What are inhibitory postsynaptic potentials (IPSPs)?

• A. Depolarizations that increase the likelihood of an action potential.
• B. Hyperpolarizations that decrease the likelihood of an action potential.
• C. Subthreshold shifts with no effect.
• D. Rapid action potential generation without a depolarization.

Answer: (B)

Inhibitory postsynaptic potentials are hyperpolarizing changes at the postsynaptic membrane that reduce the likelihood of an action potential. When inhibitory neurotransmitters (like GABA or glycine) bind to receptors, they typically increase Cl− influx or K+ efflux, pulling the membrane potential away from the threshold needed to fire a spike. Because action potentials require a depolarization to threshold, this hyperpolarization makes it harder for excitatory inputs to summate to trigger a spike, thereby dampening excitation. These potentials are graded, can be temporally and spatially summated with excitatory inputs, and serve to shape neural signaling rather than produce a spike on their own. The other options describe depolarizing or nonfunctional shifts, which do not match IPSPs.