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).
