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FIG 3. Coronal diffusion anisotropy maps of 10-day-old rat pups, showing the effect of tetrodotoxin (TTX). The untreated animal (left) shows high signal intensity in the slightly myelinated optic structures (white arrows), and the unmyelinated internal capsules (white arrowheads) show diffusion anisotropy. In the TTX-treated animal (right), diffusion anisotropy is no longer visible in the unmyelinated internal capsule. However, anisotropy (hyperintensity) still is present in the partially myelinated optic nerves, presumably because the hydrophobic myelin sheath already constitutes a physical barrier to water diffusion. This suggests that the anisotropy present before myelin results from a physiological phenomenon, such as sodium diffusion, that is paralyzed by the TTX. Note that the intensity of the entire TTX-treated brain appears different from that of the untreated animal. This intensity change is attributed to TTX-induced global cytotoxic edema, restricting water motion in gray matter as well as white matter
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