Efferent neurons and vestibular cross-talk in the frog.
S.F. Myers1, H.H. Salem1 and J.A. Kaltenbach2.
1. Biology Department, University of Michigan-Flint, Flint, MI, 2.
Department of Otolaryngology, Wayne State University, Detroit, MI.
APStracts 4:0010N, 1997.
ABSTRACT
A galvanic stimulus (30 - 120 sec. 0.3 mAmp constant current pulse) was used
to depolarize the spike generating region of horizontal and anterior canal
afferent neurons. The galvanically induced spike activity from these neurons
served as a driving input to the efferent vestibular system in the bullfrog.
Efferent-mediated effects were assessed by intracellular recordings of
posterior canal afferent spike activity, either ipsilateral or contralateral
to the driving stimulus. Ipsilateral to the driving stimulus, efferent-
mediated spike rate changes occurred in 62 (39%) of 158 posterior canal
afferent neurons. Ipsilateral efferent-mediated effects were overwhelmingly
excitatory (92%). Three percent of responding units were inhibited during
stimulus application, and 5% showed mixed responses involving 3-20 seconds of
inhibition followed by facilitation. Contralateral to the driving stimulus,
efferent-mediated spike rate changes occurred in 18 (23%) of 77 posterior
canal afferent neurons. Contralateral efferent-mediated effects were
overwhelmingly inhibitory (95%). Only one unit was facilitated during
stimulation and no mixed responses to contralateral stimulation were observed.
Analysis of the coefficient of variation in interspike intervals (CV) before
and during stimulation showed no significant efferent-mediated effects on
spike train noise. Comparisons of resting spike rates between units showing
efferent-mediated effects and those that did not were in general agreement
with previous studies. Responding units had a lower mean spike rate (6.8 ñ
0.70 SEM) than did non-responding units (10.7 ñ 0.42 SEM; p<0.001; two tailed
t-test of log-normalized data). Comparison between groups in the regularity of
their resting spike rates, as quantified by the coefficient of variation in
interspike intervals (CV), showed considerable overlap. When responding and
non-responding units with similar resting spike rates were compared,
responding units had somewhat more irregular resting spike rates than did non-
responding units (p<0.004; two-tailed, paired t-test). In most cases (77%),
the temporal pattern and general shapes of efferent-mediated responses
mirrored the driving input of the galvanically activated afferent neurons. The
other 23% of efferent-mediated responses exhibited a marked adaptation of the
response. Adapting and non-adapting units were not significantly different in
their mean resting spike rates or in the regularity of their resting spike
rates.
Received 12 August 1996; accepted in final form 16 December 1996.
APS Manuscript Number J652-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 21 January 1997