Comparison of spinal cord and ventral nerve cord

Along the body axis the nervous systems of vertebrates (left) and insects like Drosophila (horizontal view of maggot middle, and fly right; ) show common features: a complex brain is formed in the head region (green), whereas the nervous system of the body region (blue; spinal cord in vertebrates, ventral nerve cord in insects) consists of segmental units with segmental nerves. Both have a vegetal nervous system (red; not shown in Drosophila).

Transverse sections through a vertebrate spinal cord (A,C) and a larval ventral nerve cord of Drosophila (B,D) showing the synaptic areas (pale orange) and other features. A) In vertebrates, sensory neurons (grey) project through the dorsal root (sensory cell bodies lie in dorsal root ganglia) and innervate interneurons and motorneurons. Interneurons can be local interneurons (red) or neurons projecting over longer distances: ascending neurons (turquoise) project through specific tracts towards the brain region (here: typical projection of pain pathway through tractus spinothalamicus); vice versa descending interneurons (green) decline from the brain contributing to local circuits in the spinal cord [here: fibers from the pyramidal tract (tractus corticospinalis lateralis) projecting into the ventral horn in higher primates]. Motorneurons (blue) lie in the ventral horn and project through the ventral root. B) In Drosophila the principles of projections and connections are comparable, however, the spatial arrangements are very different. E.g. motorneurons project through dorsal nerve roots, sensory neurons mainly through ventral nerve roots. C,D) In the spinal cord and ventral nerve cord synapses are localised centrally (pale orange). However, in vertebrates the synaptic region (gm, gray matter) contains in addition the neuronal cell bodies (brown balls) whereas the surrounding areas are formed by myelinated ascending and descending fiber tracts (wm, white matter; turquois and green, respectively). In contrast, neuronal cell bodies are excluded from the synaptic area (np, neuropile) in the Drosophila nerve cord and are restricted to the surrounding cortex (cx, cortex). Instead ascending and descending fiber tracts lie within the synaptic neuropile. The location of sensory output regions (grey) and motor input regions (blue) in the synaptic area are swapped in dorsoventral direction.

Explanations for evolutionary origins of the above differences are given here >>>.