Analysis of fiber types in the pigeon's metapatagialis muscle. II. Effects of denervation.

The pigeon's metapatagialis muscle consists of three slips, two twitch and one tonic, and these slips are distinguishable at the gross anatomical level. Comparative studies of denervation are facilitated because the two fiber types are under the same mechanical forces, can be denervated as one muscle, and can be distinguished after denervation. Both fiber types atrophied after denervation, with the twitch fibers having a more variable response. Pathological alterations observed by light microscopy suggested that the twitch fibers were more affected by denervation than the tonus fibers. Ultrastructurally, both fiber types showed the same changes, with the twitch fibers again being more consistently altered. Proliferation of the transverse tubular system and sarcoplasmic reticulum were more marked in the tonus than twitch fibers, and the sarcoplasmic reticulum proliferated prior to the transverse tubules. Filament and fibril degeneration, peripheral and central degeneration, lysosomes and their derivatives, and satellite cell proliferation were common to both fiber types. Contracture knots were common to the denervated fibers, and were suggested to be characteristic of degenerating fibers. Degenerating motor end plates were observed, and most neurons in the fibers were naked, lacking myelin sheaths. The results are discussed in relation to the function of the neuron in maintaining the muscle, and the possibility of denervation inducing a transformation of tonic to twitch fibers.