Figure 1.

Structure of the inner ear. The inner ear comprises the endolymphatic space and perilymphatic spaces, which are filled with endolymph ([K⁺] = ∼150 mm; [Na⁺] = ∼1–10 mm) and perilymph ([K⁺] = ∼5 mm; [Na⁺] = ∼150 mm), respectively. The saccule (S) is situated in the endolymphatic system between the base of the cochlea and the rest of the vestibular labyrinth [utricle (Ut), ampulla (A), and semicircular canals (SCC)]. ES, Endolymphatic sac; ED, endolymphatic duct; OW, oval window; RW, round window; ME, middle ear. Endolymph composition of the saccule was previously thought to be completely derived from cochlear endolymph by longitudinal diffusion; the present study demonstrates that at least Na⁺ composition of the saccule is regulated locally by absorptive transport. The drawing was adapted with permission from Lim (2002) (their Fig. 2–4).