Conceptualizing the mammalian locomotor central pattern generator with modelling

Abstract

Presently, the internal organization of the mammalian locomotor central pattern generator (CPG) is unknown due to the difficulty in identifying and localizing interneurones involved in the network. The CPG was initially thought to be composed of half-centres, which set the basic locomotor rhythm by generating alternating excitation of antagonist motoneurone (MN) pools (e.g. flexors and extensors) via reciprocal inhibition (see Rybak et al. 2006 for a discussion). While this reproduced alternating activity of antagonists it failed to account for various recruitment patterns possible during locomotion. This led to a suggestion that the CPG consisted of multiple, coupled, unit burst generators allowing for flexible recruitment of MNs (Grillner, 1981). However, others stated that half-centres could likewise produce multiple recruitment patterns if additional interneuronal circuits were interposed between networks responsible for rhythm generation and MN activation (Perret, 1983). As a result, which theory better approximates the locomotor CPG has remained contentious. To circumvent the problem of identifying ensembles of neurones involved in locomotor rhythm generation, a method clearly more amenable to conceptualize the functional organization of the mammalian locomotor CPG is to employ ‘in silico’ simulations, using available experimental data, as recently done in an issue of The Journal of Physiology (Rybak et al. 2006). Whether the model by Rybak and colleagues provides an accurate description of the locomotor CPG is unknown but it offers a theoretical framework to refine and test hypotheses regarding the functional organization of the CPG. The purpose of this short review is to briefly describe and discuss some of the model's underlying assumptions, to highlight some key findings, and make a few recommendations.