Spatial organization of premotor neurons related to vertical upward and downward saccadic eye movements in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the cat

Abstract

The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) contains premotor neurons that are related to the control of vertical and torsional saccadic eye movements. In the present study, complimentary light microscopic anterograde biocytin and retrograde horseradish peroxidase experiments have been performed to determine the organization of premotor neurons in the riMLF in the cat that are related intimately to the vertical motoneuron populations in the oculomotor and trochlear nuclei. The results indicate a rostral-caudal topographic arrangement of neurons in the riMLF that is related to the target projections to vertical downward (inferior rectus and superior oblique) and vertical upward (superior rectus and inferior oblique) motoneurons, respectively, in the oculomotor and trochlear nuclei. Both the anterograde and the retrograde studies are consistent, in that they demonstrate the tendency for downward and upward riMLF neurons to be separated spatially by a distance of approximately 0.5 mm in the rostral-caudal axis of the nucleus. The riMLF projections to inferior oblique and superior oblique motoneurons are predominantly ipsilateral. Projections to inferior rectus and superior rectus motoneurons, however, are bilateral, and, presumably, they provide one means for assuring the conjugacy of vertical saccadic eye movements. Because premotor burst neurons that encode parameters for upward or downward saccades are intermingled within the riMLF, and excitatory and inhibitory premotor neurons also coexist in this region, the findings from this study suggest that subregions of the riMLF contain coexistent populations of excitatory and inhibitory neurons that are related to opposite directions of vertical eye movements. The spatial segregation of excitatory premotor neurons in the riMLF that are related to vertical upward vs. downward movements, furthermore, provides a basis for the interpretation of vertical upward and/or downward gaze palsies that might result from discrete lesions at the mesodiencephalic junction in humans.

Premotor neurons for vertical eye movements in the rostral mesencephalon of monkey and human: Histologic identification by parvalbumin immunostaining

“…Our findings are in accordance with physiologic recording experiments in monkey and cat that found upward and downward burst neurons intermingled within the riMLF (Bü ttner et al, 1977;King and Fuchs, 1979;Moschovakis et al, 1991a,b;Li et al, 1993). Anatomical experiments in cats revealed a tendency of upward neurons lying more caudally and downward neurons more rostrally (Wang and Spencer, 1996b). The authors explain the discrepancy of physiologic and anatomical results with the existence of excitatory and inhibitory premotor neu- rons with the same projection targets, but with opposite moving vectors.…”

“…The morphometric analysis shows that the majority of, if not all, medium-sized neurons within the riMLF are premotor neurons, taking into account that not all vertical premotor neurons are labelled from the TTC injection in one single experiment, that is, the burst neurons encoding vertical and torsional saccades (Bü ttner et al, 1977;King and Fuchs, 1979;Vilis et al, 1989). The predominantly ipsilateral projection to the corresponding motoneurons is in accordance with other studies applying central tracer injections into the oculomotor nucleus (Steiger and Bü ttner-Ennever, 1979;Wang and Spencer, 1996b). Additional weakly labelled cells were found in the contralateral riMLF in all cases.…”

Synaptic Control of Motoneuronal Excitability

“…The oculomotor nucleus and the trochlear nucleus receive afferents from the rostral interstitial nucleus of the medial longitudinal fasciculus (GABAergic and glutamatergic; Refs. 1180,1335) and interstitial nucleus of Cajal (654). A small number of neurons in the locus coeruleus, trigeminal sensory complex and olivary pretectal nucleus project to the oculomotor nucleus (191,455,646).…”

Projections of the caudal ventrolateral medulla to the thoracic spinal cord in the rat

“…After thoroughly rinsing in 0.1 M phosphate buffer (pH 7.2), sections were processed for the visualization of BDA label, according to the protocol of Wang and Spencer (1996). Briefly, the sections were preincubated in 1:500 avidin-HRP in 0.1% Triton X-100 phosphate buffer (0.1 M, pH 7.2) for at least 4 h, or overnight in the refrigerator.…”