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Received August 4, 2006
Revised September 21, 2006
Accepted after revision September 24, 2006
1 Department of Neurobiology and Anatomy, Drexel University
2 Spinal Cord Research Centre and Department of Physiology, University of Manitoba
3 Kogan Research Institute for Neurocybernetics, Rostov State University, Russia
* To whom correspondence should be addressed. E-mail: dave{at}scrc.umanitoba.ca.
A computational model of the mammalian spinal cord circuitry incorporating a two-level central pattern generator (CPG) with separate half-centre rhythm generator (RG) and pattern formation (PF) networks has been developed from observations obtained during fictive locomotion in decerebrate cats. Sensory afferents have been incorporated in the model to study the effects of afferent stimulation on locomotor phase switching and step cycle period and on the firing patterns of flexor and extensor motoneurones. Here we show that this CPG structure can be integrated with reflex circuits to reproduce the reorganization of group I reflex pathways occurring during locomotion. During the extensor phase of fictive locomotion, activation of extensor group I afferents increases extensor motoneurone activity and prolongs the extensor phase. This reflex prolongation may occur with or without a resetting of the locomotor cycle. These effects are reproduced in the model by controlling the degree to which sensory input affects the RG and PF circuits respectively. The same stimulation delivered during flexion produces a temporary resetting to extension without changing the timing of following locomotor cycles. The model reproduces this behaviour by suggesting that this sensory input influences the PF network without affecting the RG. The model also suggests that the different effects of flexor afferent stimulation observed experimentally (phase prolongation vs. resetting) result from opposing influences of group I and II flexor afferents on the PF and RG circuits controlling the activity of flexor and extensor motoneurones. The results of modelling provide insights into proprioceptive control of locomotion.
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