Reactive gliosis / glial scar

Reactive gliosis is the astrocyte-and-microglia arm of the foreign-body response that forms the encapsulating scar around a chronic electrode. Microglia respond first: they transform from a ramified surveillance morphology to an amoeboid activated state, migrate toward the shank within hours, and release pro-inflammatory cytokines and reactive oxygen species. Astrocytes then hypertrophy, upregulate the intermediate filament GFAP, and interweave their processes into a dense sheath tens of micrometers thick.

This glial sheath is simultaneously a diffusion barrier and a high-impedance shell. By increasing both the physical distance and the electrical resistance between surviving neurons and the recording site, it is the principal biological mechanism of chronic signal degradation. It is often described as the neural analogue of fibrotic scarring elsewhere in the body, though the cell types and molecular signals differ.