty in initiating movements. Symptoms can be alleviated by a drug called L-dopa, which converts to dopamine in the brain (Youdim & Riederer, 1997).

Excessive brain levels of dopamine are sometimes involved in the hallucinations and perceptual distortions that characterize the severe mental disorder called schizophrenia. Some antipsychotic drugs that relieve schizophrenic symptoms work by blocking dopamine receptors and reducing dopamine activity in the brain. Unfortunately, these antipsychotic drugs can also produce undesirable side effects. Because the drugs reduce dopamine in severaldifferent areas of the brain, long-term use sometimes produces symptoms that are very similar to those of Parkinson’s disease. In the chapters on psychological disorders and therapies, we’ll discuss schizophrenia, dopamine, and antipsychotic drugs in more detail.

The neurotransmitters serotonin and norepinephrine are found in many different brain areas. Serotonin is involved in sleep, moods, and emotional states, including depression. Antidepressant drugs such as Prozac increase the availability of serotonin in certain brain regions. Norepinephrine is implicated in the activation of neurons throughout the brain and helps the body gear up in the face of danger or threat. Norepinephrine also seems to be a key player in learning and memory retrieval. Like serotonin and dopamine, norepinephrine dysfunction is implicated in some mental disorders, especially depression (Nemeroff, 1998). GABA is the abbreviation for gamma-aminobutyric acid, a neurotransmitter found primarily in the brain. GABA usually communicates an inhibitory message to other neurons, helping to balance and offset excitatory messages. Alcohol makes people feel relaxed and less inhibited partly by increasing GABA activity, which reduces brain activity. Antianxiety medications, such as Valium and Xanax, also work by increasing GABA activity, which inhibits action potentials and slows brain activity. Interestingly, GABA seems to play a dual role in the brain area that regulates daily sleep–wake cycles. During the day, GABA communicates an excitatorymessage to other neurons in this brain area. At night, however, GABA communicates an inhibitory message to these same neurons (Wagner & others, 1997).

Endorphins: Regulating the Perception of Pain

In 1973, researchers Candace Pert and Solomon Snyder of Johns Hopkins University made the startling discovery that the brain contains receptor sites that are specific for the group of painkilling drugs called opiates (Pert & Snyder, 1973). Opiates include morphine, heroin, and codeine, all derived from the opium poppy. In addition to alleviating pain, opiates often produce a state of euphoria. Why would the brain have receptor sites for specific drugs like morphine? Pert, Snyder, and other researchers concluded that the brain must manufacture its own

painkillers, morphinelike chemicals that act as neurotransmitters.

Within a few years, researchers identified a number of such chemicals manufactured by the brain (Snyder, 1984). Collectively, they are called endorphins, a term derived from the phrase “endogenous morphines.” (The word endogenous means “produced internally in the body.”) Although chemically similar to morphine, endorphins are 100 times more potent. Today, it is known that endorphins are released in response to stress or trauma and that they reduce the perception of pain. Researchers have found that endorphins are implicated in the pain-reducing effects of acupuncture, an ancient Chinese medical technique that involves inserting needles at various locations in the body (Chao & others, 1999). Endorphins are alsoassociated with positive mood. For example, the “runner’s high” associated with aerobic exercise has been attributed to endorphins. In marathon runners, endorphin levels have been found to increase up to four times over their normal levels (Mahler & others, 1989).

 

How Drugs Affect Synaptic Transmission

Much of what is known about different neurotransmitters has been learned from observing the effects of drugs and other substances. Many drugs, especially those that affect moods or behavior, work by interfering with the normal functioning of neurotransmitters in the synapse. As Figure 2.6 illustrates, some drugs increase or decrease the amount of neurotransmitter released by neurons. For example, the venom of a black widow spider bite causes acetylcholine to be released continuously by motor neurons, causing severe muscle spasms. Drugs may also affect the length of time the neurotransmitter remains in the synaptic gap, either increasing or decreasing the amount available to the postsynaptic receptor. One way in which drugs can prolong the effects of the neurotransmitter is by blocking the reuptake of the neurotransmitter by the sending neuron. For example, Prozac inhibits the reuptake of serotonin, increasing the availability of serotonin in the brain. The illegal drug cocaine produces its exhilarating rush by interfering with the reuptake of dopamine (Volkow & others, 1997). Drugs can also mimic specific neurotransmitters. When a drug is chemically similar to a specific neurotransmitter, it may produce the same effect as that neurotransmitter.

It is partly through this mechanism that nicotine works as a stimulant. Nicotine is chemically similar to acetylcholine and can occupy acetylcholine receptor sites, stimulating skeletal muscles and causing the heart to beat more rapidly. Alternatively, a drug can mimic and block the effect of a neurotransmitter by fitting into receptor sites and preventing the neurotransmitter from acting. For example, the drug curare mimics acetylcholine and blocks acetylcholine receptor sites, causing almost instantaneous paralysis. The brain sends signals to the motor neurons, but the muscles can’t respond because the motor neuron receptor sites are blocked by the curare. Similarly, a drug called naloxone eliminates the effects of both endorphins and opiates by blocking opiate receptor sites.

Along with neurons, the human nervous system is made up of other types of specialized cells, called glial cells between neurons and blood vessels in the brain.

There are three basic types of neurons, each communicating different kinds of information:

Sensory neurons convey information about the environment, such as light or sound, from specialized receptor cells in the sense organs to the brain. Sensory neurons also carry information from the skin and internal organs to the brain.

Motor neurons communicate information to the muscles and glands of the body. blinking your eyes activates thousands of motor neurons.

Interneurons communicate information between neurons. By far, most of the neurons in the human nervous system are interneurons, and many interneurons connect to other interneurons.

The cell body contains the nucleus, which provides energy for the neuron to carry out its functions. The cell body also contains genetic material and other structures that are found in virtually all the cells in the body.

Extending out from the cell body are many short, branching fibers, called dendrites. Dendrites receive messages from other neurons or specialized cells.

The axon is a single, elongated tube that extends from the cell body in most, though not all, neurons. Axons carry information from the neuron to other cells in the body, including other neurons, glands, and muscles.

The axons of many, though not all, neurons are surrounded by the myelin sheath. The myelin sheath is a white, fatty covering  anufactured by special glial cells.

In the brain, as in the rest of the nervous system, information is transmitted by electrical impulses that speed from one neuron to the next. The point of communication between two neurons is called the synapse. The transmission of information between two neurons occurs in one of two ways: electrically or chemically. One neuron will transmit info to another neuron or to a muscle or gland cell by releasing chemicals called neurotransmitters.

