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.
Figure 59a shows the major ascending sensory inputs and descending motor outputs, and Figure 59b shows the main looped pathways within the CNS.
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.The motor cortex, via the lateral corticospinal and corticorubrospinal tracts, then initiates the activity of the muscles. This activity is due to the excitation of both α- and fusi-motor neurones. During this movement, there is continuous feedback from receptors in the joints, muscles and skin, which can lead to fine adjustments via local spinal and brain stem reflexes. Furthermore, there is often visual feedback which can modulate the motor outputs at the cortical and cerebellar levels. Modulations of the activity at all levels continue throughout the voluntary 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 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.