Synaptic actions of the superior colliculus on medial rectus motoneurons in the cat

Abstract

Synaptic effects of superior colliculus stimulation on medial rectus motoneurons were studied in encéphale isolé cats. Excitatory postsynaptic potentials were observed in all medial rectus motoneurons located on the side of stimulation, whereas contralateral motoneurons received mainly inhibition. The latencies of stimulus-locked excitatory and inhibitory postsynaptic potentials were in the ranges of 1.3–2.6 and 2.0–3.5 ms. respectively, i.e. on the average longer than in abducens motoneurons. Acute lesions of paramedian structures at bulbar levels did not affect the excitatory responses. Pontine transection at the level of the abducens nucleus reduced the mass response of medial rectus motoneurons, but failed to abolish short latency excitatory potentials in motoneurons studied intracellularly. The present data suggest that the shortest excitatory pathway from the superior colliculus to medial rectus motoneurons is disynaptic. The inhibitory pathway appears to contain at least one additional interneuron. The reciprocal pattern of synaptic action on antagonistic (left and right) medial rectus motoneurons indicates that collicular stimulation activated connections responsible for conjugate contraversive eye movements. According to the results of transection experiments. bulbar structures cannot be regarded as the main relay site of tectofugal effects on ocular motoneurons. Although the exact location of relay neurons could not he at present established. the observed timing of synaptic events is not inconsistent with the idea that tectal influences on medial rectus and abducens motoneurons are mediated by common internuncial cells in the parabducens region.