Final answer:
An Inhibitory Postsynaptic Potential (IPSP) causes the hyperpolarization of a postsynaptic neuron, making an action potential less likely. The correct answer is E, which states that an IPSP occurs when a post-synaptic neuron is hyperpolarized.
Step-by-step explanation:
An Inhibitory Postsynaptic Potential (IPSP) is a graded potential that occurs in the dendrites of a post-synaptic neuron in response to the binding of a neurotransmitter. When this happens, it can lead to the hyperpolarization of the postsynaptic membrane, which makes it less likely for the neuron to fire an action potential. Specifically, an IPSP occurs when certain neurotransmitters, such as GABA, bind to receptors and cause the opening of ion channels that allow negatively charged ions, like Cl−, into the cell. This influx of negative ions increases the negative charge inside the neuron, creating a more negative membrane potential—i.e., hyperpolarization.
Given these points, the correct answer to the student's question is E. occurs when a post-synaptic neuron is hyperpolarized.
EPSPs and IPSPs can both occur within a single neuron, and their combined effects determine whether the neuron will reach the threshold to fire an action potential. This process, called summation, occurs at the axon hillock and is critical for neuronal communication.