Musculi longus capitis et splenius of the rat and innervating motoneurons (author's transl)

The nuclei for the nerves of a dorsal (m. splenius) and a ventral (m. longus capitis) neck muscle of the rat were retrogradely labeled by applying horseradish peroxidase (HRP) to the respective cut muscle nerves. Motoneurons of both muscles were analyzed for their localization, diameter of perikarya, and area of dendritic arborization. The nucleus of m. longus capitis is situated dorsomedially, the nucleus of m. splenius ventromedially within the ventral horn. Thus, the relative positions of the two nuclei are inverse to those of their muscles, with the more ventral nucleus innervating the more dorsal muscle. In both nuclei the areas of dendritic arborization are very large, extending into the nuclei of other neck muscles, and also into the ipsilateral anterior funiculus. In addition, m. longus capitis motoneurons were found to send dendrites into the contralateral ventral horn, reaching the nucleus of the contralateral muscle. The size distribution of perikarya is bimodal for m. longus capitis motoneurons, but only unimodal in the case of m. splenius.