The axon hillock is the region where the cell body transitions into the axon and is crucial for initiating an action potential. This area contains a much higher concentration of voltage-gated Na+ channels compared to other parts of the neuron, making it highly sensitive to changes in membrane potential. When the membrane at the axon hillock becomes sufficiently depolarized—reaching a threshold of around -55 mV—these voltage-gated channels open, allowing sodium ions to rush in. This triggers a wave of depolarization that propagates along the axon toward the terminal, resulting in the transmission of the signal. To learn more about how the membrane plays a role in this process, visit the Membrane Page. After the depolarization phase, voltage-gated K+ channels open to restore the resting membrane potential.
Following an action potential, the membrane enters a refractory period, which is divided into two phases: the absolute refractory period and the relative refractory period. The refractory period occurs due to the behavior of the voltage-gated ion channels involved in generating the action potential, particularly sodium (Na+) and potassium (K+) channels.
During the absolute refractory period, voltage-gated Na+ channels become inactivated after opening. The inactivation gate closes, preventing further Na+ entry even if the membrane is depolarized. This prevents the initiation of a new action potential and ensures that the signal travels in only one direction along the axon, toward the axon terminal, and does not propagate backward.
The relative refractory period follows, during which some Na+ channels begin to return to their resting state, but the membrane is still hyperpolarized due to the delayed closing of voltage-gated K+ channels. A stronger-than-normal stimulus is required to initiate another action potential during this phase, ensuring that there is a short delay between successive action potentials.
This coordination between the inactivation of Na+ channels and the delayed closing of K+ channels is what creates the refractory period.
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