The sympathetic nervous system and parasympathetic nervous system comprise two divisions of the autonomic nervous system (ANS). Sympathetic fibers innervate smooth muscle, cardiac muscle, and glandular tissue. In general, stimulation via sympathetic fibers increases activity and metabolic rate. Sympathetic system stimulation is a critical component of the fight or flight response.

Most target organs and tissues are innervated by neural fibers from both the sympathetic and parasympathetic systems. The systems can act to stimulate organs and tissues in opposite ways (antagonistically). For example, sympathetic stimulation acts to increase heart rate. In contrast, parasympathetic stimulation results in decreased heart rate. The systems can also act in concert to stimulate activity (e.g., both increase the production of saliva by salivary glands, but sympathetic stimulation results in viscous or thick saliva as opposed to a watery saliva). Although they share a number of common features, the...