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Although terminology seems to indicate otherwise,
there is really only one nervous system in the body.
Although each subdivision of the system is also called
a "nervous system," all of these smaller systems belong
to the single, highly integrated nervous system. Each
subdivision has structural and functional characteristics
that distinguish it from the others. The nervous system
as a whole is divided into two subdivisions: the central
nervous system (CNS) and the peripheral nervous system
(PNS).
The Central Nervous System
The brain and spinal cord are the organs of the central
nervous system. Because they are so vitally important,
the brain and spinal cord, located in the dorsal body
cavity, are encased in bone for protection. The brain
is in the cranial vault, and
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the spinal cord is in the vertebral canal of the vertebral column.
Although considered to be two separate organs, the brain and
spinal cord are continuous at the foramen magnum. Click here
to learn more about the CNS.
The Peripheral Nervous System
The organs of the peripheral nervous system are the nerves
and ganglia. Nerves are bundles of nerve fibers, much like
muscles are bundles of muscle fibers. Cranial nerves and spinal
nerves extend from the CNS to peripheral organs such as muscles
and glands. Ganglia are collections, or small knots, of nerve
cell bodies outside the CNS.
The peripheral nervous system is further subdivided into
an afferent (sensory) division and an efferent (motor) division.
The afferent or sensory division transmits impulses from peripheral
organs to the CNS. The efferent or motor division transmits
impulses from the CNS out to the peripheral organs to cause
an effect or action. Click here
to learn more about PNS.
Finally, the efferent or motor division is again subdivided
into the somatic nervous system and the autonomic nervous
system. The somatic nervous system, also called the somatomotor
or somatic efferent nervous system, supplies motor impulses
to the skeletal muscles. Because these nerves permit conscious
control of the skeletal muscles, it is sometimes called the
voluntary nervous system. The autonomic nervous system, also
called the visceral efferent nervous system, supplies motor
impulses to cardiac muscle, to smooth muscle, and to glandular
epithelium. It is further subdivided into sympathetic and
parasympathetic divisions. Because the autonomic nervous system
regulates involuntary or automatic functions, it is called
the involuntary nervous system.
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