Sense Organs: Balance and Orientation
Embedded in the temporal bone inside the inner ear lays the vestibular system, which contains fluid-filled sacs and cavities that monitor the position and movement of the head and transmit that information to the brain. This system contains three semicircular canals, each oriented at right angles to the other two. The canals are connected to a saclike utricle, below which lies the sacule, another hollow structure.
The utricle and sacule contain receptors consisting of groups of hair-like cells, cilia, that are embedded in a gelatinous material. The gelatinous material contains many small particles of calcium carbonate called otoliths. These increase the sensitivity of the cilia. At the base of each receptor is a nerve fiber. The nerve fibers collectively carry information to the brain via cranial nerve VIII, the auditory vestibular nerve. The receptors of the saccule and utricle respond to static positions of the head. In other words, they tell the brain which way is up.
The semicircular canals also contain gelatin-embedded receptors bearing cilia. These cilia detect changes in the rate of rotation or angular movements of the head. When the head moves, the fluid in the semicircular canals presses against the hair cells, causing them to bend. Bending of the cilia triggers action potentials in the nerve fibers connected to the receptors. More rotation causes more bending, causing more action potentials. The specific direction of head movement stimulates different receptors in the canals in each of the three planes. A three-dimensional message about direction of head movement is therefore compiled and sent to the brain along the eighth cranial nerve.
Low frequency movements that a person can't control often lead to motion sickness. Most often, a person may experience motion sickness as a passenger, but not as the driver. Motion sickness is probably the result of the brain's receiving contradictory information from the eyes and the vestibular system. The eyes, fixed on the interior of the vehicle, report "no motion" to the brain, while the stimulation of the hair cells reports "motion." Destruction of the semicircular canals by antibiotics or other drugs eliminates motion sickness.
Some of the neurons of the auditory vestibular nerves synapse in the vestibular nuclei of the lower brain stem. From here, neurons synapse on the motor neurons that control the muscles that move the eyes and on nuclei in the thalamus, cerebral cortex, and other locations. Some of the neurons, however, carry information from the inner ear directly to the cerebellum, a structure that coordinates motor control. The cerebellum uses information about the position and movement of the head to regulate to regulate output from the motor cortex, helping to maintain balance.
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