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The ear houses some of the most sensitive organs in the body. The physics of sound is well understood, while the mechanics of how the inner ear translates sound waves into neurotransmitters that then communicate to the brain is still incomplete. Because the vestibular labyrinth and the auditory structure are formed very early in the development of the fetus and the fluid pressure contained within both of them is mutually dependant, a disorder in one of the two reciprocating structures affects the (2).
The vestibular system accomplishes three tasks. First, it contributes to an individualís sense of equilibrium in relation to the force of gravity and thus adds to the subjective sense of motion and spatial orientation. Second, inputs coming from the vestibular system convey information to the bodyís muscles and posture. Third, while head and body are in motion, the vestibular system controls eye movements so that images remain steady and in focus. This is called the vestibular-ocular reflex.
These tasks are accomplished through the mechnoreceptors of the three semicircular canals, the utricle and the saccule (3). Like the neighboring auditory system, each canal has hair cells that detect minute changes in fluid displacement, but unlike the auditory system, the utricle and the saccule send information to the brain regarding linear acceleration and head tilt. Shaking your head ënoí employs one of these canals. Likewise, there is a canal that detects head movement in the ëyesí position, and there is yet another semicircular canal that detects motion from moving your head from shoulder to shoulder (4). These hair cells called stereocilia are located within the crista that is in each semicircular canalís ampullae. The displacement of the hair cells opens the ion channels at their base, and the cupula moves over the utricle. These signals become neurotransmitters on their trek to the brain.
To understand the inhibitory manner of this ion displacement, letís take for example, the vertical canals. When the cupula moves away from the utricle, the canal is excited (3). When the cupula moves toward the utricle, the vertical canal is inhibited (3). Using this example, one can see how the three semicircular canals interact to produce non-conflicting information to the brain about head position. If you move your head from shoulder to shoulder, the other two semicircular canals are inhibited while the other is excited. Therefore you feel that side-to-side motion, and not a combination of directions.
As described earlier, sight is a requirement for good balance and brings into play the vestibular-ocular reflex. As you move your head to the left, for example, your left horizontal canal is stimulated, and the right horizontal canal is inhibited (7). To accomplish the task of keeping your eyes on a steady object while you are moving, the two canals send messages to the left inner eye muscle, the medial rectus, and to the right outer eye muscle, the lateral rectus (7). Your eyes, therefore, remain fixed on the object.
To understand the vestibular system is to help many of those people who have balance and vestibular disorders. One of the most publicized vestibular studies was conducted during NASAís mission in 1995. That payload included the US Senator, John Glenn, and two oyster toadfish. Because the vestibular system of the toadfish is similar to humans, it has been used in the bulk of studies on the vestibular system (5). The alteration of the vestibular system is especially acute for astronauts. Therefore NASA has significantly contributed to research in this area. According to Highstein, a researcher in this area, the vestibular system was one of the first sensory systems to develop. This makes sense because the force of gravity was one of the first conditions with which life of earth had to deal. When the vestibular system is not working, all kinds of normally functioning body functions are affected---loss of appetite, dizziness, moving vision, etc. An astronaut is subject to all of these malfunctions, and during required intense study, no time can be spared for such vestibular related illnesses.
The toadfish is also good for studying the balance disorder called Meniereís disease. An abnormal volume of fluid within the semicircular canals (6) causes this diseaseís long term characteristic feelings of vertigo and dizziness. This endolymph volume slowly increases and gives rise to hearing loss because the fluid is shared with the auditory system. Vertigo is thought to result from membrane breakage and the release of this fluid into the perilymphatic system (6). While theoretically one can reduce vertigo by simply not moving the head or body, this is quite impossible, and the resulting nausea is extremely uncomfortable.
As is shown by the paucity of indepth explanation of the vestibular system on the web, little is still known about the diseases associated with this system. Future NASA studies involving the vestibular system will no doubt shed light on some of the brain-semicircular canal neuro-connections.
1) The Collapsing Tower
3) Disorders of the Inner Ear
4) Facts about Balance
6) The Meniereís Page
7) The Vestibular System
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