Theventral nerve cord is a major structure of theinvertebratecentral nervous system. It is the functional equivalent of thevertebratespinal cord.[2] The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output.[1] Because arthropods have anopen circulatory system, decapitated insects can still walk, groom, and mate—illustrating that thecircuitry of the ventral nerve cord is sufficient to perform complex motor programs without brain input.[3]
The ventral nerve cord runs down theventral ("belly", as opposed to back) plane of the organism. It is made of nervous tissue and is connected to the brain.
Ventral nerve cord neurons are physically organized into neuromeres that process signals for each body segment.[4] 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.[4][5]
In the small wormMeara stichopi there is a pair of dorsal nerve cords instead.[6]
For the fruit flyDrosophila melanogaster, theconnectome of the ventral nerve cord (and its connections to the brain) has been reconstructed down to the level of individual neurons and synapses, for both male[7] and female[8] flies. SeeDrosophila connectome.
Like the vertebrate spinal cord, the function of the ventral nerve cord is to integrate and transmit nerve signals. It contains ascending anddescending neurons that relay information to and from the brain,motor neurons and theircentral pattern generators that project into the body and synapse onto muscles,axons fromsensory neurons that receive information from the body and environment, andinterneurons that coordinate circuitry of all of these neurons.[3] In addition to spikingneurons which transmitaction potentials, some neural information is transmitted vianon-spiking interneurons. These interneurons filter, amplify, and integrate internal and external neural signals to guide and control movement and behavior.[9]
Ventral nerve cords are found in somephyla of thebilaterians, particularly within thenematodes,annelids and thearthropods. Ventral nerve cords are well-studied withininsects, 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 separatedsegmental ganglia. In contrast, inDrosophila, the thoracic and abdominal neuromeres are contiguous and the whole ventral nerve cord is considered to be one ganglion.[5] The presumed common ancestral structure is rarely observed; instead the ventral nerve cords of most insects show extensive modification as well asconvergence. Modifications include shifts inneuromere positions, their fusion to form composite ganglia, and, potentially, their separation to revert to individual ganglia.[4] In organisms with fused neuromeres, the connectives are still there but are very reduced in length.[4]
The insect ventral nerve cord develops according to a body plan based on a segmental set of 30 paired and one unpairedneuroblasts.[10] 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.[11][12] Each neuroblast gives rise to two hemilineages: an "A" hemilineage characterized by activeNotch signalling, and a "B" hemilineage characterized by an absence of active Notch signalling.[13] Research in the fruit flyD. melanogaster suggests that all neurons of a given hemilineage release the same primaryneurotransmitter.[14]
Engrailed is atranscription factor that helps regulate the genefrazzled in order to separateneuroblasts during embryonic development. The segregation of neuroblasts is essential for the formation and development of the ventral nerve cord.[15]