Hearing / EP
Endocochlear potential (EP)

- endolymph that bathes the apical surfaces of mammalian hair cells is a unique extracellular fluid
- 150 mM K+
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1 mM Na+
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scala media is also +80 mV relative to scala tympani - this voltage difference is called the endocochlear potential (EP)
- hair cell transduction channels are non-selective cation channels
- when the transduction channels open, K+ flows into the hair cells to cause depolarization because there is a large electrochemical gradient
between the endolymph and the hair cell intracellular fluid
- concentration gradient: 150 vs. 120 mM
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electrical gradient: +80 vs. -40 mV
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the reticular lamina, as well as an extensive tight junction system, enables this electrochemical gradient to be established
- there is also a K+ recycling system that allows supporting cells to take up K+ ions extruded from the hair cells, and return them to the stria vascularis
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defects in the genes that code for the connexin proteins that form the gap junctions are the most common inherited cause of sensorineural deafness
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large driving force for K+ entry contributes to extremely low threshold of auditory hair cells
- hair cells tend to be continuously slightly depolarized relative to the "normal" neuronal potential of -70 mV
- allows for a receptor potential that follows the stimulus (depolarization and hyperpolarization)