DISCHARGE PROPERTIES OF BRAIN STEM NEURONS PROJECTING TO THE FLOCCULUS IN
THE ALERT CAT. I. MEDIAL VESTIBULAR NUCLEUS
CHERON, Guy, Miguel ESCUDERO and Emile GODAUX.
Laboratory of Neurosciences, University of Mons-Hainaut, Belgium,
Laboratory of Neurosciences, University of Sevilla, Spain.
APStracts 3:0090N, 1996.
SUMMARY AND CONCLUSIONS
1. The aim of this study was to characterize the signals transmitted by
neurons of the medial vestibular nucleus (MVN) to the middle zone of the
flocculus in alert cats. 2. Bipolar stimulating electrodes were implanted into
the middle zone of each flocculus, because this zone is known to be involved
in the control of horizontal eye movements. Correct implantation of the
stimulating electrodes was ensured by (a) recording of Purkinje cells whose
activity was related to horizontal eye movements and (b) elicitation of slow
abduction of the ipsilateral eye upon electrical stimulation. 3. The rostral
two thirds of the MVN were investigated by microelectrodes during stimulation
of both flocculi. Antidromically activated neurons were found only in the
central part of the explored area. Forty-four units were activated from the
contralateral, 8 from the ipsilateral flocculus. Neurons could never be
activated from both flocculi. 4. Neurons included in this study were MVN
neurons that had (a) to be antidromically activated from one flocculus and (b)
to modulate their firing rate during the horizontal vestibulo-ocular reflex
(VOR) elicited by sinusoidal stimulation (0.1 Hz; 10, 20, 30 or 40 deg). The
39 neurons matching both criteria were classified in two groups: 22 neurons
changed their firing rate during spontaneous horizontal eye movements (EM-
neurons), 17 modulated their activity only during head rotation and were
labeled vestibular-only neurons (VO-neurons). 5. Sufficient data were obtained
from 13 EM-neurons to allow a quantitative analysis. Among those, 12 were
activated from the contralateral and 1 from the ipsilateral flocculus. Their
sensitivity to horizontal eye position during intersaccadic fixation was 3.54
+/- 2.75 spikes.s 1/deg. Eight EM-neurons behaved as type I neurons, 5 as type
II neurons. During the slow phases of the VOR, all of these neurons combined
some head-velocity sensitivity (mean +/- SD: 1.50 +/- 0.43 spikes.s-1/deg.s-1)
with some horizontal eye-position sensitivity (3.61 +/- 2.45 spikes.s-1/deg).
Additionally, 7 of these neurons presented a sensitivity to eye velocity (1.34
+/- 0.55 spikes.s-1/deg.s-1). The phase difference between the modulation of
firing rate and eye position varied substantially between neurons. The
observed phase lead with respect to eye position ranged from 2 to 110 deg
(mean +/- SD: 41.9 +/- 31.8 deg). 6. Sufficient data were obtained from 10 VO-
neurons to allow a quantitative analysis. Among those, 9 were activated from
the contralateral and 1 from the ipsilateral flocculus. All of these neurons
behaved as type I neurons. The sensitivity to head velocity was 1.64 +/- 1.07
spikes.s-1/deg.s-1. The phase lead of the modulation of spike activity with
respect to head velocity ranged from 4.5 to 30.5 deg (mean +/- SD: 16.4 +/-
8.9 deg). 7. We conclude that the MVN provides the horizontal zone of the
flocculus (with a strong contralateral preference) with information about head
velocity (through VO-neurons and EM-neurons) and about eye velocity and
position (through EM-neurons).
Received 28 September 1994; accepted in final form 23 February 1996.
APS Manuscript Number J604-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 28 May 96