What Happens When a Neuron Is in the State of Polarization?

Neurons are responsible for the transmission of nerve impulses from one neuron to another as messages are relayed throughout the body. When a neuron is said to be in a state of polarization, it is at "resting potential" in the transmission cycle, awaiting the next stimulus to fire again. Not for long, however, since an impulse completely passes through a neuron in about seven milliseconds, setting off a chain reaction from one neuron to the next.

Chemical Basis of Polarization

• While at resting potential, a neuron has attained polarization due to a slightly greater negative electrical charge existing inside the membrane surrounding the cell body than on the outside. This is a result of excess sodium ions (Na+) existing on the outside of the cell and excess potassium ions (K+) -- combined with negatively charged protein and nucleic acid molecules contained within.

Depolarization

• A neuron is said to reach its "action potential" phase when a chemical stimulus reaches it and causes ion channels within the cell's membrane to open. This allows sodium (Na+) ions on the outside to rush inside the cell, changing the internal negative charge to a positive charge, thereby depolarizing the neuron. In the process, the neuron reaches its threshold action potential, fires, and the stimulus is transmitted.

Repolarization

• The influx of sodium (Na+) ions to the inside of the cell subsequently triggers potassium (K+) channels to open in the membrane and potassium (K+) ions rush to the outside of the cell. The cell membrane is once again polarized, but potassium (K+) and sodium (Na+) are on the wrong sides of the membrane. A neuron is unable to respond to stimuli in this reversed polarized state.

The Refractory Period

• For a neuron to return to its resting potential, a state of hyperpolarization must briefly be reached in which the outside of the neuron actually comes to have a greater amount of potassium (K+) ions than the level of sodium (Na+) ions on the inside. This signals sodium and potassium pumps within the membrane to re-establish polarization, with potassium (K+) and sodium (Na+) on the correct sides of the membrane. Within a few thousandths of a second, the internal negative electrical charge is resumed, and the neuron is once again at resting potential, returned to a state of polarization, in line to fire again.