For a neuron to fire an action potential, its membrane must reach________. a. hyperpolarization b. the threshold of excitation c. the refractory period d. inhibitory postsynaptic potential

An action potential is the rapid electrical signal that travels along the membrane of a neuron when it is stimulated. It is the neuron's way of transmitting information. The action potential is elicited once the membrane potential reaches a specific value called the threshold of excitation. This threshold is the point at which the net inward current becomes greater than the net outward current, causing depolarization of the membrane and initiation of the action potential.

a. Hyperpolarization: This refers to the process in which the membrane potential becomes more negative than the resting potential. Hyperpolarization makes it more difficult for the neuron to reach the threshold of excitation and fire an action potential. Hence, this option is incorrect. b. The threshold of excitation: This is the membrane potential value that must be reached for the neuron to generate an action potential. As mentioned previously, reaching this threshold triggers depolarization of the membrane and the propagation of the action potential. This option aligns with our understanding of action potentials, and it is the correct answer. c. The refractory period: This is the period following an action potential during which a neuron cannot initiate another action potential. The refractory period ensures that action potentials only travel in one direction and allows the neuron to recover from the previous action potential. This option is related to action potentials, but it is not the value that the membrane must reach for an action potential to be generated. Therefore, this option is incorrect. d. Inhibitory postsynaptic potential (IPSP): IPSPs are neurotransmitter-induced changes in the membrane potential that decrease the likelihood of an action potential. They are usually caused by the opening of ion channels that allow the passage of negatively charged ions into the neuron, leading to a hyperpolarization. This option is the opposite of what we're looking for, so it is also incorrect.

Based on our analysis of each option, the correct answer is: b. The threshold of excitation