Nervous mechanisms of postural control

Maintenance of body con guration 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 di erent motor behaviors and with di erent postural perturbations. Di erent aspects of postural control were considered in this study: (i) the postural motor responses during standing and locomotion, (ii) the role of supraspinal systems (corticoand reticulospinal), and (iii) the e ects of spinal cord injury on postural performance. Three animal models (lamprey, rabbit, cat), most adequate for speci c 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. Di erent damages to the spinal cord at T12 were performed in the rabbit, and their e ect 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 di erent 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 di erent: this is a re-distribution of muscle activity between the symmetrical limbs (in standing), and a recon guration 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 di erent 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 ndings suggest that the PTNs are primarily involved in the intra-limb postural coordination, i.e., in the feedback control of the projection limb