The ventral nerve cord is a major structure of the
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
. It is the functional equivalent of the
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
.
The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output.
Decapitated insects can still walk, groom, and mate, illustrating that the circuitry of the ventral nerve cord is sufficient to perform complex motor programs without brain input.
Structure
The ventral nerve cord runs down the
ventral
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
("belly", as opposed to back) plane of the organism. It contains ascending and
descending neurons that relay information to and from the brain,
motor neurons
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...
that project into the body and synapse onto muscles,
sensory neurons
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
that receive information from the body and environment, and
interneurons
Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. In ...
that coordinate circuitry of all of these neurons.
Ventral nerve cord neurons are physically organized into neuromeres that process signals for each body segment.
Anterior neuromeres control the anterior body segments, such as the forelegs, and more posterior neuromeres control the posterior body segments, such as the hind legs. Neuromeres are connected longitudinally, anterior to posterior, by fibrous nerve tracts called connectives. Pairs of hemisegments, corresponding to the left and right side of the ventral nerve cord, are connected horizontally by fibrous tracts called commissures.
In the small worm
Meara stichopi
''Meara '' is a genus of bilaterally symmetric, small aquatic worms in the phylum Acoelomorpha. This genus contains only one species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as wel ...
there is a pair of dorsal nerve cords instead.
Evolution
Ventral nerve cords are found in some
phyla of the
bilaterian
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...
s, particularly within the
nematode
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
s,
annelid
The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...
s and the
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s. Ventral nerve cords are well-studied within
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s, have been described in over 300 species covering all the major orders, and have remarkable morphological diversity. Many insects have a rope-ladder-like ventral nervous cord, composed of physically separated
segmental ganglia. In contrast, in Drosophila, the thoracic and abdominal neuromeres are contiguous and the whole ventral nerve cord is considered to be one ganglion.
The presumed common ancestral structure is rarely observed; instead the ventral nerve cords of most insects show extensive modification as well as
convergence
Convergence may refer to:
Arts and media Literature
*''Convergence'' (book series), edited by Ruth Nanda Anshen
* "Convergence" (comics), two separate story lines published by DC Comics:
**A four-part crossover storyline that united the four Wei ...
. Modifications include shifts in
neuromere
Neuromeres are morphologically or molecularly defined transient segments of the early developing brain. Rhombomeres are such segments that make up the rhombencephalon or hindbrain. More controversially, some argue that there exist early develo ...
positions, their fusion to form composite ganglia, and, potentially, their separation to revert to individual ganglia.
In organisms with fused neuromeres, the connectives are still there but are very reduced in length.
Development
The insect ventral nerve cord develops according to a body plan based on a segmental set of 30 paired and one unpaired
neuroblasts
In vertebrates, a neuroblast or primitive nerve cell is a postmitotic cell that does not divide further, and which will develop into a neuron after a migration phase. In invertebrates such as ''Drosophila,'' neuroblasts are neural progenitor cells ...
. 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
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...
'' suggests that all neurons of a given hemilineage release the same primary
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neuro ...
.
Engrailed is a
transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
that helps regulate the gene ''frazzled'' in order to separate
neuroblasts
In vertebrates, a neuroblast or primitive nerve cell is a postmitotic cell that does not divide further, and which will develop into a neuron after a migration phase. In invertebrates such as ''Drosophila,'' neuroblasts are neural progenitor cells ...
during embryonic development. The segregation of neuroblasts is essential for the formation and development of the ventral nerve cord.
See also
*
Dorsal nerve cord
The dorsal nerve cord is a unique feature to chordates, and it is mainly found in the Vertebrata chordate subphylum. The dorsal nerve cord is only one embryonic feature unique to all chordates, among the other four chordate features-- a notochord, ...
in
chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...
s
*
Supraesophageal ganglion
The supraesophageal ganglion (also "supraoesophageal ganglion", "arthropod brain" or "microbrain") is the first part of the arthropod, especially insect, central nervous system. It receives and processes information from the first, second, and th ...
, the arthropod "brain"
*
Nerve net
A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a condensation of neurons or, in more advanced forms, a central nervous syst ...
in
cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
and
echinodermata
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...
phyla
*
Hemichordates
Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and includ ...
, who have both dorsal and ventral nerve cords
References
External links
Comparison of spinal cord and ventral nerve cordInsect morphology
{{DEFAULTSORT:Ventral Nerve Cord
Annelid anatomy
Arthropod anatomy
Nematode anatomy
Invertebrate nervous system