ABSTRACT
T he nervous system can be divided in two ways. First, it can be divided ana-tomically into central and peripheral components. The central nervous system consists of the brain and spinal cord, whereas the peripheral nervous system consists of the nerve fibers that lie outside the brain and spinal cord (Figure 4.1). Second, it can be divided functionally into autonomic and somatic systems, both of which have central as well as peripheral components (Figure 4.2). The autonomic nervous system innervates organs and tissues that are not usually under voluntary control, such as cardiac muscle, smooth muscle, and glands. The autonomic nervous system has two divisions: (1) the sympathetic division, which originates in the thoracic and lumbar regions of the spinal cord, and (2) the parasympathetic division, which originates in the brain and in the sacral region of the spinal cord. Generally, the sympathetic branch is activated in emergency or stressful situations. Because of this, it is sometimes referred to as the “fight or flight” branch of the autonomic nervous system. The parasympathetic branch, on the other hand, is activated during times of inactivity and is responsible for encouraging all of those processes leading to the digestion of nutrients. Therefore, it is sometimes referred to as the “rest and digest” branch of the autonomic nervous system.
