Motoneuron organisation 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 dihydrochloride and Fast Blue was used to determine the location of the spinal nucleus of the accessory nerve in the sheep. We also considered whether in this species the sternocephalic, brachiocephalic, omotransversarius 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 C1 to C7 spinal cord segments and occupies a lateral position within the ventral horn. The most rostral portion of this column is located dorsolaterally, whereas the remaining portion from C2 to C7 occupies a ventrolateral position. At C1 and C4 levels the nucleus also possesses some cells with a medial location. All the muscles of the spinal accessory complex receive their motor innervation both from the spinal accessory nucleus and from motoneurons forming the cervical spinal nerves. A double motor innervation of these muscles is thus present in the sheep.