Invaginated membrane in crustacean tonic muscle fibers: estimates of membrane capacitance

Surface and volume densities of the invaginated membrane components of tonic muscle fibers of crayfish (Orconectes rusticus) were determined by stereological methods. Surface densities of the sarcolemma, sarcolemmal invaginations, T tubules and Z tubules were 2.0, 5.9, 26.3, and 3.3 micron2/100 micron3 of fiber, respectively, and volume densities of the sarcolemmal invaginations, T tubules, and Z tubules were 0.28, 0.29, and 0.08 micron3/100 micron3. The values obtained for surface density show that 95% of the membrane exposed to the extracellular environment is composed of invaginated membrane. Specific membrane capacitance, determined from square-wave pulse analysis and referred to the apparent area of surface sarcolemma, was 30.3 muF/cm2. However, after correction for the extent of invaginated membrane, the specific membrane capacitance was 1.6 muF/cm2.