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White matter; Diffusion; Activation energy; Myelin; Microtubules
Abstract:
Water diffusion in brain tissue can now be easily investigated using magnetic resonance (MR) techniques, providing unique insights into cellular level microstructure such as axonal orientation. The diffusive motion in white matter is known to be non-Gaussian, with increasing evidence for more than one water-containing tissue compartment. In this study, freshly excised porcine brain white matter was measured using a 125-MHz MR spectrometer (3 T) equipped with gradient coils providing magnetic field gradients of up to 35,000 mT/m. The sample temperature was varied between − 14 and + 19 °C. The hypothesis tested was that white matter contains two slowly exchanging pools of water molecules with different diffusion properties. A Stejskal–Tanner diffusion sequence with very short gradient pulses and b-factors up to 18.8 ms/μm2 was used. The dependence on b-factor of the attenuation due to diffusion was robustly fitted by a biexponential function, with comparable volume fractions for each component. The diffusion coefficient of each component follows Arrhenius behavior, with significantly different activation energies. The measured volume fractions are consistent with the existence of three water-containing compartments, the first comprising relatively free cytoplasmic and extracellular water molecules, the second of water molecules in glial processes, and the third comprising water molecules closely associated with membranes, as for example, in the myelin sheaths and elsewhere. The activation energy of the slow diffusion pool suggests proton hopping at the surface of membranes by a Grotthuss mechanism, mediated by hydrating water molecules.