The site of this chemical interplay is known as the synapse.

An axon terminal (synaptic knob) will abut another cell, a neuron, muscle fiber, or gland cell.

This is the site of transduction – the conversion of an electrical signal into a chemical signal.

 

The Nervous System

The central nervous system’s neurons in the brain and spinal cord communicate with the peripheral nervous system’s sensory and motor neurons. The peripheral nervous system has two main divisions. The somatic nervous system directs voluntary movements and reflexes. The autonomic nervous system, through its sympathetic and parasympathetic divisions, controls our involuntary muscles and glands. Like people clustering into neighborhoods, neurons cluster into working networks.

The central nervous system includes the brain and the spinal cord. The central nervous system is so critical to your ability to function that it is entirely protected by bone—the brain by your skull and the spinal cord by your spinal column. As an added measure of protection, the brain and spinal cord are suspended in cerebrospinal fluid to protect them from being jarred. The central nervous system is aptly named. It is central to all your behaviors and mental processes. And it is the central processing center—every action, thought, feeling, and sensation you experience is processed through the central nervous system. The most important element of the central nervous system is, of course, the brain, which acts as the command center. We’ll take a tour of the human brain in a later section. Think of the spinal cord as an old-fashioned but very busy telephone switchboard, handling both incoming and outgoing messages. Sensory receptors send messages along sensory nerves to the spinal cord, then up to the brain. To activate muscles, the brain sends signals down the spinal cord, which are relayed out along motor nerves to the muscles.

Most behaviors are controlled by your brain. However, the spinal cord can produce spinal reflexes—simple, automatic behaviors that occur without any brain involvement. One of the simplest spinal reflexes involves a three-neuron loop of rapid communication—a sensory neuron that communicates sensation to the spinal cord, an interneuron that relays information within the spinal cord, and a motor neuron leading from the spinal cord that signals muscles to react.

Spinal reflexes are crucial to your survival. The additional few seconds that it would take you to consciously process sensations and decide how to react could result in serious injury. Spinal reflexes are also important as indicators that the neural pathways in your spinal cord are working correctly. That’s why physicians test spinal reflexes during neurological examinations by tapping just below your kneecap for the knee-jerk spinal reflex or scratching the sole of your foot for the toe-curl spinal reflex.

Reflex - a simple, automatic, inborn response to a sensory stimulus.

 

The Peripheral Nervous System

The peripheral nervous system is the other major division of your nervous system. The word peripheral means “lying at the outer edges.” Thus, the peripheral nervous system comprises all the nerves outside the central nervous system that extend to the outermost borders of your body, including your skin. The communication functions of the peripheral nervous system are handled by its two subdivisions: the somatic nervous system and the autonomic nervous system. The somatic nervous system takes its name from the Greek word soma, which means “body.” It plays a key role in communication throughout the entire body. First, the somatic nervous system communicates sensory information received by sensory receptors along sensory nerves to the central nervous system. Second, it carries messages from the central nervous system along motor nerves to perform voluntary muscle movements. All the different sensations that you’re experiencing right now are being communicated by your somatic nervous system to your spinal cord and on to your brain. When you perform a voluntary action,such as turning a page of this book, messages from the brain are communicated down the spinal cord, then out to the muscles via the somatic nervous system. The other subdivision of the peripheral nervous system is the autonomic nervous system. The word autonomic means “self-governing.” Thus, the autonomic nervous system regulates involuntary functions, such as heartbeat, blood pressure, breathing, and digestion. These processes occur with little or no conscious involvement. This is fortunate, because if you had to mentally command your heart to beat or your stomach to digest the pizza you had for lunch, it would be difficult to focus your attention on anything else.

However, the autonomic nervous system is not completely self-regulating. By engaging in physical activity or purposely tensing or relaxing your muscles, you can increase or decrease autonomic activity. Emotions and mental imagery also influence your autonomic nervous system. Vividly imagining a situation that makes you feel angry, frightened, or even sexually aroused can dramatically increase your heart rate and blood pressure. A peaceful mental image can lower many autonomic functions. The involuntary functions regulated by the autonomic nervous system are controlled by two different branches: the sympathetic and parasympathetic nervous systems. These two systems control many of the same organs in your body but cause them to respond in opposite ways. In general, the

sympathetic nervous system arouses the body to expend energy, and the parasympathetic nervous system helps the body conserve energy.

The sympathetic nervous system is the body’s emergency system, rapidly activating bodily systems to meet threats or emergencies. When you are frightened, your breathing accelerates, your heart beats faster, digestion stops, and the bronchial tubes in your lungs expand. All these physiological responses increase the amount of oxygen available to your brain and muscles. Your pupils dilate to increase your field of vision, and your mouth becomes dry, because salivation stops. You begin to sweat in response to your body’s expenditure of greater energy and heat. These bodily changes collectively represent the fight-or-flight response—they physically prepare you to fight or flee from a perceived danger. Whereas the sympathetic nervous system mobilizes your body’s physical resources, the parasympathetic nervous system conserves and maintains your physical resources. It calms you down after an emergency. Acting much more slowly than the sympathetic nervous system, the parasympathetic nervous system gradually returns your body’s systems to normal. Heart rate, breathing, and blood pressure level out. Pupils constrict back to their normal size. Saliva returns, and the digestive system begins operating again. Although the sympathetic and parasympathetic nervous systems produce opposite effects, they act together, keeping the nervous system in balance. Each division handles different functions, yet the whole nervous system works in unison so that both automatic and voluntary behaviors are carried out smoothly.

Sympathetic nervous system -  branch of the autonomic nervous system that produces rapid physical arousal in response to perceived emergencies or threats.

Parasympathetic nervous system -  branch of the autonomic nervous system that maintains normal bodily functions and conserves the body’s physical

Sympathetic

“Fight or flight”

“E” division

Exercise, excitement, emergency, and embarrassment

Parasympathetic

“Rest and digest”

“D” division

Digestion, defecation, and diuresis

 

Endocrine glands communicate information from one part of the body to another by secreting messenger chemicals called hormones into the bloodstream.

The hormones circulate throughout the bloodstream until they reach specific hormone receptors on target organs or tissue. By interacting with the nervous system and affecting internal organs and body tissues, hormones regulate physical processes and influence behavior in a variety of ways.

Metabolism, growth rate, digestion, blood pressure, and sexual development and reproduction are just some of the processes that are regulated by the endocrine hormones. Hormones are also involved in emotional response and your response to stress.

Nervous & Endocrine System

Similarities:

They both monitor stimuli and react so as to maintain homeostasis.

