Peripheral Nervous System

The peripheral nervous system consists of the sensory receptors such as
those that recognize touch or heat in the skin or visual stimuli in the retina
of the eye, and the nerves which communicate the stimuli to the brain. The
peripheral nervous system is often subdivided into two parts, according to
function: the somatic portion, which recognizes stimuli in the external environment
such as on the skin, and the autonomic portion, which recognizes
changes in the internal environment, such as hormone or mineral concentrations
in the bloodstream.
The somatic portion of the peripheral nervous system in humans consists
of twelve pairs of nerves which originate in the brain and which transmit
sensory input from the body. For example, nerve endings in the retina of
the eye transmit images to the brain; sensory fibers in the face transmit impulses
affecting the skin or teeth. An additional thirty-one pairs of nerves
emerge from the spinal cord, subdivide into branches, and innervate various
regions of the body.
The autonomic nervous system maintains homeostasis, or constancy,
within the body. For example, receptors measure heart rate, body temperature,
and the activity of hormones within the bloodstream and tissues. Any
abnormality or change results in a signal sent to the brain.
The most notable of the functions of the autonomic nervous system occur
in the sympathetic and parasympathetic systems. The sympathetic arm
of the system is primarily associated with the stimulation tissues and organs.

For example, during times of stress, hormones are released that increase the
heart rate, constrict blood vessels, and stimulate the sweat glands, a phenomenon
often referred to as “fight or flight.” By contrast, the parasympathetic
system counteracts these effects, decreasing the heart rate, dilating
blood vessels, and decreasing the rate of sweating. 568