Influence of Potassium and Calcium on Motor Discharges

Abstract

Physico-chemical factors within neurones must be important in controlling their activity, both spontaneous and evoked. Impinging nerve impulses are the main secondary control in the latter case; the condition of the fluid milieu of the cells in the former. The prolonged motor discharges which give maintained rebound postures following stimulation of the deep cerebellar nuclei 1 are a favorable case of evoked activity grading slowly into spontaneous activity. Since K+ and Ca++ are able respectively to increase and decrease potentials in the brain, 2 these ions were tested for effects on intensity and duration of the motor sequelae of cerebellar stimulation.

A stimulating electrode was placed by the Horsley-Clark instrument in the nucleus fastigius of cats under light nembutal anesthesia. A stimulus was chosen to give a good, but not maximal response. During stimulation (up to 3 sec.) the crossed foreleg is flexed at the elbow; to pass into strong extension on cessation of the stimulus. This after-extension was timed and its intensity judged semiquantitatively by feel. After several repetitions had yielded constant times, 1 or 2 cc. of isotonic KCl, CaCl₂ or Na citrate were injected into the carotid artery and observations continued within 30 seconds. In each of 3 tests with K, responses were prolonged beyond those earlier or later by an average of 230%. (The first 3 responses in each series are averaged.) Likewise, Ca++ consistently decreased response duration on the average by 51%. Sodium citrate increased durations 10 to 20%. Intensity fairly paralleled duration, being increased with K+ and decreased with Ca++.

In any series of tests under “control” conditions the first 1 or 2 responses, after a rest, may be shorter than later ones; and still later responses are again shorter. After salt injections, in each case the third test, about 1 minute after the injection, gave the extreme value—longest after K or citrate, shortest after Ca. Table I gives 2 single series and averages.