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.