Nervous mechanisms of postural control

Abstract

Maintenance of body confguration and orientation in space (postural control) is a vital motor function. A general goal of the present study was to analyze nervous feedback mechanisms for the maintenance of posture during different motor behaviors and with different postural perturbations. Different aspects of postural control were considered in this study: (i) the postural motor responses during standing and locomotion, (ii) the role of supraspinal systems (cortico- and reticulospinal), and (iii) the effects of spinal cord injury on postural performance. Three animal models (lamprey, rabbit, cat), most adequate for specifc aims, were used.

Control of body orientation in horizontal plane was studied in lampreys. Important elements of the postural system in lamprey are the reticulospinal (RS) neurons. They are driven by vestibular input and transmit commands for postural corrections to the spinal cord. By means of chronically implanted electrodes, responses of RS neurons to lateral turns were studied. It was found that the majority of RS neurons are dynamically activated by a contralateral turn, and these responses are caused mainly by input from the contralateral labyrinth. It was suggested that these RS signals, when arriving to the spinal cord, will cause a turn of the lamprey in the opposite direction and thus will restore the initial orientation of the lamprey in the horizontal plane.

Impairment and recovery of postural control after spinal cord lesions were studied in rabbits. Different damages to the spinal cord at T12 were performed in the rabbit, and their effect on the postural performance was characterized for the task of standing and keeping balance on the tilting platform. It was found that postural control recovered in a few weeks after the lateral and dorsal hemisection, but did not recover after ventral hemisection, suggesting a decisive role of ventral pathways for postural function.

Lateral stability in different motor behaviors was studied in cats. Postural reactions to the same postural perturbation (lateral push) during standing and during walking were compared. It was found that the basic mechanisms for balance control in these two forms of behavior are largely different: this is a re-distribution of muscle activity between the symmetrical limbs (in standing), and a reconfguration of the base of support due to a lateral step (in walking).

Role of motor cortex in limb coordination during maintenance of equilibrium was studied in cats. The activity of pyramidal tract neurons (PTNs) of the motor cortex is known to strongly correlate with corrective responses to dynamic postural perturbations (lateral tilting of the supporting platform), suggesting that PTNs contribute to formation of these postural corrections. The corrections are caused by tilt-related somatosensory input from limbs. The aim of this study was to reveal a sensory origin of PTNs commands. In chronically instrumented cats, the activity of individual PTNs was recorded during the postural task of standing and keeping balance on the tilting platform. By suspending different numbers and combinations of limbs above the platform (abolishing tilt-related sensory input from them), it was shown that the pattern of PTN responses was determined primarily by the sensory input from the projection limb. These fndings suggest that the PTNs are primarily involved in the intra-limb postural coordination, i.e., in the feedback control of the projection limb and, to a lesser extent, in the coordination of postural activity within a girdle and between the two girdles.

Role of motor cortex in postural adaptations to the environment was studied in cats. When standing or walking on an inclined plane, cats easily adapt their posture to the support inclination, by inducing asymmetry in the confguration of the left and right limbs. The activity of individual PTNs during standing and walking on the inclined plane was recorded. A positional response (i.e., an increase of activity either with ipsi- or with contra-tilt) was observed in many PTNs, either in both tasks, or only in one of them. It was suggested that these PTNs contribute to modifcations of the limb confguration necessary for postural adaptation, and there are both common and separate cortical mechanisms underlying this adaptation in the two motor tasks.