Biological data questions the support of the self inhibition required for pattern generation in the half center model

Abstract

Locomotion control in mammals has been hypothesized to be governed by a central pattern generator (CPG) located in the circuitry of the spinal cord. The most common model of the CPG is the half center model, where two pools of neurons generate alternating, oscillatory activity. In this model, the pools reciprocally inhibit each other ensuring alternating activity. There is experimental support for reciprocal inhibition. However another crucial part of the half center model is a self inhibitory mechanism which prevents the neurons of each individual pool from infinite firing. Self-inhibition is hence necessary to obtain alternating activity. But critical parts of the experimental bases for the proposed mechanisms for self-inhibition were obtained in vitro, in preparations of juvenile animals. The commonly used adaptation of spike firing does not appear to be present in adult animals in vivo. We therefore modeled several possible self inhibitory mechanisms for locomotor control. Based on currently published data, previously proposed hypotheses of the self inhibitory mechanism, necessary to support the CPG hypothesis, seems to be put into question by functional evaluation tests or by in vivo data. This opens for alternative explanations of how locomotion activity patterns in the adult mammal could be generated.

A model for self-organization of sensorimotor function: spinal interneuronal integration

“…The steadily increasing number of subtypes of neurons identified via transcriptomes ( 74 , 75 ) may account for the many functions that were not part of our simulations such as ascending connectivity and autonomic regulation. Similarly, we did not attempt to model interneurons that constitute locomotor central pattern generators, which have been associated with distinctive transcriptomes ( 76 ) but whose circuit function remains contentious ( 77 ). Within those organizing rules of nature, however, there is room for muscle-by-muscle adaptation of interneuronal connectivity to reflect the mechanical dynamics of the musculoskeletal systems as determined by the experiences of nurture.…”

Whole Body Coordination for Self-Assistance in Locomotion

“…These species obviously have bodies of very different biomechanical properties, different movement patterns, and consequently, they require different neuromechanical pattern generators. This is hard to explain without assuming that the spinal cord circuitry ‘phenotype’ is to a large extent shaped by the spinal neuronal network learns plant biomechanics as an essential part of its early development (Kohler et al, 2020 ; Enander et al, 2021 , 2022 ). This can be achieved by a neural network that initially is a passive follower to the anatomically predefined biomechanical dynamics and corresponding sensor activation patterns during locomotion.…”

The Bcm rule allows a spinal cord model to learn rhythmic movements

“… 2014 ; Kohler et al. 2020 ). Furthermore, the normal locomotor behavior of animals in nature is much more varied and flexible to the circumstances of the terrain than the current CPG models can explain.…”

“…However, without drug manipulation, spinal neurons do not intrinsically generate rhythms in adult animals in vivo (Spanne etal. 2014;Kohler et al 2020). Furthermore, the normal locomotor behavior of animals in nature is much more varied and flexible to the circumstances of the terrain than the current CPG models can explain.…”