Differences:

The NS is a rapid, fast-acting system whose effects do not always persevere. 

The ES acts slower and its actions are usually much longer lasting.

The central nervous system includes the brain and the spinal cord.

This is the primary internal communication network of the body; divided into the central nervous system and the peripheral nervous system.

 

As the human fetus develops, brain cells multiply, differentiate, and migrate to their final locations.  By the fourth week of prenatal development, new neurons are being generated at the rate of 500,000 per minute. By 24 weeks of prenatal age, the brain has nearly its full complement of neurons. After birth, the neurons grow in size and continue to develop new dendrites. Myelin forms on neuron axons in key areas of the brain, such as those involved in motor control. Axons also grow longer, and the branching at the ends of axons becomes more dense. By adulthood, the fully mature human brain weighs about three pounds.

 

The Endocrine System

The endocrine system, one of the body’s communication systems, is a kindred system to the nervous system. Its glands release hormones at a slower rate than neurotransmitters, resulting in a longer lasting effect. The feeling outlasts the thought. However, the two systems are so closely interconnected that the distinction between them is sometimes difficult to decipher.

Endocrine glands communicate information from one part of the body to another by secreting messenger chemicals called hormones into the bloodstream. The hormones circulate throughout the bloodstream until they reach specific hormone receptors on target organs or tissue. By interacting with the nervous system and affecting internal organs and body tissues, hormones regulate physical processes and influence behavior in a variety of ways. Metabolism, growth rate, digestion, blood pressure, and sexual development and reproduction are just some of the processes that are regulated by the endocrine hormones. Hormones are also involved in emotional response and your response to stress. Endocrine hormones are closely linked to the workings of the nervous system. For example, the release of hormones may be stimulated or inhibited by certain parts of the nervous system. In turn, hormones can promote or inhibit the generation of nerve impulses. Finally, some hormones and neurotransmitters are chemically identical. The same molecule can act as a hormone in the endocrine system and as a neurotransmitter in the nervous system. In contrast to the rapid speed of information transmission in the nervous system, communication in the endocrine system takes place much more slowly. Hor mones rely on the circulation of the blood to deliver their chemical messages to target organs, so it may take a few seconds or longer for the hormone to reach its target organ after it has been secreted by the originating gland. The signals that trigger the secretion of hormones are regulated by the brain, primarily

by a brain structure called the hypothalamus. The hypothalamus serves as the main link between the endocrine system and the nervous system. The hypothalamus directly regulates the release of hormones by the pituitary gland, a pea-sized gland just under the brain. The pituitary’s hormones, in turn, regulate the production of other hormones by many of the glands in the endocrine system. This is why the pituitary gland is often referred to as the body’s master gland. Under the direction of the hypothalamus, the pituitary gland controls hormone production in other endocrine glands.

The pituitary gland also produces some hormones that act directly. For example, the pituitary produces growth hormone, which stimulates normal skeletal growth during childhood. In nursing mothers, the pituitary produces both prolactin, the hormone that stimulates milk production, and oxytocin, the hormone that produces the let-down reflex, in which stored milk is “let down” into the nipple. Interestingly, the pituitary gland can also secrete endorphins to reduce the perception of pain. Another set of glands, called the adrenal glands, is of particular interest to psychologists. The adrenal glands consist of the adrenal cortex, which is the outer gland, and the adrenal medulla, which is the inner gland. Both the adrenal cortex and the adrenal medulla produce hormones that are involved in the human stress response. On stress, hormones secreted by the adrenal cortex also interact with the immune system, the body’s defense against invading viruses or bacteria. The adrenal medulla plays a key role in the fight-or-flight response, described earlier. When aroused, the sympathetic nervous system stimulates the adrenal medulla. In turn, the adrenal medulla produces epinephrine and norepinephrine.

As they circulate through the bloodstream to the heart and other target organs, epinephrine and norepinephrine complement and enhance the effects of the sympathetic nervous system. These hormones also act as neurotransmitters, stimulating activity at the synapses in the sympathetic nervous system. The action of epinephrine and norepinephrine is a good illustration of the long-lasting effects of hormones. If you’ve noticed that it takes a while for you to calm down after a particularly upsetting or stressful experience, it’s because of the lingering effects of epinephrine and norepinephrine in your body.

 

The Brain

Clinical observations have long revealed the general effects of damage to various areas of the brain. But CT and MRI scans now reveal brain structures, and EEG, PET, and functional MRI recordings reveal brain activity. By surgically lesioning or electrically stimulating specific brain areas, by recording the brain’s surface electrical activity, and by displaying neural activity with computer-aided brain scans, neuroscientists explore the connections among brain, mind, and behavior.

 

Older Brain Structures

The brainstem begins where the spinal cord swells to form the medulla, which controls heartbeat and breathing. Within the brainstem, the reticular formation controls arousal. Atop the brainstem is the thalamus, the brain’s sensory switchboard. The cerebellum, attached to the rear of the brainstem, coordinates muscle movement.

Between the brainstem and cerebral cortex is the limbic system, which is linked to memory, emotions, and drives. One of its neural centers, the amygdala, is involved in responses of aggression and fear. Another, the hypothalamus, is involved in various bodily maintenance functions, pleasurable rewards, and the control of the hormonal system.

 

The Hindbrain

The hindbrain connects the spinal cord with the rest of the brain. Sensory and motor pathways pass through the hindbrain to and from regions that are situated higher up in the brain. Sensory information coming in from one side of the body crosses over at the hindbrain level, projecting to the opposite side of the brain. And outgoing motor messages from one side of the brain also cross over at the hindbrain level, controlling movement and other motor functions on the opposite side of the body.

This crossover accounts for why people who suffer strokes on one side of their brain experience muscle weakness or paralysis on the opposite side of their body. Our friend Asha, for example, suffered only minor damage to motor control areas in her brain. However, because the stroke occurred on the left side of her brain, what muscle weakness she did experience was localized on the right side of her body, primarily in her right hand.

Three structures make up the hindbrain—the medulla, the pons, and the cerebellum. The medulla lies directly above the spinal cord and contains centers active in the control of such vital autonomic functions as breathing, heart rate, and digestion. Because the medulla is involved in such critical life functions, damage to it can result in death. The medulla also controls a number of vital reflexes, such as swallowing, coughing, vomiting, and sneezing. Above the medulla is a swelling of tissue called the pons, which represents the uppermost level of the hindbrain. Bulging out behind the pons is the large cerebellum. On each side of the pons, a large bundle of axons connects it to the cerebellum. The word pons means “bridge,” and the pons is a bridge of sorts: Information from various other brain regions located higher up in the brain is relayed to the cerebellum via the pons. The cerebellum functions in the control of balance, muscle tone, and coordinated muscle movements. It is also involved in the learning of habitual or automatic movements and motor skills, such as typing, writing, or gracefully backhanding a tennis ball.

