Chapter 2. Prehension

228 THE PHASES OF PREHENSION
different numbers of lamellae. We saw earlier that cutaneous
paciniform receptors also are found subcutaneous to the glabrous,
palmar skin. These fast adapting paciniform corpuscles, both in skin
and joints, may be conveying information related to compliance, both
in passive palmar tissue and in joint stiffness, as a function of
cocontraction activity. As well, they may provide information about
motion and reactive forces during object manipulation.
Proprioceptors provide information about the relative position of
body segments to one another and about the position of the body in
space, including information about mechanical displacements of
muscles and joints. Integrated with other efferent and receptor
information, joint and muscle receptors are critically important during
both the free motion phase discussed in Chapter 5 and the compliant
motion, force generating phase, of interest here in Chapter 6. When
the hand is in contact with objects, proprioceptors contribute to
information about both object and body: skin stretch, joint motion,
mechanical deformations, compliance, geometry of contact, and
interactions with the object in a hand-centered coordinate system. It is
possible that the functional roles of joint, muscle and skin receptors
are fundamentally different during free motion and compliant motion
phases. Thus, there may be differences in the information
communicated by their afferent activity during the two phases, in the
patterns of integration of these sensory signals at segmental levels and
with descending motor commands, hence in the patterns of motor unit
recruitment, and control of muscles by the CNS.
6.2 Active Touch and Sensorimotor Integration
Phillips (1986, p. 1) refers to the the glabrous skin of the
fingertips as the somesthetic macula12, noting that “our explorations of
tactual and visual space require active movements of the ‘sensifacient
fields’ to centre their respective maculae on objects of interest”.
Comparisons and contrasts between the eye and the hand were earlier
traced to Charles Bell (1833), who discussed what is currently known
as ‘foveation’ in which there is active control of the eye to place an
object on the fovedmacula of the retina. Such comparisons and
contrasts should be expanded further, including the roles of reflexes,
postural support, automatic and voluntary control, efference copy or
outflow, somatic and autonomic innervation, accommodation and
12The macula lutea of the eye contains the fovea which is the area of greatest
visual acuity.