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1 Physiol./Biophysics, Mt. Sinai Med Schl, New York, New York, USA; Phase Five Communications Inc., New York, New York, USA
2 Physiol./Biophysics, Mt. Sinai Med Schl, New York, New York, USA
* To whom correspondence should be addressed. E-mail: elizabeth.cropper{at}mssm.edu.
During rhythmic behaviors sensori-motor transmission is often regulated so that there are phasic changes in afferent input to follower neurons. We study this type of regulation in the feeding circuit of Aplysia. We characterize effects of the B4/5 interneurons on transmission from the mechanoafferent, B21, to the radula closer motor neuron B8. In quiescent preparations, B4/5 induced PSPs can block spike propagation in the lateral process of B21 and inhibit afferent transmission. B4/5 are, however, active during the retraction phase of motor programs, i.e., when mechanoafferent transmission to B8 presumably occurs. To determine whether mechanoafferent transmission is necessarily inhibited when B4/5 are active, we characterize the B4/5 firing frequency during retraction and show that, for the most part, it is low (below 15 Hz). There is, therefore, a low probability that spike propagation will be inhibited. The relative ineffectiveness of low frequency activity isnot simply a consequence of insufficient PSP magnitude because a single PSP can block spike propagation. Instead, it is related to the fact that PSPs have a short duration. When B4/5 fire at a low frequency, there is, therefore, a low probability that afferent transmission in the lateral process of B21 can be inhibited. In conclusion, we demonstrate that afferent transmission will not always be affected when a neuron that exerts inhibitory effects is active. Although a cell may be ineffective when it fires at a low frequency, ineffectiveness is not necessarily a consequence of spike frequency per se. Instead it may be due to spike timing.
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