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axon:see nervous systemnervous system,
network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system.
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, junction between various signal-transmitter cells, either between two neurons or between a neuron and a muscle or gland. A nerve impulse reaches the synapse through the axon, or transmitting end, of a nerve cell, or neuron.
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a neurite, or axis cylinder; the process of a nerve cell along which neural impulses proceed from the cell body to innervated organs and other nerve cells.
Only one axon branches off each neuron, or nerve cell. The nutrition and growth of the axon depends on the neuron body; upon transection of the axon, its peripheral portion dies, but the central portion preserves its viability. Large animals possess axons—for example, those proceeding from the spinal cord to the extremities—that may reach a length of one meter or more when their diameter is several millimicrons (mμ). In some animals—for example, squid and fish—huge axons are found which measure hundreds of mμ in thickness. In axoplasm—that is, the protoplasm of axons—there are extremely thin fibrils, known as neurofibrils, as well as mitochondria and the endoplasmic network. Depending on whether axons are covered with a myelic (fatty) membrane or not, they are known as medullated or nonmedullated nerve fibers. The structure of the membranes and the diameters of the axons that constitute the nerve fiber are the factors that determine the rate of stimulus transmission along the nerve. The terminal sections of the axon, or terminals, branch off and make contact with other nerve, muscle, and gland cells. Stimuli are transmitted through these contact points, which are known as synapses. A nerve is a collection of axons.