Disorders of the inner ear Flashcards
(97 cards)
homeostasis is defined as
The ability of an organism or a cell to maintain internal equilibrium by adjusting its physiological processes
inner ear homeostasis is
“The process by which chemical equilibrium of inner ear fluids and tissues is maintained”
what is necessary for proper inner ear function?
a tight control of ion movement across the cell membranes
inner ear functions include?
-hair cell function
- regulation of extracellular endolumpatic and periplymph
-conduction of nerve impulse
what are major ion s involved in the inner ear homeostasis ?
sodium and potassium
what other ions help in the inner ear
chloride
calcium
ion homeostatis is controlled by
numerous ion channels and transporters in plasma membrane of cells, especially cells lining the scala media
-they regulate various genes
- and are an active transport of H2o across celll membrane is needed
what must happen before sound is perceived
-Before sound can be perceived it has to be converted to electrical impulses in the auditory nerve, a process mediated by the cilia of the inner ear hair cells
-The sound-induced excitatory deflection of the stereocilia causes a mechanoelectrical transduction (MET) current to depolarize the hair cells and initiate action potentials in the auditory nerve or vestibular nerve in response to acceleration/gravity changes
what must endolymph and periplymp maintain?
-The endolymph and perilymph must maintain their specific ion concentration for maximum sensitivity of hair cells
-Significant hearing loss occurs when either of these support systems fail
inner ear potentials are :
-The hair cell has an intracellular potential of -80 mV relative to the surrounding perilymph
-The high K+ concentration in the endolymph creates a +80 mV endocohlear potential (EP) that couples with a -80 mV hair cell intracellular potential to create a differential potential of +160 mV
-80 mV – (-80mV) = 160 mV
-This arrangement of potentials between endolymph and perilymph is necessary for hair cell depolarization when stereocilia are deflected by the travelling wave
-The EP in the vestibular system is only ~+5 to +10 mV resulting in a much smaller total potential difference
Stereocilia are laterally displaced either by
-Shearing movement of the tectorial membrane (outer hair cells) or
-Motion of the endolymphatic fluid (inner hair cells)
how does potassium play a role in ion homeostasis ?
-The flow of K+ ions is down an electrical gradient that brings K+ into the cell from endolymph and then out the base of the cell into the perilymph
-If the entire hair cell was surrounded by endolymph the hair cell would not function
-The K+ ion concentration inside and outside the cell would not allow for a electrochemical gradient
-When these two fluids are same, transduction is compromised resulting in hearing loss/vestibular dysfunction
-This situation occurs during Meniere’s disease when membranous labyrinth rupture allows mixing of endolymph and perilymph
look at slide 9
what other 2 things are involved with ion homeostatis ?
Aquaporins and water transport
what are aquaporins?
-Aquaporins are proteins embedded in the cell membrane that regulate the flow of water between cells and play a central role in water homeostasis in both plants and animals
-They are part of the blood-brain barrier and probably the blood-labyrinth barrier
how does water play a part in ion homeostasis
-Water easily diffuses through cell membranes but that is not enough to maintain osmotic homeostasis in the inner ear
-Active water transport b/w cells occurs through aquaporin channels
what kind of effects can happen if we have defects involving aquaporin?
-Defects involving aquaporin genes are associated with several human diseases
-Viral infections in the lungs can shut down aquaporin function causing inflammation and edema
-Bacterial and viral labyrinthitis may be caused by cochlear aquaporin dysfunction
what is the most caused genetic hearing loss?
-Endolymphatic xerosis caused by various genetic anomalies is believed to be the cause of most genetic hearing loss in humans, for example
1)Connexin 26 genes
-abnormalities result in abnormal connexin gap junction proteins
-responsible by itself >50% of nonsyndromic hl
KCNE1 and KCNQ1
-Produce proteins that make up K+ channels on the apical stria
-Their absence leads to reduced endolymph and associated hearing loss seen in Jervell-Lange-Nielsen (JLNs) syndrome
hearing loss can result from what? (in terms of ion homeostasis?)
-Hearing loss can result from increased or decreased activity of the strial process
-Increased K+ transport in the endolymph or increased endolymph production
-endolymphatic hydrops
-too much endolymph
Decreased K+ transport in the endolymph or decreased endolymph production
-endolymphatic xerosis
-JLNS and connexin
temporary disorders of ion homeostasis
-Several cochlear and vestibular disorders are transient and recover spontaneously, such as
-Sudden-onset hearing loss
-Diuretic otoxicity (e.g., Furesmide) recovery after stopping drugs
-Autoimmune labyrinthitis, which can have transient symptoms
-Meniere’s disease, which exhibits intermittent symptoms
IMPORTANT:
The fact that these disorders manifest temporary hearing loss and recovery indicates that the damage is not to the hair cells but to the ion homeostatic process
how can HL look like in cochlear disorders?
Hearing loss – constant or fluctuating
-Any severity but always sensorineural, unless superimposed on a middle ear issue, which then results in a mixed hearing loss
how can their speech thresholds appear with cochlear disorders
Difficulty with speech perception – may not match h. loss levels
what is loudness recruitment associated with in cochlear disorders?
Loudness recruitment – abnormal loudness growth typically seen with presbycusis and high frequency hearing loss
how do patients ears feel in cochlear disorders?
Aural fullness – feeling plugged up or “water” in the ears
