The scala media of the cochlea is separated from the scala vestibuli by which membrane, and what is the ionic composition of endolymph within it?
- A Reissner's membrane; high K+, low Na+ ✓
- B Basilar membrane; high K+, low Na+
- C Basilar membrane; high Na+, low K+
- D Reissner's membrane; high Na+, low K+
Correct answer: A. Reissner's membrane; high K+, low Na+
Explanation
The scala media (cochlear duct) is separated from the scala vestibuli above by Reissner's (vestibular) membrane, while the basilar membrane separates it from the scala tympani below. Endolymph within the scala media is unique extracellular fluid with high K+ (~150 mEq/L) and low Na+ (~1 mEq/L), maintained by active transport via the stria vascularis; this composition generates the endocochlear potential (+80 mV) critical for hair cell mechanotransduction. The other scalae contain perilymph, which has high Na+ and low K+, resembling extracellular fluid.
Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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