start stage end stage motor neuron primary neuron primary motor neuron TAO:0009244 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. ZFA:0009244 CL:0000533 primary motoneuron zebrafish_anatomy ZFA:0000572 unknown segmentation:1-4 somites