THE PLANNING OF MOVEMENT BY THE CEREBRAL CORTEX
Motor Cortex
It is a region of the frontal lobe consisting of Area 4 (often referred to as primary motor cortex
or M1) and Area 6 (see Fig. 14.7). There is speculation that area 6 might be specialized for
skilled voluntary movement. It was later shown that electrical stimulation of area 6 could evoke
complex movements of either side of the body.
Wilder Penfield found two somatotopically organized motor maps in area 6:
o One in a lateral region called the premotor area (PMA)
innervate proximal motor units
o One in a medial region called the supplementary motor area (SMA)
Innervate distal motor units directly
The Contributions of Posterior Parietal and Prefrontal Cortex
A mental body image seems to be generated by somatosensory, proprioceptive, and visual
inputs to the posterior parietal cortex. Two areas of interest in the posterior parietal cortex are:
Area 5 – target of inputs from the primary somatosensory cortical areas 3, 1, and 2
Area 7 – target of higher-order visual cortical areas such as MT
Patients with lesions in these areas show bizarre abnormalities of body image and the
perception of spatial relations
The parietal lobes are extensively interconnected with regions in the anterior frontal lobe that in
humans are thought to be important for abstract though, decision making, and anticipating the
consequences of action.
These „prefrontal‟ areas and the posterior parietal cortex send axons that converge on
area 6 and represent the highest levels of the motor control hierarchy
Area 6 lies at the junction where singles encoding what actions are desired are
converted into signals that specify how the action will be carried out
Neuronal Correlates of Motor Planning
Cells in the SMA typically increase their discharge rates about a second before the execution of
a hand or wrist movement. An important feature of this activity is that it occurs in advance of the
movements of either hand.
This means that supplementary areas of the two hemispheres are closely linked via the
corpus callosum
Therefore, movement deficits following an SMA lesion on side is particularly pronounced
for tasks required the coordinated actions of two hands
In humans, a selective inability to perform complex (but not simple) motor acts is called
apraxia
Consider the expression “Ready, set, go.”
The readiness depends on activity in the parietal and frontal lobes along with
contributions from brain centers that control levels of attention and alertness “Set” may reside in the supplementary and premotor areas, where movement strategies
are devised and held until they are executed
“Go” appears to be implemented with participation of a major subcortical input to area 6.
See Figure 14.9
Weinrich and Wise monitored the discharge of a neuron in the PMA as a monkey performed a
task requiring a specific arm movement to a target
The neuron in the PMA began firing if the instruction was to move the arm left and it
continued to discharge until the trigger stimulus came on and the movement was
initiated
If the instruction was to move to the right, this neuron did not fire (presumably another
population of PMA cells became active under this condition)
The activity of this PMA neuron reported the direction of the upcoming movement and continued
to do so until the movement was made
THE BASAL GANGLIA
The major subcortical input to area 6 arises in a nucleus called the ventral lateral (VL) nucleus
located in the dorsal thalamus. The input to this location, called VLo, arises from the basal
ganglia which in turn are targets of the cerebral cortex (i.e. frontal, prefrontal and parietal).
There is a loop where information cycles from the cortex through the basal ganglia and
thalamus and then back to the cortex (see Fig. 14.1
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