The well-developed feedback system between the hand and the brain, with continuous proprioceptive information of position, and tactile input that is coordinated with memory systems in the brain, is a prerequisite for regulation of grip force and grip speed. 16 The feedback from a functioning sense of touch is also essential for body awareness. 17
Sensory perception is based on stimuli of mechanoreceptors and free nerve endings in the skin, resulting in an afferent signal pattern that has to be interpreted and processed in brain cortex. Sensory palsy may be a result of lesions or pathologic processes at many levels, such as sensory receptors in the hand, peripheral nerves, spinal cord, and brain cortex. The discriminative touch, one aspect of tactile gnosis, is dependent on cutaneous mechanoreceptors sensitive to pressure, vibration, or stretching. The physiology, function, and distribution of the mechanoreceptors in subepidermal, dermal, and subcutaneous layers of the volar glabrous skin of the hand have been well defined and described in numerous investigations. 18-24
Fast-adapting (FA) mechanoreceptors are sensitive to fast mechanical changes in the skin such as vibration and friction, while slowly adapting (SA) mechanoreceptors respond to sustained stimuli and skin stretch. Among FA mechanoreceptors, vibrations below 50 Hz are mediated mainly by Meissner ’ s corpuscles (FA I) located superficially in the dermal papillae. Meissner ’ s corpuscles have small peripheral, well-defined receptive fields in the fingers, with sharp borders. Vibrations above 50 Hz are mediated mainly
by the Pacinian corpuscles, which are located in the subcutaneous layers. Pacinian corpuscles represent a system of FA mechanoreceptors type II (FA II). In humans, the Pacinian corpuscles cluster close to nerves and vessels at the metacarpophalangeal joints, the total mean number in a human hand being around 300. 24
Among SA mechanoreceptors, Merkel ’ s end-organs SA type I, SA I) respond to sustained stimuli in the skin, such as pressure and stretching. These receptors respond especially to indentations that are produced when an edge cuts through the receptive fields.