Action potential

Action potential refers to the electrical impulses by which neurons transmit signals from cell to cell. These very short pulses of energy are essential for generating the body's responses to stimuli. An action potential is triggered by a reverse in the cell's polarization that results from information received by the cell. Action potential is an essential part of the communication process between neurons and the other cells in the body.

Background

The human body contains about one hundred billion neurons, or brain cells. Each neuron needs to communicate with each other and with other parts of the body for the body to function. This communication is accomplished by means of electrical impulses that travel from cell to cell.

It was not until the early twentieth century that scientists developed a way of staining brain cells so that they could be effectively studied under a microscope. When they did, researchers discovered that neurons have three main parts. These are the soma, or main part of the cell that contains DNA and all the essential material for the cell to survive and function; the axons, or long tendril-like projections from the soma that carry the electrical impulses; and the dendrites, which are very fine tendrils located at the ends of the axons. The dendrites help make the connections between neurons and also detect changes in the environment around the cells.

The cells are enclosed in a cell membrane that acts like the cell's skin. Among other functions, the membrane keeps certain substances inside the cell and others outside, and controls whether these substances can pass through from inside to out. When scientists were first able to view neurons under the microscope, they learned these cells somehow generated electrical impulses that travelled from inside the cell membrane to the outside, but they did not know exactly how the process worked.

In 1939, British neurophysicists Alan Lloyd Hodgkin (1914 – 1998) and Andrew Huxley (1917 – 2012), began experiments to determine how the impulses travel in brain cells. They used the axon of a giant squid for their experiments because the squid axon is large—as much as one millimeter in diameter—making it far easier to work with than the cells in any other animal. Using an electrode inserted into the squid axon, the researchers were able to determine the difference the charge made between the inside and outside of the cell.

Hodgkin and Huxley began to investigate a theory put forth years earlier that nerve impulses were triggered by an exchange in the positive-negative balance—the ionization—of the cell caused by the potassium ions inside the cell and the sodium ions outside. They tested this theory by placing one electrode in the squid axon to introduce the charge and a second to measure its effect. Through these experiments, they were able to prove that a change in ionization was the trigger for releasing the electrical impulses and forming the action potential of the cell.

In the 1950s, Australian neurophysicist Sir John Eccles (1903 – 1997) was able to measure these electrical impulses and show how the brain cells received and transmitted the signals. For their work in determining the inner workings of neurons and action potential, Eccles, Huxley, and Hodgkin shared the 1963 Nobel Prize in Physiology.

Overview

The research of Eccles, Huxley, Hodgkin, and others determined that impulses were generated by the action of ion channels that allow ions to travel between the inside and outside of the cell membrane. In neurons, some of these channels act as gates to allow potassium and sodium to move back and forth, which affects the positive-negative balance of the cell. Other channels called receptors are located on the cell's dendrites. They respond to special chemicals called neurotransmitters to allow passage of sodium ions into the cell. Still other channels are pumps, like the sodium-potassium pumps that force sodium ions out of the neurons and potassium ions in.

The inside of the neuron usually maintains a negative charge because of molecules known as anions. These anions have a very strong negative ionization and are too large to pass outside the cell. When the cell has this negative charge, the sodium-potassium pumps are not at work and the gates that allow sodium and potassium in and out are closed; the axon is said to be at rest. However, because it is charged and ready to work, it is said to have resting potential.

Just as a battery-powered device is ready to be activated by flipping the switch, the neurons only need to be turned on. This happens when neurotransmitters attach to the dendrites, triggering the receptors to open the ion gates and allow more sodium ions into the cell. This changes the level of the negative charge inside the cell. When it reaches a certain threshold, more gates open and more sodium ions are allowed in. Eventually enough are inside that the positive-negative balance between the inside and outside of the cell is reversed.

The cell wants to maintain its negative charge inside, however, so it opens gates to allow some of the positively charged ions out. Potassium ions are allowed out until the cell is once again negatively charged. However, these potassium ions are needed inside for the cell's health. Now the sodium-potassium pumps are activated to push sodium out and potassium in, restoring the cell to its resting potential.

The entire process, from the point when the neurotransmitters trigger the gates until the pumps have completed their work, is action potential. It lasts about one one-thousandth of a second. It takes a series of these short pulses to transfer an impulse from cell to cell. Each action potential triggers the release of a calcium ion, which in turn activates the neurotransmitters that open the ion gates and keeps the process moving. Once a cell has been triggered, it must reset to its full negative resting potential before it can be activated to action potential again.

Bibliography

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