Neuropil is the feltwork of unmyelinated neuronal processes (axonal and dendritic) within the gray matter of the central nervous system.
Traditionally, when pathologists look at brain tissue they concentrate on neurons, glial cells and axons (especially in white matter, which is mostly composed of axons and glial cells). Neurons are the cells that perform the actual processing of information. Glial cells have 'supporting' functions in a very loose sense of the word. Also, most brain tumors arise from glial cells.
On a coarse scale, nervous tissue (ignoring blood vessels etc.) is composed of the cell bodies of neurons and glial cells and their processes or protrusions. For neurons, these are dendrites, dendritic spines and axons. Dendrites collect input from other neurons, which is processed by the neuron (in both its dendrites and its cell body) and propagated to other cells via axons, which act as long-distance cables. At the end of an axon synapses are formed, serving as chemical junctions to other cells.
The neuropil is what is left when you take away the cell bodies of neurons and glia of gray matter, i.e. the tangle of dendrites, axons and glial processes. White matter is generally not considered to be a part of the neuropil.
Reference
- Neuropil: Roche Encyclopedia of Medicine, Dictionary Barn.
