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Membrane particles and transmission at the triad
Abstract:
The structure of the membranes limiting the sarcoplasmic reticulum (SR) and transverse (T) tubules in frog and fish muscle fibers has been studied by freeze fracture. Emphasis is placed on the structure of the membranes at the triad, where thin sections have previously shown that rows of regularly disposed "feet" join SR and T exposed fracture faces allows the following conclusions. 1) The SR membrane is continuous and identical in appearance along the whole sarcomere. Thus the SR is a single uninterrupted compartment and it is likely that the major function of the reticulum, calcium accumulation is performed by the membrane limiting the lateral sacs of the triad, as well as the longitudinal tubules. 2) At the level of the junction with the T tublue, the SR presents a strikingly different number, size and arrangement of particles and pits. This distinct portion of the SR membrane extends father than the area covered by the junctional "feet" and no correlation can be found between the disposition of particles within the membrane and that of the feet on the membrane surface. 3) The T system membrane presents few prominent particles on its junctional face, but these are far less numerous than the feet. 4) Thus, no visible preformed channels exist between SR and T system lumina and it is suggested that direct electrical coupling between the two membranes during excitation is unlikely.
Insights
Freeze fracture microscopy reveals the sarcoplasmic reticulum (SR) membrane is continuous. However, distinct structural differences at the triad junction suggest no direct electrical coupling between the SR and transverse tubules.
Area of Science:
- Muscle physiology
- Cell biology
- Membrane biophysics
Background:
- The sarcoplasmic reticulum (SR) and transverse (T) tubules are crucial for muscle excitation-contraction coupling.
- Previous studies using thin sections indicated specialized structures, "feet," at the junction between SR and T tubules.
Purpose of the Study:
- To investigate the detailed structure of SR and T tubule membranes at the triad junction using freeze fracture.
- To elucidate the relationship between membrane structure and potential communication pathways between SR and T tubules.
Main Methods:
- Freeze fracture electron microscopy was employed to examine the membrane structures of SR and T tubules in frog and fish muscle fibers.
- Analysis focused on the junctional regions, particularly the triad, comparing SR and T tubule membrane faces.
Main Results:
- The SR membrane exhibits a continuous and uniform structure throughout the sarcomere, indicating a single compartment.
- The SR membrane at the T tubule junction displays unique particle and pit arrangements, distinct from other regions and not correlated with the "feet."
- The T tubule membrane has fewer particles on its junctional face compared to the "feet."
Conclusions:
- The sarcoplasmic reticulum functions as a unified compartment for calcium storage and release.
- The observed membrane structures at the triad do not support the existence of preformed channels between the SR and T tubules.
- Direct electrical coupling between the SR and T system membranes during muscle excitation is considered unlikely based on structural evidence.