A physiological study of vestibular and prepositus hypoglossi neurones projecting to the abducens nucleus in the alert cat.

Abstract

SUMMARY1. Vestibular and prepositus hypoglossi (PH) neurones projecting to the abducens (ABD) nucleus were recorded in the alert cat. Their discharge characteristics were analysed to ascertain the origin of the horizontal eye position signal present in ABD neurones.2. Neurones were classified according to: their location with respect to the ABD nucleus; their antidromic activation from the ABD nucleus; the synaptic field potential they induced in the ABD nucleus with the spike-triggered averaging technique; and their activity during spontaneous and vestibularly induced eye movements.3. Vestibular neurones projecting to the ABD nucleus were located in the rostral medial vestibular nucleus. They were excitatory on the contralateral and inhibitory on the ipsilateral ABD neurones. Both types of premotor vestibular neurone showed a firing rate weakly related to eye position, increasing for eye fixations in the contralateral on-direction, and decreasing with ipsilateral fixation. Position sensitivity during eye fixations was (means + S.D.) 1-8 +09 spikes s-' deg-' for excitatory neurones and 2-2 + 1-3 spikes s-I deg-' for inhibitory neurones. Firing rate exhibited a high variability during eye fixations. Their responses during saccades in the off-direction were characterized by a pause that, although less defined, was occasionally present during saccades in the on-direction. Eye velocity sensitivity during spontaneous saccades in the on-direction was 0 17 +0.15 spikes s8-deg-' s-' for excitatory neurones and 0.15 + 0-07 spikes s-' deg-' s-' for inhibitory vestibular neurones. During sinusoidal head stimulation at 0-2 Hz, vestibular neurones showed a type I discharge rate with a phase lead over eye position of 86-0 + 14-1 deg for excitatory and 80-2 + 12-5 deg for inhibitory neurones. Position sensitivity during vestibular stimulation did not differ significantly from values obtained for spontaneous eye movements. However, the velocity sensitivity of premotor vestibular neurones during head rotation was significantly higher (I1 6 + 0-2 spikes s-1 deg-' s-' for excitatory and 1-5 + 0-3 spikes s-I deg-1 s-' for inhibitory neurones) than during spontaneous eye movements.4. PH neurones projecting to the ABD nucleus were located in the rostral one- MS 1118M. ESCUJDERO AND OTHERS third of the nucleus. These neurones were excitatory on the ipsilateral and inhibitory on the contralateral ABD nucleus. Their firing rates were correlated mainly with eye position, increasing for abducting eye positions of the ipsilateral eye and decreasing with adduction movements. Eye position sensitivities were 8 2 + 2-9 and 7-9 + 2-9 spikes s-' deg-' for excitatory and inhibitory neurones, respectively. Variability of the firing rate during eye fixations was lower than that shown by premotor vestibular neurones. During saccades, premotor PH neurones showed low eye velocity sensitivity (0 15 + 021 for the excitatory and 0 32 + 028 spikes s-' deg-' s-' for the inhibitory group) with low coefficients of correlation. During vestibular sinusoidal st...

A Cholinergic Synaptically Triggered Event Participates in the Generation of Persistent Activity Necessary for Eye Fixation

“…A glass microelectrode was advanced through the cerebellum toward the abducens nucleus, which was identified by the recording of the antidromic field potential induced by electric stimulation of the ipsilateral VIth nerve. The PH nucleus was localized in the same sagittal plane and posterior to the abducens nucleus, as confirmed by recording the characteristic firing discharge of PH neurons during spontaneous and vestibularly induced eye movements (Escudero et al, 1992). Injections were performed by means of glass micropipettes with tip diameters of 7-8 m, filled with the corresponding drug dissolved in 0.1 M PB, pH 7.4.…”

“…Eye movements were stored on an eight-channel videotape recording system and fed into a computer for offline analysis. Additional details of this chronic preparation were published previously (Escudero et al, 1992;Moreno-Ló pez et al, 1996.…”

“…1). Intracellular recordings (n ϭ 125) were made from neurons located in the rostral third of the nucleus, because this is the place where principal cells projecting to the abducens nucleus and other oculomotor structures are located (McCrea and Baker, 1985a;Escudero et al, 1992). Recorded neurons did not present action potentials spontaneously at resting potential values (Ϫ66.6 Ϯ 2.5 mV).…”

“…The synaptic feedback among those neurons seems necessary for the temporal integration (Aksay et al, 2003), and the sustained firing rate observed in the integrator could be explained by changes in the amplitude or rate of the synaptic inputs to the area I position-related neurons, rather than by neuronal intrinsic membrane properties (Aksay et al, 2001). Considering that position-related neurons of the goldfish area I are equivalent to "principal" cells identified in the cat PH nucleus (McCrea and Baker, 1985a,b;Escudero et al, 1992), it could be suggested that a similar circuit for the temporal integration is present in the latter neuronal center (Aksay et al, 2003). Additional information is needed regarding synaptic mechanisms underlying the integration process.…”

Differential Sensorimotor Processing of Vestibulo-Ocular Signals during Rotation and Translation

“…Specifically, to incorporate motor projections from the PH (Fig. 10 A, dashed line) (McCrea and Baker, 1985;Langer et al, 1986;Escudero and Delgado-Garcia, 1988;Spencer et al, 1989;Escudero et al, 1992;McFarland and Fuchs, 1992), a collateral PH output projection to the EM I cell type was included in the model of Figure 10 B to maintain the appropriate dynamic processing of canal signals in the RVOR. In addition, projections to and from visuomotor areas have been included (for details, see Green andGaliana, 1998, 1999;Green, 2000).…”