Difference Between Nerve Fibers And Nerve Membranes
Nerve fibers that transmit impulses from parts of the body to the brain or spinal cord are called afferent fibers (from the Latin affere, meaning “to carry to or in”), or sensory fibers. Fibers that transmit impulses from the brain or spinal cord to body structures are known as efferent (from efferre, meaning “to carry out or away”), or motor, fibers.
Bundles of fibers (that is, axons) that come from nerve-cell bodies and lead from the brain and spinal cord are known as nerves. They extend to various parts of the body. There is two-way communication along the nerves connected with the spine, since these nerves bear both afferent and efferent fibers. Some of the nerves leading from the brain also have such mixed fibers.
Many of the nerve fibers within the spinal cord and brain are covered by a layer of white tissue called the myelin sheath. These sheaths act as insulators, keeping the nerve fibers electrically charged.
In the nerve fibers that come from the brain and spinal cord, the myelin sheath is enclosed by a thin, semitransparent protective membrane called the neurilemma. As it approaches its end, the nerve fiber first loses its myelin covering and then its neurilemma. At its end the axon almost always divides into two or more terminal branches, each of which splits into more terminals. These groups of naked fibers at the tips of the nerves make contact with the sense organs, muscles, or glands.
Within the brain and spinal cord, nerve fibers that are cut can never be restored. But the fibers that extend out beyond the central nervous system can regenerate: the portion of the fiber that has been cut off from the cell body disintegrates until only the neurilemma is left as an empty tube. The rest of the fiber, connected with the cell body, remains intact. Eventually the fiber sends out branches, which enter the empty sheath of neurilemma and make their way through its entire length.