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Key:

• Z-disc: Protein structure that separates adjacent sarcomeres.
• I-band (Isotropic band): Region containing thin filaments (actin) only.
• A-band (Anisotropic band): Region containing thick filaments (myosin) and overlapping thin filaments.
• H-zone (Heller zone): Region within the A-band containing thick filaments (myosin) only. It may appear lighter due to the absence of overlapping thin filaments.

This diagram illustrates the organization of a sarcomere, the basic contractile unit of skeletal muscle.

The sliding filament theory of muscle contraction is a widely accepted explanation for how muscles generate force and produce movement at the molecular level. Proposed by Andrew Huxley and Hugh Huxley in the 1950s, this theory describes the interaction between the proteins actin and myosin within muscle fibers during contraction. The key principles of the sliding filament theory are as follows:

1. Muscle Structure:

   • Muscle fibers are composed of myofibrils, which in turn are made up of repeating units called sarcomeres.
   • Sarcomeres contain thick filaments composed of the protein myosin, which have globular heads, and thin filaments composed of the protein actin, along with other associated proteins like tropomyosin and troponin.

2. Resting State:

   • In the resting state, actin and myosin filaments partially overlap but are not directly interacting.
   • Regulatory proteins (tropomyosin and troponin) cover the active sites on the actin filaments, preventing myosin binding.

3. Initiation of Contraction:

   • When a muscle is stimulated by a nerve impulse, calcium ions (Ca2+) are released from the sarcoplasmic reticulum (SR) into the sarcomere.
   • Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the active sites on the actin filaments, exposing them.

4. Cross-Bridge Formation:

   • With the active sites exposed, the myosin heads (cross-bridges) bind to the actin filaments, forming cross-bridges.
   • ATP (adenosine triphosphate) bound to the myosin head is hydrolyzed to ADP (adenosine diphosphate) and inorganic phosphate (Pi), releasing energy that causes the myosin head to change its conformation and pivot, pulling the thin filaments towards the center of the sarcomere.

5. Power Stroke and Sliding Filaments:

   • This conformational change in the myosin head is called the power stroke, during which the myosin head pivots, pulling the thin filaments (actin) towards the center of the sarcomere.
   • As the myosin heads bind, pivot, and release, they pull the actin filaments past the myosin filaments in a sliding motion, leading to sarcomere shortening and muscle contraction.

6. Relaxation:

   • When nerve stimulation ceases, calcium ions are actively transported back into the sarcoplasmic reticulum.
   • With the decrease in calcium concentration, troponin releases calcium ions, allowing tropomyosin to cover the active sites on the actin filaments again.
   • As a result, cross-bridge formation ceases, and the muscle relaxes back to its resting length.

Overall, the sliding filament theory describes how the interaction between actin and myosin filaments within sarcomeres leads to muscle contraction by sliding past each other, resulting in the shortening of muscle fibers and the generation of force.

(a) All mammals (except a few) have seven cervical vertebrae. (b) The number of phalanges in each limb of humans is fourteen.

(a) Atlas/axis: Pivot joint (b) Carpal/metacarpal of thumb: Saddle joint (c) Between phalanges: Hinge joint (d) Femur/acetabulum: Ball and socket joint