POSTSYNAPTIC POTENTIAL

(PSP) a transient change in the electric polarization of the membrane of a nerve cell (neuron). The result of chemical transmission of a nerve impulse at the synapse (neuronal junction), it can lead to the firing of a new impulse. When an impulse arrives at the synapse from an activated neuron (presynaptic neuron), a chemical substance called a neurotransmitter is released and causes the opening of channel-shaped molecules in the membrane of the resting neuron (postsynaptic neuron). Ions flowing through the channels create a shift in the resting membrane polarization, which usually has a slightly more negative charge inside the neuron than outside. Hyperpolarization-that is, an increase in negative charge on the inside-constitutes an inhibitory PSP, because it draws the neuron away from the firing of an impulse. Depolarization-a decrease in negative charge-constitutes an excitatory PSP because, if it brings the neuron to the critical threshold potential, it can excite the generation of a nerve impulse (action potential). The PSP is a graded potential; that is, its degree of hyperpolarization or depolarization varies according to the activation of ion channels. The ability to integrate multiple PSPs at multiple synaptic connections into an overall membrane potential is an important property of neurons, as it enables the nervous system to respond coherently to multiple stimuli. The equivalent of the PSP at nerve-muscle synapses is called the end-plate potential. See also action potential; synapse.