<P> Graded potentials are changes in membrane potential that vary in size, as opposed to being all - or - none . They arise from the summation of the individual actions of ligand - gated ion channel proteins, and decrease over time and space . They do not typically involve voltage - gated sodium and potassium channels . These impulses are incremental and may be excitatory or inhibitory . They occur at the postsynaptic dendrite as a result of presynaptic neuron firing and release of neurotransmitter, or may occur in skeletal, smooth, or cardiac muscle in response to nerve input . The magnitude of a graded potential is determined by the strength of the stimulus . </P> <P> Graded potentials that make the membrane potential less negative or more positive, thus making the postsynaptic cell more likely to have an action potential, are called excitatory postsynaptic potentials (EPSPs). Depolarizing local potentials sum together, and if the voltage reaches the threshold potential, an action potential occurs in that cell . </P> <P> EPSPs are caused by the influx of Na or Ca from the extracellular space into the neuron or muscle cell . When the presynaptic neuron has an action potential, Ca enters the axon terminal via voltage - dependent calcium channels and causes exocytosis of synaptic vesicles, causing neurotransmitter to be released . The transmitter diffuses across the synaptic cleft and activates ligand - gated ion channels that mediate the EPSP . The amplitude of the EPSP is directly proportional to the number of synaptic vesicles that were released . </P>

Which part of the neuron undergoes graded potentials