Figure 2

Diffusion measurements using point-source paradigm and a model of extracellular space. A. Recording setup for diffusion measurements using RTI and IOI methods. Either a brain slice (shown here) or anesthetized animal rests on the stage of a compound microscope. For RTI, TMA⁺ is iontophoresed from the injection micropipette and detected with an ISM about 100 μm away. The resulting voltage-versus-time curve is digitized and converted to concentration-versus-time using a personal computer (PC). By fitting Eq. (2) or a related equation, D _effective (and hence λ) and α are measured. For IOI, a fluorescent molecule, (3 kDa MW dextran here) is pressure injected and a time-series of images captured using a 10× objective (obj.) and a charge-coupled device (CCD) camera. A solution of the diffusion equation (similar to Eq. (3)) is fitted to the intensity profiles and D _effective extracted. B. Diffusion curves with TMA⁺ in normal and thick rat cortical slices. Separation between source and ISM was 100 μm. In normal (400 μm) slices typical values of λ and α were measured. In thick slices (1000 μm), representing ischemic brain tissue, λ increased and α decreased, (modified with permission from [24]). C. Monte Carlo computer simulation of diffusion in 3D medium containing concave dead spaces. Brain tissue modeled from cubes surrounded by a thin atmosphere of ECS and with a cubic cavity at one corner. This cavity was surrounded by three cubes and associated ECS creating a dead-space. Volume fraction was varied from 0.05 to 0.90 with α _C equal to 0.05, 0.10, and 0.20. Filled symbols show simulation results and continuous lines came from Eq. (4) with β = 3. Modified Maxwell relationship (Eq. (4) with α _C = 0) derived from simulations using cells without cavities also shown (modified with permission from [34]).