Final answer:
An inhibitory postsynaptic potential (IPSP) is a graded potential in the postsynaptic membrane that results in hyperpolarization and makes an action potential less likely to occur.
Step-by-step explanation:
An inhibitory postsynaptic potential (IPSP) is a graded potential in the postsynaptic membrane that results in hyperpolarization and makes an action potential less likely to occur. It is caused by the opening of Cl- channels, where Cl- ions enter the cell and hyperpolarize the membrane.