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NEUROSCIENCE |
1 Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, PA 19129, USA
2 Spinal Cord Research Centre and Department of Physiology, University of Manitoba, Winnipeg, Manitoba R3E 3J7, Canada
3 A. B. Kogan Research Institute for Neurocybernetics, Rostov State University, Rostov-on-Don 344090, Russia
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 muscle group I afferents increases extensor motoneurone activity and prolongs the extensor phase. This extensor phase prolongation may occur with or without a resetting of the locomotor cycle, which (according to the model) depends on 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 muscle nerve afferent stimulation observed experimentally (phase prolongation versus resetting) result from opposing influences of flexor group I and II 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.
(Received 4 August 2006;
accepted after revision 24 September 2006;
first published online 28 September 2006)
Corresponding author D. A. McCrea: Spinal Cord Research Centre, University of Manitoba, 730 William Avenue, Winnipeg, Manitoba, R3E3J7 Canada. Email: dave{at}scrc.umanitoba.ca
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