Dorsal nerve cord

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The dorsal nerve cord is an anatomical feature found in chordate animals, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates. It is one of the five embryonic features unique to all chordates, the other four being a notochord, a post-anal tail, an endostyle, and pharyngeal slits.

The dorsal nerve cord is located dorsal to the notochord and thus also to the gut tube (hence the name). It is formed from clustered neuronal differentiation at the axial region of the ectoderm, known as the neural plate. During embryonic development, the neural plate first invaginates longitudinally to form the neural groove, whose edges (neural folds) fuse over to form a hollow neural tube. This is an important feature as it distinguishes chordates from other invertebrate phyla such as annelids and arthropods, who have solid nerve cords that are located ventral to the gut tube and often separated into a ladder-like series of segmental ganglia. The process by which neural tube is performed from the ectoderm is called neurulation. The evolutionary explanation to this adaptation from a solid cord to a hollow tube is unknown.

In vertebrates, the dorsal nerve cord (and the subsequent neural tube) gives rise to the brain (via vesicular enlargements at the rostral end) and spinal cord, which together form a highly centralized central nervous system. All the structures developed from the dorsal nerve cord are covered by meninges and enclosed by the bony (sometimes cartilaginous) axial endoskeleton, namely the cranium (hence a synonym word for vertebrates, the craniates) and the spinal canal. The hollow cavity inside the neural tube is filled with cerebrospinal fluid (CSF), and further develops into the cerebral ventricles, aqueduct and central canal, which communicate with the also CSF-filled subarachnoid space via the median and lateral apertures.^[1]