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Biological Aspects of Psychology
-------------------------------Before you read the next sentence, close your eyes and touch your nose. This task is easy,
but it is not simple. To get the job done, your brain used specific nerves to tell your
eyelids to close. It used other nerves to tell your hand to extend a finger and then sent a
series of messages that moved your arm in just the right direction until it received a
message that your finger and your nose were in contact. This example illustrates that
everything you do—including how you feel and think—is based on some kind of
biological activity in your body, especially in your brain. This chapter tells the story of
that activity, beginning with the neuron, one of the body's most basic biological units. We
describe how neurons form systems capable of receiving and processing information and
translate it into behavior, thoughts, and biochemical changes. We have organized our
presentation as follows:
1
The Nervous System 61
Cells of the Nervous System Action Potentials
Synapses and Communication Between Neurons
Organization and Functions of the Nervous System
The Peripheral Nervous System: Keeping in Touch with the World 67
The Somatic Nervous System The Autonomic Nervous System
The Central Nervous System: Making Sense of the World 69
The Spinal Cord The Brain
Thinking Critically: What Can fMRI Tell Us About Behavior and
Mental Processes?
Focus on Research Methods:
Manipulating Genes in Animal
Models of Human Disease The Cerebral Cortex The Divided Brain in a Unified Self
Plasticity in the Central Nervous System
Linkages: Human Development and the Changing Brain
The Chemistry of Psychology 95
Three Classes of Neurotransmitters
The Endocrine System: coordinating the internal World 99
The immune System: Defending the Body 102
LINKAGES SUMMARY
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(SIDE NOTE)biological psychology The psychological specialty focused on the
physical and chemical changes that cause, and occur in response to, behavior and mental
processes.
(END SIDE NOTE)
Do you drink coffee or cola? Do you like beer or wine? Are you still unable to quit
smoking?
If so, you know that caffeine, alcohol, and nicotine can change the way you feel. The
effects of these substances are based largely on their ability to change the chemistry of
your brain. There are many other examples of how our mental experiences, and our
identity as individuals, are rooted in biological processes. Each year, millions of people
who suffer anxiety, depression, and other psychological disorders take prescription drugs
that alter brain chemistry in ways that relieve their distress. Millions of others have the
resilience to bounce back, without drugs, from even the worst experiences life can throw
at them, and find meaning and fulfillment in their lives. To what extent are these negative
and positive reactions rooted in biological differences among people?
The importance and impact of biological processes stems from the fact that brain cells,
hormones, genes, and other biological factors are related to everything you think and feel
and do, from the fleeting memory you had a minute ago to the anxiety or excitement or
fatigue you felt last night to the movements of your eyes as you read right now. In this
chapter, we describe these biological factors in more detail. Reading it will take you into
the realm of biological psychology, which is the study of the cells and organs of the body
and the physical and chemical changes involved in behavior and mental processes. It is
here that we begin to consider the relationship between your body and your mind, your
brain and your behavior.
It is a complex relationship. Scientific psychologists are no doubt correct when they say
that every thought, every feeling, and every action are represented somehow in the
nervous system and that none of these events could occur without it. However, we must
be careful not to oversimplify or overemphasize biological explanations in psychology.
Many people assume, for example, that if a behavior or mental process has a strong
biological basis, it is beyond our control—that "biology is destiny." Accordingly, many
smokers don't even try to quit, simply because they are sure that their biological addiction
to nicotine will doom them to failure. This is not necessarily true, as millions of exsmokers can confirm. The fact that all behavior and mental processes are based on
biological processes does not mean that they can be fully understood through the study of
biological processes alone.
In fact, reducing all of psychology to the analysis of brain chemicals would seriously
underestimate the complexity of the interactions between our biological selves and our
psychological experiences, between our genes and our environments. Just as all behaviors
and mental processes are influenced by biology, all biological processes are influenced
by the environment. We will see later that the experiences we have can change our brain
chemistry and even our brain anatomy—often by affecting whether certain aspects of our
genetic makeup are expressed. Examples of experience-dependent gene expression are
discussed in the appendix on behavioral genetics, but for now, just consider height. Your
height is strongly influenced by genetics, but how tall you actually become depends
heavily on nutrition and other environmental factors (Tanner, 1992). Hereditary and
environmental influences also combine to determine intelligence, personality, mental
disorders, and all our other characteristics.
In short, understanding behavior and mental processes requires that we combine
information from many sources, ranging from the activity of cells and organ systems to
the activity of individuals and groups in social contexts. This chapter focuses on the
biological level, not because it reveals the whole story of psychology but because it tells
an important part of that story.
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FIGURE 3.1
Three Functions of the Nervous System
The nervous system's three main functions are to receive information (input), integrate
that information with past experiences (processing), and guide actions (output). When the
alarm clock goes off, this person's nervous system, like yours, gets the message,
recognizes what it means, decides what to do, and then takes action—by getting out of
bed or perhaps hitting the snooze button. (GRAPHIC DELETED)
(SIDE NOTE) nervous system A complex combination of cells whose primary function
is to allow an organism to gain information about what is going on inside and outside the
body and to respond appropriately.
