EXCITATORY AND INHIBITORY INPUTS FROM SACCULAR AFFERENTS TO SINGLE
VESTIBULAR NEURONS IN THE CAT
Y. UCHINO, H. SATO and H. SUWA
Department of Physiology, Tokyo Medical College
6-1-1 Shinjuku, Shinjuku-ku, Tokyo 160, Japan.
APStracts 4:0083N, 1997.
ABSTRACT
Connections from saccular afferents to vestibular neurons were studied by
means of intracellular recordings of excitatory (E) and inhibitory (I)
postsynaptic potentials (PSPs) in vestibular neurons after focal stimulation
of the saccular macula in decerebrated cats. Focal stimulation was given to
the saccular macula in two ways, in which the polarity of stimulus current via a pair of electrodes was changed. In group A, one of the electrodes was
inserted into the ventral and the other into the dorsal edge of the saccular
macula. The focal stimulation was across the striola, so that the reversal of
morphological polarization in hair cells was bridged by the pulse stimulus. In 22/36 vestibular neurons tested, the stimulation of the saccular macula evoked monosynaptic (ó1.2 ms) EPSPs, including EPSP-IPSP sequences, with one polarity of stimulation, and disynaptic (ò1.5 ms) IPSPs when the polarity of the
stimulus current was changed. In 14 /36 neurons, the response pattern was the
same regardless of the stimulus polarity; EPSPs (12/36) or IPSPs (2/36). In
group B, a pair of electrodes was inserted into the dorsal edge of the
saccular macula, so that the striola was not bridged by the current stimulus.
In all of the vestibular neurons tested, the response pattern was always the
same regardless of the polarity; mono- (22/31) and disynaptic (3/31) EPSPs or
disynaptic IPSPs (6/31). In addition, the saccular nerve was stimulated after
removing the macula in some cats (group C). The stimulation of the saccular
nerve evoked EPSPs in 62 vestibular neurons (including EPSP-IPSP sequences in
31 neurons) and IPSPs in 19 vestibular neurons. Convergence between the
saccular nerve and other vestibular nerves was studied by the intracellular
recording of PSPs. Fifty-six percent (18/32) of the saccular-activated neurons had excitatory and/or inhibitory potentials evoked after stimulation of the
utricular nerve and the horizontal and anterior semicircular canal nerves, and 44% (19 /43) of the neurons received inputs from the posterior semicircular
canal nerve. The results support the hypothesis that saccular afferents from
one population of hair cells activate vestibular neurons monosynaptically, and that afferents from another population of hair cells located on the opposite
side of the striola appear to project to the same vestibular neurons
disynaptically via inhibitory interneurons. Neural circuits from saccular
afferents to vestibular neurons, which we term cross-striolar inhibition, may
thus provide a mechanism for increasing the sensitivity to vertical linear
acceleration. The circuit described is provided not only with high
sensitivity, but also with input noise-resistant characteristics.
Received 13 Decemeber 1996; accepted in final form June 13 1997.
APS Manuscript Number J0971-6
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 15 July 1997