Function of the Utricle and Saccule in the Maintenance of Static Equilibrium
It is especially important that the hair cells are all ori-ented in different directions in the maculae of the utri-cles and saccules, so that with different positions of the head, different hair cells become stimulated. The “pat-terns” 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 dysequilib-rium when the body leans from the precise upright position.
Detection of Linear Acceleration by the Utricle and Saccule Maculae. When the body is suddenly thrust forward—that is, when the body accelerates—the statoconia, which have greater mass inertia than the surrounding fluid, fall backward on the hair cell cilia, and informa-tion of dysequilibrium is sent into the nervous centers, causing the person to feel as though he or she were falling backward. This automatically causes the person to lean forward until the resulting anterior shift of the statoconia exactly equals the tendency for the stato-conia to fall backward because of the acceleration. At this point, the nervous system senses a state of proper equilibrium and leans the body forward no farther. Thus, the maculae operate to maintain equilibrium during linear acceleration in exactly the same manner as they operate during static equilibrium.
The maculae do not operate for the detection of linear velocity. When runners first begin to run, they must lean far forward to keep from falling backward because of initial acceleration, but once they have achieved running speed, if they were running in a vacuum, they would not have to lean forward. When running in air, they lean forward to maintain equilib-rium only because of air resistance against their bodies; in this instance, it is not the maculae that make them lean but air pressure acting on pressure end-organs in the skin, which initiate appropriate equilib-rium adjustments to prevent falling.