Biological Systems MCAT Practice Exam

What effect does an excitatory postsynaptic potential (EPSP) have on the postsynaptic neuron?

1. It polarizes the next neuron

2. It decreases the chance of action potential firing

3. It depolarizes the next neuron

4. It inhibits neurotransmitter release

The correct answer is: It depolarizes the next neuron

An excitatory postsynaptic potential (EPSP) has a crucial role in the functioning of neurons, particularly in how they communicate with one another. An EPSP is a change in the postsynaptic membrane potential that makes the neuron more likely to fire an action potential. This occurs when neurotransmitters bind to receptors on the postsynaptic neuron, leading to the opening of ion channels that allow positively charged ions (such as sodium) to flow into the neuron. As a result, the membrane potential of the postsynaptic neuron becomes less negative (or more positive), which is termed depolarization. This depolarization moves the membrane potential closer to the threshold needed to generate an action potential. If the EPSPs are strong enough or occur in rapid succession, they can summate and entirely shift the membrane voltage past the threshold, triggering an action potential. In contrast, if a postsynaptic potential were to polarize the neuron, it would move the membrane potential further away from the threshold, making it less likely to fire. Similarly, decreasing the chance of action potential firing would not describe the activity of an EPSP, which is fundamentally about increasing that likelihood. Lastly, inhibiting neurotransmitter release is not relevant to the function of