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1. As sound waves enter the ear, they travel through the outer ear, the external auditory canal, and strike the eardrum causing it to vibrate.
The central part of the eardrum is connected to a small bone of the middle ear called the malleus (hammer). As the malleus vibrates, it transmits the sound vibrations to the other two small bones or ossicles of the middle ear, the incus and stapes.
As the stapes moves, it pushes a structure called the oval window in and out. This action is passed onto the cochlea, which is a fluid-filled snail-like structure that contains the receptor organ for hearing.
The cochlea contains the spiral organ of Corti, which is the receptor organ for hearing. It consists of tiny hair cells that translate the fluid vibration of sounds from its surrounding ducts into electrical impulses that are carried to the brain by sensory nerves.
2. A change in head position—typically turning the head over on the pillow on first awakening in the morning, or tipping the head backward to reach a high shelf—triggers most episodes of this disorder. BPPV (Benign paroxysmal positional vertigo) usually develops when calcium particles (otoconia) that are normally embedded in one part of the inner ear (the utricle and saccule) are displaced and move into another part of the inner ear (most commonly the posterior semicircular canal). The inner ear contains three semicircular canals, which help with balance. The posterior canal, unlike the anterior and horizontal canals, is in the best position to receive most of the loose particles through gravity during the night. As they collect, they form a chalky sludge and may further form into a mass that exaggerates the movement of fluid in the canal when the head changes position. The result is overstimulation of nerve receptors (hair cells) inside the posterior canal, making the brain feel as if the head were moving much faster or differently than it is.
Particles may be displaced from the utricle and saccule as people age. Or, displacement may be caused by ear infections, injury, prolonged bed rest, ear surgery, head injury, or blockage of an artery to the inner ear.
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