Evolutionary and developmental understanding of the spinal accessory nerve.

The vertebrate spinal accessory nerve (SAN) innervates the cucullaris muscle, the major muscle of the neck, and is recognized as a synapomorphy that defines living jawed vertebrates. Morphologically, the cucullaris muscle exists between the branchiomeric series of muscles innervated by special visceral efferent neurons and the rostral somitic muscles innervated by general somatic efferent neurons. The category to which the SAN belongs to both developmentally and evolutionarily has long been controversial. To clarify this, we assessed the innervation and cytoarchitecture of the spinal nerve plexus in the lamprey and reviewed studies of SAN in various species of vertebrates and their embryos. We then reconstructed an evolutionary sequence in which phylogenetic changes in developmental neuronal patterning led towards the gnathostome-specific SAN. We hypothesize that the SAN arose as part of a lamprey-like spinal nerve plexus that innervates the cyclostome-type infraoptic muscle, a candidate cucullaris precursor.