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The nervous system is composed of all nerve tissues in the body. The functions of nerve tissue are to receive stimuli, transmit stimuli to nervous centers, and to initiate response. The central nervous system consists of the brain and spinal cord and serves as the collection point of nerve impulses. The peripheral nervous system includes all nerves not in the brain or spinal cord and connects all parts of the body to the central nervous system. The peripheral (sensory) nervous system receives stimuli, the central nervous system interprets them, and then the peripheral (motor) nervous system initiates responses.
The somatic nervous system controls functions that are under conscious voluntary control such as skeletal muscles and sensory neurons of the skin.
The autonomic nervous system, mostly motor nerves, controls functions of involuntary smooth muscles, cardiac muscles, and glands. The autonomic nervous system provides almost every organ with a double set of nerves - the sympathetic and parasympathetic. These systems generally but not always work in opposition to each other.
The sympathetic system activates and prepares the body for vigorous muscular activity, stress, and emergencies. While the parasympathetic system lowers activity, operates during normal situations, permits digestion, and conservation of energy.
The two systems generally act in opposition to each other. For example, a stimulation by the sympathetic system on the heart would increase contractions, while a stimulation by the parasympathetic system would decrease heart contractions. Where dual control of an organ exists, both systems operate simultaneously although one may be operating at a higher level of activity than the other. The operation is similar to the operation of a car with both the accelerator and brake pedals depressed.
In the peripheral nervous system, a chemical neurotransmitter carries the nerve impulses from neuron to neuron across a synapse (space between neurons). The neurotransmitters are acetylcholine, norephinephrine, serotonin, and others.
Nerves that release acetylcholine are called cholinergic nerves. Cholinergic nerves are part of the parasympathetic system, somatic motor nerves, preganglionic sympathetic nerves* and central nervous system. (*The nerve that carries the message from the central nervous system to a ganglion - junction for a group of nerve cells - is a preganglionic nerve.)
Nerves that release norepinephrine are called adrenergic nerves. Adrenergic nerves are part of the postganglionic sympathetic nerve system** and parts of the central nervous system. (**A nerve that carries the impulse from the ganglion to the effecter cell is a postganglionic nerve.)
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