Chapter 2. Prehension

174 THE PHASES OF PREHENSION
grasp types in Chapter 2.
5.4.4 Getting to grasp: Control by the CNS
Neural pathways controlling the proximal and distal muscle groups
have a different organization. The proximal musculature used in
reaching is controlled bilaterally by descending brain stem and corti-
cospinal pathways (Brinkman & Kuypers, 1972; Lawrence &
Kuypers, 1968a, b). Examinations of complete split brain monkeys
demonstrated a clear dissociation between exclusively contralateral
control of independent finger movements by the contralateral hemi-
sphere, and bilateral control of more proximal reaching movements
(Brinkman & Kuypers, 1972). In monkeys, cutting the pyramidal
pathway does not affect reaching or collective finger movements to
viewed objects, but does disrupt control of independent finger
movements (Lawrence & Kuypers, 1968a, b). The independent
finger movements required in pad opposition are controlled via the
corticospinal tract in primates (Muir, 1985; Muir & Lemon, 1983;
Tower, 1940). For palm opposition, or grasping with all the fingers
flexing in unison to oppose the palm or palmar surface of the hand, the
intact pyramidal system is not essential (Lawrence & Kuypers, 1968a,
b). It may be that the brain stem pathways controlling collective finger
movements do not provide the same sensitivity and refinement for
motor control as the corticospinal system, nor the same fine calibration
with visual information about intrinsic object properties such as size.
With respect to the motor cortex, we discussed earlier the work of
Georgopoulos et al., who showed that a population vector represent-
ing the summation of directional preferences for individual neurons
predicted well the direction of arm movements. Relevant to grasping
is the finding that specific corticospinal neurons fire during perfor-
mance of pad opposition (precision grip) between index finger and
thumb, not during a palm opposition (power grip; Muir & Lemon,
1983). Using a cross correlational analysis between individual dis-
charges of motor cortex neurons and the electromyograms (EMGs) of
the contralateral intrinsic and extrinsic hand muscles, they identified a
subpopulation of pyramidal tract neurons principally related to the
small, intrinsic hand muscles16. These pyramidal tract neurons were
active prior to contact and during the force applying phase of pad op-
position. The motor cortex discharge in relation to forces produced
161ntrinsic hand muscles do not cross the wrist, i.e., their origins and insertions are
in the hand. Appendix A provides figures to clarify.