Final answer:
An initial stimulus, such as a neurotransmitter or physical force, triggers Na+ ion channels to open, leading to depolarization and the potential triggering of an action potential. This is followed by further depolarization, repolarization, and potential hyperpolarization due to the sequential opening and closing of voltage-gated Na+ and K+ channels, regulated by ion concentration gradients.
Step-by-step explanation:
The shift from the resting membrane potential to the threshold that triggers an action potential is caused by an initial stimulus. This stimulus can be a neurotransmitter binding to a ligand-gated Na+ channel or a physical stimulus to a mechanically gated Na+ channel. When these channels open, Na+ ions flow into the cell due to the higher external concentration, causing the inside of the cell to become less negative. This state is known as depolarization, which, if it reaches a certain level, will trigger the action potential.
During an action potential, voltage-gated channels play a critical role. First, voltage-gated Na+ channels open, causing further depolarization. Once the membrane potential peaks, these channels inactivate, and voltage-gated K+ channels open, allowing K+ to exit the cell, which leads to repolarization. Hyperpolarization may occur momentarily before the membrane potential returns to the resting state, maintained by the Na+/K+ pump. An action potential can only be generated again once the Na+ channels are reset from their inactivated state, and a stronger stimulus may be required if the K+ efflux during hyperpolarization is still ongoing.