One of the oddest artifacts of astocytes in particular is that despite their morphological complexity, they do not overlap in “normal” development. They have varying density by region and by individual, but the cells themselves maintain discrete local environments. This coupled with every developed astrocyte having unique properties serves to answer one of the big outstanding questions I’ve had – how do cells signal other cells in different parts of the nervous system?
I don’t think I’ve ever seen a good explanation from this, and most traditional neuroscience models have the signal rectifying forward throughout the brain until it magically decides to stop. It seems possible that a significant function of astrocytes is a representative map of the entire nervous system, and their heterogeneity is what allows specific addressing of various brain regions.
The signal itself propogates through the circuitry, and when it reaches the astrocyte population it’s targeted, the astrocytes respond with calcium dynamics to suppress further propagation of the wave. Individually, each astrocyte is responsible for a certain domain worth of inputs, and they transfer the normalized/condensed contents of their domain as a reference/pointer to cells further up the chain which are responsible for collecting those inputs, and so on up to the highest level place/space/object cells in the hippocampus and brainstem.
This mechanic gives granular control to the top level maps because they are essentially a linked list, making it efficient to touch or modify any particular piece of data in the cognitive stream. Because each astrocyte represents a unique space in a nervous system, they also represent an interconnected map of the nervous system as well.
The interesting thing about this construct is that it gives us a mechanic for the cumulative effects that we see physiologically, as well as the top level cognitive “experience”. It also accommodates the difference between “conscious” (e.g. maps at the “highest” levels) and “unconscious” processes which are underlying component maps.
Edit:
Further Reading –
Evolution of astrocytes: From invertebrates to vertebrates – Astrocytes look like they developed as an artifact of nervous system complexity. Also, didn’t realize that we still can’t completely differentiate astrocytes from other cell types, giving further weight to the conceit that all nervous system cells are of the same root, rather than discrete evolutionary adaptations. It also suggests that astrocytes may not have non-overlapping local environments, it might be an artifact of the inability to properly stain for the full range of astrocyte heterogeneity. It’s still highly probable they do, but this needs to be re-verified each time a new stain is developed.