Structural dynamics of frog muscle during isometric contraction

Area of Science:

• Biophysics
• Muscle Physiology

Background:

• Muscle contraction is a complex process involving coordinated molecular events.
• Understanding the dynamic changes within muscle fibers during contraction is crucial for elucidating muscle function.

Purpose of the Study:

• To investigate the dynamic behavior of scattering elements within actively contracting muscle.
• To characterize the nature of internal muscle movements during isometric tetanus using laser light scattering.

Main Methods:

• Measurement of intensity fluctuation autocorrelation functions of laser light scattered by contracting muscle.
• Analysis of scattering patterns at various points in the scattered field.
• Experiments conducted on whole sartorius muscle and bundles of semitendinosus muscle fibers.

Main Results:

• Reproducible autocorrelation functions were obtained, dependent on muscle physiological state and scattering geometry.
• Autocorrelation functions exhibited large amplitudes and decay rates varying with the contraction-relaxation cycle.
• Scattering geometry dependence suggested independent, random changes in axial positions and polarizability of elements ~0.5 µm (sarcomeres/bands).
• A model proposed random fluctuating axial velocities (avg. 20 nm/ms) and local polarizability fluctuations in scattering elements.

Conclusions:

• Actively contracting muscle exhibits dynamic internal movements at the sarcomere level.
• These movements contribute a fluctuating component to the overall tension developed during isometric tetanus.
• The findings provide insights into the mechanical and optical properties of muscle during activation.