Inner ear sensory organs differ in the way their hair cell stereocilia bundles are mechanically bent. The hair cells in each organ are grouped in one of three types of sensory epithelium. The maculae and the cristae are the sensory epithelium of the vestibular system (balance) and the organ of Corti is the sensory epithelium of the cochlea. There are two maculae (the saccule and the utricle), three cristae and one organ of Corti on each side of the head. The hair cell stereocilia of each macula are linked to an overlying structure whose movement causes them to bend. Tilting your head or slamming on your car's brakes cause the overlying mass in the maculae to move relative to the hair cells, bending the stereocilia bundle and activating the afferent nerve fibers connected to the hair cells. Your brain interprets the incoming information and tells the muscles of the body what to do without your awareness. The stereocilia in a cristae are bent in response to movement of the fluid in a semicircular canal (see Figure 4 in Organ of Corti - The Temple of Hearing). The fluids are set in motion by rotation about the axis of the canal. For instance, when watching a tennis match you rotate your head to follow the ball. This causes the fluid in the "horizontal canal" to move relative to the crista and the nerve fibers are stimulated. The end result is that the muscles of your eyes move (again, without your conscious attention) so as to stabilize the visual field. While the hair cells in the vestibular end organs remain fixed relative to the bony cavity in which they are located, the hair cells in the organ of Corti move (or vibrate) in response to sound. Organ of Corti hair cell stereocilia insert into an overlying structure (the tectorial membrane) which vibrates less than the hair cells, and the differential movements bend the stereocilia bundle. This arrangement can vibrate at frequencies of between 20 cycles/sec to 20,000 cycles/sec for humans.
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