Which statement best contrasts EPSPs and IPSPs?

• A. EPSP hyperpolarizes membrane; IPSP depolarizes
• B. EPSP involves inhibitory synapses; IPSP excitatory
• C. EPSP only occurs in motor neurons; IPSP only in sensory neurons
• D. EPSP increases likelihood of firing; IPSP decreases likelihood

Answer: (D)

Understanding how excitatory and inhibitory postsynaptic potentials influence a neuron's firing helps you see the contrast clearly. An EPSP depolarizes the postsynaptic membrane, bringing it closer to the threshold and making an action potential more likely. An IPSP hyperpolarizes the membrane (or increases conductance to resist change), pushing the membrane away from threshold and making firing less likely. So the best contrast is that EPSPs increase the likelihood of firing, while IPSPs decrease it. The other options mix up the directions of change or overlook that both types can occur in many neuron types, not just motor or sensory ones.