Final answer:
Cells cannot be stimulated to conduct an electrical impulse during the absolute refractory period, which happens after an action potential is initiated and is due to the inactivation of voltage-gated Na+ channels.
Step-by-step explanation:
Neurons and muscle cells cannot be stimulated to conduct an electrical impulse, no matter how strong the stimulus, during the absolute refractory period. This period follows the initiation of an action potential and is characterized by the inactivation of voltage-gated Na+ channels, preventing any subsequent action potential from occurring. This refractory phase is essential to ensure that each action potential is a separate, all-or-nothing event and aids in the directional propagation of the impulse along the axon. During the absolute refractory period, the inactivated state of these sodium channels does not allow for another depolarization, regardless of the stimulus strength. This period is followed by the relative refractory period, during which a new action potential can only be initiated by a stimulus that is stronger than normal due to potassium ions (K+) flowing out of the cell. This flow maintains the cell's hyperpolarized state, contributing to resetting the excitability of the cell.