Structural changes in single muscle fibers after stimulation at a low frequency

Area of Science:

• Muscle Physiology
• Cellular Ultrastructure
• Excitation-Contraction Coupling

Background:

• Muscle fatigue involves complex cellular mechanisms.
• Understanding the ultrastructural basis of muscle contraction is crucial.

Purpose of the Study:

• To investigate ultrastructural changes in Xenopus laevis muscle fibers during direct stimulation.
• To correlate these changes with muscle tension decline and fatigue levels.

Main Methods:

• Single muscle fibers from Xenopus laevis were directly stimulated at 1 Hz.
• Fibers were chemically fixed at various twitch counts and fatigue levels.
• Ultrastructural analysis was performed using electron microscopy and compared to resting fibers.

Main Results:

• Subtle ultrastructural changes were observed, including increased mitochondrial matrix width and slight swelling in the myosin lattice.
• The transverse tubule (T system) and sarcoplasmic reticulum remained intact.
• Increased electron density in terminal cisternae (TC) and more frequent T-tubule to TC (T-TC) connecting pillars were noted in stimulated fibers.

Conclusions:

• Observed structural changes correlate with the number of twitches, not fatigue level.
• Increased TC density and T-TC pillar formation are likely integral to normal excitation-contraction coupling mechanisms.