lecture recording on 17 March 2025 at 18.29.46 PM

Terminal Cisternae and Triads

• Terminal cisternae are structures adjacent to T-tubules in muscle cells.

• Groups of these structures, along with T-tubules, form triads.

• Triads play a crucial role in calcium ion communication essential for muscle contraction.

Muscle Contraction Mechanism

• Skeletal muscle contraction is initiated by action potentials traveling from the nervous system to muscle cells.

• Action potentials propagate through T-tubules, leading to calcium release from the sarcoplasmic reticulum (SR).

• This calcium release triggers the contraction of myofibrils and therefore the entire muscle.

Skeletal Muscle Contraction Dynamics

• In muscles, even at rest, skeletal muscles hold posture and keep the body balanced.

• The contraction occurs in a coordinated manner across multiple muscle fibers, enhancing movement effectiveness.

• Muscle contraction involves various structural levels: myofibrils → muscle cells (myocytes) → fascicles → entire muscle organs.

Myofibrils and Myofilaments

• Myofibrils consist of stacked units called sarcomeres, composed of thick (myosin) and thin (actin) filaments.

• Sarcomeres are the smallest functional units where muscle contraction occurs. They are characterized by:

  • A Band: Region with thick myosin filaments.

  • I Band: Region with only thin actin filaments.

Cross-Bridge Cycle

1. Myosin heads bind to exposed sites on actin filaments, forming cross-bridges.

2. The myosin heads perform a power stroke powered by ATP, pulling actin filaments towards the center of the sarcomere, shortening the muscle.

3. Myosin detach from actin when a new ATP molecule binds.

4. Myosin heads reset to their original position, ready to bind again if calcium is present.

Role of Calcium and ATP

• Calcium ions are key regulators in muscle contraction; they bind to the troponin-tropomyosin complex, uncovering binding sites for myosin.

• ATP is essential for both cross-bridge formation and relaxation. Muscle fatigue occurs when ATP availability diminishes.

Relaxation of Muscle

• Muscle relaxation occurs after stimulation ceases: calcium ions are reabsorbed, restoring troponin-tropomyosin blockage over actin binding sites.

• Myosin heads detach, and muscle fibers return to a resting state.