section_path: Anatomy and morphology/Central nervous system
section_name: Central nervous system
paragraph_text: Central nervous system
  
  The central nervous system of vertebrates is based on the hollow dorsal nerve cord running along the dorsal aspect of the notochord. Of particular importance and unique to vertebrates is the presence of neural crest cells, which are progenitor cells critical to coordinating the functions of cellular components. Neural crest cells migrate through the body from the dorsal nerve cord during development, and initiate the formation of neuronal ganglia and structures such as the jaws and skull. The peripheral nervous system forms when neural crest cells branch out laterally from the dorsal nerve cord and migrate together with the mesodermal somites to innervate the various different structures that develop in the body.
  
  The vertebrates are the only chordate group with neural cephalization, and their neural functions are centralized towards a series of vesicular enlargements in the head, which give rise to a brain. A slight swelling of the anterior end of the nerve cord is found in invertebrate chordates such as lancelets (a sister subphylum known as the cephalochordates), though it lacks eyes and other complex special sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalization.
  
  The rostral end of the neural tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), which are further differentiated in the various vertebrate groups. Two laterally placed retinas and optical nerves form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss. The forebrain is more well-developed in most tetrapods and subdivided into the telencephalon and diencephalon, while the midbrain dominates in many fish and some salamanders. Vesicles of the brain are usually bilaterally symmetrical, giving rise to the paired cerebral hemispheres in mammals.
  
  The resulting anatomy of the central nervous system with a single hollow nerve cord dorsal to the gut tube, headed by a series of (typically paired) brain vesicles, is unique to vertebrates. Other invertebrates with well-developed central nervous systems such as arthropods and cephalopods, have a ventral nerve cord made of segmental ganglia on the opposite side of the gut tube, with a split brain stem circumventing the foregut around each side to form a brain on the dorsal side of the mouth.
  
  Another distinct neural feature of vertebrates is the axonal / dendritic myelination in both central (via oligodendrocytes) and peripheral nerves (via neurolemmocytes). Although myelin insulation is not unique to vertebrates — many annelids and arthropods also have myelin sheath formed by glia cells, with the Kuruma shrimp having twice the conduction velocity of any vertebrates — vertebrate myelination is annular and non- fenestrated, and the combination of myelination and encephalization have given vertebrates a unique advantage in developing higher neural functions such as complex motor coordination and cognition. It also allowsvertebrates to evolve larger sizes while still maintaining considerable body reactivity, speed and agility (in contrast, invertebrates typically become sensorily slower and motorically clumsier with larger sizes), which are crucial for the eventual adaptive success of vertebrates in seizing dominant niches of higher trophic levels in both terrestrial and aquatic ecosystems.