ABSTRACT
Perception of the orientation of the body and of its movement within a spatial co-ordinate system, having six degrees of freedom (three linear and three angular) is achieved through the integration of information provided by several sensory systems. Pre-eminent is vision, but in darkness humans can still determine their orientation to the gravitational vertical, maintain their postural equilibrium, detect whole-body movements and be aware of the relative position and movement of their limbs. The non-visual systems that contribute to the perception of orientation and movement of the body embrace the vestibular, somatosensory, proprioceptive and auditory systems. The transduction of the physical stimulus in all these sensory systems is achieved by mechanoreceptors, though each sensory system exhibits a high degree of morphological and functional adaptation to specific physical stimuli. Thus, the vestibular receptors are functionally adapted to transduce angular and linear accelerations of the head; the somatosensory receptors transduce forces acting, principally, on the skin and hence the acceleration acting on the body through the surfaces in which it is in contact; proprioceptive receptors transduce muscle length and tension, and the cochlear receptors respond to oscillatory changes in ambient pressure.
