EPSP stands for excitatory postsynaptic potential. Excitatory postsynaptic potentials (EPSPs) are associated with a transmitter-induced increase in Na+ and K+ conductance of the synaptic membrane, resulting in net entry of positive charge carried by Na+ and membrane depolarization. During this potential, the postsynaptic membrane is depolarized temporarily.
An excitatory postsynaptic potential occurs when there is an unexpected flow of positive ions into the cell. It can also be caused when positive ions outflow reduces.
An excitatory postsynaptic potential causes depolarization most times because it has changed the membrane potential of the cell. When there are larger EPSPs, they caused more membrane depolarization. Therefore, they cause the postsynaptic cell to reach the threshold needed to fire an action potential. This usually causes a voltage change from 70 mV to -69.5 mV.
IPSPs, which is the opposite of Excitatory postsynaptic potentials, are caused when positive ions outflow increases or when negative ions outflow increases in the postsynaptic cells.