Jerky, uncoordinated movements can result from damage to the cerebellum. Simple movements, such as walking or standing upright, may become difficult or impossible. The cerebellum is also one of the brain areas affected by alcohol consumption, which is why a person who is intoxicated may stagger and have difficulty walking a straight line or standing on one foot. (This is also why a police officer will ask a suspected drunk driver to execute these normally effortless movements.) At the core of the medulla and the pons is a network of neurons called the reticular formation, or the reticular activating system. The reticular formation is composed of many groups of specialized neurons that project up to higher brain regions and down to the spinal cord. The reticular formation plays an important role in regulating attention and sleep.

 

The Midbrain

The midbrain is an important relay station that contains centers important to the processing of auditory and visual sensory information. Auditory sensations from the left and right ears are processed through the midbrain, helping you orient toward the direction of a sound. The midbrain is also involved in processing visual information, including eye movements, helping you visually locate objects and track their movements. After passing through the midbrain level, auditory and visual information is relayed to sensory processing centers farther up in the forebrain region, which will be discussed shortly. A midbrain area called the substantia nigra is involved in motor control and contains a large concentration of dopamine-producing neurons. Substantia nigra means “dark substance,” and as the name suggests, this area is darkly pigmented. The substantia nigra is part of a larger neural pathway that helps prepare other brain regions to initiate organized movements or actions. In the section on neurotransmitters, we noted that Parkinson’s disease involves symptoms of abnormal movement, including difficulty initiating or starting a particular movement. Many of those movement-related symptoms are associated with the degeneration of dopamine-producing neurons in the substantia nigra.

 

The Forebrain

Situated above the midbrain is the largest region of the brain: the forebrain. In humans, the forebrain, also called the cerebrum, represents about 90 percent of the brain. The size of the forebrain has increased during evolution, although the general structure of the human brain is similar to that of other species (Clark & others, 2001). Many important structures are found in the forebrain region, but we’ll begin by describing the most prominent—the cerebral cortex.

 

 

The Cerebral Cortex

The outer portion of the forebrain, the cerebral cortex, is divided into two cerebral hemispheres. The word cortex means “bark,” and much like the bark of a tree, the cerebral cortex is the outer covering of the forebrain. A thick bundle of axons, called the corpus callosum, connects the two cerebral hemispheres. The corpus callosum serves as the primary communication link between the left and right cerebral hemispheres. The cerebral cortex is only about a quarter of an inch thick. It is mainly composed of glial cells and neuron cell bodies and axons, giving it a grayish appearance—which is why the cerebral cortex is sometimes described as being composed of gray matter. Extending inward from the cerebral cortex are white myelinated axons that are sometimes referred to as white matter. These myelinated axons connect the cerebral cortex to other brain regions. Numerous folds, grooves, and bulges characterize the human cerebral cortex. The purpose of these ridges and valleys is easy to illustrate. Imagine a flat, three-foot by threefoot piece of paper. You can compact the surface area of this piece of paper by scrunching it up into a wad. In much the same way, the grooves and bulges of the cerebral cortex allow about three square feet of surface area to be packed into the small space of the human skull.

Each hemisphere of the cerebral cortex—the neural fabric that covers the hemispheres—has four geographic areas: the frontal, parietal, occipital, and temporal lobes. Small, well-defined regions within these lobes control muscle movement and receive information from the body senses. However, most of the cortex—its association areas—is uncommitted to such functions and is therefore free to process other information.

Some brain regions serve specific functions. The brain divides its labor into specialized subtasks and then integrates the various outputs from its neural networks. Thus, our emotions, thoughts, and behaviors result from the intricate coordination of many brain areas. Language, for example, depends on a chain of events in several brain regions. If one hemisphere is damaged early in life, the other will pick up many of its functions, thus demonstrating the brain’s plasticity. The brain becomes less plastic later in life. Frequently, however, nearby neurons can partially compensate for damaged ones, as when a person recovers from a stroke or brain injury.

Many psychological processes, particularly complex ones, involve multiple brain structures and regions. Even seemingly simple tasks—such as carrying on a conversation or catching a ball—involve the smoothly coordinated synthesis of information among many different areas of your brain. How is information communicated and shared among these multiple brain regions? Many brain functions involve the activation of neural pathways that link different brain structures. Neural pathways are formed by groups of neuron cell bodies in one area of the brain that project their axons to other brain areas. These neural pathways form communication networks and circuits that link different brain areas. As a result, damage to one area of the brain may disrupt many neural pathways and affect many different functions. Thus, even though we’ll talk about brain centers and structures that are involved in different aspects of behavior, the best way to think of the brain is as an integrated system. As part of our tour, let’s start with an overview of the methods that have been used to study the human brain.

 

Each cerebral hemisphere can be roughly divided into four regions, or lobes: the temporal, occipital, parietal, and frontal lobes. Each lobe is associated with distinct functions. Located near your temples, the temporal lobe contains the primary auditory cortex, which receives auditory information. At the very back of the brain is the occipital lobe. The occipital lobe includes the primary visual cortex, where visual information is received. The parietal lobe is involved in processing bodily, or somatosensory, information, including touch, temperature, pressure, and information from receptors in the muscles and joints. A band of tissue on the parietal lobe, called the somatosensory cortex, receives information from touch receptors in different parts of the body. Each part of the body is represented on the somatosensory cortex, but this representation is not equally distributed. Instead, body parts are represented in proportion to their sensitivity to somatic sensations. For example, your hands and face, which are very responsive to touch, have much greater representation on the somatosensory cortex than do the backs of your legs, which are far less sensitive to touch. If body areas were actually proportional to the amount of representation on the somatosensory cortex, humans would resemble the misshapen. The largest lobe of the cerebral cortex, the frontal lobe, is involved in planning, initiating, and executing voluntary movements. The movements of different body parts are represented in a band of tissue on the frontal lobe called the primary motor cortex. The degree of representation on the primary motor cortex for a particular body part reflects the diversity and precision of its potential movements.

