Neuropil · Lobeworks/17

Neuropil is nervous tissue made of a dense tangle of axons, dendrites and glial processes, with few or no cell bodies, in which neurons make most of their synapses; in an insect brain it forms the core of every region, while the cell bodies sit in a rind around it.

Neuropil. Neuropil is nervous tissue made of a dense tangle of axons, dendrites and glial processes, with few or no cell bodies, in which neurons make most of their synapses; in an insect brain it forms the core of every region, while the cell bodies sit in a rind around it.

In an electron micrograph neuropil is the field of grey that looks like a cut bundle of spaghetti: hundreds of cross-sectioned processes, each outlined in dark membrane, some carrying vesicles and mitochondria. In vertebrates the grey matter of the cortex is largely neuropil too, though cell bodies are scattered through it. Insect neuroanatomy names the brain by its neuropils (the mushroom bodies, the antennal lobes, the central complex), each a compartment where particular circuits meet.

It is where a connectome is made. Almost every synapse in a fly brain lies in neuropil, which is why plotting all detected synapses draws the outline of the neuropils.

It is the hardest tissue to segment. Processes thinner than 100 nm run in every direction, so it is where flood-filling networks and proofreaders spend most of their effort.

Questions: What does neuropil look like under an electron microscope? Like the cut end of a bundle of spaghetti of many thicknesses. Each strand is a neurite cut across, outlined in dark membrane, some carrying vesicles and mitochondria, and the dark specks among them are synapses. There are almost no cell bodies: in an insect they sit in a rind outside the neuropil, which is why a plot of every synapse in a brain draws its neuropils. Why is neuropil the hardest tissue to segment? Because its processes are thinner than 100 nm, packed together and run in every direction. A membrane that failed to stain for a few nanometres can let a segmentation pour from one neurite into the next, and a process running along the plane of the cut is smeared across one section. Flood-filling networks and proofreaders spend most of their effort there.