The neural control of interlimb coordination during mammalian locomotion

Abstract

Neuronal networks within the spinal cord directly control rhythmic movements of the arms/forelimbs and legs/hindlimbs during locomotion in mammals. For an effective locomotion, these networks must be flexibly coordinated to allow for various gait patterns and independent use of the arms/forelimbs. This coordination can be accomplished by mechanisms intrinsic to the spinal cord, somatosensory feedback from the limbs, and various supraspinal pathways. Incomplete spinal cord injury disrupts some of the pathways and structures involved in interlimb coordination, often leading to a disruption in the coordination between the arms/forelimbs and legs/hindlimbs in animal models and in humans. However, experimental spinal lesions in animal models to uncover the mechanisms coordinating the limbs have limitations due to compensatory mechanisms and strategies, redundant systems of control, and plasticity within remaining circuits. The purpose of this review is to provide a general overview and critical discussion of experimental studies that have investigated the neural mechanisms involved in coordinating the arms/forelimbs and legs/hindlimbs during mammalian locomotion.

Keywords: central pattern generator; interlimb coordination; locomotion; propriospinal; somatosensory; spinal cord injury; supraspinal.

Fig. 1.

Schematic representation of the neural control of interlimb coordination. Each limb is controlled by its own spinal locomotor central pattern generator (CPG). Left-right coordination at cervical and lumbar levels is mediated by commissural interneuronal pathways. Descending and ascending propriospinal pathways, with homolateral and diagonal projections, coordinate cervical and lumbar CPGs. The propriospinal pathways can be composed of neurons with long or short axonal projections. Supraspinal inputs and somatosensory feedback from the limbs access the spinal CPGs via commissural and propriospinal pathways. The arrows can represent an excitatory or inhibitory influence. E, extensor; F, flexor; LF, left forelimb; LH, left hindlimb; RF, right forelimb; RH, right hindlimb.

Fig. 2.

Schematic representation of the effects of various spinal lesions on the neural control of forelimb-hindlimb coordination. Shown are the effects of a mid-thoracic lateral hemisection (A), contralaterally placed lateral hemisections at mid-thoracic and low thoracic levels (B), and contralaterally placed lateral hemisections at upper cervical and mid-thoracic levels (C) on the neural control of interlimb coordination depicted in Fig. 1. The dashed lines for diagonal pathways indicate a weakening of these projections with the various lesions. CPG, central pattern generator; E, extensor; F, flexor; LF, left forelimb; LH, left hindlimb; RF, right forelimb; RH, right hindlimb.