Follow the electrical and chemical changes undergone to transmit an impulse through the human nervous system

NARRATOR: Healthy nerve cells contain a balance of ions, ions being positively or negatively charged atoms. Because of these ions, the movement of nerve impulses through individual nerve cells involves both chemical and electrical changes. Before a neuron transmits an impulse, its outside layer is composed of electrically positive ions, its inside layer is composed of electrically negative ions. When the neuron is at rest, there is no movement of chemicals into or out of the cell. When the neuron is stimulated, electrical and chemical changes occur. At the stimulated point, the outside of the nerve cell becomes negative and the inside becomes positive. The ions change places. As soon as the impulse passes, the stimulated point returns to its original electrical and chemical state. The passage of the nerve impulse along the cell causes a similar pattern of changes throughout the neuron. Before individual parts of the cell can pass another impulse, they must rest. In some neurons, this recovery period is as brief as a thousandth of a second.