Gravity mechanisms in tonic motor system. Neurophysiological and muscle aspects

Abstract

Nowadays it is widely believed that the animal motor system historically evolved under the powerful pressure of gravity forces. By the late 1970s, many manifestations of the microgravity effects on the motor system had already been known. At the same time, the basic sensorimotor relationships during exposure to zero-gravity remained unexplored. The article considers the main results of the studies of the scientific school of I.B. Kozlovskaya regarding the roles of the gravitational forces in the functioning of the tonic motor system in humans and other mammals. In these studies, it was demonstrated that the muscle tonic system is relatively independent and possesses its own structures and mechanisms at every level—from receptors to effectors. The support afferent input plays the main role in the regulation of the postural tonic system. The withdrawal of the support afferentation leads to the decline of the tonic motor units activity in extensor muscles and the alteration of the motor units recruitment patterns in the spinal cord. The decline of the tonic activity of the extensor motoneurons triggers on the development of the sensorimotor effects of microgravity including atony and muscle atrophy.