STANDARD TERMS AND GROUPING
Anatomically, the neurons are arranged in a system-atic and logical
manner in the brain and spinal cord, with neurons having the same or similar
functions invariably grouped together. Many standard terms describe these areas
and groupings:
Ganglia.A collection of cell bodies
of neurons(e.g., Preganglionic nerves of the sympathetic and parasympathetic
nerves synapse with postganglionic neurons in a region located outside the
spinal cord and brain). The collection of cell bodies of the post-ganglionic
neurons of one region is known as gan-glia. Another example is the dorsal root ganglion, a collection of
the cell bodies of the unipolar sensory neurons that lies just outside the
spinal cord.
Centers, located in the CNS, are collections
of cellbodies of neurons having the same function. For ex-ample, the vasomotor
center in the brain has cell bodies of neurons involved in regulating the
activities of the smooth muscles in the walls of blood vessels. If the boundary
of a center can be distinctly made out in anatomic sections of the brain, it is
referred to as the nucleus. The
hypothalamus, for example, has many nuclei, some controlling sleep, some
appetite.
Nerve.A nerve is a collection of
axons of motorneurons, dendrites of sensory neurons, and axons/ dendrites of
autonomic fibers bundled together by connective tissue in the PNS. A specific
nerve may or may not contain all three types of nerve fibers (i.e., motor,
sensory, and autonomic). Nerves leading to or from the brain and brain stem are
the cranial nerves, and those
leading from and to the spinal cord are thespinal
nerves.
Tracts are bundles of the axons of
neurons havingthe same function and destination. For example, the spinothalamic
tract carries pain impulses from the body to the thalamus. Many tracts, if
present in the same region of the CNS, are referred to as columns. For example, the sensations of touch and pressure are
carried by neurons whose axons lie in the posterior as-pect of the spinal cord,
the dorsal column.
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