Nerve cells in the nervous system send signals called action potentials, during which the soma, or cell body of the neuron, sends an electrical signal down the axon. When not signaling, a neuron is hyperpolarized, meaning it has a negative charge when compared to the outside. When an action potential signal travels across the axon, it causes the cell to depolarize, or become more positively charged. After the signal ends, the cell goes through repolarization, where it returns to its original negative polarization.
A neuron is made up of a soma, or cell body, from which dendrites reach out like branches of a tree. At one end of the neuron, there is a long cable called the axon, which ends in the synaptic buttons. Excitatory and inhibitory signals travel from other neurons to the dendrites and cell body, and these signals are summed at the axon hillock, which rests just before the beginning of the axon. These signals can hyperpolarize or depolarize the cell. Repolarization returns the cell to its resting state.
Hyperpolarization, depolarization, and repolarization of a neuron are all caused by the flow of ions, or charged molecules, in and out of the cell. When a cell is at rest, these ion channels remain closed, however, when the membrane potential reaches a certain point, called the threshold potential, they open. The cell body receives messages from other cells which either depolarize or hyperpolarize the cell, and if enough messages are received, the cell will reach the threshold potential.
When the threshold potential is reached, potassium and sodium channels open, allowing positively charged potassium and sodium ions to enter the cell. At the same time, chloride channels allow negatively charged chloride ions to leave the cell. This causes depolarization, where the cell is less negatively charged than when it is at rest.
After the action potential depolarizes the cell, it begins the process of repolarization. The sodium and potassium channels close, blocking positively charged ions from entering the cell. At the same time, the negatively charged chloride ions return to the cell.
The first part of repolarization is called the refractory period, and there are two stages of this phase, the absolute refractory period and the relative refractory period. During the absolute refractory period, the cell refuses to generate another action potential. During the relative refractory period, it is possible for the cell to generate another action potential, however it takes a larger than usual signal. This refractory period of repolarization occurs because there is a hyperpolarization of the cell due to the influx of potassium ions after an action potential has passed.
