Chapter 2. Prehension

72 THE PHASES OF PREHENSION
organization and planning of the upcoming movement. These
processes include activity in motivational systems, sensory systems
and motor systems. Figure 4.3 shows how these three systems must
be coordinated during the delay interval, to process the ‘GO’ signal
and prepare the effectors. There could be some preparation of spinal
networks prior to and immediately after receipt of the visual signal to
go; that is, this model of the brain systems need not be a strictly serial
processing model whereby the stimulus must be processed prior to
any motor processing.
It is noteworthy that the motivational systems are viewed
traditionally to act through two separate, independent motor systems,
the somatic motor system and the autonomic nervous system (Kandel
& Schwartz, 1985). In the planning and control of hand movements,
attention had been directed almost exclusively to the somatic nervous
system; however, the sympathetic nervous system prepares the body
for action, from innervation of the ciliary muscles of the lens of the
eye to the eccrine sweat glands of the hand (discussed in Chapter 6).
Although the CNS consists of billions of individual nerve cells,
decades of neuroscience research have shown that they combine into
regions, some of which are shown in Figure 4.4. Kalaska and
Crammond (1992) summarize much of the research, suggesting that
each area is concerned with motor planning at a different level of ab-
straction. Sensory information from the skin and muscles can act at
multiple levels: within the spinal cord, at the same or other segmental
levels; acscending through the spinal cord and brainstem with synaptic
relays in the dorsal column nuclei and the thalamus; and in the so-
matosensory and motor areas of the cerebral cortex. Brainstem and
subcortical regions (e.g., basal ganglia, red nucleus, cerebellum) are
distinguished from regions in the cerebral cortex, the large hemi-
spheres that fill up most of the cranial cavity. In the frontal lobe of
cerebral cortex, motor areas have been distinguished, including the
primary motor cortex (Ml, or Brodmann’s Area 4), supplementary
motor cortex (MII, or SMA), and premotor cortex (PM, or Area 6).
In the parietal lobe behind the central sulcus, are found primary so-
matosensory cortex (SI, or Areas 3a, 3b, l and 2), secondary so-
matosensory cortex (SII), and posterior parietal areas (Area 5 and 7).
One way to determine the functionality of CNS preparatory pro-
cesses is by recording electrical potentials at the surface of the brain.
It has been noted that different localized areas are active during this
period prior to movement. For example, Deecke, Heise, Kornhuber,
Lang, and Lang (1984) observed changes in potentials beginning
about 800 ms before onset of movement in rapid finger or hand