Membrane Systems of Crab Fibers

An electron microscopic study of internal and surface-connectedmembrane systems of leg muscle of the crab shows that thereare three kinds of surface-connected membrane systems in additionto an intracellular sarcoplasmic reticulum (SR). One is a systemof large infoldings of the sarcolemma referred to as clefts.These are longitudinally-oriented, flattened infoldings of boththe plasma membrane and the fibrous sheath of the fiber, andwere probably seen earlier with the light microscope. Extendinginto the fiber both from these clefts and from the free fibersurface are two systems of tubules of much smaller caliber,the Z tubules and the A tubules. The Z tubules are located,as their name indicates, near the Z lines of the myofibrils,and are thought to be attached to them mechanically. The A tubulesare found in pairs, near the ends of each A band, and are closelybound to the SR in two-part structures called dyads. Local-activationexperiments, like those done earlier by Huxley and Taylor, suggestthat the A tubules are involved in excitation-contraction coupling;no such experimental suggestion of function exists for the Ztubules.