Cortical Motor Areas
A number of cortical areas are involved
with the control of movement, including the primary motor cortex (see Chapter
39), premotor cortex (PMC), supplementary motor area (SMA) and several adjacent
areas in the anterior cingulate cortex. In addition, there are other areas that
play specific roles in the cortical control of movement, including the frontal
eye fields (see Chapter 56) and posterior parietal cortex (see Chapter 34).
This chapter briefly discusses the organization of the motor cortical areas and
their relative roles in movement control, while the next chapter concentrates
on the primary motor cortex.
Primary motor cortex
The primary motor cortex (MsI)
is that part of the cerebral cortex that produces a motor response with the
minimum electrical stimulation. It corresponds to Brodmann’s area 4 and lies
just in front of the central sulcus and projects to the motor neurones (MNs) of
the brainstem via the corticobulbar tracts and to the MNs of the spinal
cord directly via the corticospinal tract (CoST) and indirectly
via the subcortical extrapyramidal tracts. Indeed, MsI is closely associated with the pyramidal tract
(even though 60–70% of it originates in other cortical areas) and so has a role
in the control of distal musculature and fine movements (see Chapter 37).
Other cortical areas
A range of other cortical areas are
involved in the control of movement, including the PMC (corresponding to
the lateral part of Brodmann’s area 6); the SMA (corresponding to the
medial aspect of Brodmann’s area 6); a number of motor areas centred on the anterior
cingulate cortex on the medial aspect of the frontal lobe; the frontal
eye fields (corresponding to Brodmann’s area 8); and the posterior
parietal cortex (especially Brodmann’s area 7).
Some of these areas have specialist
functions such as the frontal eye fields with eye movement control (see Chapter
56) and the posterior parietal cortex with the visual control of movement (see
Chapter 34). The remaining areas in the frontal lobe are involved with more
complex aspects of movement. Most of these other cortical areas therefore occupy a higher level in the motor hierarchy
than MsI, and their connections and functions are summarized in the figure and
Table 38.1 (see also Chapter 35).
The PMC refers to a specific area
of Brodmann’s area 6, and like the primary motor cortex has an input directly
to the spinal MNs via the corticospinal or pyramidal tract. This area therefore
occupies two levels of the motor hierarchy as it also has a role in the
planning of movement (see Chapter 35). In contrast, the SMA lies medial to the
PMC, and has a much more clearly defined role in the planning of movements
especially in response to sensory cues. Furthermore, it is now clear that the
SMA is part of a much larger number of higher order motor cortical areas that
lie along the medial side of the frontal cortex and which are involved in the
planning of movements more than their execution. It is these cortical areas
that receive the predominant outflow of the basal ganglia (see Chapter 41), which helps explain the
abnormal movements that are seen
with diseases of this area of the brain (see Chapter 42). For example, in Parkinson’s
disease there is a slowness and poverty of movement that is associated
with underactivation of these cortical areas, a situation that is rectified by
the administration of antiparkinsonian medication or successful neurosurgical
interventions.
