Perception, perfusion and posture

Abstract

Upright posture is a fundamental and critical human behaviour that places unique demands on the brain for motor and cardiovascular control. This thesis examines broad aspects of sensorimotor and cardiovascular function to examine their interdependence, with particular emphasis on the physiological processes operating during standing. How we perceive the forces our muscles exert was investigated by contralateral weight matching in the upper limb. The muscles on one side were weakened to about half their strength by fatigue and by paralysis with curare. In two subjects with dense large-diameter neuropathy, fatigue made lifted objects feel twice as heavy but in normal subjects it made objects feel lighter. Complete paralysis and recovery to half strength also made lifted objects feel lighter. Taken together, these results show that although a signal within the brain is available, this is not how muscle force is normally perceived. Instead, we use a signal that is largely peripheral reafference, which includes a dominant contribution from muscle spindles. Unlike perception of force when lifting weights, it is shown that we perceive only about 15% of the force applied by the legs to balance the body when standing. A series of experiments show that half of the force is provided by passive mechanics and most of the active muscle force is produced by a sub-cortical motor drive that does not give rise to a sense of muscular force through central efference copy or reafference. In addition to feeding perceptual centres of the brain, an efference copy of the motor drive also feeds cardiovascular centres to generate a pressor response during volitional contractions, the effect of sub-cortical motor output on the central pressor response was determined. It is shown that the postural leg muscles depend strongly on their high perfusion pressure during orthostatic posture but because the motor drive from the balance system does not generate a central pressor response they do not receive the benefit of increased perfusion pressure to offset the loss of contractility as they fatigue.