Your psychological life depends on not only your brain, but also communications systems that link your brain to the rest of your body. The body’s primary communication system is the nervous system, the complete collection of neurons that transmits signals among the parts of the body.

The brain, our focus so far in this chapter, is one part of the overall nervous system. Let’s learn about the rest of the nervous system by examining its two main parts, the central and peripheral nervous systems.

The central nervous system gets its name from its location; it is found in the center of the body. The central nervous system consists of two main parts, the brain and the spinal cord. We have already discussed the brain, so let’s look at the spinal cord.

The spinal cord is a bundle of neurons and glial cells that extends from the brain stem down to the bottom of the spine. These cells run through the bones of the spine, which protects them from damage (Figure 3.23).

The spinal cord participates in two-way communications between the brain and body:

Some bodily actions do not require signals that are sent to the brain. People possess reflexes, which are automatic, involuntary responses to external stimulation (Figure 3.24). For example, the leg movement that occurs when a doctor taps you on the knee during a physical examination is a reflex. (By comparison, play-acting that same leg movement is not a reflex.) Reflex actions are executed by neurons in the spinal cord. Sensory information reaches an interneuron, which is a neuron that relays information to a motor neuron that, in turn, puts the body into action.

The peripheral nervous system is found, as the name suggests, in the periphery of the body—that is, away from the body’s center. All neurons outside of the central nervous system are part of the peripheral nervous system.

There are two ways of classifying the various parts of the peripheral nervous system. One method is by their physical location. Cranial nerves are those parts of the peripheral nervous system found in the head. These nerves extend out from the bottom of the brain and connect to structures in the head, such as the eyes, nose, and tongue. Spinal nerves extend from the spinal cord to the body’s neck, torso, and limbs (Figure 3.25).

The second way of classifying the nerves of the peripheral nervous system is functionally, that is, in terms of what they do. The somatic nervous system provides the brain-to-periphery communications that enable you to control your bodily movement. For instance, when deciding to pass a soccer ball to your teammate rather than shooting on goal, the messages from your brain that adjust the movements of your leg and foot to get the ball to your teammate are carried along the somatic nervous system. The autonomic nervous system, on the other hand, provides the communications that control bodily functions that generally are not under your control; they occur without your even thinking about them. As you rush upfield deciding whether to pass or shoot, your heart beats faster than when you are at rest, your sweat glands are more active, your breathing is faster, and the pupils of your eyes are wider. But you don’t make conscious decisions about altering your heart rate, breathing, sweating, and pupil dilation; these changes occur automatically, through signals carried by the autonomic nervous system.

The autonomic nervous system contains two divisions, that is, two subsystems that perform different tasks. The sympathetic nervous system prepares you for action. It activates the biological systems required for rapid activity, such as increased heart rate. This activation enables “fight” or “flight” responses—actions to confront a threat or to flee from it. Revisiting the soccer example above, the sympathetic nervous system was responsible for activating biological systems as you rushed up the field.

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But imagine what happens after you pass to your teammate: She scores, the game ends, your team wins, and now your body responds differently. You relax, rest, and may find that you need a bathroom break. Your body’s parasympathetic nervous system is swinging into action. The parasympathetic nervous system is the part of the autonomic nervous system that maintains normal functioning of the body when you are not under threat or stress. It activates “basic housekeeping” functions such as digestion and elimination of bodily waste, and reduces heart rate and blood pressure to low, baseline levels of activity.

The different roles of the sympathetic and parasympathetic nervous systems explain everyday examples in which psychological factors influence physical functioning. For instance, the demands of travel can be stressful, and travelers often experience “traveler’s constipation.” During the stress of travel, the sympathetic nervous system is relatively more active than usual—and the parasympathetic system less active, which interferes with normal digestive system functioning.