Function of the Cerebellum in Overall Motor Control

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The nervous system uses the cerebellum to coordinate motor control functions at three levels, as follows: 1. The vestibulocerebellum. This consists principally of the small flocculonodular cerebellar lobes (that lie under the posterior cerebellum) and adjacent portions of the vermis. It provides neural circuits for most of the body's equilibrium movements.

2. The spinocerebellum. This consists of most of the vermis of the posterior and anterior cerebellum plus the adjacent intermediate zones on both sides of the vermis. It provides the circuitry for coordinating mainly movements of the distal portions of the limbs, especially the hands and fingers.

3. The cerebrocerebellum. This consists of the large lateral zones of the cerebellar hemispheres, lateral to the intermediate zones. It receives virtually all its input from the cerebral motor cortex and adjacent premotor and somatosensory cortices of the cerebrum. It transmits its output information in the upward direction back to the brain, functioning in a feedback manner with the cerebral cortical sensorimotor system to plan sequential voluntary body and limb movements, planning these as much as tenths of a second in advance of the actual movements. This is called development of "motor imagery" of movements to be performed.

Vestibulocerebellum—Its Function in Association with the Brain Stem and Spinal Cord to Control Equilibrium and Postural Movements

The vestibulocerebellum originated phylogenetically at about the same time that the vestibular apparatus in the inner ear developed. Furthermore, as discussed in Chapter 55, loss of the flocculonodular lobes and adjacent portions of the vermis of the cerebellum, which constitute the vestibulocerebellum, causes extreme disturbance of equilibrium and postural movements.

We still must ask the question, what role does the vestibulocerebellum play in equilibrium that cannot be provided by other neuronal machinery of the brain stem? A clue is the fact that in people with vestibulo-cerebellar dysfunction, equilibrium is far more disturbed during performance of rapid motions than during stasis, especially so when these movements involve changes in direction of movement and stimulate the semicircular ducts. This suggests that the vestibulocerebellum is especially important in controlling balance between agonist and antagonist muscle contractions of the spine, hips, and shoulders during rapid changes in body positions as required by the vestibular apparatus.

One of the major problems in controlling balance is the amount of time required to transmit position signals and velocity of movement signals from the different parts of the body to the brain. Even when the most rapidly conducting sensory pathways are used, up to 120 m/sec in the spinocerebellar afferent tracts, the delay for transmission from the feet to the brain is still 15 to 20 milliseconds. The feet of a person running rapidly can move as much as 10 inches during that time. Therefore, it is never possible for return signals from the peripheral parts of the body to reach the brain at the same time that the movements actually occur. How, then, is it possible for the brain to know when to stop a movement and to perform the next sequential act, especially when the movements are performed rapidly? The answer is that the signals from the periphery tell the brain how rapidly and in which directions the body parts are moving. It is then the function of the vestibulocerebellum to calculate in advance from these rates and directions where the different parts will be during the next few milliseconds. The results of these calculations are the key to the brain's progression to the next sequential movement.

Thus, during control of equilibrium, it is presumed that information from both the body periphery and the vestibular apparatus is used in a typical feedback control circuit to provide anticipatory correction of postural motor signals necessary for maintaining equilibrium even during extremely rapid motion, including rapidly changing directions of motion.

Spinocerebellum—Feedback Control of Distal Limb Movements by Way of the Intermediate Cerebellar Cortex and the Interposed Nucleus

As shown in Figure 56-8, the intermediate zone of each cerebellar hemisphere receives two types of information when a movement is performed: (1) information from the cerebral motor cortex and from the midbrain red nucleus, telling the cerebellum the intended sequential plan of movement for the next few fractions of a second, and (2) feedback information

Motor cortex

Motor cortex

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Essentials of Human Physiology

Essentials of Human Physiology

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