(END SIDE NOTE)
We begin by considering the nervous system, the billions of cells that make up your
brain, your spinal cord, and other nerve fibers. The combined activity of these cells tells
you what is going on inside and outside your body and allows you to make appropriate
responses. For example, if you are jabbed with a pin, your nervous system gets the
message and immediately causes you to flinch. But your nervous system can do far more
than detect information and execute responses. When information about the world
reaches your brain, that information is processed—meaning that it is combined with
information about past experiences and current wants and needs—so that you can make a
decision about how to respond. The chosen action is then taken (see Figure 3.1). In other
words, your nervous system displays the characteristics of an information-processing
system: It has input, processing, and output capabilities.
The processing capabilities of the nervous system are especially important, not only
because the brain interprets information, makes decisions, and guides action but also
because the brain can actually adjust the impact of incoming information. This
phenomenon helps explain why you can't tickle yourself. In one study, simply telling
ticklish people that they were about to be touched on the bottom of their feet caused
activation in the brain region that receives sensory information from the foot (Carlsson et
al., 2000). The anticipation of being touched made these people all the more sensitive to
that touch. However, when they were asked to touch the bottoms of their own feet, there
was far less advance activation of this brain region, and they did not overreact to their
touch. Why? The explanation is that when the brain plans a movement, it also predicts
which of its own touch-detecting regions will be affected by that movement. So
predictable, self-controlled touches, even in a normally "ticklish" spot, reduce activation
of the sensory regions associated with that spot (Blakemore, Wolpert, & Frith, 2000).
The nervous system is able to do what it does partly because it is made up of cells that
communicate with each other. Like all cells in the body—indeed, like all living cells—
those in the nervous system can respond to various kinds of signals. Many of the signals
that cells respond to come in the form of chemicals released by other cells. So even as
various cells specialize during prenatal development to become skin, bone, hair, and
other tissues, they still "stay in touch" through chemical signals. Bone cells, for example,
add or lose calcium in response to hormones secreted in another part of the body. Cells in
the bloodstream respond to viruses and other invaders by destroying them. We'll focus
first on the cells of the nervous system, because their ability to communicate is the most
efficient and complex.
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The Nervous System
We begin our exploration of the nervous system at the "bottom," with a description of the
individual cells and molecules that compose it. Then we consider how these cells are
organized to form the structures of the human nervous system.
• Cells of the Nervous System
Brain tissue appears to be a hopelessly complex web of interconnecting fibers, but about
a hundred years ago, the great Spanish anatomist Ramon y Cajal (pronounced "kaHALL) argued that the nervous system is made up of separate cells, called neurons.
When the invention of the electron microscope allowed scientists to see individual
neurons, and the gaps between them, Cajal's idea was confirmed. Neurons are cells that
are specialized to rapidly respond to signals and to quickly send signals of their own. For
many years communication in the nervous system was thought to take place only
between neurons, but this neuron doctrine has turned out to be incorrect (Bullock et al.,
2005). Non-neuronal cells perform important communication functions, too. These cells
are called glial cells because glial means "glue," and scientists had long believed that they
did no more than hold neurons together. However, we now know that glial cells also help
neurons communicate by directing their growth, keeping their chemical environment
stable, providing energy, and secreting chemicals to help restore damage (Fellin et al.,
2004). So without glial cells, neurons could not function. Further, glial cells are capable
of the signature functions of neurons, including releasing chemicals that influence
neurons, responding to chemicals from neurons, and changing in response to experience
(Auld & Robitaille, 2003; Ge et al., 2006). So the nervous system is actually made up of
two major types of cells—neurons and glial cells—that, together, allow the system to
carry out its complex signaling tasks so efficiently.
• Common Features of Neurons Figure 3.2 shows three features that neurons share with
almost every other kind of cell in the body. First, neurons have an outer membrane that
acts like a fine screen, letting some substances pass in and out while blocking others.
Second, nervous system cells have a cell body, which contains a nucleus (only red blood
cells have no nucleus). The nucleus carries the genetic information that determines how a
cell will function. Third, nervous system cells contain mitochondria (pronounced "mytoh-CON-dree-ah"), which are structures that turn oxygen and glucose into energy. This
process is especially vital to brain cells. Although the brain accounts for only 2 percent of
the body's weight, it consumes more than 20 percent of the body's oxygen (Sokoloff,
1981). All of this energy is required because brain cells transmit signals among
themselves to an even greater extent than do cells in the rest of the body.
Besides being similar to other cells in the human body, neurons are also amazingly
similar to the cells in all living organisms, from bacteria to plants to humans. For
example, bacteria, plant cells, and brain cells all synthesize similar proteins when they are
subjected to reduced oxygen or elevated temperatures. Because of this similarity, we can
learn something about human brain cells by studying cells in much simpler organisms.
For example, research on the cells in worms recently provided clues to the causes of
Alzheimer's disease (Cohen et al., 2006).
(SIDE NOTE) neuron Fundamental unit of the nervous system; nerve cell. glial cells
Cells in the nervous system that hold neurons together and help them communicate with
one another.
(END SIDE NOTE)
• Special Features of Neurons However, neurons have three special features that enable
them to communicate signals efficiently. The first is their structure. Although neurons
come in many shapes and sizes, they all have long, thin fibers that extend outward from
the cell body (see Part A of Figure 3.2). When these fibers get close to other neurons,
communication between the cells can occur. The intermixing
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