Ultrastructure of nerve endings in the stapedius muscle of the squirrel monkey.

The stapedius muscle of the squirrel monkey (Saimiri sciureus) was studied electron microscopically with regard to neuromuscular junctions, muscle-tendon junctions and tendons. Two types of neuromuscular junctions were identified. The first type which was predominant in number showed well-developed primary and secondary synaptic clefts. They were almost the same with those of other mammalian skeletal muscles. The second type showed a flat-shaped junction with fewer secondary synaptic clefts. Muscle-tendon junctions were characterized by many interdigitations between the muscle fibers and tendon. The tendon was composed of collagen fibers, approximately 70-100 nm in diameter. Muscle spindles were not present, while nerve endings resembling the Golgi tendon organ were found adjacent to the tendon and the muscle-tendon junctions. These findings suggest that a feedback system may exist and regulate the fine contractile function of this muscle.