Chapter 2. Prehension

222 THE PHASES OF PREHENSION
which, in turn, can lead to sweat, due to the thermal properties of the
system. At the same time, mechanoreceptors provide information for
possible sudosensory integration, also leading to eccrine gland
secretion. Either way, the mechanical properties of sweat on skin, as
a boundary lubricator, are creating adhesion and preventing slip, thus
enhancing the frictional component of the force without adding extra
wear and tear on the fingers. Pursuing the analogy to gripping by
automobile tires, a slight dampness improves the grip of both hand
and tire, while wetness leads to a loss of grip in both. Napier (1980)
noted that sweating is the most important feature which complements
the mechanical and sensory functions of the papillary ridges.
6.1.3 Sensory receptors and afferent control by the central
nervous system
Skin, muscles and joints contain receptors that transduce natural
stimulus energy into electrical energy. It is of considerable historical
import that in the nineteenth and twentieth centuries, physiologists,
psychologists and neuroscientists who have studied these receptors
have tended to focus on either skin receptors as critical for
somatosensory processes and exteroception, or muscle and joint
receptors as critical for motor processes, proprioception and the
control of movement. Notable exceptions to this include Vallbo,
Johansson, Westling, and their colleagues in Umea, Sweden, who
have focussed on cutaneous mechanoreceptors with a view to their
role in motor control. For grasping objects, skin receptors contribute
to information about skin stretch, joint motion, contact with objects,
mechanical deformations and interactions with the object in a hand
centered coordinate system. Integrated with other efferent and
receptor information, skin receptors can provide information about the
relative location, orientation, configuration and deformation of the
hand in body centered, hand centered and object centered coordinate
sy s tems.
There are about 17,000 cutaneous mechanoreceptors innervating
the glabrous skin of one hand (Vallbo & Johansson, 1984). Glabrous
skin contains at least four distinct mechanoreceptors, shown
schematically in skin in Figure 6.6. Meissner corpuscles are located
in primary or secondary dermal papillae, at limiting ridges. Ovoid in
form, their long axis runs tranverse to the skin surface. Merkel's
receptors or disks are also located in dermal papillae, at intermediate
ridges. Pacinian corpuscles are located in subcutaneous tissue, and
are comprised of onion-like rings of connective tissue surrounding the