The muscle-tendon junctions of fast and slow fibres in the garter snake: ultrastructural and stereological analysis and comparison with other species.

The muscle-tendon junctions of fast and slow muscle fibres from the costocutaneous muscles of garter snakes have been analyzed by scanning and transmission electron microscopy and by a stereological protocol. The junctions of the two fibre types are morphologically similar, and consist of anastomosing processes which increase the surface area of the muscle-tendon interface and significantly reduce the stress at the junction when the fibres contract. The muscle-tendon junctions of these fibres are analogous to adhesive joints between stiff adherends and fibre-reinforced composite materials. Stereological estimates of the area of surface membrane devoted to force transmission, relative to the cross-sectional area of myofilaments (the 'interfacial ratio'), have produced values of about 14 for the fast fibres and about 16 for the slow fibres. This difference is small but statistically significant. Interfacial ratios of about 14 and 10 were previously determined for fast and slow fibres, respectively, of both chickens and mice. Thus the slower fibres of the snake differ from those of the mouse and chicken, in that their interfacial ratio is greater than that of fast fibres.