Logarithmic Time Course of Sensory Adaptation in Electrosensory Afferent
Nerve Fibers in a Weakly Electric Fish.
Xu, Zhian, Jeremy R. Payne, Mark E. Nelson.
Department of Molecular & Integrative Physiology and The Beckman Institute,
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801.
APStracts 3:0082N, 1996.
SUMMARY AND CONCLUSIONS
1. We recorded single unit activity from individual primary electrosensory
afferent axons in the posterior branch of the anterior lateral line nerve
(pALLN) of gymnotid weakly electric fish, Apteronotus leptorhynchu s. We
analyzed the responses of P-type (probability-coding) afferent fibers to
externally applied amplitude step changes in the quasi-sinusoidal transdermal
potential established by the fish's own electric organ discharge (EOD). 2. In
response to AM step increases in transdermal potential, the firing rate of P-
type afferents exhibited an abrupt increase followed by an initially rapid and
subsequently more gradual decay back toward the baseline level. Afferent
responses continued to adapt slowly throughout the duration of prolonged step
stimuli lasting more than 100 ? s. The time course of sensory adaptation was
similar for all units tested. 3. We introduce a new functional form for
describing the time course of sensory adaptation in which the change in firing
rate D r decays logarithmically with time: Dr(t)=A/(Bln(t)+1 ). This
logarithmic form accurately describes the adaptation time course of P-type
afferents over 5 decades in time, from milliseconds to hundreds of seconds,
with only two free parameters. Using a non-linear least-squares fitting
technique, we obtained a mean value of the parameter B, which characterizes
the adaptation time course, of 0.149 (s.d. =0.028, n =49). 4. We compare
logarithmic fits with traditional multiexponential and power law forms and
demonstrate that the logarithmic form yields a better characterization of P-
type afferent responses. This analysis helps explain the variability in
previously reported adaptation time constants in gymnotid P-type afferents
which have ranged from 0.2 to 3.4 ? s. 5. We tested the linearity of P-type
afferent responses using positive and negative AM steps of varying amplitudes.
Aside from non-linearities associated with rectification (firing rates cannot
be negative) and saturation (firing rates cannot exceed the EOD frequency), we
found that P-type afferent responses scaled linearly with stimulus amplitude.
6. Based on the observed linearity, we predict the frequency domain response
characteristics of P-type afferents and find that the predicted gain and phase
are in good agreement with experimental measurements using sinusoidal AM
stimuli over a range of AM frequencies from 1 to 100 Hz. Thus the logarithmic
parameterization of the step response appears to accurately capture the
response dynamics of P-type afferents over a wide range of behaviorally-
relevant AM frequencies. 7. We conclude that the temporal filtering properties
of pyramidal cells in the medullary electrosensory nucleus, the electrosensory
lateral line lobe (ELL), need to be reevaluated in light of the logarithmic
adaptation time course in the periphery and we discuss implications for the
role of P-type afferents in driving a feedback gain control mechanism that
regulates ELL pyramidal cell responsiveness .
Received 2 February 1996; accepted in final form 14 April 1996.
APS Manuscript Number J85-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 May 96