Thursday, November 26, 2009

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. 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 vestibulocerebellar
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.

ShareThis

Google+ Badge