Thus, it’s not surprising that almost one-third of the primary motor cortex is devoted to the hands and another third is devoted to facial muscles. The disproportionate representation of these two body areas on the primary motor cortex is reflected in the human capacity to produce an extremely wide range of hand movements and facial expressions. The primary sensory and motor areas found on the different lobes represent just a small portion of the cerebral cortex. The remaining bulk of the cerebral cortex consists mostly of three large association areas. These areas are generally thought to be involved in processing and integrating sensory and motor information. For example, the prefrontal association cortex, situated in front of the primary motor cortex, is involved in the planning of voluntary movements. Another association area includes parts of the temporal, parietal, and occipital lobes. This association area is involved in the formation of perceptions and in the integration of perceptions and memories.

 

Studying the Brain. The Toughest Case to Crack

Imagine how difficult it would be to try to figure out how something works without being able to open it, take it apart, or watch it operate. Such has long been the challenge faced by scientists investigating the workings of the human brain. Because the brain is encased entirely by bone, it has been impossible to directly observe a normal, living brain in action. One early approach to mapping brain functions involved examining the bumps on a person’s skull. As you can see in Science Versus Pseudoscience Box 2.1, this approach was not very successful. Another obstacle is the complexity of the brain itself—complex not only in its enormous number of interconnected neurons, but also in the intricate structures, regions, and pathways formed by those neurons. But scientists are not an easily discouraged lot.

Some of the oldest methods of studying the brain are still commonly used. In the case study, researchers systematically observe and record the behavior of people whose brains have been damaged by illness or injury. Case studies of individuals with brain damage have provided valuable insights into behavior in such areas as memory, speech, emotion, movement, and personality. As you’ll see later, the knowledge gained from such observations allowed scientists to begin mapping the functions of the brain. However, generalizing results from a case study must be done cautiously. By their very nature, case studies usually focus on unusual situations or behaviors— in this case, brain disease or injury. Because these behaviors or situations are out of the ordinary, they may not reflect typical behavior. Another potential limitation to using case studies in brain research is that injuries to the brain are rarely limited to specific, localized areas or contained within well-defined anatomical boundaries. It’s often difficult to be sure exactly which brain area is responsible for specific behavioral problems. In addition, many brain areas are linked to other brain areas, and damage in one area may disrupt functioning in another, otherwise normal area.

A related research method involves producing lesions—surgically altering, removing, or destroying specific portions of the brain—and observing subsequent behavior. In humans, lesions are sometimes produced for medical reasons, such as when part of the brain is surgically altered or removed to relieve uncontrollable seizures. Following such medical treatment, researchers can study the behavioral effects of the lesions. Lesions are sometimes produced in animals to systematically investigate the behavioral effects of damage in specific brain areas.

Researchers have also studied the behavioral effects of electrically stimulating specific brain areas. This procedure usually involves implanting tiny electrified disks or wires, called bipolar electrodes, into a specific brain area. Electrical stimulation causes activation of the neurons in the area around the tip of the electrode and usually produces the opposite behavioral effect of a lesion in the same brain area.

The invention of the electroencephalograph allowed scientists to record the brain’s electrical activity through the use of large, disk-shaped electrodes placed harmlessly on a person’s scalp. The graphic record of the brain’s electrical activity that this instrument produces is called an electroencephalogram, abbreviated as EEG. Modern electroencephalographs provide sophisticated computerized analyses of the brain’s electrical activity, recording the electrical activity of the brain from millisecond to millisecond. As technology has become more advanced, so have the tools used to study the brain. In the Focus on Neuroscience, we take a look at the new imaging techniques that allow neuroscientists to see the human brain at work.

 

The Developing Brain

New Neurons Throughout Life? Our guided tour will follow the same general sequence that the brain follows in its development before birth. The human brain begins as a fluid-filled neural tube that forms about two weeks after conception. Gradually, the neural tube expands and develops into separate, fluid-filled cavities, called ventricles, which are at the core of the fully developed brain. Cerebrospinal fluid is manufactured in the ventricles by special glial cells. We noted previously that cerebrospinal fluid acts as a shock absorber for the central nervous system and cushions the brain.

As the human fetus develops, brain cells multiply, differentiate, and migrate to their final locations. By the fourth week of prenatal development, new neurons are being generated at the rate of 500,000 per minute. By 24 weeks of prenatal age, the brain has nearly its full complement of neurons. These neurons will continue to function for decades throughout the person’s lifespan.

The fetal brain is constantly changing, forming as many as 2 million synaptic connections per second. Connections that are used are strengthened, while unused connections are pruned (Rakic, 1995). Progressively, the three major regions of the brain develop: the hindbrain, the midbrain, and the forebrain. Over the course of fetal development, the forebrain structures eventually come to surround and envelop the hindbrain and midbrain structures.

At birth, the infant’s brain weighs less than a pound and is only about one-fourth the size of an adult brain. After birth, the neurons grow in size and continue to develop new dendrites. Myelin forms on neuron axons in key areas of the brain, such as those involved in motor control. Axons also grow longer, and the branching at the ends of axons becomes more dense. By adulthood, the fully mature human brain weighs about three pounds.

For many years, scientists believed that people and most animals did not experience neurogenesis—the development of new neurons—after birth.With the exception of birds, tree shrews, and some rodents, it was thought that the mature brain could lose neurons but could not grow new ones. But new studies offered compelling evidence that persuaded most neuroscientists to abandon that dogma (Gross, 2000).

First, research by psychologist Elizabeth Gould and her colleagues (1998) showed that adult marmoset monkeys were generating a significant number of new neurons every day in the hippocampus, a brain structure that plays a critical role in the ability to form new memories. Gould’s groundbreaking research provided the first demonstration that new neurons could develop in an adult primate brain. Could it be that the human brain also has the capacity to generate new neurons in adulthood? Researchers Peter Eriksson, Fred Gage, and their colleagues (1998) provided evidence that it does. The subjects were five adult cancer patients, whose agesranged from the late fifties to the early seventies. These patients were all being treated with a drug to determine whether tumor cells are multiplying. The drug is incorporated into newly dividing cells and colors the cells. Under fluorescent light, this chemical tracer can be detected in the newly created cells. The reasoning was that if new neurons were being generated, the drug would be present in their genetic material.

Within hours after each patient died, an autopsy was performed and the hippocampus was removed and examined. The results were unequivocal. In each patient, hundreds of new neurons had been generated since the drug had been administered, even though all the patients were over 50 years old (see photo at left). The conclusion? Contrary to the traditional scientific view, the hippocampus has the capacity to generate new neurons throughout the lifespan (Eriksson & others, 1998; Kempermann & Gage, 1999).

