Ionic permeability changes occurring at excitatory receptor membranes of chemical synapses

Abstract

ALTHOUGH the ionic basis for generation of action potentials in nerve and muscle cells is now well understood¹, there is still much uncertainty about the changes in ionic permeability which result in the generation of graded depolarisations at receptor membranes of excitatory chemical synapses². The equilibrium potential for these graded depolarisations (E_R) is usually between −20 and +10 mV. This suggests an underlying increase in sodium permeability (P_Na) and potassium (P_K) and/or chloride (P_Cl) permeability, for in most nerve and muscle cells, the sodium equilibrium potential (E_Na) is very positive (+50 to +75 mV) and the potassium and chloride equilibrium potentials (E_K and E_Cl) are very negative (−40 to −100 mV). There is evidence that an increase in P_Na does occur during activation of excitatory synaptic membranes², but there are few such membranes for which there is evidence for an increase in either P_K or P_Cl. Here we review and analyse the data obtained from previous investigations on the ionic basis of the graded depolarisations occurring at excitatory synapses. We demonstrate that the application of the Goldman–Hodgkin–Katz equation^3,4 to previous data suggests that an increase in P_K as well as P_Na probably occurs at most if not all excitatory synapses.