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Application of horseradish peroxidase to lesions of the muscles and the central nervous system of larval zebrafish Brachydanio rerio was used to identify several types of neurons present in the spinal cord. The spinal cord was found to contain three distinct motoneuronal types: primary and secondary motoneurons that innervate the axial muscles, and pectoral fin motoneurons that innervate the muscles of the pectoral girdle. The cell types are similar to those described in larvae of other anamniote vertebrates. The axial muscles of a given hemisegment are innervated by two or three primary motoneurons and a larger number of secondary motoneurons in the corresponding spinal segment, whereas fin muscles are innervated by a pool of motoneurons spanning several spinal segments.
Paul Z. Myers (Sat,) studied this question.