<P> If the EPSP is not large enough to trigger an action potential, the membrane subsequently repolarizes to its resting membrane potential . This shows the temporary and reversible nature of graded potentials . </P> <P> Graded potentials that make the membrane potential more negative, and make the postsynaptic cell less likely to have an action potential, are called inhibitory post synaptic potentials (IPSPs). Hyperpolarization of membranes is caused by influx of Cl or efflux of K. As with EPSPs, the amplitude of the IPSP is directly proportional to the number of synaptic vesicles that were released . </P> <P> The resting membrane potential is usually around--70 mV . The typical neuron has a threshold potential ranging from--40 mV to--55 mV . Temporal summation occurs when graded potentials within the postsynaptic cell occur so rapidly that they build on each other before the previous ones fade . Spatial summation occurs when postsynaptic potentials from adjacent synapses on the cell occur simultaneously and add together . An action potential occurs when the summated EPSPs, minus the summated IPSPs, in an area of membrane reach the cell's threshold potential . </P>

Where do most graded local potentials form in a neuron