NERVE CONDUCTION

The process by which impulses are carried by nerve cells.Our knowledge of nerve conduction has gone through a number of stages. Early thinkers, such as Descartes, believed that impulses in the form of “animal spirits” were carried through the body in hollow tubes. In 1791 Gal- vani used the newly developed Leyden jar to make a frog leg twitch, but he mistakenly concluded that the animal tissue itself generated electricity. In 1800 Volta showed that electricity of this type could be obtained without placing animal tissue in the circuit, and therefore concluded that it was the property of inorganic substances. Nevertheless the association of nerve impulse with electricity continued, and in 1834 Johannes Muller asserted that it was clearly electrical in nature, although he felt that its speed could not be measured because it was close to the speed of light. A few years later his student, DuBois-Reymond, adumbrated the present theory of nerve conduction by suggesting the concept of polarization and by insisting that the impulse was not instantaneous but actually finite and measurable. As Boring (1950) points out, “It was these experiments of Du- Bois that brought the nervous impulse out of the mystic realm of animal spirits and the pneumatics of the soul into the realm of materialistic science.”DuBois’ work prompted his friend Helmholtz to measure the speed of nerve conduction in 1850. Shortly after, in 1866, Bernstein discovered that the outside of the membrane of the fiber is positively charged during its resting state, while the inside is negatively charged. He described the impulse itself as a “wave of negativity” generated by changes occurring along the cell membrane. When the nerve fiber is stimulated, the negative charge spreads to the outside, sweeping along the fiber as a wave of depolarization. The speed of this impulse, or “spike potential” as it is now called, has been found to vary with the thickness of the fiber. Most fibers are between .001 and .02 mm. in diameter. Recent measurements have shown that the rate of conduction, expressed in meters per second, is six times the diameter in thousandths of a millimeter. Thus, a fiber with a diameter of .010 mm. will conduct at the rate of sixty meters per second.In 1899 F. Gotch and G. J. Burch showed that for a brief period after a nerve fires it cannot be fired again. This period, during which it is recovering its excitability, is called the refractory phase. In 1912 E. D. Adrian and K. Lucas plotted the curve of recovery and found an absolute refractory period during which no stimulation no matter how intense will fire the fiber, and a relative refractory period occurring directly afterward, during which an intense stimulus will set it off. These two investigators also discovered the all-or- none principle, which states that a stimulus which is above the threshold of a given fiber will excite the entire fiber, and if it is below the threshold strength it will not excite it at all.