The physiology and cell biology of paraneurons

This paper reviews the sequence of events in stimulus-secretion coupling in neurons and paraneurons which can be divided into three main steps: the reception of the stimuli, the conduction of excitation, and the extrusion of messenger substances. One physiological criterion of the paraneuron is that its conductile region is believed to be short enough to transmit the net electrogenesis to the output portion. This contrasts with the neuron in which the depolarization in the input region can initiate spikes which in turn propagate via the axonal membrane to the output portion. The final step of the coupling, the extrusion of the secretory products by exocytosis, is common between neurons and paraneurons. The intracellular messengers acting rather directly in exocytosis are [Ca2+]c and diacylglycerol; both of these seem to be under physiological control. An increase in [Ca2+]c may also act on mitochondria to accelerate the synthesis of ATP, which may be used as the principal energy source and substrate for diacylglycerol-induced protein phosphorylation. In contrast to the common mechanism in the extrusion step of messenger substances, there is a wide diversity in the reception step in the variety of chemical and physical stimuli. This diversity is recognized not only among various neurons and paraneurons but also among the same type of cell in different species and at different stages of adaptation.