Chapter 2. Prehension

314 CONSTRAINTS AND PHASES
fine, supple, and does not impede mobility in flexion. The palmar
skin, unlike the dorsal skin, is covered with epidermal ridges and
exocrine sweat glands, and contains malleable 'fat pads'. At the distal
ends of the fingers and thumb are the specialized pulps (Glicenstein &
Dardour, 1981). Shearing forces at the pulps are resisted by strong
interdigitating folds between the epidermis and dermis, where
interpapillary ridges prevent the epidermis from sliding over the
dermis (Quilliam, 1978). The dermis is further anchored to the
connective tissue around the bone of the distal phalanges with fibrous
connections. It is particularly noteworthy that the trajectory followed
by the fingers during thumb opposition brings the finger pulps into the
same plane as the thumb pulp (Kapandji, 1982). In perturbation
studies of rapid pinch movements, Cole and Abbs (1987) observed
that subjects consistently brought the finger pulps into contact
although that was not part of the task instructions. For subjects to do
this in response to the thumb perturbation, it required the reciprocal
adjustments at two index finger joints, an adjustment more complex
than a single joint one. The reason for this could be due to a higher
level goal (see Section 8.3).
In terms of the nervous system, its anatomy and physiology create
temporal and spatid physical limitations. The types of receptors (both
cutaneous and proprioceptive), and their spatial and temporal response
characteristics, serve as constraints in the control process. Studies
analyzing the motor response to tactile sensory information show low
level interactions. Torebjork et al. (1978) presented evidence that a
tonic vibration reflex can cause the fingers to flex. In this study, they
placed a small motor against the finger, causing the finger to vibrate.
They found that all subjects increased their finger flexion force against
a small plate, in a frequency-dependent way. Torebjork et al. argued
that the signals from particular skin mechanoreceptors could be
involved in such a motor response. Numerous tactile receptors are
found in the finger pulps, more so than most other parts of the body,
thus giving the CNS much information about the object with which
they come into contact (Vallbo 8z Johansson, 1984). These
mechanoreceptors are classified by their adaptation response to
sustained skin deformation and the structure of their receptive fields.
Receptors having small and well-defined receptive fields are especially
dense in the finger pulps. Westling and Johansson (1987) observed
that at higher grip forces (when the skin is less compliant) these
receptors are less responsive. In terms of proprioception, receptors in
the muscles, tendons, and joint capsules signal the CNS about the
current state of the limb (McCloskey, 1978). An interesting result