EP

Endocochlear potential (EP)

endolymph that bathes the apical surfaces of mammalian hair cells is a unique extracellular fluid
- 150 mM K+
- 1 mM Na+
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
1. concentration gradient: 150 vs. 120 mM
2. electrical gradient: +80 vs. -40 mV
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
- defects in the genes that code for the connexin proteins that form the gap junctions are the most common inherited cause of sensorineural deafness
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)