Neuromuscular relationships in a muscle having segregated motor endplate zones. I. Anatomical and physiological...

The cranial portion of the rat anterior gracilis muscle is innervated by the obturator nerve at two discrete motor endplate zones of which the distal is supplied by well-defined branches of the nerve. Prior to the use of this muscle in a study of motoneuron sprouting, further morphological and physiological studies have shown that its fibers vary in length but a number traverse the whole muscle or are long enough to extend through both endplate zones. The distribution of muscle fiber types is typical of a rat fast-twitch muscle, and each fiber is innervated at a single endplate. Myography showed that the spinal cord segments, which may contribute to the muscle's innervation, are L2, 3, and 4, of which L3 is constant and predominant, and that denervation of the distal endplate zone leads to a 50% reduction of the maximum isometric tension developed by the muscle. Antidromic stimulation of nerves supplying the distal endplate zone produced contraction of the proximal part of the muscle and, following similar antidromic stimulation, intracellular recordings made at proximal zone endplates showed the presence of endplate potentials. It was concluded from these data that endplates at both the proximal and distal zones can form part of the same motor unit.