Distinguish EPSP from IPSP.

• A. EPSP is depolarizing; IPSP is hyperpolarizing.
• B. EPSP is hyperpolarizing; IPSP is depolarizing.
• C. Both are depolarizing, but EPSP is larger.
• D. Both are hyperpolarizing, but IPSP lasts longer.

Answer: (A)

Signals at a synapse can either push the postsynaptic membrane toward firing or away from it. An excitatory postsynaptic potential depolarizes the membrane, making the inside less negative and closer to the threshold, usually by opening channels that let positive ions (primarily Na+, and sometimes Ca2+) enter. An inhibitory postsynaptic potential hyperpolarizes the membrane or counteracts depolarization, making the inside more negative or less responsive to excitatory input, typically through Cl- influx or K+ efflux. Because EPSPs move the potential toward threshold and IPSPs move it away, the correct statement is that EPSP is depolarizing and IPSP is hyperpolarizing. The other options misstate the direction of change or the effect on excitability.