Which term describes a local depolarization that can sum with other inputs to reach threshold?

• A. EPSP
• B. Graded potential
• C. IPSP
• D. Resting potential

Answer: (A)

A depolarizing input that can add together with other signals to reach the firing threshold is an excitatory postsynaptic potential. This is the local change at the postsynaptic membrane caused by excitatory neurotransmitters that open channels allowing positive ions to enter, making the inside less negative. Because these depolarizations are graded, their size varies with input strength, and they can summate both over time (temporal summation) and across different synapses (spatial summation). When enough EPSPs accumulate, the membrane potential reaches the threshold to trigger an action potential.

In contrast, an IPSP would hyperpolarize the membrane and move it further from threshold; resting potential is just the baseline electrical state with no active input; and while the broader category of graded potentials includes EPSPs, the specific description here points to the excitatory postsynaptic potential.