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
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
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
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
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