The Postsynaptic Electrical Events Produced by Chemically Transmitting Inhibitory Synapses

Abstract

Activation of chemical inhibitory synapses always produces an increased electrical conductance of the subsynaptic membrane, which is the principal agent in suppressing impulse generation. With the vertebrate central nervous system there is in addition an associated increase in the resting membrane potential; but in invertebrates there is usually no hyperpolarization, the increased conductance alone effecting inhibition by tending to stabilize the membrane potential close to its resting level. Nevertheless it will be seen that the ionic mechanisms responsible for the increased conductances in all these chemical inhibitory actions belong to a category that is distinguished both by exclusion of sodium ions and by a high conductance for chloride and/or potassium ions (Chapter XI). The potential changes that eventuate are, of course, merely a resultant of the net movements of potassium and chloride ions down their electrochemical gradients. It is therefore justifiable to classify all these postsynaptic inhibitory actions together. Just as with excitatory synapses it has become customary to designate inhibitory synaptic action by the potential it produces, the inhibitory postsynaptic potential, IPSP, rather than by the less easily demonstrable inhibitory postsynaptic current that produces the potential. It is important to recognize that the IPSP may be in either the hyperpolarizing or the depolarizing direction in accord with the electrochemical gradients, or, of course, it may be virtually zero.