![]() ![]() Many insects have a rope-ladder-like ventral nervous cord, composed of physically separated segmental ganglia. Ventral nerve cords are well-studied within insects, have been described in over 300 species covering all the major orders, and have remarkable morphological diversity. Ventral nerve cords are found in some phyla of the bilaterians, particularly within the nematodes, annelids and the arthropods. These interneurons filter, amplify, and integrate internal and external neural signals to guide and control movement and behavior. In addition to spiking neurons which transmit action potentials, some neural information is transmitted via non-spiking interneurons. It contains ascending and descending neurons that relay information to and from the brain, motor neurons that project into the body and synapse onto muscles, axons from sensory neurons that receive information from the body and environment, and interneurons that coordinate circuitry of all of these neurons. Like the vertebrate spinal cord, the function of the ventral nerve cord is to integrate and transmit nerve signals. In the small worm Meara stichopi there is a pair of dorsal nerve cords instead. 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. 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. Ventral nerve cord neurons are physically organized into neuromeres that process signals for each body segment. It is made of nervous tissue and is connected to the brain. The ventral nerve cord runs down the ventral ("belly", as opposed to back) plane of the organism. Because arthropods have an open circulatory system, 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. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord is a major structure of the invertebrate central nervous system. Right, a cross section of the ventral nerve cord, illustrating sensory input and motor output. ![]() Left, a schematic of the Drosophila central nervous system, including the brain and ventral nerve cord. ![]()
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