MOTONEURON ORGANIZATION OF THE MUSCLES OF THE SPINAL ACCESSORY COMPLEX OF THE SHEEP INVESTIGATED WITH THE FLUORESCENT RETROGRADE TRACER TECHNIQUE

Retrograde transport of the fluorescent tracers Diamidino Yellow dihyd rochloride and Fast Blue was used to determine the location of the spi nal nucleus of the accessory nerve in the sheep. We also considered wh ether in this species the sternocephalic, brachiocephalic, omotransver sarius and trapezius muscles, i.e. the muscles of the spinal accessory complex, are supplied by more than one population of motoneurons. The spinal accessory nucleus extends as a single column of neurons from C 1 to C7 spinal cord segments and occupies a lateral position within th e ventral horn. The most rostral portion of this column is located dor solaterally, whereas the remaining portion from C2 to C7 occupies a ve ntrolateral position. At C1 and C4 levels the nucleus also possesses s ome cells with a medial location. All the muscles of the spinal access ory complex receive their motor innervation both from the spinal acces sory nucleus and from motoneurons forming the cervical spinal nerves. A double motor innervation of these muscles is thus present in the she ep.