Final answer:
The 'all-or-none' event refers to the action potential either occurring fully or not at all, with the same amplitude regardless of stimulus strength. Stronger stimuli cause more frequent action potentials, not larger ones.
Step-by-step explanation:
The term 'all-or-none' event refers to the principle that the action potential in a neuron either happens completely or not at all. This digital-like behavior means that once the threshold of excitation is reached, an action potential will be triggered and will propagate down the axon without diminishing in strength. The action potential occurs at a constant amplitude; a stronger stimulus won't create a larger potential but may trigger more frequent action potentials.
During an action potential, specific ion channels open and close in a sequence. The initial influx of Na+ ions causes depolarization, which, if it reaches the threshold voltage, prompts further Na+ channels to open. Following depolarization, K+ channels open, allowing K+ ions to exit the cell, leading to repolarization. These events are not gradational; either the conditions for an action potential are met and the process unfolds predictably, or they are not met and no action potential occurs.