Fig. 1.

Electrochemical profile of the cochlea. a Structure of the cochlea. The composition of the human ear and a cross section of the cochlea are illustrated in the upper left and right panels, respectively. The lower panel depicts the tissue and cellular components of the cochlea. The electrochemical properties of the endolymph and perilymph and the possible circulation of K⁺ between the two are also shown. Note that the intrastrial space in the stria vascularis is emphasized with an expansion, although it is a narrow extracellular space of 15 nm in width. The locations of the five types of fibrocytes in the spiral ligament are indicated by roman numerals. The boxed region is enlarged in b. EP endocochlear potential, OSC outer sulcus cells. Reproduced and modified from [74]. b Cellular components of the lateral wall. Ion channels and transporters that are likely to play key roles in the maintenance of unidirectional K⁺ transport across the lateral wall and endocochlear potential (EP) are shown in the upper panel. The lateral wall comprises the marginal cell layer and the syncytial layer that results from interconnections between fibrocytes (FCs) in the spiral ligament and the basal and intermediate cells (BC and IC, respectively) in the stria vascularis. Note that the basolateral and apical surfaces of the syncytial layer are provided primarily by fibrocytes and intermediate cells, respectively. MC marginal cells, NKCC1 Na⁺,K⁺,2Cl⁻-cotransporter type 1, ClC ClC-K/barttin type Cl⁻ channel, TJ tight junction. Reproduced and modified from [74]. c Potential and [K⁺] properties of the lateral wall under physiological conditions. The top panel displays the properties averaged from multiple measurements. v _SB, v _SA, v _MB, and v _MA indicate the membrane potentials across the basolateral and apical surfaces of the syncytial layer and the basolateral and apical surfaces of the marginal cell layer, respectively. Shown in the lower panel is a representative trace of the measurement in the cochlea of a live guinea pig. In this experiment, a microelectrode sensitive to potential (red) and [K⁺] (blue) was inserted from the perilymph (PL) into the endolymph (EL) across the lateral wall. Potentials in each compartment were measured with respect to the PL, which is defined to be 0 mV in in vivo assays. Spike-like variations of [K⁺] represent the passage of numerous fibrocytes interconnected with each other by gap junctions. Within this region, the compartment located next to the intrastiral space (IS) likely represents the properties of the syncytial layer (SY) (i.e., v _SB and [K⁺] inside the syncytium; indicated by arrows) [1, 116]. The wedges filled with gray below the lower panel indicate the periods during which the electrode was inserted toward the EL. MC marginal cells. Reproduced and modified from [115]