Nerve Cord: Definition, Function, and Vertebrate Development

Nerve cord terminology differs by clade:

Invertebrates (Ventral Nerve Cord)

• Protostomes (arthropods, annelids, mollusks): Paired ventral longitudinal cords with segmental ganglia. "Ladder-like."
• Tunicates (urochordates): Larva has dorsal hollow nerve cord (chordate trait); adult loses it (sessile).

Chordates (Dorsal Hollow Nerve Cord)

• Defining feature of phylum Chordata (with notochord, pharyngeal slits, post-anal tail, endostyle).
• Embryology: Neural plate → neural groove → neural tube (neurulation).
• Rostral end → brain (vesicles: prosencephalon, mesencephalon, rhombencephalon).
• Caudal → spinal cord.
• Neural crest → PNS, melanocytes, facial bone, etc.

Vertebrates

• Neurulation completes; neural tube differentiates into CNS.
• Ventral horn = motor neuron cell bodies; dorsal horn = sensory processing.
• Dorsal root ganglia (neural crest) = sensory neuron cell bodies.

The original description conflates invertebrate ventral cords with vertebrate dorsal cords. Key distinction: dorsal hollow vs. ventral solid/ganglionated.