Structurally, the neuropil is a concentrated region of synapses that consists of primarily unmyelinated axons, dendrites, and glial cells (the supportive cells of the nervous system). It is relatively devoid of neuronal cell bodies and myelinated axons, but it is a region where signals are processed. The term neuropil is derived from Greek meaning “nerve felt.” This is because of all the integrated fibers found within the neuropil, thus making the region look like a piece of felt. The neuropil is a generic term that defines several parts of the central nervous system and is not concentrated in just one area or another. However, the largest amount of neuropil is found in the outer layer of the cerebral cortex—which is the outermost portion of the brain—and the olfactory bulb—which is responsible for the detection and discrimination of odors.
The cerebral cortex is a six-layered structure with each layer being numbered with the corresponding Roman numeral: I—VI. Although it is layered, meaning there are clear divisions of neuron layers, the majority of the cortex contains neuropil. The outermost layer is the molecular layer and is also called layer I. Alternating layers of neurons that have either a granular or pyramidal shape follow the molecular layer, thus these layers are called granular and pyramidal layers. Layer VI is polymorphic, meaning it consists of several cell types and is the deepest layer. Within layers II—V, the granular or stellate cells act as the principal interneurons (neurons between the primary neurons), and their projections do not leave the local cortex. The pyramidal cells act as the primary output neurons and typically do send projections out of the cortex. These projections frequently contact other cortical areas of the brain or subcortical areas. The neuropil can be found within and between each cortical layer. Thus the neuropil within the cerebral cortex is an amalgam of stellate cell axons projecting from one layer to another; the glial cells in the region; and the dendrites of the principal cells.
In the olfactory bulb, there are also very specific layers of neurons that are easily identifiable. Specifically, the outer surface of the olfactory bulb is made up of several circular regions called glomeruli (plural; glomerulus is the singular form). It is these glomeruli that make up some of the olfactory bulb’s neuropil by the axons of the olfactory sensory neurons, the dendrites of the mitral cells, and the short dendrites and axons of the periglomerular cells. It is at this region where an odorant’s perception begins to be processed by the brain.
Finally, there are other brain regions that are composed of neuropil such as the retina. The retina consists of 10 identifiable layers of neurons and both the inner and outer plexiform layers have regions of neuropil. Similar to the olfactory bulb, the neuropil regions in the retina begin the processing of vision and what is being seen or perceived.
At the turn of the 21st century, neuroscientists researched the target amount of neuropil to neurons that is needed for a functioning central nervous system. They found that the volume of the tissue should contain three-fifths of neuropil for an optimally wired brain (Chklovskii et al., 2002). This information is useful for neuroscientists who are developing computer programs to better understand the brain’s circuitry as well as for developing artificial intelligence.
Jennifer L. Hellier
See also: Axon; Cerebral Cortex; Nerves; Olfactory Bulb; Olfactory Sensory Neurons; Olfactory System; Retina; Visual System
Chklovskii, Dmitri B., Thomas Schikorski, & Charles F. Stevens. (2002). Wiring optimization in cortical circuits. Neuron, 34(3), 341—347.
Purves, Dale, et al. (Eds.). (2004). Neuroscience (3rd ed.). Sunderland, MA: Sinauer Associates.