Physiology of paraneurons.

An attempt is made in this review to elucidate the functional characteristics of paraneurons. The secretory functions of neuron, neurosecretory cell and paraneuron consist of three main phases: secretion, transmission and reversion. Secretion can be divided into a steady state and an activated state. A paraneuron behaves characteristically in the active phase of secretion and in the phase of transmission. There are three main events in the activated phase of secretion ("stimulus-extrusion coupling") in paraneurons, reception of stimulus, conduction of excitation, and extrusion of secretory substances. These three events seem to correspond to the functional characteristics in the three different regions of the paraneuron membrane. A stimulus-induced change in conformation in the input region of paraneurons may result in the inward movement of Na+ and Ca++. The inward movement of Na+ sometimes coincides with a graded depolarization which may occasionally generate all-or-nothing, short-duration action potential possibly in the intermediate region. A rise in [Ca++]i, whether it coincides with Na-dependent depolarization or not, may initiate extrusion of granules. If eccytosis is the mode of extrusion of secretory granules of paraneurons, the cell membranes at the output region should be rearranged by fusion with the membranes of granules. Ionic permeability of the output region might decrease during eccytosis if ionic permeability of the granule membrane is lower than that of other regions. Somatostatin in most cases and insulin in a special case may act locally on the adjacent target cells, and such a mode of transmission may come into the category of paracrine secretion.