Motor Control And
The Cerebellum
Motor control is defined as the control of movements by the
body. These movements can be both influenced and guided by the many sensory
inputs that are received, or can be triggered by sensory events. They can also
be triggered by the need to move using internal mechanisms. The major division
of the body into sensory and motor functions is artificial, because almost all
motor areas in the central nervous system (CNS) receive sensory inputs.
The organization and physiology of
motor systems have been represented as a number of hierarchical structures,
but these must be viewed with caution, as they are again artificial and, by
necessity, oversimplified.
Voluntary movements can be
summarized as follows. Exactly where the idea of a movement is initiated is
unknown, but it is thought to be in the areas of the cortex other than the
primary sensory or primary motor cortices (the association cortex) and
possibly the basal ganglia. At this stage, sensory information relating
to the intended movement is analysed in the posterior parietal cortex.
This sensory information mainly comes from the visual and sensory
cortex.
The posterior parietal cortex
activates the supplementary motor area and the premotor cortex.
This excitation also causes the basal ganglia loop and the cerebrocerebellar
loop to be excited and to lead to a degree of amplitude setting and
coordination of the activity. The supplementary motor area and the premotor
cortex then initiate activity in the motor cortex. In addition, the
premotor cortex initiates, via the anterior corticospinal tract and the
connections to the brain stem ventromedial pathways, any postural
adjustments needed for the movement.
Figure 59d shows the anatomical
sites of the principal motor and sensory centres, and Figure 59c shows the
relative size of the areas in the motor cortex represented by the different
parts of the body (the motor homunculus).
The term upper motor neurones refers
to those neurones that are wholly in the CNS motor pathways. These descending
motor pathways are divided into the pyramidal tracts, which originate in
the cerebral cortex, and the extrapyramidal tracts, which originate in
the brain stem. The pyramidal tracts descend through the internal
capsule and terminate in the brain stem. One small group of fibres (the corticobulbar
tract) terminates on cranial motor nuclei and is involved in controlling
eye, facial and masticatory muscles. Another larger group of fibres (the corticospinal
tract) descends directly from the cortex to the grey matter of the spinal
cord but, as it passes through the brain stem, it divides into two. Approximately 85% of the fibres cross
over the midline (decussate)
and descend as the lateral corticospinal tract, terminating directly
onto the α- and fusi-motor neurones. Some of the fibres do not terminate
directly onto the motor neurones but excite interneurones instead. These
interneurones can be either excitatory or inhibitory in nature.
The other 15% of
corticospinal neurones, the anterior corticospi- nal tract, do not
decussate and remain ipsilateral, eventually terminating in the upper thoracic
spinal cord, and project bilaterally onto the motor neurones and interneurones
that innervate the muscles of the upper trunk and neck.
The extrapyramidal tract neurones
project to the spinal cord, where they synapse mainly onto interneurones. There
are two groups: the ventromedial pathways, which terminate in the motor
pools of the axial and proximal limb muscles, and the dorsolateral pathways,
which terminate in the motor pools of the distal limb muscles. The ventromedial
pathways comprise the vestibulospinal tract, which receives neurones
from the vestibular system and is involved in the reflex control of balance,
the tectospinal tract, which is involved in the coordination of eye and
body movements, and the reticulospinal tract, which is concerned with
regulating the excitability of extensor muscle reflexes. The dorsolateral
pathways comprise mainly the rubrospinal tract, which originates in
the red nucleus in the mid- brain and projects to similar motor
neurone pools as those served by the corticospinal tracts, and are involved
with the reflex control of flexor muscles.
The cerebellum
The cerebellum is anatomically
distinct from the rest of the brain and is connected to the brain stem by thick
strands of afferent and efferent fibres through three (cerebellar)
peduncles. Its primary function is the coordination and learning of
movements, and it is made up of three functional and anatomical structures: the
spinocerebellum, which is involved in the control of musculature and
posture; the cerebrocerebellum, which is involved in the coordination
and planning of limb movement; and the vestibulocerebellum, which is
involved with posture and the control of eye movements. The spinocerebellum receives
both sensory inputs from the spinal cord and motor inputs from the cerebral
cortex. It regulates ongoing movements of axial and distal muscles, by
comparison of the descending inputs with the ascending sensory feedback, and
regulates muscle tone. The cere- brocerebellum receives inputs from the
cerebral cortex, particularly the premotor cortex, and is primarily involved in
the planning and initiation of movements, particularly involving the visual
system. The vestibulocerebellum receives inputs and sends outputs to the
vestibular nuclei in the medulla, and is involved in the regulation of balance,
posture and the control of eye movements.
The cerebellum functions by acting
as a comparator, comparing sensory and motor inputs and achieving
coordinated movements that are both smooth and accurate. It can also function
as a timing device in which it converts descending motor signals into a
sequence of coordinated and smooth events. Finally, it can store motor
information and regularly update it; therefore, given the right sequence of
events, it can lead to the initiation of
accurate learnt movements.
