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  1. The vestibular apparatus is adjacent and continuous with the cochlea and contains a bony labyrinth in which the membranous labyrinth is fitted
    1. between the bony and membranous labyrinths is perilymph; viscous endolymph fills the membranous labyrinth
      1. endolymph has a very high concentration of potassium similar to intracellular fluid; perilymph surrounding the labyrinth has the composition of extracellular fluid
      2. endolymph drains into the dural sinus via the endolymphatic duct while perilymph drains into the CSF via the perilymphatic duct
    2. labyrinths are divided into 5 divisions
      1. utricle
      2. saccule
      3. 3 semicircular canals (posterior, superior and horizontal) detecting angular acceleration; each filled with endolymph
    3. at the end of each canal is an enlargement called the ampula; the epithelium inside each ampulla is thickened in a region called the crista which contains the vestibular transducers; the transducers are hair cells similar to the ones in the cochlea; the saccule and utricle also have a region of epithelium called the macula (macula is equivalent to the ampulla of the semicircular canals) which contain a dense aggregate of hair cells; the utriclar macula is oriented horizontally and the sacular macula is oriented vertically
      1. the cilia of the hair cells in each ampulla are embedded in a structure called the cupula which is a gelatinous capsule that hangs down from the roof of the ampulla; movement of the viscous endolymph displaces the cupula and bend the stereocilia opposite to the direction of head rotation
        1. the hair cells of the vestibular apparatus have one kinocilium and many stereocilia (microvilli)
        2. the hair cells are connected to bipolar cells in the internal auditory meatus
        3. vestibular ganglion lies in the fundus of the internal auditory meatus and sends neurons to the vestibular nuclei of the brainstem and the flocculonodular lobe of the cerebellum
        4. Meniere’s disease is an inner ear disease associated with increased endolymphatic fluid pressure, unilateral hearing loss (low tones early in disease and then high tones later in the disease), horizontal nystagmus occurring during an attack, and low pitched tinnitus; Remember: HTN (hypertension) and Meniere’s (Hearing, Tinnitus, Nystagmus)
  2. Macula is the sensory organ of the utricle and saccule which contains hair cells with cilia that are embedded in a gelatinous layer containing calcium carbonate otoliths; when the hairs bend forward there is increased Na conductance causing depolarization; when they bend backward there is decreased Na conductance causing hyperpolarization; e.g. turning the head to the right bends the stereocilia in the opposite direction so that the horizontal semicircular canal on the right depolarizes with hyperpolarization occurring on the left because the stereocilia are bent away
    1. benign positional vertigo is probably due to cuprolithiasis (displacement of otoliths) in the posterior semicircular duct; characterized by recurrent attack of rotational vertigo with changes in head position
    2. labyrinthitis with vertigo is a common feature of salicylate, aminoglycoside, quinine and alcohol damage; aspirin will often make tinnitus worse as well
  3. The utricle and saccule are the static sensors and respond principally to changes in linear acceleration (particularly that caused by gravity) and control posture and balance
    1. tilting the head forward maximally stimulates hair cells in the utricle while tilting the head to the side maximally stimulates hair cells in the saccule
    2. cold water calorics do not test the utricle and saccule
  4. cold water calorics
    1. used to stimulate the horizontal semicircular canal but may be used to test each labyrinth separately with different positioning of the patient
    2. In a comatose patient, the patient’s eyes will deviate toward the side where the cold water is injected without any nystagmus
    3. in normal patients, cold water injected into the external auditory canal will result in nystagmus to the opposite side with past pointing and falling to the same side
      1. warm water will reverse those findings (Remember: COWS – Cold Opposite, Warm Same)
  5. spinning
    1. spinning to the right and then stopping will cause a fast phase nystagmus to the left (slow phase to the right), past pointing to the right, and a sensation of turning to the left; spinning to the right produces the opposite effects
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