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
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.)