Action Potential and How Neurons Fire

Nerve cell, artwork

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A neuron (a nerve cell) is the basic building block of the nervous system. When neurons transmit signals through the body, part of the transmission process involves an electrical impulse called an action potential.

This process, which occurs during the firing of the neurons, allows a nerve cell to transmit an electrical signal down the axon (a portion of the neuron that carries nerve impulses away from the cell body) toward other cells. This sends a message to the muscles to provoke a response.

For example, say you want to pick up a glass so you can take a drink of water. The action potential plays a key role in carrying that message from the brain to the hand.

Prior to the Action Potential

When a neuron is not sending signals, the inside of the neuron has a negative charge relative to the positive charge outside the cell.

Electrically charged atoms known as ions maintain the positive and negative charge balance. Calcium contains two positive charges, sodium and potassium contain one positive charge, and chloride contains a negative charge.

When at rest, the cell membrane of the neuron allows certain ions to pass through while preventing or restricting other ions from moving. In this state, sodium and potassium ions cannot easily pass through the membrane. Chloride ions, however, are able to freely cross the membrane. The negative ions inside the cell are unable to cross the barrier.

The resting potential of the neuron refers to the difference between the voltage inside and outside the neuron. The resting potential of the average neuron is around -70 millivolts, indicating that the inside of the cell is 70 millivolts less than the outside of the cell.

At this point, the brain has not yet sent the message to the hand to pick up the glass, but the neuron is ready to receive the signal.

During the Action Potential

You’ve decided that you are thirsty and would like a drink of water. Your brain starts the chain of events to send a message to the muscles in your hand that you need to pick up the glass.

When a nerve impulse (which is how neurons communicate with one another) is sent out from a cell body, the sodium channels in the cell membrane open and the positive sodium cells surge into the cell.

Once the cell reaches a certain threshold, an action potential will fire, sending the electrical signal down the axon. The sodium channels play a role in generating the action potential in excitable cells and activating a transmission along the axon.

Action potentials either happen or they don't; there is no such thing as a "partial" firing of a neuron. This principle is known as the all-or-none law

This means that neurons always fire at their full strength. This ensures that the full intensity of the signal is carried down the nerve fiber and transferred to the next cell and that the signal does not weaken or become lost the further it travels from the source.

The message from the brain is now traveling down the nerves to the muscles in the hand.

After the Action Potential

After the neuron has fired, there is a refractory period in which another action potential is not possible. The refractory period generally lasts one millisecond.

During this time, the potassium channels reopen and the sodium channels close, gradually returning the neuron to its resting potential. Once the neuron has "recharged," it is possible for another action potential to occur and transmit the signal down the length of the axon.

Through this continual process of firing then recharging, the neurons are able to carry the message from the brain to tell the muscles what to do—hold the glass, take a sip, or put it down.

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