CHAPTER 2: NEUROSCIENCE AND 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.
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 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.
The Brain
The Tools of Discovery
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 Cerebral Cortex
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, welldefined 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.
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.