An IPSP is inhibitory because
a. It hyperpolarizes the postsynaptic membrane.
b. It reduces the amount of neurotransmitter released by the presynaptic terminal.
c. It prevents calcium ion entry into the presynaptic terminal.
d. It changes the threshold of the neuron.

Verified step by step guidance

1. Step 1: Understand what an IPSP (Inhibitory Postsynaptic Potential) is. It is a change in the postsynaptic membrane potential that makes the neuron less likely to fire an action potential.
2. Step 2: Recall that hyperpolarization means the inside of the neuron becomes more negative relative to the outside, moving the membrane potential further from the threshold needed to trigger an action potential.
3. Step 3: Recognize that IPSPs typically result from the opening of ion channels that allow negatively charged ions (like Cl⁻) to enter or positively charged ions (like K⁺) to leave the cell, causing hyperpolarization.
4. Step 4: Evaluate each option in the question: (a) hyperpolarizes the postsynaptic membrane, (b) reduces neurotransmitter release presynaptically, (c) prevents calcium entry presynaptically, and (d) changes the threshold of the neuron.
5. Step 5: Conclude that the defining feature of an IPSP is the hyperpolarization of the postsynaptic membrane, which inhibits the generation of an action potential.

Key Concepts

Inhibitory Postsynaptic Potential (IPSP)

An IPSP is a change in the postsynaptic membrane potential that makes the neuron less likely to fire an action potential. It typically involves hyperpolarization, where the inside of the neuron becomes more negative, moving the membrane potential further from the threshold needed to trigger firing.

Hyperpolarization of the Postsynaptic Membrane

Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential, often due to the influx of negatively charged ions or efflux of positive ions. This change inhibits neuronal firing by increasing the distance from the action potential threshold.

Neurotransmitter Release and Presynaptic Modulation

Neurotransmitter release is controlled by calcium ion entry into the presynaptic terminal. While modulation of neurotransmitter release affects synaptic strength, IPSPs are primarily postsynaptic events and do not directly involve reducing neurotransmitter release or altering calcium entry presynaptically.