Role of the motor cortex in the control of visually triggered gait modifications

Abstract

One important aspect of locomotor control is the ability of an animal to make anticipatory gait modifications to avoid obstacles, by stepping either around them or over them. This paper reviews some of the evidence that suggests that the motor cortex is one of the principal structures involved in the control of such anticipatory gait modifications in cats, in particular when they are triggered by a visual signal. Evidence for this statement is provided both from experiments in which the motor cortex has been lesioned or inactivated and from studies in which the activity of motor cortical neurones has been recorded during locomotor tasks in which visual information is required to ensure the correct positioning of the paw or an appropriate modification of the limb trajectory. Inactivation of small regions of the motor cortex with the GABA agonist muscimol results in changes in the limb trajectory so that cats hit an obstacle instead of stepping over it as they do normally. A similar disruption of the hindlimb trajectory is seen following lesions of the spinal cord at T13 that interrupt the corticospinal tract. The results from cell recording studies are complementary in that they show that the activity of many identified pyramidal tract neurones increases when the cat is required to modify the forelimb or hindlimb trajectory to step over obstacles. We suggest that the major function of this increased discharge frequency is to regulate the amplitude, duration, and temporal pattern of muscle activity during the gait modification to ensure an appropriate modification of limb trajectory. We further suggest that different groups of pyramidal tract neurones are involved in regulating the activity of groups of synergistic muscles active at different times in the gait modification. For example, some groups of pyramidal tract neurones would be involved in ensuring the appropriate and sequential activation of the muscle groups involved in the initial flexion of the elbow, while others would be active prior to the repositioning of the paw on the support surface. We discuss the possibility that the motor cortical activity seen during locomotion is the sum result of a feedforward signal, which provides visuospatial information about the environment, and feedback activity, which signals, in part, the state of the interneuronal pattern generating networks in the spinal cord. The way in which the resulting descending command may interact with the basic locomotor rhythm to produce the gait modifications is discussed.

Forebrain control of locomotor behaviors

“…However, goal-directed locomotor activity requires non-limbic systems. Indeed, motor cortical neurons significantly increased their discharge rate when walking animals had to accurately overcome obstacles (Drew et al, 1996). This accuracy requires a visuomotor gait modification so that a precise foot placement can be achieved (Georgopoulos and Grillner, 1989).…”

Basal ganglia efferents to the brainstem centers controlling postural muscle tone and locomotion: a new concept for understanding motor disorders in basal ganglia dysfunction

“…The majority of motor cortical neurons significantly increase their discharge rate when a walking subject has to overcome obstacles accurately (Drew et al, 1996). This accuracy requires a precise, visually-initiated, gait modification, in which the subject must modify their limb trajectory for each step, so that an appropriate foot placement can be achieved (Georgopoulos and Grillner, 1989).…”

Activity- and use-dependent plasticity of the developing corticospinal system☆

“…We studied visually-guided locomotion, which has been shown in cats to depend on an intact CS tract (Drew et al 1996;Drew et al 2002). Moreover, neurons in M1 modulate their activity in association with an animal's guided responses (Drew, 1993).…”