GRADED POTENTIALS IN NEURONS
A, Ion movements. Excitatory and inhibitory neurotransmissions are processes by which released neurotransmitter, acting on postsynaptic membrane receptors, elicits a local or regional perturbation in the membrane potential:(1) toward 0 (depolarization, excitatory postsynaptic potential; EPSP) via an inward flow of Na+ caused by increased permeability of the membrane to positively charged ions; or (2) away from 0 (hyperpolarization, inhibitory postsynaptic potential; IPSP) via an inward flow of Cl− and a compensatory outward flow of K+ caused by increased membrane permeability to Cl−.
Following the action of neurotransmitters on the postsynaptic membrane, the resultant EPSPs and IPSPs exert local influences that dissipate over time and distance but contribute to the overall excitability and ion distribution in the neuron. It is unusual for a single excitatory input to generate sufficient EPSPs to bring about depolarization of the initial segment of the axon above threshold so that an action potential is fired. However, the influence of multiple EPSPs, integrated over space and time, may sum to collectively reach threshold. IPSPs reduce the ability of EPSPs to bring the postsynaptic membrane to threshold. B, EPSPs, IPSPs, and current flow. EPSP- and IPSP-induced changes in postsynaptic current (red) and potential (blue).