When can a nerve cell membrane depolarize?

A nerve cell membrane can depolarize when there is a sufficient stimulus that brings the membrane potential to a threshold level, typically around -55mV. This process is generally described by the concept of summation, where excitatory postsynaptic potentials (EPSPs) can add together to reach this threshold.

When the summation of individual action potentials, or more accurately, synaptic potentials, exceeds the membrane threshold, voltage-gated sodium channels open. This results in an influx of sodium ions, which leads to depolarization of the membrane. Therefore, option C accurately captures the physiological basis for depolarization: the combined effect of all incoming synaptic signals must reach a critical level to initiate an action potential.

Other options do not contribute to membrane depolarization under typical circumstances. Closed sodium channels mean that no sodium can enter the cell, thus preventing depolarization. A resting membrane threshold of -70mV indicates that the cell is at rest and has not yet reached the threshold for action potential generation. Opening chloride channels generally causes hyperpolarization, making the interior of the cell more negative and moving it further away from the threshold necessary for depolarization.