Neuroanatomical and neurophysiological bases of postural control

At this stage of our study, we do not know yet the relative contribution of each pathway that descends from the cerebral cortex, the cerebellar nuclei, and the SN to the control of pontomedullary reticuloreticular and reticulospinal neurons. Nor do we know the resultant activation patterns of reticulospinal neurons and those of axial and proximal motoneurons in relation to postural fixation and equilibrium. In dealing with the physiology of posture, such as the maintenance or change of body position, we are dealing with the emergent properties of the total system that are not described by the sum of the properties of its parts. Learning how a system subcomponent behaves in a carefully defined and constrained set of circumstances gives us no guarantee that the subcomponent will behave according to the same rules if the circumstances were changed (59). Further behavioral studies in animal and human subjects, especially using natural posture and movements, plus studies aimed at elucidating the fine architecture of the target system, including identification of acting neurotransmitters at each synaptic level of a system subcomponent, are now needed to confirm existing hypotheses (10, 37, 47) and elaborate new ones related to the control of posture and movement.