The accessory nerve nucleus in the baboon.

In the savanna baboon, Papio cynocephalus, the accessory nerve nucleus was identified by using a mixture of 20% free horseradish peroxidase (HRP) and 2.5% HRP conjugated to wheat germ agglutinin (WGA) in a 5% aqueous detergent solution (Nonidet P-40). Following surgical exposure of the appropriate nerve branch to the sternocleidomastoid or trapezius muscle, the nerve was transected, placed in an Argyle tubing collar, and bathed in 5-10 microliter of the tracer. After a 48-hour survival time and vascular perfusion-fixation, 40-micron sections of the lower medulla oblongata and the cervical spinal cord were treated according to the tetramethyl benzidine (TMB)-HRP method of Mesulam (J. Histochem. Cytochem. 26: 106-117, 1978). The accessory nucleus extends as a distinct column of neurons from lower medullary levels into the rostral part of C5. One to ten labeled cells were present in each section, and all labeled neurons were located on the side of the bathed nerve. The rostral portion of the accessory nucleus occupies a central position, its intermediate portion occupies a lateral position, and its caudal portion occupies a central position within the ventral horn. All labeled neurons were confined to Rexed's lamina IX, ranged from 15 to 75 micron in diameter, and were either distinctly round (oval) or stellate in shape. Neurons within the baboon accessory nucleus supplying the sternocleidomastoid muscle were located from lower medullary to upper C2 spinal cord levels, while those supplying the trapezius muscle extended from C2 to C5.