The organization of acromiodeltoid and spinodeltoid motor nuclei in rat spinal cord.

The retrograde intraaxonal transport of fluorescent dyes (10% Bisbenzimide or 2% Nuclear Yellow) was used to identify the motoneuron populations that control two prime movers of the rat shoulder joint, the acromiodeltoid (Ad) and spinodeltoid (Sd) muscles. The Ad and Sd motor nuclei were both comprised of an average 30-40 motoneurons distributed between spinal segments C5 and C7, in lateral regions of Rexed's lamina IX. Although there was considerable overlap among them, the center of the Ad motor nucleus was consistently located more rostral and medial than that for the Sd motor nucleus.