Chapter 2. Prehension

112 THE PHASES OF PREHENSION
and temperature. ProDrioceptors provide information about the rela-
tive position of body segments to one another and about the relative
position and motion of the body in space, including information about
mechanical displacements of muscles and joints. Proprioceptive
receptors traditionally include joint receptors, tendon receptors, and
muscle spindles. For our purposes, we exclude vestibular
contributions to proprioception, acknowledging their critical
contributions. Integrated with other efferent and receptor information,
joint and muscle receptors are critically important during both free
motion and compliant motion. Skin mechanoreceptors are also
important in feedback control for force regulation and for providing
information concerning motion about the underlying joints (skin
receptors are discussed in Chapter 6). For grasping objects, these all
contribute to information about skin stretch, joint motion, contact with
objects, object properties, compliance, mechanical deformations and
interactions with the object.
Once a task plan such as the CCP is constructed, it must be carried
out by the CNS. The neural model of Paillard, as was seen in Figure
4.4, separated the programming (trajectory planning) and execution of
the movement from the task plan. A key player between plan and
execution is the motor cortex. Over a hundred years of neurological
stimulation of cortical areas of the brain have indicated that the motor
cortex is topographically organized; that is, there is a body
representation arranged in an orderly fashion, from toes to head, as
seen in Figure 5.2. This is true for the somatosensory cortex as well.
Those parts of the body requiring fine control, such as the hands and
the face, have a disproportionately large representation. An important
question is what is the function of motor cortex and whether muscles,
movements or forces are being represented. In general, it has been
observed that single neurons can influence several muscles, and also
that individual muscles can be represented more than once in motor
cortex. For example, Strick and Preston (1979) found two
anatomically separate hand-wrist representations in the motor cortex.
Are these representations functional? Muir and Lemon (1983)
recorded neurons in the hand area of the motor cortex while recording
EMGs from the first dorsal interosseus muscle (a muscle active both in
pad and palm opposition). They found that motor cortex neurons
responded for pad opposition, but not for palm opposition. As noted
in Chapter 4, however, the motor cortex is only one of many
distributed regions in the CNS. The neural control of grasping is
examined in greater detail in this chapter.
The coordinated program suggests that a target location is sent to