| Term: | primary motor neuron secondary motor neuron |
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| Note: | This page represents a term created by the combination ("post-composition") of two ontology terms. For more information on the individual terms, click the hyperlinked name. |
| Name: | primary motor neuron |
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| Synonyms: | primary motoneuron |
| Definition: | Motor neurons that arise during the first day of development. Primary motoneurons have larger diameter cell bodies and axons than secondary motoneurons and are positioned more dorsally and medially than secondary motoneurons. Primary motoneurons are present in a single bilateral pair of clusters in each spinal segment and innervate axial muscle fibers in the corresponding pair of myotomes. Axons make a conspicuous loop around the Mauthner axon before entering the ventral root. |
| Ontology: | Anatomy Ontology [ZFA:0009244] |
| Name: | secondary motor neuron |
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| Synonyms: | secondary motoneuron |
| Definition: | Motor neurons that develop several hours later than primary motoneurons and sprout axons several hours after the primary motor growth cones have extended to the periphery. They have smaller cell bodies and are located more ventrally and laterally in the spinal cord than primary motoneurons. Axons usually project directly out of the spinal cord with no apparent contact with the Mauthner axon and are positioned lateral to the Mauthner axon in their path to the ventral root. Secondary motoneurons have smaller fields, which can overlap one another, and innervate many fewer muscle fibers than primary motoneurons. |
| Ontology: | Anatomy Ontology [ZFA:0009247] |