Excitatory postsynaptic potential
The Excitatory (causing) postsynaptic potential (EPSP) ( engl. Excitatory postsynaptic potential) is a local, gradual change of the membrane potential at the post-synaptic membrane of nerve cells , comprising a action potential triggers the postsynaptic element or contributing to the release.
The potential is triggered by the release of a certain amount of an excitatory neurotransmitter and the activation of transmitter-sensitive ion channels , which are usually permeable to sodium and potassium ions at the same time.
In general , these local and graduated potentials depolarize the postsynaptic membrane. With intracellular derivation of the membrane potential, the EPSP presents itself as a depolarization of the soma membrane as a result of the passive expansion and the summation of potentials. The size of the EPSP is not only dependent on the amount of the released transmitter, but also on the previous size of the membrane potential.
With increasing, z. B. experimentally generated (pre) depolarization of the membrane, the EPSP is smaller, d. That is, if the membrane is already depolarized from its resting potential , the amplitude of the postsynaptic excitatory potential becomes smaller with increasing pre-depolarization and finally equals zero (the reversal potential for the excitatory potentials is reached). With further pre-depolarization, a potential with the opposite sign is reached.
The EPSP is therefore by no means always a depolarization, but drives the membrane towards a certain equilibrium potential, which is mostly far below the resting potential. The ion mechanism at work here is of a complex nature. In addition to EPSP, in which an increased membrane conductivity ( membrane permeability ) for sodium and potassium ions is observed, there are also those with reduced conductivity. Here it is assumed that the triggering mechanism is the closure of “leakage” channels for potassium ions.