The Nerve Impulse or Action Potential

Nerve cells are excitable; that is, they are specialized to detect changes in their surroundings. If a change reaches a certain intensity or threshold of stimulation, it may disturb the membrane's resting state and trigger a nerve impulse.

Changes in temperature, pressure, chemical concentration, or electrical condition, for example, can stimulate various nerve cells.

Once the threshold is reached, the portion of the cell membrane that is being stimulated undergoes a sudden change in permeability.

Channels that are highly selective for sodium ions open in the membrane, and allow the ions to pass through. As the membrane becomes more permeable to sodium, the ions rapidly diffuse inward.

This movement is aided by the fad that the sodium ions arc attracted by the negative electrical condition on the other side of the membrane.

As the sodium ions rush inward, the membrane loses its electrical charge and is said to be depolarized: but as more and more ions enter, the inside of the cell becomes relatively positive.

At almost the same time, channels open in the membrane that allow potassium ions to pass through more readily than before, and as potassium ions diffuse outward the outside of the membrane becomes positively charged once more.

Then the ion channels close, the membrane becomes less permeable to sodium and potassium, and its active transport mechanism reestablishes the original concentrations of sodium and potassium ions. As a result, the resting potential returns.

This sequence of changes, which takes about 1/1000th second is called an action potential, and when it occurs it stimulates other action potentials in adjacent portions of the cell membrane. These in turn stimulate still other areas, and a wave of action potentials moves away in all directions from the point of stimulation to the ends of the nerve fiber. This transmission of action potentials along a fiber constitutes a nerve impulse.

In the muscle fiber, stimulation that occurs at the motor end plate triggers an impulse to travel over the surface of the fiber and down into its transverse tubules.