An action potential repolarization is one of the stages that occur when a nerve impulse is transmitted. Nerve impulses are electrochemical messages that are sent along the length of a neuron, or nerve cell, from the dendrites to the terminal buds of the axon. The messages are said to transmit electrochemically because chemicals within the body cause an electrical signal to move through the neuron. The stages that a neuron undergoes are resting, action potential depolarization and action potential repolarization.
Within the nervous system, electrically charged ions play the main role in causing a nerve impulse to be sent. In particular, the specific ions that are involved are sodium, potassium, chloride and calcium. Sodium and potassium both have a positive charge of +1, while calcium has a positive charge of +2. Chloride ions are negatively charged (-1).
When a neuron is resting, or a nerve impulse isn’t being sent, the cell membrane controls what ions are within the cell and which ones are kept outside of it. As a result, the inside of a neuron has a negative charge when compared to the area surrounding it. Ion channels and pumps within the membrane counter the effects of diffusion of the ions to maintain a resting potential of approximately -70 millivolts (mV).
For a nerve impulse to be sent, an action potential must occur within the neuron. Depolarization of the neuron is necessary for an action potential to be sent along the nerve cell. This means that there is an exchange of ions across the cell membrane of the neuron, which is caused by some type of stimulation. When the neuron is stimulated, the sodium channels within the membrane open, which allows the sodium ions to rush into the nerve cell. As more sodium ions enter the neuron, the inside of the cell becomes more and more positive compared to the outside of it.
A threshold of at least -55 mV must be reached for an action potential to be sent. If the threshold is not attained, then an action potential, or nerve impulse, is not sent. When the threshold is reached, most of the sodium ion channels open so the charge within the cell reaches +30 mV. This spike is the action potential, which is an electrical impulse that travels the length of the nerve cell. The action potential stimulates the next section of the neuron, while the previous section begins to undergo action potential repolarization.
During action potential repolarization, the relative charge of the neuron is returned to its resting state. Once +30 mV is reached, the potassium channels begin to open and the sodium channels close. When the potassium channels open, potassium rushes out of the cell to try and balance out the charges on either side of the membrane. The potassium channels remain open until the resting potential of -70 mV is attained.