The spaces between membrane bilayers within PNS myelin as characterized by X-ray diffraction.

X-ray diffraction data from a wide range of animals characterize the two spaces in the peripheral nervous system (PNS) and the central nervous system (CNS) myelins. The 'extracellular space' is nearly constant from species to species while the 'cytoplasmic space' is more variable. A profile of swollen rat sciatic myelin shows material projecting from the bilayer into the extracellular space. Interdigitating Po molecules therefore are proposed to separate neighboring myelin bilayers across the extracellular space in PNS.