Structure of the primate oculomotor burst generator. II. Medium-lead burst neurons with downward on-directions

Abstract

1. To investigate the morphology and physiology of vertical medium-lead burst neurons with downward on-directions (DMLBs), we impaled midbrain axons and recorded their discharge patterns in relation to spontaneous saccades of alert, behaving squirrel monkeys. Selected axons were injected with horseradish peroxidase and morphologically characterized. 2. DMLBs emitted bursts of impulses that preceded rapid eye movements by approximately 5 ms. Parameters of the burst (duration and number of spikes) were highly correlated with parameters of the saccadic eye movement (duration and amplitude of the downward displacement of the eyes). 3. Somata of DMLBs were recovered in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF, n = 14), and in the interstitial nucleus of Cajal (NIC, n = 2). Fibers originating from riMLF DMLBs projected, usually ipsilaterally, to the NIC as well as in the inferior rectus and the superior oblique subdivisions of the oculomotor complex. The axons of NIC DMLBs projected to the ipsilateral riMLF, NIC, and the mesencephalic reticular formation but not to the oculomotor complex. 4. Our data demonstrate that some DMLBs can provide extraocular motoneurons of both eyes with the pulse of activity they display during downward saccades. In addition, such neurons can supply the NIC with one of the signals that this nucleus is thought to integrate to extract an estimate of the vertical eye position. Finally, our data demonstrate the existence of DMLBs that do not establish direct connections with oculomotoneurons.

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

“…In contrast to previous findings in the monkey (Moschovakis et al, 1991a), in which bilateral projections of riMLF neurons have been observed in relation to inferior oblique motoneurons, in the present study, riMLF neurons in the cat project bilaterally only to inferior rectus and superior rectus motoneurons, albeit modestly to the contralateral side. The extent to which these disparate findings might represent species differences is unclear.…”

“…Consistent with previous studies (Graybiel and Hartwieg, 1974;Steiger and Biittner-Ennever, 1979;Shiraishi, 1983a, 1985;Labandeira-Garcia, et al, 1989;Moschovakis et al, 1990Moschovakis et al, , 1991aCarpenter et al, 1992;Li et al, 1993), the riMLF projections to the vertical motoneurons in the oculomotor and trochlear nuclei are predominantly ipsilateral. In contrast to previous findings in the monkey (Moschovakis et al, 1991a), in which bilateral projections of riMLF neurons have been observed in relation to inferior oblique motoneurons, in the present study, riMLF neurons in the cat project bilaterally only to inferior rectus and superior rectus motoneurons, albeit modestly to the contralateral side.…”

“…Morphological (Graybiel, 1977;Steiger and ButtnerEnnever, 1979;Buttner, 1978,1988;Labandeira-Garcia et al, 1989;Moschovakis et al, 1990Moschovakis et al, , 1991a and physiological (Schwindt et al, 1974;Buttner et al, 1977;King and Fuchs, 1979;Shiraishi, 1983a,b, 1985;Hepp et al, 1989;Vilis et al, 1989;Moschovakis et al, 1990Moschovakis et al, , 1991aCrawford and Vilis, 1992;Li et al, 1993;Moschovakis and Highstein, 1994;Shiraishi et al, 1994) studies have established the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) as the location of premotor neurons that control vertical saccadic eye movements.The riMLF is located rostral to the interstitial nucleus of Cajal (INC) at the mesodiencephalic junction and occupies the dorsomedial part of the field H of Forel (Graybiel, 1977;Buttner, 1978, 1988;Steiger and Buttner-Ennever, 1979;Buttner-Ennever et al, 1982;Crossland et al, 1994). Premotor burst neurons that discharge in relation to vertical upward and downward saccades are intermingled within the riMLF (Buttner et al, 1977;King and Fuchs, 1979;Vilis et al, 1989;Moschovakis et al, 1990Moschovakis et al, , 1991a.…”

“…Premotor burst neurons that discharge in relation to vertical upward and downward saccades are intermingled within the riMLF (Buttner et al, 1977;King and Fuchs, 1979;Vilis et al, 1989;Moschovakis et al, 1990Moschovakis et al, , 1991a. Neurons that establish excitatory or inhibitory synaptic connections with vertical motoneurons in the oculomotor and trochlear nuclei also coexist within the region of the riMLF (Schwindt et al, 1974;Shiraishi, 1983a,b, 1985;Li et al, 1993;Robinson et al, 1994).…”

Decline of Vertical Gaze and Convergence with Aging

“…Those for downward gaze project only to the ipsilateral oculomotor nucleus [17]. Thus, the upward gaze is supported by more complex pathways, so these anatomical differences between the upward and downward systems might account for the distinct age-associated changes of the vertical gaze mechanism.…”