The Nervous System. Nerve Potentials

When considering a nerve network, it is easy to talk about impulses traveling along various pathways through the network, but of what, exactly, do those impulses consist? The ancient doctrine of a "spirit" flowing through the nerves had been smashed by Haller and Gall; but it arose again almost at once, albeit in a new form, when the Italian anatomist, Luigi Galvani (1737-98), discovered, in 1791, that the muscles of a dissected frog could be made to twitch under electrical stimulation. He declared there was such a thing as "animal electricity" produced by muscle.

This suggestion, in its original form, was not correct, but properly modified it proved fruitful. The German physiologist, Emil Du Bois-Reymond (1818-96), wrote a paper on electric fishes while still a student, and this initiated in him a lifelong interest in the electrical phenomena within tissues. Beginning in 1840, he set about refining old instruments and inventing new ones, instruments with which he might detect the passage of tiny currents in nerve and muscle. He was able to show that the nerve impulse was accompanied by a change in the electrical condition of the nerve. The nerve impulse was, in part at least, electrical in nature, and certainly electricity was as subtle a fluid as the old believers in a nervous "spirit" could have wished.

Electrical changes not only moved along the nerve but along muscles as well. In the case of a muscle undergoing rhythmic contractions, as was true of the heart, the electric changes were also rhythmic. In 1903, the Dutch physiologist, Willem Einthoven (1860-1927), devised a very sensitive "string galvanometer" capable of detecting extremely faint currents. He used it to record the rhythmically changing electric potentials of the heart through electrodes placed on the skin. By 1906, he was correlating the "electrocardiograms" (EKG) which he was recording, with various types of heart disorders.

A similar feat was performed in 1929 by the German psychiatrist, Hans Berger (1873-1941), who attached electrodes to the skull and recorded the rhythmically changing potentials that accompany brain activity. The "electroencephalograms" (EEC) are extremely complicated and hard to interpret. However, there are easily noted changes where extensive brain damage exists, as when tumors are present. Also the old "sacred disease" of epilepsy reveals itself in the form of changes in the EEC.

Electric potentials cannot, however, be the entire answer. An electrical impulse traveling along a nerve ending cannot, of itself, cross the synaptic gap between two neurons. Something else has to cross and initiate a new electrical impulse in the next neuron. The German physiologist, Otto Loewi (1873-1961), demonstrated, in 1921, that the nerve impulse involved a chemical change as well as an electrical one. A chemical substance, set free by the stimulated nerve, crossed the synaptic gap. The particular chemical was quickly identified by the English physiologist, Henry Hallet Dale (1875- ), to be a compound called "acetylcholine."

Other chemicals have since been discovered to be related to nerve action in one fashion or another. Some have been found which will produce the symptoms of mental disorders. Such neurochemistry is as yet in its infancy, but it is hoped that it will eventually represent a powerful new means of studying the human mind.