Chapter 2. Prehension

Chapter 5 - Movement Before Contact 121
paradoxical; i.e., it fires after the motor cortex. This is puzzling
because this parietal region is believed to be upstream from the motor
cortex, yet it fires after the motor cortex, according to the upcoming
movement direction (see also Georgopoulos, 1990). Kalaska and
Crammond speculate that Area 5 in posterior parietal cortex could be
contributing simultaneously both to kinesthetic perception and to
motor control of limb kinematics.
At levels below the motor cortex, there is neural integration at the
spinal level. Movements initiated by supraspinal structures engage
spinal ‘reaching’ circuits. Propriospinal neurons are selectively en-
gaged during reaching movements and receive monosynaptic inputs
from several supraspinal sources, via corticospinal, reticulospinal,
rubrospinal and tectospinal tracts (Illert, Lundberg, Padel, & Tanaka,
1978), sending outputs to target motoneurons. In addition, they send
ascending collaterals to the lateral reticular nucleus (Alstermark,
Lindstrom, Lundberg, & Sybirska, 1981) which projects to the
cerebellum, reflecting on going activity to cerebellum about the
evolving movement. Sectioning the output of these propriospinal neu-
rons at C3-C4 levels in the cat results in abnormal reaching, with nor-
mal grasping. Similar results are obtained with removal of the corti-
cospinal input to the propriospinal neurons (Alstermark et al., 1981).
Rapid responses to visual perturbation of direction of reaching in cats
(from 83- 1 18 ms) has been attributed to descending influences on C3-
C4 from a retino- tectospinal or retino-tectoreticulospinal pathways
(Alstermark, Gorska, Lundberg & Pettersson, 1990). Related results
have been obtained with tetraplegic humans, with complete symmetri-
cal sections who have normal grasping but abnormal reaching; volun-
tary shoulder control may be used to compensate for paralysis of
muscles controlling the elbow, so that the hand can be transported to
the appropriate location (Popovic, Popovic & Tomovic, 1993;
Popovic, personal communication, May 13, 1993).
At the level of the motor command, various control variables have
been suggested in the history of motor control, such as muscle length
(Merton, 1953), muscle stiffness (Houk, 1978), and resting length
between agonist/antagonist (Feldman, 1986) (see Nelson, 1983 for an
excellent review). The mass-spring model (Bernstein, 1967; Bizzi,
1980; Feldman, 1986) argues that the CNS takes advantage of the
viscoelastic properties of muscles, which act as tunable springs. A
spring follows the mass-spring law:
F=KaX (1)