Extracellular space structure revealed by diffusion analysis.
Nicholson C, Sykova E
Dept of Physiology and Neuroscience, New York University Medical Center,
NY 10016, USA.
The structure of brain extracellular space resembles foam. Diffusing
molecules execute random movements that cause their collision with membranes
and affect their concentration distribution. By measuring this distribution,
the volume fraction (alpha) and the tortuosity (lambda) can be estimated.
The volume fraction indicates the relative amount of extracellular space
and tortuosity is a measure of hindrance of cellular obstructions. Diffusion
measurements with molecules <500 Mr show that alpha approximately 0.2
and lambda approximately 1.6, although some brain regions are anisotropic.
Molecules > or =3000 Mr show more hindrance, but molecules of 70000 Mr
can move through the extracellular space. During stimulation, and in pathophysiological
states, alpha and lambda change, for example in severe ischemia alpha =
0.04 and lambda = 2.2. These data support the feasibility of extrasynaptic
or volume transmission in the extracellular space.