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Neuropil is the brain tissue that lies between axons.

Traditionally when pathologists haved look at brain tissue they have concentrated on neurons (the active functioning cells of the brain), glial cells or supporting cells, and long axons (especially in white matter which is mostly composed of axons). This is because the structures are relatively large and easy to see. Also, most brain tumours arise from glial cells.

Neuropil is the stuff between axons, and it is fair to say that, traditionally, it has been largely ignored. A useful analogy is to imagine a neuron as the trunk of a tree, and imagine the branchs as axons. Neuropil could then be regarded at the twigs and leaves at the end of the branches. It we were to study a forest we would be tempted to look at the big things first (trunks and branches) but we now know that most of the real activity in the forest is happening in the leaves (growth and photosynthesis).

Most synaptic activity in the brain occurs in neuropil. It is highly structured material, consisting of elements (the dendritic spines) which are of the order of a few micrometres in size (a fraction of the size of a red blood cell). It is difficult to study because of its complexity and small scale, and because it is extremely fragile. When a body tissue dies, it self-destructs or autolyses and turns into mush. Structures on this scale can autolyse within seconds, making it impossible to obtain useful information about the fine structure of neuropil from autopsies of brain material.

It has been suggested that many mysterious brain pathologies are caused by damage to, or defects in, this tissue.

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