Ventral nerve cord


The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The VNC coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output

Structure

The ventral nerve cord usually consists of segmental ganglia with the nerve cords running down the ventral plane of the organism. Ventral nerve cords from anterior to posterior are made up of segmental ganglia that are connected by a tract of nerve fibers passing from one side to the other of the nerve cord called commissures . The complete system bears some likeness to a rope ladder. In some animals the bilateral ganglia are fused into a single large ganglion per segment.

Evolution

Ventral nerve cords are found in some phyla of the bilaterians, particularly within the nematodes, annelids and the arthropods. They are well-studied within insects, and VNCs have been described in over 300 species covering all the major orders. The presumed common ancestral structure is rarely observed; instead the VNCs of most insects show extensive modification as well as convergence. Modifications include shifts in neuromere positions, their fusion to form composite ganglia, and, potentially, their separation to revert to individual ganglia.

Development

The insect VNC develops according to a body plan based on a segmental set of 30 paired and one unpaired neuroblasts. A neuroblast can be uniquely identified based on its position in the array, its pattern of molecular expression, and the suite of early neurons that it produces. Each neuroblast gives rise to two hemilineages: an "A" hemilineage characterized by active Notch signalling, and a "B" hemilineage characterized by an absence of active Notch signalling. Research in the fruit fly D. melanogaster suggests that all neurons of a given hemilineage release the same primary neurotransmitter.
"engrailed" is a transcription factor that helps regulate the gene "frizzled" in order to separate neuroblast during embryonic development. The segregation of neuroblast is essential for the formation and development of the ventral nerve cord.