THE CONTROL OF POSTURE BY THE CENTRAL NERVOUS SYSTEM

Abstract

This paper, in itself a review, describes the 2 components of the walking reflex, one dynamic, the other static, progression and posture. Theories of the neural mechanisms utilized in stepping movements are briefly considered. The pathways in the central nervous system involved in the postural reflex are poorly understood and a mass of rather conflicting data has to be weighed without giving any very convincing evidence. Sherrington''s studies of the characteristics of the decerebrate preparation opened this subject for modern investigation. The source of the afferent impulses underlying tone is found in the proprioceptive endings in the muscles themselves. Magnus has placed particular stress on the importance of impulses from the vestibular mechanism and the deep musculature of the neck. The portion of the central nervous system where these impulses are correlated into reflex patterns is not clearly understood. Certainly, in mammals, these correlations are made in the brain stem and very little in the spinal cord. Many experiments must be considered that weigh the importance of the cerebellum, mid-brain and fore-brain in the production and control of postural tone. These experiments, stimulating or removing portions of the nervous system, aim either at increasing or decreasing postural tone. Technical difficulties in these investigations arise, in general, from 2 causes. First, the pathways producing and those inhibiting or controlling the postural reflex may be both stimulated or injured by the same procedure, for example by severing the superior peduncle of the cerebellum. And again, the reflex arcs involved undoubtedly form long circuits which may be stimulated or broken at many points with the same equivocal results. Moreover, different investigators, each working on isolated phases of this same problem do not agree as to a definition of decerebrate rigidity. Decerebrate rigidity must include all the physiological abnormalities resulting from a transection of the brain stem at a definite level. It is unfortunate that no satisfactory quantitative method of measuring extensor tonus is applicable to many of the experimental procedures. Almost all portions of the central nervous system appear to have a part in the nice control of the postural reflex and with the progressive development of the brain this control is constantly assumed by higher levels.