Presence of unmyelinated axons in the spinal root of the feline accessory nerve

The spinal root of the accessory nerve was studied electron microscopically at different levels in 3 adult cats. It was found that this nerve contains several unmyelinated axons. In the main nerve trunk the proportion of unmyelinated axon profiles was 27% at the level of the foramen magnum. In juxtamedullary root fascicles near the PNS/CNS transition the proportion of unmyelinated axons was lower and these axons tended to occupy superficial positions in the fascicles. No intrafascicular unmyelinated axons were found in the immediate vicinity of the PNS/CNS transition but bundles of unmyelinated axons occurred in the surrounding pia mater. The findings suggest that unmyelinated axons in the spinal accessory nerve contribute to the pial vasomotor and/or sensory innervation.