Is the capacity to generate new neurons limited to just the hippocampus? Not according to later research by Gould and her colleagues (1999b), which showed that adult macaque monkeys continually develop new neurons that migrate to multiple brain locations. These brain areas are involved in sophisticated cognitive abilities, including memory, learning, and decision making. In the next section, we’ll continue our guided tour of the brain. Following the general sequence of the brain’s development, we’ll start with the structures at the base of the brain and work our way up to the higher brain regions, which are responsible for complex mental activity.

 

Commonly Used Brain-Imaging Techniques

Positron emission tomography, or a PET scan, generates images of the brain’s activity by tracking the brain’s use of a radioactively tagged compound, such as glucose, oxygen, or a particular drug. An invasive procedure, PET involves injecting participants with a radioactive substance before the scan. The PET scan then measures the amount of the radioactively tagged substance used in thousands of brain areas while the person engages in some type of mental activity. Over the course of several minutes, the information is collected, analyzed, and averaged by computer. In the resulting color-coded images, the areas of greatest brain activity are indicated by red and yellow colors. Magnetic resonance imaging, or MRI, is a noninvasive procedure that provides highly detailed images of the body’s internal structures, including the brain. MRI is very versatile, producing thin “slice” images of body tissue from virtually any angle. As the person lies motionless in a long magnetic tube, powerful but harmless magnetic fields bombard the brain or other body area. In response to these magnetic fields, the molecules of the body generate electromagnetic signals, which are analyzed by computer to create the highly detailed images. Tissues with high concentrations of water, such as fat, appear lighter in color, while bone and other tissues with less water appear darker. Functional magnetic resonance imaging, or fMRI, provides moment-by-moment images of the brain’s changing activity. Using the same scanning hardware as an MRI, fMRI also tracks changes in the brain’s blood flow and oxygen levels. Compared to PET scans, fMRI produces a much sharper picture and can detail much smaller brain structures. Another advantage of fMRI is that it provides a picture of brain activity averaged over seconds rather than the several minutes required by PET scans. Because fMRI is a noninvasive procedure, researchers can repeatedly scan a single subject. In an fMRI image, the areas of greatest brain activity are indicated by red and orange colors.

 

Brain scan images have become so commonplace in news articles and popular magazines that it’s easy to forget just how revolutionary brain imaging technology has been to the field of neuroscience (Posner & DiGirolamo, 2000). Shown above are the three types of brain-imaging techniques most commonly used in psychological research— PET scans, MRI, and functional MRI, which is abbreviated fMRI. The descriptions explain how each brain-imaging technique works and the kind of information it provides. How Psychologists Use Brain-Imaging Technology Like other scientific data-gathering methods, brain imaging is used for both descriptive and experimental research. A descriptive study utilizing brain scans might compare the brain structure or functioning of one carefully defined group of people with another. For example, MRI scans were used to compare London taxi drivers, who are required to have an encyclopedic knowledge of the city streets to pass their licensing exam, with matched participants who were not London taxi drivers (Maguire & others, 2000). The MRI scans showed that part of a brain structure involved in spatial memory, the hippocampus, was significantly larger in the experienced taxi drivers than in the control subjects. The size of the hippocampus was also positively correlated with the length of time the participants had been driving taxis in London. One implication of this study is that structures in the adult human brain can change in response to learning and environmental demands—an important topic that we’ll explore later in the chapter. Brain-imaging technology is also used in experimental research, especially in an important new field called cognitive neuroscience. Integrating contributions from psychology, neuroscience, and computer science, cognitive neuroscience is the study of the neural basis of cognitive processes.

 

How is brain imaging used in cognitive neuroscience research?

In a typical experiment, a brain scan is taken during a control task or condition, such as lying down with eyes closed. In the top row of PET scans in the image on page 63, the control condition is shown in the middle PET scan. In that particular study, the control condition consisted of resting while staring at a fixed point. The control scan is compared to brain scans taken while the participant is exposed to the experimental treatment or performing the experimental task. In the top row of PET scans, the first PET scan is the treatment task, which is labeled “Stimulation.” More specifically, the treatment task in that study was looking at a flickering checkerboard pattern. The difference between the PET scans is calculated to determine the brain activity that can be attributed to the experimental condition (Gusnard & Raichle, 2001). In the chapters to come, you’ll see several examples of experimental research that use brain-imaging techniques.

 

Potential Limitations of Brain-Imaging Studies

As technological advances occur, brain-imaging technology continues to improve, offering increasingly detailed pictures of the intact living brain. Nevertheless, brain-imaging research has some limitations. When you consider the results of brain-imaging studies, including those presented in this textbook, keep the following points in mind:

1. Most brain-imaging studies involve small groups of subjects. Because of the limited availability of sophisticated equipment and the high cost of brain-imaging technology, brain-scan research tends to involve small groups of subjects, often as few as few as a dozen or less. As is true with any research that involves a small number of participants, caution must be exercised in generalizing results to a wider population.

2. Most brain-imaging studies involve simple aspects of behavior. Human behavior is extraordinarily complex, and even seemingly simple tasks involve the smooth coordination of multiple brain regions. Reading this paragraph, for example, activates visual, language, memory, and auditory centers in your brain. As psychologist William Uttal (2001) observes, “The more complex the psychological process, the less likely it is that a narrowly circumscribed [brain] region uniquely associated with that process will be found.”

3. Knowing what brain area is involved may tell us little about the psychological process being investigated. Knowing the brain location of a psychological process does not necessarily translate into an understanding of that process. For example, identifying a particular brain structure as being involved in fear does little to explain our psychological experience of fear (Miller & Keller, 2000). Snapshots of brain activity can only be interpreted within the context of psychological knowledge about the behavior being studied.

 

Looking at Brain-Scan Images

What should you notice when you look at a brain scan image? First, read the text description carefully, so that you understand the task or condition that is being measured. Second, when a controlcondition brain scan is shown, carefully compare the control scan with the treatment scan, noting how the two scans differ. Third, keep the limitations of brain-scan technology in mind, remembering that human experience is much too complex to be captured by a single snapshot of brain activity. Although brain-imaging research has its limitations, the advent of sophisticated imaging technology has revolutionized our understanding of the human brain. But brain-imaging technology has also revealed just how much remains to be discovered about the most complex piece of matter known to exist in the universe—the human brain.

 

Our Divided Brain

Clinical observations long ago revealed that the left cerebral hemisphere is crucial for language. Experiments on people with a severed corpus callosum have refined our knowledge of each hemisphere’s special functions. Separately testing the two hemispheres, researchers have confirmed that in most people the left hemisphere is the more verbal, and that the right hemisphere excels in visual perception and the recognition of emotion. Studies of healthy people with intact brains confirm that each hemisphere makes unique contributions to the integrated functioning of the brain.

