ABSTRACT
Introduction: Extracellular Space................................................................................................. 168
Brief History ............................................................................................................................. 169
Volume Fraction and Width..................................................................................................... 169
Hindrance and Tortuosity ......................................................................................................... 170
Bulk Flow ................................................................................................................................. 170
Volume Transmission and Drug Delivery ............................................................................... 170
Tools and Methods to Study Extracellular Space in Real Time................................................. 171
Diffusion Measurements Reveal Extracellular Space Properties ............................................ 171
The Diffusion Equation and Its Meaning ................................................................................ 171
Choice of Molecular Probes for Extracellular Space .............................................................. 173
Delivery of Molecular Probes: Iontophoresis and Pressure Ejection...................................... 175
Iontophoresis......................................................................................................................... 176
Pressure Ejection .................................................................................................................. 178
Detection of Molecular Probes: Ion-Selective Microelectrodes, Integrative
Optical Imaging and Carbon-Based Microelectrodes.......................................................... 179
Ion-Selective Microelectrodes.............................................................................................. 179
Integrative Optical Imaging ................................................................................................. 181
Carbon-Based Microelectrodes ............................................................................................ 182
Diffusion Measurements Using Dual Microdialysis Probes.................................................... 184
Choice of Brain Tissue Preparation ............................................................................................. 185
Tissue Slices and Slabs ............................................................................................................ 185
Effects of Boundary Conditions on Diffusion Measurements in Brain Tissue....................... 185
In Vivo Preparations................................................................................................................. 190
Specialized Software for Point-Source Diffusion Analysis......................................................... 191
Software for Intophoresis and Pressure Ejection-VOLTORO, Walter and Wanda................ 191
Software for Integrative Optical Imaging-Vida and Ida....................................................... 191
Diffusion Properties of Brain Extracellular Space ...................................................................... 192
Brain under Physiological Conditions ..................................................................................... 192
Brain during Reversible Osmotic Challenge ........................................................................... 193
Brain in Pathological States ..................................................................................................... 194
Monte Carlo Simulation of Diffusion in 3D Media .................................................................... 194
MCell and DReAMM............................................................................................................... 195
Three-Dimensional Media Composed of Convex Cells .......................................................... 195
Dwell-Time Diffusion Theory.................................................................................................. 196
Three-Dimensional Media Containing Concave Cells and Lakes .......................................... 197
Future Directions .......................................................................................................................... 198
References .................................................................................................................................... 199
The extracellular space (ECS) may be imagined as a system of interconnected channels demarcated
by cellular membranes (Figure 10.1) and filled with an ionic solution, primarily NaCl, along with
macromolecules of the extracellular matrix, negatively-charged proteoglycans and glycosamino-
glycans (Margolis and Margolis 1993; Ruoslahti 1996; Novak and Kaye 2000). The ECS mediates
intercellular communication (Nicholson 1979) and the transport of nutrients and metabolites
(Sykova
´
et al. 2000), and it forms a reservoir of ions that establishes the resting potentials of
cells and mediate fluxes across the membranes. It also may serve to deliver therapeutic substances
to cells (Ulbrich et al. 1997; Saltzman 2001). All these processes are primarily mediated by
diffusion. The ECS diffusive properties are thus critical for neurotransmission over short and
FIGURE 10.1 Electron micrograph of a small region of the cerebral cortex of a rat with a prominent synapse.
The black areas between cells indicate the ECS, which may have been reduced in size as a consequence of the
histological processing. The asterisk indicates an extension of a dendritic profile into a dendritic spine. The
spine is joined by a synaptic cleft to a pre-synaptic terminal containing several small round vesicles filled with
transmitter molecules. The presynaptic membrane and vesicles, cleft and immediate post-synaptic membrane
constitute the synapse. Some of the round profiles abutting the dendrite are probably unmyelinated axons cut in
cross-section. The scale bar represents a distance of about 1 mm. (Reproduced from Nicholson, C., Rep. Prog.
Phys., 64, 815-884, 2001.)
