What term describes the slight hyperpolarization of the postsynaptic cell, moving the membrane potential further from threshold?

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

Answer: (D)

Inhibitory postsynaptic potential describes a small hyperpolarizing change in the postsynaptic membrane, which pushes the membrane potential further away from the threshold for firing an action potential. This makes the neuron less likely to reach the voltage needed to open the voltage-gated Na+ channels that start an action potential. Mechanistically, IPSPs often arise when inhibitory neurotransmitters (like GABA or glycine) open Cl− channels to let negative ions in, or open K+ channels to let positive ions out, both driving the cell toward a more negative interior.

In contrast, excitatory postsynaptic potentials depolarize toward threshold, bringing the neuron closer to firing. Action potentials themselves are the all-or-nothing events that occur if threshold is reached, while graded potentials (including IPSPs and EPSPs) are the smaller, variable changes that can summate to influence whether the threshold is reached. The key idea here is that the hyperpolarizing shift of the membrane potential reduces excitability by moving away from the threshold.