Neuronal activity related to vertical eye movement in the region of the interstitial nucleus of Cajal in alert cats

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Abstract
Summary (1) Discharge characteristics of neurons in the region of the interstitial nucleus of Cajal (INC) were studied in alert cats during spontaneous or visually induced eye movement and sinusoidal vertical (pitch) rotation. Activity of a majority of cells (n = 68) was closely related to vertical eye position with or without bursting activity during on-direction saccades. They were called vertical burst-tonic (n = 62) and tonic (n = 6) neurons. Mean discharge rates for individual cells when the eye was near the primary position ranged from 35 to 133 (mean 75) spikes/s with a coefficient of variation (CV) ranging from 0.04 to 0.29 (mean 0.15). Average rate position curves were linear for the great majority of these cells with a mean slope of 3.9 ± 1.2 SD spikes/s/deg. (2) The burst index was defined as the difference in discharge rate between maximal rate during an on-direction saccade and the tonic rate after the saccade. The values of mean burst index for individual cells ranged from 8 to 352 (mean 135) spikes/s. Tonic neurons had a burst index lower than 60 spikes/s and were distributed in the lower end of the continuous histogram, suggesting that burst-tonic and tonic neurons may be a continuous group with varying degrees of burst components. During off-direction saccades, a pause was not always observed, although discharge rate consistently decreased and pauses were seen when saccades were made further in the off-direction toward recruitment thresholds. Significant positive correlation was observed between average discharge rate during off- as well as on-direction saccades and tonic discharge rate after saccades for individual cells, which was not due to cats making saccades mainly from the primary position. (3) During pitch rotation at 0.11 Hz (±10 deg), burst-tonic and tonic neurons had mean phase lag and gain of 128 (±13 SD) deg and 4.2 (±1.7 SD) spikes/s/deg/s2 relative to head acceleration. During pitch rotation of a wide frequency range (0.044–0.495 Hz), the values of phase lag were mostly constant (120–140 deg), while simultaneously recorded vertical VOR showed the mean phase lag of 178 deg. Vertical eye position sensitivity and pitch gain (re head position) showed significant positive correlation. (4) Comparison of the discharge characteristics of vertical burst-tonic and tonic neurons with those of secondary vestibulo-ocular neurons (Perlmutter et al. 1988) and extraocular motoneurons (Delgado-Garcia et al. 1986) in alert cats suggests that signals carried by burst-tonic and tonic neurons are partially processed signals in vertical VOR and saccades, and different from oculomotor signals. (5) The INC region also contained many cells that did not belong to the above groups but whose activity was clearly modulated by pitch rotation (called pitch cells for the present study, n = 44). Many (n = 23) showed some correlation with vestibular quick phases, and some (n = 12) with visually elicited eye movement, although they showed significantly lower and more irregular discharge rates than burst-tonic and tonic neurons (mean discharge rate when the eye was near the primary position 34, range 3–91, spikes/s; mean CV 0.61, range 0.15–1.7). During pitch rotation they showed the mean phase lag and gain of 119(±26 SD) deg and 3.2(±2.1 SD) spikes/s/deg/s2. Some cells showed a much lower phase lag of about 90 deg. (6) More than half the burst-tonic, tonic and pitch cells tested were antidromically activated by stimuli applied to the ponto-medullary medial longitudinal fasciculus at the level of abducens nucleus, while none of them were activated from the inferior olive, suggesting that vertical eye position signals carried by some burst-tonic and tonic neurons are carried to the lower brainstem.

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