Biological psychology is the scientific study of the biological bases of behavior and mental processes. This area of research is also called biopsychology. Both terms emphasize the idea of a biological approach to the study of psychological processes.

Biological psychology is one of the scientific disciplines that makes important contributions to neuroscience—the scientific study of the nervous system. As neuroscientists, biopsychologists bring their expertise in behavior and behavioral research to this scientific endeavor.

Some of the other scientific disciplines that contribute to neuroscience include physiology, pharmacology, biology, and neurology.

Communication throughout the nervous system takes place via neurons—cells that are highly specialized to receive and transmit information from one part of the body to another.

By the end of the 1700s it had already been well established that injury to one side of the brain could produce muscle paralysis or loss of sensation on the opposite side of the body. By the early 1800s, animal experiments had shown that specific functions would be lost if particular brain areas were destroyed. And, as discussed in Box 2.1 on phrenology, scientists were beginning to debate the notion of cortical localization—the idea that particular areas of the human brain are associated with particular functions.

In the 1860s, more conclusive evidence for cortical localization was gathered by a French surgeon and neuroanatomist named Pierre Paul Broca. Broca treated a series of patients who had great difficulty speaking but could comprehend written or spoken language. Subsequent autopsies of these patients revealed a consistent finding—brain damage to an area on the lower left frontal lobe. Today, this area on the left hemisphere is referred to as Broca’s area, and it is known to play a crucial role in speech production.

About a decade after Broca’s discovery, a young German neurologist named Karl Wernicke discovered another area in the left hemisphere that, when damaged, produced a different type of language disturbance. Unlike Broca’s patients, Wernicke’s patients had great difficulty understanding spoken or written communications. They could speak quickly and easily, but their speech sometimes made no sense. They sometimes used meaningless words or even nonsense syllables, though their sentences seemed to be grammatical. In response to the question “How are you feeling?” a patient might say something like, “Don’t glow glover. Yes, uh, ummm, bick, bo chipickers the dallydoe mick more work mittle.” Autopsies of these patients’ brains revealed consistent damage to an area on the left temporal lobe that today is called Wernicke’s area.

The discoveries of Broca and Wernicke provided the first compelling clinical evidence that language and speech functions are performed primarily by the left cerebral hemisphere. If similar brain damage occurs in the exact same locations on the right hemisphere, these severe disruptions in language and speech are usually not seen.

The notion that one hemisphere exerts more control over or is more involved in the processing of a particular psychological function is termed lateralization of function. Speech and language functions are lateralized on the left hemisphere. Generally, the left hemisphere exerts greater control over speech and language abilities in virtually all right-handed and the majority of left-handed people.

The language disruptions demonstrated by Broca’s and Wernicke’s patients represent different types of aphasia. Aphasia refers to the partial or complete inability to articulate ideas or understand spoken or written language because of brain injury or damage. There are many different types of aphasia. People with Broca’s aphasia find it difficult or impossible to produce speech, but their comprehension of verbal or written words is relatively unaffected. People with Wernicke’s aphasia can speak, but they often have trouble finding the correct words and have great difficulty comprehending written or spoken communication. In more severe cases of Wernicke’s aphasia, speech can be characterized by nonsensical, meaningless, incoherent words, as in the example given earlier. At the beginning of this chapter, we described the symptoms experienced by our friend Asha in the weeks before and the months following her stroke. Asha, who is right-handed, experienced the stroke in her left hemisphere. About three days after her stroke, an MRI brain scan showed where the damage had occurred: the left temporal lobe.

Asha experienced many symptoms of Wernicke’s aphasia. Talking was difficult, not because Asha couldn’t speak, but because she had to stop frequently to search for the right words. Asha was unable to name even simple objects, like the cup on her hospital dinner tray or her doctor’s necktie. She recognized the objects but was unable to say what they were. She had great difficulty following a normal conversation and understanding speech, both in English and in her native language, Tulu. Asha also discovered that she had lost the ability to read. She could see the words on the page, but they seemed to have no meaning. Paul brought some of their Christmas cards to the hospital. Asha recalls, “When I realized I couldn’t read the Christmas cards, I thought my life was over. I just lost it. I remember crying and telling the nurse, ‘I have a doctorate and I can’t read, write, or talk!’ ” When we visited Asha in the hospital, we brought her a Christmas present: a portable cassette tape player with headphones and some tapes of relaxing instrumental music. Little did we realize how helpful the music would be for her. One tape was a recording of Native American flute music called Sky of Dreams. The music was beautiful and rather unusual, with intricate melodies and unexpected, complex harmonies. Although it was very difficult for Asha to follow normal speech, listening to Sky of Dreams was an entirely different experience. As Asha explained: I tried cranking up the music very high and it soothed me. I could sleep. At the time, the flute music seemed to be just perfectly timed with the way my brain was working. It was tuning out all the other noises so I could focus on just one thing and sleep. So I would play the music over and over again at a very high level. I did that for a long time because my mind was so active and jumbled that I couldn’t think.

Asha’s language functions were severely disrupted, yet she was able to listen to and appreciate instrumental music—even very complex music. Why? At the end of the next section, we’ll offer a possible explanation for what seems to have been a disparity in Asha’s cognitive abilities following her stroke.

 

Cutting the Corpus Callosum. The Split Brain

Since the discoveries by Broca and Wernicke, the most dramatic evidence illustrating the independent functions of the two cerebral hemispheres has come from a surgical procedure called the split-brain operation. This operation is used to stop or reduce recurring seizures in severe cases of epilepsy that can’t be treated in any other fashion. The procedure involves surgically cutting the corpus callosum, the thick band of axons that connects the two hemispheres. What was the logic behind cutting the corpus callosum? An epileptic seizure typically occurs when neurons begin firing in a disorganized fashion in one region of the brain. The disorganized neuronal firing quickly spreads from one hemisphere to the other via the corpus callosum. If the corpus callosum is cut, seizures should be contained in just one hemisphere, reducing their severity or eliminating them altogether. This is exactly what happened when the split-brain operation was first tried in this country in the 1940s (Springer & Deutsch, 1998). Surprisingly, cutting the corpus callosum initially seemed to produce no noticeable effect on the patients, other than reducing their epileptic seizures. Their ability to engage in routine conversations and tasks seemed to be unaffected. On the basis of these early observations, some brain researchers speculated that the corpus callosum served no function whatsoever (see Gazzaniga, 1995). One famous psychologist, Karl Lashley, joked that its primary function seemed to be to keep the two hemispheres from sagging (Hoptman & Davidson, 1994). In the 1960s, however, psychologist and neuroscientist Roger Sperry and his colleagues began unraveling the puzzle of the left and right hemispheres. Sperry and his colleagues used the apparatus shown in Figure below to test the abilities of split-brain patients. They would direct a split-brain subject to focus on a point in the middle of a screen, while briefly flashing a word or picture to the left or right of the midpoint. In this procedure, visual information to the right of the midpoint is projected to the person’s left hemisphere, and visual information to the left of the midpoint is projected to the right hemisphere. Behind the screen several objects were hidden from the split-brain subject. The subject could reach under a partition below the screen to pick up the concealed objects but could not see them (Sperry, 1982). In a typical experiment, Sperry projected the image of an object concealed behind the screen, such as a hammer, to the left of the midpoint. Thus, the image of the hammer was sent to the right, nonverbal hemisphere. If a split-brain subject was asked to verbally identify the image flashed on the screen, he could not do so and often denied that anything had appeared on the screen. Why? Because his verbal left hemisphere had no way of knowing the information that had been sent to his right hemisphere.

