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GENERAL PHYSIOLOGY OF EXCITABLE TISSUES

All cells can pass from a state of physiological rest to one of excitation in response to stimuli. However, the term excitable tissue refers specifically to nerve and muscular tissues in which excitation generates an electrical impulse travelling along the cell membrane.

Excitation is characterized by an aggregate of electrical, temperature, chemical, functional, and structural changes in the living cell, of which the bioelectrical phenomena are the most important.

BIOELECTRICAL PHENOMENA

HISTORICAL OUTLINE

The theory of “animal electricity”, i.e. of electrical phenomena occurring in living tissues, arose in the latter half of the 18th century. Soon after the invention of the Leyden jar it was shown that certain fish (electric rays and electric eels) stunned their prey with a strong electric shock. Priestley surmised at that time that the transmission of a nerve impulse was a flow of “electrical fluid” along the nerve, while Bertholon attempted to develop a theory of medicine attributing the cause of disease to an excess or deficit of this fluid in the organism.

An attempt to produce a consistent theory of “animal electricity” was made by Galvani in his well-known treatise on electrical forces in muscular motion (1791). In experiment to study the physiological influence of electrical discharges in a current-generating

machine, and of atmospheric electricity during thunderstorms, Galvani used a preparation of the hind legs of a frog linked with the spine. After suspending it on a copper hook from the iron railing of a balcony, he noticed that the muscles of the frog's legs, swinging in the wind, contracted each time they touched the railing. He concluded from that that the twitching of the legs was caused by “animal electricity” generated in the spinal cord and transmitted through the metal conductors (hook and railing) to the leg muscles.

His experiments were repeated by Volta (1792), who found that the phenomena described by Galvani had not been caused by “animal electricity”; the source of the current in Galvani's experiments had not been the spinal cord of the frog but the circuit formed by the two different metals, copper and iron. To refute Volta’s objections, Galvani carried out another experiment, without using metal, and showed that if the skin was stripped from the frog’s legs, and the sciatic nerve severed at the point where its roots came out of the spinal cord, separating the nerve along the thigh to the shin, then bringing the nerve into contact with the exposed shin muscles would cause the latter to contract. Du Bois-Reymond called that experiment the chief true experiment of neuromuscular physiology.