Dorsal Hollow Nerve Cord

Nerve Cord foundation for nervous system

The dorsal hollow nerve cord is fundamental to all vertebrates and it serves as the basis for their elaborate central nervous system.  This nerve cord extends beyond animals with vertebrae to also include other chordates in which the vertebrates represent the major part.  Among these animals are the fish-like lancelets, or amphioxus, and tunicates.  The invertebrate tunicates, or sea squirts, seems to be a highly unlikely organism to have a dorsal hollow nerve cord as its sedentary lifestyle and primitive body plan appears to have no need for such a sophisticated nervous system arrangement.  In fact the adult has no such structure and the nerve cord appears only in its mobile larval form.  The lancelet retains its nerve cord but its anterior end does not elaborate into anything one might consider a brain and its head has only rudimentary, unpaired sense receptors. 

Embryo with early neurulation 

The nervous system is made up of cells that give life awareness of its surroundings and, in its highest implementation, a consciousness of the self and a sense of wonder and perspective of the self within all existence.  Imagine that.  Cells are working together to produce the power of thought.  This alone makes creation of nerve cells the most extraordinary development of advanced life forms. 

Development of the nerve cord involves a complex choreography of various types of cells moving with synchronization in both time and space.  It begins at a very early stage of the embryo, during the blastula, when cells first begin to divide and differentiate into what will become three distinct layers of cells.  The outermost layer, the ectoderm, proceeds to form the skin, anterior and posterior parts of the digestive tract as well as much of the nervous system, including the eyes and ears.  The innermost layer, the endoderm, provides lining for the gut and the glands associated with the digestive tract.  The respiratory surfaces of vertebrates also originate from endoderm.  The last of the three layers to usually differentiate is the mesoderm, or middle layer.  Products of mesoderm include the muscles, skeleton, connective tissue and the circulatory and urogenital systems. 

Nerve cord among key chordate characteristics

Neurulation begins when mesodermal cells, called chordamesoderm, collect to form the notochord which becomes the embryo’s body axis.  Presence of the chordamesoderm induces the ectoderm overlying the notochord to develop two longitudinal folds, creating a mid-dorsal furrow between them.  The crests of the two folds grow towards one another, forcing the furrow deeper into the dorsal mesoderm that lies adjacent to the notochord.  These neural folds fuse together to make a tube of isolated ectoderm beneath the surface of the embryo.  This neural tube becomes the basis for the central nervous system. 

During the formation of the neural tube within the embryo of vertebrates another group of cells differentiate themselves from the ectoderm.  Arising in the area between the developing neural tube and the closing ectoderm overhead is a distinct group called neural crest cells.  These cells have great evolutionary importance because they are responsible for the creation of most every characteristic that sets vertebrates apart from all other organisms.  They disperse laterally and ventrally from their point of origin to settle and differentiate into a variety of forms throughout the body.  These migrating neural crest cells become the basis for most of the peripheral nervous system.  They form the autonomic system and several endocrine glands.  They are responsible for much of the head’s skeleton and connective tissue as well as other elements that make up the nervous system.

Anterior nerve cord elaborates into vertebrate brain

A nervous system has developed among insects and other invertebrates but no organism comes close to the refinement of its abilities to comprehend its surroundings and provide reasoned solutions to confronted problems like the power of the nervous systems exhibited among the higher vertebrates.

Chordata: Link to Invertebrates

Tunicate larva and tadpole

The major grouping of animals, or phylum, known as Chordata is of particular interest because it contains all vertebrate animals (subphylum Vertebrata) as well as provides a historic relationship with a number of organisms, like starfish, that show no identifiable similarities with mammals seen in the zoo.  What ancestor we have in common with this invertebrate would have to go back many millions of years.  Starfish belong to the phylum Echinodermata.  This grouping is made up of sea urchins, sea cucumbers and other marine animals in addition to the starfish.  Their link to us probably goes back well over five hundred million years, to the early Paleozoic era – near the beginning of fossils that have been discovered with the unaided eye. 

There is no clue about vertebrate origins when you examine an adult echinoderm.  Their morphology lacks any of the characteristics we associate with modern vertebrates – such as a brain and an internal skeleton.  The relationship with vertebrate ancestors is found in the larval stage of animals similar to these.  The larva of the sea squirt, the tunicates, has some of the hallmark characteristics found in Chordates – ancestors to the vertebrates.  These include a notochord (which serves as a backbone), a dorsal nerve cord and segmented muscles – all characteristics found in today’s vertebrate animals. 

Adult sea squirts

How juvenile traits are retained by adult descendants is explained by the process called paedomorphosis.  For chordates to descend from the larval form of some ancestral echinoderm would require the larva’s reproductive organs to mature prior to reaching the adult stage.  Descendants of this process might retain these larval characteristics into adulthood if they prove beneficial to the animal’s survival. 

The notochord is a flexible, rodlike structure that extends the length of the body providing the animal an axis for muscle attachment and giving the animal the undulating movement needed for propulsion through water.   For vertebrates the notochord appears during embryonic development and becomes the basis for the vertebrae. 

The dorsal nerve cord enables the development of a central nervous system and the enlargement of the anterior end into what becomes the organism’s brain.  This allows for a more sophisticated body plan based on greater awareness of one’s surroundings and the means to quickly move to more suitable locations and to pursue prey. 

Segmented musculature controlled by a centralized nervous system improves coordination and to enable a response time rapid enough to effectively pursue prey or to avoid the lunge of a predator.  These elements, found in the ancient fossils of the first chordates, are elaborated upon over time to include a postanal tail – a muscular appendage extending beyond the anus that significantly improves the animal’s means of propulsion.

The earliest chordates undoubtedly had pharyngeal pouches that were perforated and would be used for filter feeding.  These slits enabling water to pass through would later be the basis for respiration, gills that enabled the exchange of gases between the animal and the water.  Embryonic pharyngeal pouches give rise to the Eustachian tube, the middle ear and other devices in land-based vertebrates. 

X-Ray Tetra reveals vertebra

Other chordate characteristics that would be increasingly elaborated upon over time would include an endoskeleton, paired appendages, a complete digestive tract and a ventral heart with a closed blood system to go with it.  The body plan worked because it proved to be extremely adaptable and the resulting vertebrates were able to widely diversify in form and function to fit the demands of most of the world’s habitats.