Differential Effects of Direct Current on Insect Ganglia

Abstract

1. The passage of small direct currents transversely through the intact thoracic ganglia of the cockroach affects the flexor-extensor balance of the legs in such a way that the leg on the negative side flexes while that on the positive side extends. 2. With longitudinal stimulation, ascending currents bring about flexion of both legs and descending currents encourage extension. 3. Preparations in which only a single nerve is attached to the ganglion do not show these differential effects when the polarizing current is applied between ganglion and nerve. The negative electrode excites when it is on the ganglion, and the positive inhibits irrespective of whether the nerve supplies flexor or extensor muscles. Intermittent discharge of impulses may occur before the spontaneous discharge is completely inhibited when the positive electrode is on the ganglion. 4. At least three different responses have been recorded from some of the leg muscles under these conditions and they are excited in a ‘functional order’. That is, the ‘slow’ response accompanied by a tonic contraction occurs at a lower threshold than the ‘faster’ twitch systems. 5. A hypothesis is put forward to interpret these apparently contradictory results. It is suggested that the polarizing current influences the gradient of potential along oriented internuncials which are stimulated at low threshold in the intact preparation. These internuncials connect regions of the neuropile containing flexor and extensor synaptic endings.