The vestibular apparatus, is the sensory organ for detecting sensations of equilibrium.
It is encased in a system of bony tubes and chambers located in the petrous portion of the temporal bone, called the bony labyrinth.Within this system are membranous tubes and chambers called the membranous labyrinth. The membranous labyrinth is the functional part of the vestibular apparatus It is composed mainly of the cochlea (ductus cochlearis); three semicircular canals; and two large chambers, the utricle and saccule. The cochlea is the major sensory organ for hearing and has little to do with equilibrium. However, the semicircular canals, the utricle, and the saccule are all integral parts of the equilibrium mechanism.
Function of the Utricle and Saccule in the Maintenance of Static Equilibrium
It is especially important that the hair cells are all oriented in different directions in the maculae of the utricles and saccules, so that with different positions of the head, different hair cells become stimulated.The “patterns” of stimulation of the different hair cells apprise the brain of the position of the head with respect to the pull of gravity. In turn, the vestibular, cerebellar,
and reticular motor nerve systems of the brain excite appropriate postural muscles to maintain proper equilibrium.This utricle and saccule system functions extremely effectively for maintaining equilibrium when the head is in the near-vertical position. Indeed, a person
can determine as little as half a degree of dysequilibrium when the body leans from the precise upright position.
“Predictive” Function of the Semicircular Duct System in the Maintenance of Equilibrium.
Because the semicircular ducts do not detect that the body is off balance in the forward direction, in the side direction, or in the backward direction, one might ask:What is the semicircular ducts’ function in the maintenance of equilibrium? All they detect is that the person’s head is beginning
or stopping to rotate in one direction or another. Therefore, the function of the semicircular ducts is not to maintain static equilibrium or to maintain equilibrium during steady directional or rotational movements. Yet loss of function of the semicircular ducts does cause a person to have poor equilibrium when attempting to perform rapid, intricate changing body movements.
We can explain the function of the semicircular ducts best by the following illustration: If a person is running forward rapidly and then suddenly begins to turn to one side, he or she will fall off balance a fraction of a second later unless appropriate corrections are made ahead of time. But the maculae of the utricle and saccule cannot detect that he or she is off balance
until after this has occurred. The semicircular ducts,however, will have already detected that the person is turning, and this information can easily apprise the central nervous system of the fact that the person will fall off balance within the next fraction of a second or so unless some anticipatory correction is made.
In other words, the semicircular duct mechanism predicts that dysequilibrium is going to occur and thereby causes the equilibrium centers to make appropriate anticipatory preventive adjustments. In this way, the person need not fall off balance at all before he or she begins to correct the situation.
Removal of the flocculonodular lobes of the cerebellum prevents normal detection of semicircular duct signals but has less effect on detecting macular signals.
It is especially interesting that the cerebellum serves as a “predictive” organ for most rapid movements of the body, as well as for those having to do with equilibrium.