Activity of commissural interneurons in spinal cord of Xenopus embryos

Abstract

Horseradish peroxidase- (HRP) filled microelectrodes were used to examine the anatomy and physiology of commissural interneurons, a morphologically defined class of spinal cord interneuron in X. laevis embryos. Commissural interneurons have unipolar cell bodies in the dorsal half of the spinal cord. Their dendrites lie in the mid to ventral parts of the lateral tracts and their axons cross the cord ventrally, T branch and ascend and descend on the opposite side of the cord. Recordings were made from animals immobilized in tubocurarine and responding to natural stimulation with 3 patterns of fictive motor activity. During episodes of fictive swimming, commissural interneurons are phasically excited to fire 1 spike/cycle in phase with motor discharge on the same side and receive a mid/cycle inhibitory postsynaptic potential (IPSP) in phase with motor discharge on the opposite side. Rhythmic activity is superimposed on a background depolarization. During periods of synchrony, phasic excitatory input doubles in frequency so that cells fire with half the swimming cycle period. The background depolarization is generally stronger than during swimming. During periods of fictive struggling, evoked by electrical stimulation of the skin, commissural interneurons fire a burst of spikes per cycle, cells being relatively hyperpolarized when motoneurons on the opposite side are active. In response to ipsilateral skin stimulation, some cells receive an IPSP at a latency of 12-20 ms. This precedes the onset of fictive locomotion. The anatomy and activity of commissural interneurons is suitable for a reciprocal inhibitory role.