Action Potentials

The action potential, a specific change in charge across the cell membrane, is the primary way that electrical signals are propagated in the nervous system.

Neurons typically have a resting potential of about negative 70 millivolts. When they receive signals, such as neurotransmitters, their membrane potential can hyperpolarize, decrease or depolarize, increase.

When a neuron is depolarized to the threshold potential, the point where an action potential is triggered, voltage-gated sodium channels open, because of the activity of the sodium potassium pump. There is a greater concentration of sodium ions outside of neurons and a greater concentration of potassium ions inside. Therefore when the sodium channels open, sodium rushes in, down its gradient.

The influx of positive charge rapidly increases the membrane potential to around plus 40 millivolts, the peak of the action potential. Sodium channels then inactivate, preventing more sodium from flowing in. Also, voltage-gated potassium channels now open, allowing potassium to flow out down its gradient, reducing the membrane potential. The membrane becomes briefly hyperpolarized, called the refractory period, greatly reducing the chance of a new action potential until the resting potential is restored.

In myelinated axons the action potential is regenerated at each node of Ranvier, allowing the signal to be reliably and rapidly transmitted over long distances.