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. 2016 Jan 13;115(4):1836–1859. doi: 10.1152/jn.01077.2015

Fig. 13.

Fig. 13.

Diagrams showing relationship of JPAX to JPIM and JMAX to JMIM (A), comparative sizes of JMIM conduits (B), proposed coupling of KV1 to Cx29 rosettes (C), proposed intramyelinic pathways for K+ along the JMIM conduits that link proximal and distal JPIM collars (D), and myelin unwrapped from its axon (E). A: distribution of NaV channels (red) at nodes of Ranvier, intercellular contactin/Caspr proteins (green) that link paranodal loops (beige) to the axolemma (blue), and axonal KV1 channels (dark blue circles) concentrated in the JPAX region and in 2 thin bands along the JMAX. Cx29 channels (yellow rosettes) are restricted to the JPIM and JMIM. The cross section in the blue box in A is diagrammed in B. B: cytoplasmic expansions of myelin on both sides of the mesaxon (i.e., the paired JMIM) are continuous with the stair-stepped cytoplasmic expansions of the SLIs. (See Figs. 2–4 for immunofluorescence images.) The JMAX (purple arrow to purple segment of axolemma), opposite the inner mesaxon, is defined by its content of KV1 rosettes (blue ovals), whereas the JMIM (aqua arrow to aqua line segments) is defined by its high density of Cx29 rosettes (yellow ovals) and by tight junctions linking the inner tongue with the second layer of myelin (Fig. 10). C: models for tight vs. loose coupling of Cx29 to KV1 in mammals (C1 vs. C2) and their corresponding freeze-fracture images (C3 vs. C4). Cx29 channel (yellow) aligns precisely with KV1.1-containing channel (blue). It is not yet determined whether the channels are in molecular contact (C1) or whether there are tether proteins loosely linking the 2 channels (C2), where they may contribute to a molecular seal. D and E: wrapped (D) and unwrapped myelin (E) showing the cytoplasmic expansions (beige cylinders) surrounding the axolemma (blue), with its densely packed NaV channels (red) at nodes of Ranvier. Axolemmal KV1 channels (dark blue rosettes) are concentrated in the JPAX and in 2 thin bands within the JMAX. Cx29 channels (yellow rosettes) are concentrated in the JPIM and JMIM. SLIs are stair-stepped expansions of myeloplasm that usually extend from the juxtamesaxonal expansions to outermost cytoplasmic myelin.