[Organization of pectoral muscle motor neurons in the rat. Contribution to the study of the axillary arch (Achselbogen)].

In spite of their role of the forelimb activity, the organization of the mammalian pectoral muscles remains unclear. In order to get more information, we have studied, qualitatively and quantitatively, the motor pools of the various pectoral muscles in the rat (musculus pectoralis major, musculus pectoralis minor and musculus panniculus carnosus). The method applied was the retrograde transport of horseradish peroxidase by the peripheral nerves supplying the various muscles. The results provided the precise localization within the ventral horn of the motor neurons from the pectoral muscles which were all in the most ventral part of the gray matter. Three distinct motor pools were observed, which confirms the functional identity of each muscle. The motor neurons of m. panniculus carnosus exhibited some particularities. They were 4 times more numerous than those from m. pectoralis major, they occupied the whole ventral edge of the ventral horn, from its medial part to its lateral tip. The cell size was different from that of the other muscles. These results were discussed, according to the role of this muscle, which links the forelimb to the whole trunk. In mammals, m. panniculus carnosus constitutes, with musculus latissimus dorsi, the axillary arch (Achselbogen). A small area of overlapping between the motor pools of these two muscles was observed. It was suggested that the muscular fibers from the axillary arch were innervated by the same, well-defined spinal area, independent of the muscle from which they originate.