<P> EPSPs, like IPSPs, are graded (i.e. they have an additive effect). When multiple EPSPs occur on a single patch of postsynaptic membrane, their combined effect is the sum of the individual EPSPs . Larger EPSPs result in greater membrane depolarization and thus increase the likelihood that the postsynaptic cell reaches the threshold for firing an action potential . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (February 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (February 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> EPSPs in living cells are caused chemically . When an active presynaptic cell releases neurotransmitters into the synapse, some of them bind to receptors on the postsynaptic cell . Many of these receptors contain an ion channel capable of passing positively charged ions either into or out of the cell (such receptors are called ionotropic receptors). At excitatory synapses, the ion channel typically allows sodium into the cell, generating an excitatory postsynaptic current . This depolarizing current causes an increase in membrane potential, the EPSP . </P>

​action potentials and excitatory postsynaptic potentials (epsps) are similar in that both