Determinants of anisotropic water diffusion in nerves

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Abstract
We report NMR diffusion measurements of water in three central nervous system models, namely the nonmyelinated olfactory, and the myelinated trigeminal and optic nerves of the spotted and long-nosed garfish. A similar degree of anisotropy of the average diffusion coefficients (DNMR) is observed for all three freshly excised nerve types (DNMR parallel)/DNMR-(perpendicular) is 3.6 ± 1.2, 3.2 ± 0.9, and 2.6 ± 0.4 for the olfactory, trigeminal, and optic nerves, respectively). The anisotropy of DNHR for the nonmyelinated olfactory nerve argues strongly that myelin is not a necessary determinant of diffusional anisotropy in ordered axonal systems (even though it may contribute when present). Garfish nerves treated with vinblastine, in order to depolymerize microtubules and inhibit fast axonal transport, also exhibit diffusional anisotropy {DNMR(parallel)/DNMR (perpendicular) is 2.6 ± 0.4,2.8 ± 0.8, and 2.2 ± 0.7 for the olfactory, trigeminal, and optic nerves, respectively) thus excluding a significant role for microtubules and fast axonal transport in that observed anisotropy.