Biophysics of Junctional Transmission

Abstract

The salient features of this compact review of recent work are the following concepts for which substantial evidence is adduced (1) Synaptic and neuromyal transmission in vertebrates is for the most part mediated by chemical transmission although direct electric transmission may occur in certain situations, especially in invertebrates. (2) The role of Ca in facilitating the discharge of terminal units by a pre-junctional impulse may be related to its action in setting up and propagating an action potential. It is probable that the agent for transport of Na ions across excitable membranes is combined with a Ca ion. (3) The potentiating effect of pre-junctional activity is associated with an increased membrane potential in the terminal region of afferent fibers. (4) It is probable that all terminals of a single nerve fiber release the same chemical mediator. It is possible that direct inhibition always involves an interneuron all of whose terminals release an inhibitory transmitter. (5) The reaction of the post-junctional receptive surface shows chemical specificity suggestive of an enzyme and can be depressed by various substances in a manner similar to competitive inhibition of enzymes. The membrane changes elicited by junctional activity are of two types: (1) excitatory action involving nonselective increase of permeability toward all ions on either side of the membrane and (2) inhibitory action involving an increase of permeability toward such small ions (notably chloride) as are near electrochemical equilibrium across the resting membrane. Inhibition also involves some interference with excitatory junctional action, presumably due to chemical competition between excitatory and inhibitory transmitters.