Local diffusion in the extracellular space of the brain

The brain extracellular space (ECS) is a vast interstitial reticulum of extreme morphological complexity, composed of narrow gaps separated by local expansions, enabling interconnected highways between neural cells. Constituting on average 20% of brain volume, the ECS is key for intercellular communication, and understanding its diffusional properties is of paramount importance for understanding the brain. Within the ECS, neuroactive substances travel predominantly by diffusion, spreading through the interstitial fluid and the extracellular matrix scaffold after being focally released. The nanoscale dimensions of the ECS render it unresolvable by conventional live tissue compatible imaging methods, and historically diffusion of tracers has been used to indirectly infer its structure. Novel nanoscopic imaging techniques now show that the ECS is a highly dynamic compartment, and that diffusivity in the ECS is more heterogeneous than anticipated, with great variability across brain regions and physiological states. Diffusion is defined primarily by the local ECS geometry, and secondarily by the viscosity of the interstitial fluid, including the obstructive and binding properties of the extracellular matrix. ECS volume fraction and tortuosity both strongly determine diffusivity, and each can be independently regulated e.g. through alterations in glial morphology and the extracellular matrix composition. Here we aim to provide an overview of our current understanding of the ECS and its diffusional properties. We highlight emerging technological ad-vances to respectively interrogate and model diffusion through the ECS, and point out how these may contribute in resolving the remaining enigmas of the ECS.

Automated summary

The brain extracellular space (ECS) is a complex interstitial reticulum that plays a crucial role in intercellular communication. Diffusion is the primary mechanism for the transportation of neuroactive substances within the ECS. Recent nanoscopic imaging techniques have revealed the dynamic nature of the ECS and its heterogeneous diffusivity across brain regions. The geometry of the ECS and the properties of the interstitial fluid and extracellular matrix determine the diffusivity, which can be regulated independently.