Saylor URL: http://www.saylor.org/books Saylor.org
The autonomic nervous system (ANS) is the division of the PNS that governs the internal
activities of the human body, including heart rate, breathing, digestion, salivation, perspiration,
urination, and sexual arousal. Many of the actions of the ANS, such as heart rate and digestion,
are automatic and out of our conscious control, but others, such as breathing and sexual activity,
can be controlled and influenced by conscious processes.
The somatic nervous system (SNS) is the division of the PNS that controls the external aspects of
the body, including the skeletal muscles, skin, and sense organs. The somatic nervous system
consists primarily of motor nerves responsible for sending brain signals for muscle contraction.
The autonomic nervous system itself can be further subdivided into
thesympathetic and parasympathetic systems (see Figure 3.19 "The Autonomic Nervous
System"). The sympathetic division of the ANS is involved in preparing the body for behavior,
particularly in response to stress, by activating the organs and the glands in the endocrine
system. Theparasympathetic division of the ANS tends to calm the body by slowing the heart and
breathing and by allowing the body to recover from the activities that the sympathetic system
causes. The sympathetic and the parasympathetic divisions normally function in opposition to
each other, such that the sympathetic division acts a bit like the accelerator pedal on a car and the
parasympathetic division acts like the brake.