Freeze-fracture of frog slow tonic fibers. Structure of surface and internal membranes.

Slow frog fibers from frog cruralis were examined by freeze-fracture and compared with twitch fibers. Major differences in membrane architecture between slow and twitch fibers are in the shape and disposition of T tubules, the shape of junctional regions, the frequency of peripheral couplings and of Ca pump particles. Aggregates of ACh receptors on the surface membrane are distinguishable from peripheral couplings on the basis of size and shape of intramembranous particles.