However, if a split-brain subject was asked to use his left hand to reach under the partition for the object that had been displayed, he would correctly pick up the hammer. This was because his left hand was controlled by the same right hemisphere that saw the image of the hammer. Sperry’s experiments reconfirmed the specialized language abilities of the left hemisphere that Broca and Wernicke had discovered more than a hundred years earlier. But notice, even though the split-brain subject’s right hemisphere could not express itself verbally, it still processed information and expressed itself nonverbally: The subject was able to pick up the correct object. Over the last four decades, researchers have gained numerous insights about the brain’s lateralization of functions by studying split-brain patients, using brainimaging techniques with normal subjects, and other techniques (Gazzaniga, 1998).

On the basis of this evidence, researchers have concluded that—in most people— the left hemisphere is superior in language abilities, speech, reading, and writing. In contrast, the right hemisphere is more involved in nonverbal emotional expression and visual-spatial tasks (Corballis & others, 2002). Deciphering complex visual cues, such as completing a puzzle or manipulating blocks to match a particular design, also relies on right-hemisphere processing (Gazzaniga, 1995). And the right hemisphere excels in recognizing faces and emotional facial cues, reading maps, copying designs, and drawing (Heller & others, 1998; Reuter-Lorenz & Miller, 1998). Finally, the right hemisphere shows a higher degree of specialization for musical appreciation or responsiveness—but not necessarily for musical ability, which involves the use of the left hemisphere as well (Springer & Deutsch, 1998). Figure summarizes the research findings for the different specialized abilities of the two hemispheres for right-handed people. As you look at the figure, it’s important to keep two points in mind. First, the differences between the left and right hemispheres are almost always relative differences, not absolute differences. In other words, both hemispheres of your brain are activated to some extent as you perform virtually any task (Beeman & Chiarello, 1998; Chabris & Kosslyn, 1998). In the normal brain, the left and right hemispheres function in an integrated fashion, constantly exchanging information (Banich, 1998). Thus, the hemisphere that typically displays greater activation or exerts greater control over a particular function. Second, many functions of the cerebral hemispheres, such as those involving the primary sensory and motor areas, are symmetrical. They are located in the same place and are performed in the same way on both the left and the right hemisphere. Given the basic findings on the laterality of different functions in the two hemispheres, can you speculate about why Asha was unable to read or follow a simple conversation but could easily concentrate on a complex piece of music? Why were her language abilities so disrupted, while her ability to focus on and appreciate music remained intact after her stroke? A plausible explanation has to do with the location of the stroke’s damage on Asha’s left temporal lobe. Because language functions are usually localized on the left hemisphere, the stroke produced serious disruptions in Asha’s language abilities. However, her right cerebral hemisphere sustained no detectable damage. Because one of the right hemisphere’s abilities is the appreciation of musical sounds, Asha retained the ability to concentrate on and appreciate music.

 

Plasticity. The Malleable Brain

In our exploration of the biological foundations of behavior, we’ve traveled from the activities of individual neurons to the complex interaction of the billions of neurons that make up the human nervous system, including the brain. Crucial to the development of a scientific understanding of brain functioning were two areas—localization and lateralization. Phrenology’s incorrect interpretation of bumps on the head helped trigger scientific debate on the notion of localization—that different functions are localized in different brain areas. The early clinical evidence provided by Broca and Wernicke, and the later split-brain evidence provided by Sperry and his colleagues, confirmed the idea of lateralization—that some functions are performed primarily by one cerebral hemisphere. The ideas of localization and lateralization are  complemented by another theme evident in this chapter—integration. Despite the high degree of specialization

in the human nervous system, the smooth functioning of the nervous system demands an equally high degree of integration and harmony.Your ability to process new information and experiences, your memories of previous experiences, your sense of who you are and what you know, your actions and reactions—all are products of your brain working in harmony with the rest of your nervous system. The story of Asha’s stroke illustrated what can happen when this harmony is disrupted. Your physical survival and conscious experience are mediated by the delicate balance of chemicals and the complex, intricate connections in your nervous system. Asha survived her stroke, but many people who suffer strokes do not. Of those who do survive a stroke, about one-third are left with severe impairments in their ability to function.

What happened to Asha? Fortunately, Asha’s story has a happy ending. She was luckier than many stroke victims—she was young, strong, and otherwise healthy. Asha’s recovery was also aided by her high level of motivation, willingness to work hard, and sheer will to recover. After being discharged from the hospital, Asha began months of intensive speech therapy. Her speech therapist assigned a great deal of homework that consisted of repeatedly pairing pictures with words, objects with words, and words with objects. Asha set a very high goal for herself: to return to teaching at the university by the fall semester.With the help of her husband, Paul, and her mother, Nalini, who traveled from India to stay with her for four months, Asha made significant gains. With remarkable determination, Asha reached the goal she had set for herself. She returned to her teaching and research at the university the following fall semester. Today, more than three years after her stroke, you would be unable to detect any signs of impairment indicating that Asha had sustained significant brain damage. Asha’s story also illustrates the brain’s incredible ability to shift functions from damaged to undamaged areas, a phenomenon called functional plasticity. Even more astonishing, the uninjured brain has the ability to change and grow throughout life. As researchers have discovered, the brain can literally change its structure in response to the quality of environmental stimulation. In the chapter Application, we’ll take a look at how researchers have documented the remarkable structural plasticity of which the brain is capable. And you’ll learn how you can use the research findings on brain plasticity to enhance your own dendritic potential!