The most important of these transmitters in the autonomic nervous system are acetylcholine and norepinephrine. Like other nerves, those of the autonomic nervous system convey their messages to the appropriate end organs (blood vessels, viscera, etc.) by releasing transmitter substances to which the receptors of the target cells are responsive. This division performs tasks such as controlling the bladder, slowing down heart rate, and constricting eye pupils. The parasympathetic division of the autonomic nervous system supports homeostasis and conserves physical resources. This division is responsible for tasks such as relaxing the bladder, increasing heart rate, and dilating eye pupils. The sympathetic division of the autonomic nervous system regulates flight-or-fight responses.
This system is further divided into two systems: the sympathetic system and the parasympathetic system. The autonomic nervous system is the branch of the peripheral nervous system that controls involuntary actions, such as pulmonary respiration (breathing), heart rate and the force of contraction of the heart, and vasoconstriction and dilation (widening and narrowing of blood vessels) that impact blood pressure. This system is described as voluntary because the responses can be controlled consciously, although reflex reactions of skeletal muscles are an exception. The somatic nervous system controls skeletal muscle as well as external sensory organs such as the skin. The peripheral nervous system has two branches: the somatic nervous system and the autonomic nervous system. The peripheral nervous system involves all of the nerves outside of the brain and spinal cord that carry messages to and from the CNS.
The CNS receives sensory information from the peripheral nervous system and controls the body’s responses. The central nervous system (CNS) is composed of the brain and spinal cord.
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