The CPGs for Limbed Locomotion-Facts and Fiction.

Sten Grillner,Alexander Kozlov

doi:10.3390/ijms22115882

Sten Grillner, Alexander Kozlov

Open Access

https://doi.org/10.3390/ijms22115882

Copy DOI

Abstract

The neuronal networks that generate locomotion are well understood in swimming animals such as the lamprey, zebrafish and tadpole. The networks controlling locomotion in tetrapods remain, however, still enigmatic with an intricate motor pattern required for the control of the entire limb during the support, lift off, and flexion phase, and most demandingly when the limb makes contact with ground again. It is clear that the inhibition that occurs between bursts in each step cycle is produced by V2b and V1 interneurons, and that a deletion of these interneurons leads to synchronous flexor–extensor bursting. The ability to generate rhythmic bursting is distributed over all segments comprising part of the central pattern generator network (CPG). It is unclear how the rhythmic bursting is generated; however, Shox2, V2a and HB9 interneurons do contribute. To deduce a possible organization of the locomotor CPG, simulations have been elaborated. The motor pattern has been simulated in considerable detail with a network composed of unit burst generators; one for each group of close synergistic muscle groups at each joint. This unit burst generator model can reproduce the complex burst pattern with a constant flexion phase and a shortened extensor phase as the speed increases. Moreover, the unit burst generator model is versatile and can generate both forward and backward locomotion.

Highlights

The graceful locomotor movements that we observe as a cheetah runs after a prey or a ballet dancer performs utilize most parts of the nervous system, from the spinal cord to the cortex and cerebellum
There is a locomotor network of neurons coordinating the pattern of muscle activation in each step cycle, a central pattern generator network (CPG)
The overall conclusion so far is that we have information on many subtypes of interneurons, but no overall understanding of the integrated CPG network that coordinates the whole limb with its complex motor pattern, except that V1 and V2b interneurons are required for the generation of the flexor–extensor alternation

Summary

Introduction

The graceful locomotor movements that we observe as a cheetah runs after a prey or a ballet dancer performs utilize most parts of the nervous system, from the spinal cord to the cortex and cerebellum. There is a locomotor network of neurons coordinating the pattern of muscle activation in each step cycle, a central pattern generator network (CPG). This network is subject to a profound sensory control from receptors/afferents activated by the limb movements [3,4,5,6]. The overall conclusion so far is that we have information on many subtypes of interneurons, but no overall understanding of the integrated CPG network that coordinates the whole limb with its complex motor pattern, except that V1 and V2b interneurons are required for the generation of the flexor–extensor alternation.

Facts regarding the Limb CPG for Locomotion—Summary

Conclusions