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

Abstract

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.

Dynamic Motor Compensations with Permanent, Focal Loss of Forelimb Force after Cervical Spinal Cord Injury

“…7C, C') and that the three MN subnuclei preserved the topographical distribution described for normal animals. 46 Furthermore, double-labeled neurons were not found (Fig. 7D), indicating that each fascicle maintained its independent innervation.…”

“…It was necessary to investigate whether significant MN death occurred in the whole TB motor nucleus or in the subnucleus of the lateral fascicle, which is predominantly formed by large motor units and muscle fibers, 46 because either of those phenomena will produce a substantial reduction of elbow extensor force after C6 hemisection. At the end of the follow-up, animals received neuronal tracers in the right foreleg to identify the TB MNs in the injured side of the spinal cord.…”

“…The surgical procedures for both types of neural tracing have been described in detail elsewhere. 46 The animals were sacrificed 3.5 days later for histological processing. An additional group of eight animals with hemisection at the middle part of C6 (1 mm rostral to the usual lesion site) was used to investigate if nerve axotomy or the longitudinal extent of the lesion influenced the MN counts.…”

“…We have previously compared three different methods (namely simple, absolute, and stereological) for counting the MNs in each of the TB fascicles. 46 The simple method consisted in counting all labeled cell profiles with identifiable nucleus or at least three dendrites in every tissue section. We found that this method was more efficient and accurate than the stereological estimation for this purpose and, hence, in the present study, we chose the simple counting to compare the relative numbers of MNs in the different experimental conditions.…”

“…Each of the TB fascicles is an independent neuromuscular compartment with different number, size, and type of motor units and muscle fibers. 46 The segment C7 contains approximately 60% of TB MNs, and C7 together with C8 account for 90% of the total MN pool. The three MN subnuclei intermingle in C7, which contains approximately 90% of the MNs that innervate the lateral and medial fascicles and about 56% of the MNs of the long head.…”