ANATOMY AND PHYSIOLOGY OF SACCADIC BURST NEURONS IN THE SQUIRREL MONKEY (OCULOMOTOR, VESTIBULAR, RETICULAR FORMATION, INTRACELLULAR)

Abstract

Electrophysiological studies in the cat have demonstrated separate populations of excitatory and inhibitory saccadic burst neurons (EBNs and IBNs) that project monosynaptically to the abducens nucleus. In the present study, saccadic burst neurons were studied in the alert squirrel monkey using intraaxonal recording and injection of horseradish peroxidase. Two anatomically distinct populations of burst neurons projecting to the abducens nucleus were found, analogous to EBNs and IBNs in the cat. All neurons fire exclusively during saccades, and have a discharge pattern similar to that of medium lead burst neurons described previously in the cat and monkey. In most neurons the saccadic burst begins 5-15 msec before saccade onset. Linear relationships were found between burst duration and saccade duration, number of spikes in the burst and saccade amplitude, and firing frequency and instantaneous velocity. The firing of each neurons shows the closest relationship with the amplitude of the saccade component in a particular direction. For all neurons, this on-direction is in the ispilateral hemifield, with a vertical component which may be either upward or downward.;EBNs are located in the caudal pontine reticular formation and have major axonal projections to the ipsilateral abducens nucleus. EBNs also project ispilaterally to the medial vestibular nucleus, nucleus prepositus, and regions of the pontine and medullary reticular formation. IBNs are located in the rostral medullary reticular formation, and have major projections to the contralateral abducens nucleus. Additional contralateral projections are present to nucleus prepositus, the vestibular nuclei, and regions of the pontine and medullary reticular formation. EBNs have a heavier projection to prepositus than do IBNs, whereas IBNs have a more extensive projection to the vestibular nuclei. EBN terminations in the vestibular nuclei are confined to the medial vestibular nucleus, and are concentrated in the medial part of the nucleus. Vestibular projections of IBNs are distributed throughout the medio-lateral extent of the medial vestibular nucleus, and also include regions of the superior, lateral, and descending vestibular nuclei. These results demonstrate a reciprocal organization of burst neuron input to abducens in the monkey similar to that found in the cat, and support a major role for these neurons in generating the oculomotor activity in motoneurons as well as in other classes of premotor neurons.