Detection and characterization of a sensory microganglion associated with the spinal accessory nerve: a scanning laser confocal microscopic study of the neurons and their processes.

The spinal accessory nerve has been generally thought to be a cranial nerve with purely motor function, innervating the trapezius and sternocleidomastoid muscles. The present study identified clusters of sensory neurons consistently associated with this cranial nerve in adult rats. Either a single microganglion or several dispersed microganglia were found that adhered to the spinal root of the nerve, to small vessels, or were free within the subarachnoid space. The neurons of the ganglion had axons that joined the spinal root of the nerve proximal to its exit from the skull. Additional branches appeared to have an intracranial distribution within the arachnoid of the brainstem and along its vessels. Several findings suggest that the function of the ganglion is sensory and not autonomic. First, the architectural features of neurons within the ganglion (including their size, pseudounipolar morphology, and the lack of synaptic contacts) are similar to those of neurons in other sensory ganglia. Second, substance P and calcitonin gene-related peptide coexist within neurons of the microganglion, whereas markers for the major transmitters found in autonomic ganglia in rats are absent. Third, the expression of peptides in neurons of the ganglion was sensitive to neonatal capsaicin treatment. Finally, neurons within the ganglion were filled with a retrogradely transported dye after injection of the dye into the cervical spinal cord. Although the function of the ganglion is not known, its features are consistent with a role in nociception from the muscles of the spinal accessory complex, and it may be involved in headaches that have an occipital distribution.

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.

Morphology and distribution of the motor neurons of the accessory nerve (nXI) in the Japanese toad: a cobaltic lysine study.

Motoneurons supplying the accessory nerve (nXI) of the Japanese toad were retrogradely labelled by applying the cobaltic lysine to the cut end of the nerve. They had morphological characteristics and a distribution pattern similar to those of the rostral spinal motoneurons rather than the branchial motoneurons. We propose that the anuran nXI is equivalent to the so-called spinal portion of the nXI of other vertebrates, and both should be regarded as part of the rostral spinal nerves rather than the nerve accessory to the vagus nerve.

Sensory pathways in the spinal accessory nerve.

We obtained samples of spinal accessory nerve from patients undergoing radical surgery for tumours or nerve grafting in the neck. These were analysed by light and electron microscopy for the type of fibre. All contained fibres consistent with non-proprioceptive sensory function including pain.

Central-peripheral transitional zone of the spinal accessory nerve in the rat.

The spinal accessory nerve rootlets emerge from the lateral aspect of the upper five segments of the cervical spinal cord underlying the nerve trunk. They cross the lateral funiculus of the cord with a slight rostral inclination. Here some pursue a relatively straight course while others have a dorsal convexity. The transitional zones may be classified into three distinct types, related to their orientation as they traverse the glia limitans to emerge as free rootlets. The fibres in Type 1 rootlets bend sharply rostrally on reaching the glia limitans. Type 2 rootlets turn ventrally to run in the glia limitans in the transverse plane of the cord before emerging. Type 3 rootlets are found only at C1. Their fibres initially turn caudally in the glia limitans and then loop rostrally. The morphology of the central-peripheral transitional zones of the spinal accessory rootlets closely resembles that of cervical ventral rootlets, and is therefore correlated with the motor function of these rootlets rather than with their intermediate location between the ventral and dorsal cervical rootlets.

Ultrastructural studies of the nucleus ambiguus in cat and monkey following injection of HRP into the vagus nerve.

The fine structure of retrogradely labelled neurons in the nucleus ambiguus (NA) has been examined in cat and monkey (Macaca fascicularis) following injections of horseradish peroxidase (HRP) into the vagus nerve. Many retrogradely labelled neurons were observed in the NA although unlabelled neurons were also present within the boundaries of the NA as identified by the distribution of retrogradely labelled cells. In both species a wide range of sizes of labelled neurons (20-60 microns in long axis) was observed from rostral to caudal levels of the NA. Large labelled neurons were generally oval or spindle-shaped while smaller neurons were oval in cross-section. Unlabelled neurons observed among labelled NA neurons tended to be smaller on average than the labelled neurons and ranged in size from 15 to 30 microns in long axis. The unlabelled neurons typically had nuclei which were more invaginated than those of the labelled neurons. Quantitative analyses of the synaptic organization in the NA revealed high proportions of terminals containing round vesicles and making asymmetrical or symmetrical contact with the somata in both monkey and cat. Terminals containing pleomorphic vesicles and making symmetrical contact with somata were slightly less numerous. Retrogradely labelled neurons exhibited a positive correlation between the size of neuron and density of synapses on the surface. There tended to be a greater synaptic density on monkey NA neurons than on cat NA neurons of comparable size.

Ultrastructural changes in the nerve fiber population of anastomosed vagal and spinal accessory nerves in the sheep.

BACKGROUND: The ultrastructure of the vagal and spinal accessory nerves was studied 1) in normal sheep and 2) in sheep in which an experimental crossed-nerve anastomosis had been made by sectioning the supranodose vagal and spinal accessory nerves, then suturing the distal end of the vagal nerve to the distal end of the spinal accessory nerve, and allowing time for regeneration to occur. This study was carried out in order to analyze the modifications liable to occur when this technique is used and to specify the origin and the nature of the fibers that colonize the spinal accessory nerve. METHODS: The study was performed in 4- to 5-month-old-sheep. After the surgical procedure, the animals were housed indoors during 1 year until their sacrifice by fixative perfusion. Then, nerve samples were dissected out, processed for electron microscopy, examined, and systematically photographed. After printing, the diameters of the nerve fibers were determined. RESULTS: In sheep, the ratios of nonmyelinated to myelinated fibers (NF/MF) in the infranodose and supranodose vagal nerve and accessory spinal nerve were 1.21, 1.67, and 3.21, respectively. In both parts of the vagal nerve, the myelinated fibers had a unimodal diameter distribution around a peak of 4 microns; whereas, in the spinal accessory nerve, they were distributed bimodally, and 53% had values of 15-18 microns. After making the above anastomosis, the centrifugal vagal fibers degenerated, and the NF/MF ratios increased in the centripetal infranodose vagal nerve, in the reinnervating supranodose vagal nerve, and in the reinnervated spinal accessory nerve (approximately 1.87, 1.72, and 6.04, respectively). In all of these nerves, the myelinated fibers had a unimodal distribution with a peak at 4 microns, as in the vagal nerve of normal sheep. CONCLUSIONS: These results reveal the large part taken by the nonmyelinated fibers in the nerve fiber population of the vagal nerve and support the vagal origin of the fibers reinnervating the spinal accessory nerve.