Artificial Stimulation of Nerve Cells

When an electrical stimulus is applied to a nerve cell from an external source, current flows from the positive stimulating electrode (anode) into the neuron, and exits at the negative electrode (cathode). The nerve fiber below the cathode is depolarized and an action potential is generated there if the threshold potential is reached.

The conduction velocity of a nerve can be measured by placing two electrodes on the skin along the course of the nerve at a known distance from each other, then stimulating the nerve (containing multiple neurons) and recording the time it takes the summated action potential to travel the known distance. The conduction velocity in humans is normally 40 to 70 m ■ s-1. Values below 40 m ■ s-1 are considered to be pathological.

Accidental electrification. Exposure of the body to high-voltage electricity, especially low-frequency alternating current (e.g., in an electrical outlet) and low contact resistance (bare feet, bathtub accidents), primarily affects the conduction of impulses in the heart and can cause ventricular fibrillation (^ p. 200).

Direct current usually acts as a stimulus only when switched on or off: High-frequency alternating current (> 15 kHz), on the other hand, cannot cause depolarization but heats the body tissues. Diathermy works on this principle.

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