Distribution and morphology of abducens motoneurons innervating the lateral rectus and retractor bulbi muscles in the frog Rana ridibunda.

The distribution and morphology of the neurons innervating the lateral rectus and retractor bulbi muscles of the frog Rana ridibunda were studied by means of retrograde axonal transport of horseradish peroxidase (HRP). The neurons are located in the rhombencephalon in two distinct groups which lie dorsolateral and lateral to the fasciculus longitudinalis medialis and represent the main and accessory abducens nuclei, respectively. From HRP injections into the two muscles, it can be concluded that multipolar neurons located rostrally in the main abducens nucleus innervate the lateral rectus muscle. The retractor bulbi muscle receives its innervation from bipolar, fusiform neurons located in the caudal portion of the main abducens nucleus and throughout the rostrocaudal extent of the accessory abducens nucleus.