Membrane properties and contraction of single muscle fibers in the mouse.

LUFF, A. R., AND H. L. ATWOOD. Membrane properties and contraction of single muscle fibers in the mouse. Am. J. Physiol. 222(6) : 14351440. 197 1 .-Isometric twitch contractions of single fibers in the soleus (SOL) and extensor digitorum longus (EDL) muscles of the mouse were recorded by means of a sensitive mechanoelectric transducer. Propagated action potentials and membrane electrical constants were also measured. Some fibers from each muscle were examined under an electron microscope. All measured parameters showed a wider range of values in the SOL than in the EDL. However, the mean time-to-peak tension for SOL fibers was significantly longer than that for EDL fibers; mean specific membrane resistance was significantly higher and membrane capacitance was lower. The action potentials of SOL fibers were of about the same amplitude as those of EDL fibers, but of slightly greater duration. The transverse tubular system and sarcoplasmic reticulum were more fully developed in EDL fibers. The differences in membrane resistance and capacitance can be attributed to ultrastructural features. Although differences in contraction speed are largely attributable to biochemical differences, it is likely that the membrane properties described above relate directly to the different dynamic properties.