The Central and Peripheral Nervous Systems
At the most general level, the nervous system can be divided into several parts (Figure 3.25). The central nervous system (CNS) includes the brain and spinal cord, working as an integrated unit. All nerves elsewhere in the body are part of the peripheral nervous system (PNS), and virtually all of the nerves in the peripheralnervous system connect to the CNS via the spinal cord. This is part of the reason why damage to the cord is so dangerous, and why the cord is protected by the bones and connective tissue of the spine. Of course, the brain, too, is well protected. It’s cov-ered, first, in a shell of bone (the skull) and three layers of tough membranes (the
meninges). It’s also floating in a bath of cerebrospinal fluid that (among other things) acts as a shock absorber when the head moves abruptly this way or that.
The peripheral nervous system itself has two distinguishable parts. The somaticnervous system (SNS) includes all the (efferent) nerves that control the skeletalmuscles as well as the (afferent) nerves that carry information from the sense organs to the CNS. The other division—the autonomic nervous system (ANS)—includes all the (efferent) nerves that regulate the various glands in the body as well as those that regulate the smooth muscles of the internal organs and blood vessels. (The name “smooth muscles” refers to how these muscles look when observed under a micro-scope; this is in contrast to the skeletal muscles, which look striped.) The ANS also includes (afferent) nerves that bring the CNS information about these various internal systems.
Finally, the autonomic nervous system is itself divided into two parts: the sympa-thetic branch, which tends to “rev up” bodily activities in preparation for vigorousaction, and the parasympathetic branch, which tends to restore the body’s internal activities to normal after the action has been completed (Figure 3.26).* These divisions of the ANS act reciprocally; excitation of the sympathetic branch leads to an increased heart rate, while excitation of the parasympathetic branch leads to cardiac slowing. Sympathetic activation produces a slowing down of peristalsis (rhythmic contractions of the intestines), so that we’re not using energy for digest-ing when we’re on the run; parasympathetic activation does the opposite—it speeds up peristalsis.