The objective of this study was to investigate the effects of extended suprathreshold vibratory stimulation on the sensitivity of slowly adapting type 1 (SA1), rapidly adapting (RA), and Pacinian (PC) afferents. To that end, an algorithm was developed to track afferent absolute (I0) and entrainment (I1) thresholds as they change over time. We recorded afferent responses to periliminal vibratory test stimuli, which were interleaved with intense vibratory conditioning stimuli during the adaptation period of each experimental run. From these measurements, the algorithm allowed us to infer changes in the afferents' sensitivity. We investigated the stimulus parameters that affect adaptation by assessing the degree to which adaptation depends on the amplitude and frequency of the adapting stimulus. For all three afferent types, I0 and I1 increased with increasing adaptation frequency and amplitude. The degree of adaptation seems to be independent of the firing rate evoked in the afferent by the conditioning stimulus. In the analysis, we distinguished between additive adaptation (in which I0 and I1 shift equally) and multiplicative effects (in which the ratio I1/I0 remains constant). RA threshold shifts are almost perfectly additive. SA1 threshold shifts are close to additive and far from multiplicative (I1 threshold shifts are twice the I0 shifts). PC shifts are more difficult to classify. We used an integrate-and-fire model to study the possible neural mechanisms. A change in transducer gain predicts a multiplicative change in I0 and I1 and is thus ruled out as a mechanism underlying SA1 and RA adaptation. A change in the resting action potential threshold predicts equal, additive change in I0 and I1 and thus accounts well for RA adaptation. A change in the degree of refractoriness during the relative refractory period predicts an additional change in I1 such as that observed for SA1 fibers. We infer that adaptation is caused by an increase in spiking thresholds produced by ion flow through transducer channels in the receptor membrane. In a companion paper, we describe the time-course of vibratory adaptation and recovery for SA1, RA, and PC fibers.
. Timecourse of vibratory adaptation and recovery in cutaneous mechanoreceptive afferents. J Neurophysiol 94: 3037-3045, 2005; doi:10.1152/jn.00001.2005. Extended suprathreshold vibratory stimulation applied to the skin results in a desensitization of cutaneous mechanoreceptive afferents. In a companion paper, we describe the dependence of the threshold shift on the parameters of the adapting stimulus and discuss neural mechanisms underlying afferent adaptation. Here we describe the time-course of afferent adaptation and recovery. We found that absolute and entrainment thresholds rise and fall exponentially during adaptation and recovery with time constants that vary with fiber type. slowly adapting type I (SA1) afferents adapt most rapidly, and pacinian (PC) afferents adapt most slowly, whereas rapidly adapting (RA) afferents exhibit intermediate rates of adaptation; SA1 fibers also recover more rapidly from adaptation than RA and PC fibers. We also showed that threshold adaptation is accompanied by a shift in the timing of the spikes within individual cycles of the adapting stimulus (i.e., a shift in the impulse phase). We invoked an integrate-and-fire model to explore possible mechanisms underlying afferent adaptation. Finally, we found that the time-course of afferent adaptation is more rapid than that of its psychophysical counterpart, as is the time-course of recovery from adaptation, suggesting that central factors play a role in the psychophysical phenomenon. I N T R O D U C T I O NExtended exposure to a suprathreshold vibratory stimulus applied to the skin results in a reversible decrement in the sensitivity of cutaneous mechanoreceptive afferents (Bensmaïa et al. 2005). In a companion paper, we discussed the dependence of afferent adaptation on the parameters of the conditioning stimulus. Specifically, we showed that the shift in the absolute (I 0 ) and entrainment (I 1 ) thresholds increases as the adapting amplitude or frequency increases. Furthermore, as the effects of adaptation on I 0 and I 1 were found to be approximately additive, particularly in rapidly adapting (RA) fibers, we speculated that adaptation operates on the spiking threshold of the afferent rather than on the transducer sensitivity of the receptor.The time-course along which afferent adaptation and recovery operate is largely unknown, in part because the phenomenon itself has been difficult to study for reasons discussed in the companion paper (Bensmaïa et al. 2005). Recording from RA afferents in monkeys and cats, Whitsel et al. (2000) observed a slight drop in mean spike rate within the first few seconds of a vibratory stimulus. The observed decline in responsivity was accompanied by a slight increase in the phase angle of the entrained response within the first 100 -500 ms of the stimulus. Recording the responses from PC fibers in cat mesentery, O'Mara et al. (1988) found that the spike rates recovered to their preadaptation levels along approximately exponential time-courses, with time constants Ͻ30 s. In this study, we investi...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.