Action Potential
The neuron (nerve cell) is a type of cell that is specialized to transmit electrical signals. The branch-like structures allow dendrites and axon terminals to make many unique connections with other neurons. The axon’s job is to transmit a nerve pulse from the axon’s cell body to its terminus within the neuron. Signals in the axon are passed as a cascade of electrochemical events down the axon. To achieve the cascade the neuron must perform a series of actions starting from a polarized resting state, to a depolarized state, and then back to the resting state, passing the signal on down the axon.
Resting State
In its resting state an axon has more positive charges (due mostly to Na+ ions) outside the cell in the surrounding fluid than inside the cell, giving it an electrical action potential across the cell membrane of around -70mV; it is more negative inside the cell than out (due to Cl- ions). In this state, sodium ions (Na+) and potassium ions (K+) cannot passively diffuse across the membrane because voltage gated sodium channels and voltage gated potassium channels are closed. These channels can only be opened when the electrical charge within the axon potential is below the channels’ opening thresholds.
Stimulation and Depolarization
A signal from the dendrite changes the voltage difference at the top of the axon past a certain level called the threshold potential. When this occurs, the first set of voltage-gated Na+ channels to open. The open Na+ channels allow Na+ ions to passively diffuse into the axon. This causes a localized depolarization in the axon from -70 mv to +55 mv.
Once depolarized, the influx of Na+ ions is quickly stopped by the closing of the inactivation gate – a part of the voltage gated sodium channel that has a natural affinity for blocking the open pore.
Repolarization and Downstream
Although the Na+ channel is closed, the localized depolarization causes the local voltage-gated K+ channels to open; K+ passively diffuses out of the axon. When this occurs, action potential is locally restored to near-resting state. Immediately downstream however, the depolarization stimulates adjacent Na+ channels to open, passing the action potential along down the axon.
This process cascades down the axon and Na+ and K+ ions continue to diffuse in and out of the membrane all along its length until the nerve pulse is delivered to the axon terminal then out of the neuron.
The Na+ and K+ gradient is restored and maintained by a continually running Na+/K+ pump that uses ATP to transport Na+ out of the axon and K+ into the axon.
