Compartmentalization in the triceps brachii motoneuron nucleus and its relation to muscle architecture

Each fascicle of the triceps brachii (TB) can be independently recruited during movement execution. To investigate the anatomical basis of this selective control and to gain insight into its functional role in adult rats, we carried out a triple retrograde tracing of the motoneurons (MNs) innervating each TB head, and performed muscle ATPase histochemistry and histology to correlate the size of the MN pools with the number and type of muscle fibers innervated. No double-labeled MNs were found, demonstrating that each TB head is innervated by a completely independent MN subnucleus. Absolute cell counts determined that the long fascicle had the largest MN subnucleus, followed by the medial and the lateral fascicles. MNs of the three fascicles intermingled extensively in the rostral part of the spinal motor column, while the caudal part of the column comprised mostly MNs innervating the long fascicle. Muscle histology and average innervation ratios estimated from absolute MN counts showed that the medial head was predominantly formed by small type I fibers and motor units (69 fibers/MN). In contrast, the lateral fascicle comprised a great quantity of large type IIb fibers and motor units (179 fibers/MN), whereas the long head consisted of a more balanced mixture of fiber types and motor units (99 fibers/MN). Taking into account the mechanical and physiological heterogeneity of the TB, our findings suggest that each fascicle may be considered an independent muscle with specific functional roles.