Realistic Simulation of the Aplysia Siphon-Withdrawal Reflex Circuit: Roles of Circuit Elements in Producing Motor Output. Lieb Jr., J.R. and W.N. Frost. Department of Neurobiology and Anatomy, University of Texas Houston Health Science Center, Houston, Texas 77225.
APStracts 3:0248N, 1996.
The circuitry underlying the Aplysia siphon-elicited siphon-withdrawal reflex has been widely used to study the cellular substrates of simple forms of learning and memory. Nonetheless, the functional roles of the different neurons and synaptic connections modified with learning have yet to be firmly established. In this study, we constructed a realistic computer simulation of the best understood component of this network, in order to better understand how the siphon-withdrawal circuit works. We used an integrate-and-fire scheme to simulate four neuron types (LFS, L29, L30, L34) and ten synaptic connections. Each of these circuit components was individually constructed to match the mean or typical example of its biological counterpart, based on group measurements of each circuit element. Once modeled, the free parameters of each were fixed and not subject to further manipulation. The biological LFS motor neurons respond to sensory input with a brief phasic burst followed by a long-lasting period of tonic firing. We found that the LFS neurons in the model network responded to sensory input in a qualitatively similar fashion. By selectively removing different circuit elements we determined their respective contributions to the phasic and tonic components of the LFS firing pattern. We found that: 1) the monosynaptic sensory neuron-to-motor neuron pathway contributed only to the phasic component, 2) the polysynaptic pathway (specifically the slow components of the L29-LFS fast/slow EPSPs) determined the overall duration of the tonic component, 3) the inhibitory L30 neurons exerted a significant braking action on the flow of excitatory information through the circuit, but lost their ability to reduce the duration of LFS firing at high stimulus intensities due to the intrinsic nature of the L30 current vs. frequency relationship, and 4) two circuit components, L34, and the electrical coupling between L29 and L30, were found to have little impact when subtracted from the network. These results represent a detailed dissection of the functional roles of the different elements of the siphon- elicited siphon-withdrawal circuit in Aplysia . Because many vertebrate and invertebrate circuits perform similar tasks and contain similar information processing elements, aspects of these results may be of general significance for understanding the function of motor networks. In addition, because several sites in this network store learning-related information, these results are relevant for elucidating the functional significance of the distributed storage of learned information in Aplysia .

Received 13 May 1996, accepted in final form 1 November 1996.
APS Manuscript Number J390-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996