Types of muscle fibers in the extraocular muscles of birds

Abstract

Eye muscles of the quail, pigeon, sparrow and canary were examined histologically to see if their fibers are differentiated into two or more types, as has been described for mammals. In each oblique and rectus muscle examined, an area along the orbital border was found to be occupied by fibers of small cross-sectional area, and a more extensive central portion predominantly by large-caliber fibers. These fiber populations were separable on the basis of levels of α-glycerophosphate dehydrogenase and reduced nicotinamide adenine dinucleotide diaphorase activity, and to a lesser degree by glycogen content. All fibers stained strongly with the myosin adenosine triphosphatase reaction, the level of the activity being comparable to that in fast-twitch, mammalian limb muscles. The differences in features of location, size and histochemical reaction indicate that the rectus and oblique muscles in birds contain at least two types of fibers. The quadratus and pyramidalis muscles, which serve to advance the nictitating membrane, did not show segregation into zones of fibers, although in the pyramidalis two intermixed fibers were generally of larger size in the pyramidalis. Both fiber types stained darkly with myosin adenosine triphosphatase, suggesting that they were fast-twitch fibers.