Zusammenfassung
Diffusion plays a crucial role in brain function. The spaces between
cells can be likened to the water phase of a foam and many substances
move within this complicated region. Diffusion in this interstitial
space can be accurately modelled with appropriate modifications of
classical equations and quantified from measurements based on novel
micro-techniques. Besides delivering glucose and oxygen from the
vascular system to brain cells, diffusion also moves informational
substances between cells, a process known as volume transmission.
Deviations from expected results reveal how local uptake, degradation
or bulk flow may modify the transport of molecules. Diffusion is
also essential to many therapies that deliver drugs to the brain.
The diffusion-generated concentration distributions of well-chosen
molecules also reveal the structure of brain tissue. This structure
is represented by the volume fraction (void space) and the tortuosity
(hindrance to diffusion imposed by local boundaries or local viscosity).
Analysis of these parameters also reveals how the local geometry
of the brain changes with time or under pathological conditions.
Theoretical and experimental approaches borrow from classical diffusion
theory and from porous media concepts. Earlier studies were based
on radiotracers but the recent methods use a point-source paradigm
coupled with micro-sensors or optical imaging of macromolecules labelled
with fluorescent tags. These concepts and methods are likely to be
applicable elsewhere to measure diffusion properties in very small
volumes of highly structured but delicate material.
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