Notes:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-06-skeletal-muscle-fiber.html

Inhibition and Activation of Actin

The interaction between actin and myosin is described as being under the strict control of two regulatory proteins wrapped around the actin filament: tropomyosin and the troponin complex. In a resting, relaxed muscle, tropomyosin molecules physically cover the active binding sites on the actin strands, completely inhibiting myosin cross-bridges from attaching and preventing contraction. The troponin complex which consists of Troponin I (which binds to actin), Troponin T (which binds to tropomyosin), and Troponin C (which binds to calcium) acts as the sensory trigger for this process. Activation occurs when an action potential causes a massive release of calcium ions into the sarcoplasm. Calcium rapidly binds to Troponin C, inducing a conformational change in the entire troponin-tropomyosin complex that pulls the tropomyosin deeper into the actin groove. Uncovering these active sites instantly breaks the inhibition, allowing myosin heads to bind with actin, hydrolyze ATP, and execute the power stroke to generate muscle tension.

• Pure actin binds strongly with myosin heads.
• Troponin-tropomyosin complex blocks actin-myosin binding.
• Active sites are physically covered in relaxed muscle.
• Troponin-tropomyosin inhibition must be removed for contraction.
• Calcium ions inhibit the troponin-tropomyosin complex.
• Calcium binds to troponin C (up to four ions).
• Troponin undergoes a conformational change.
• Tropomyosin moves deeper into the actin groove.
• Uncovered active sites attract myosin heads.

The Walk-Along Theory

• Activated actin attracts myosin cross-bridge heads.
• Mechanism is called the walk-along or ratchet theory.
• Myosin head attaches to an active site.
• Head tilts toward the arm, creating a power stroke.
• Power stroke drags the actin filament along.
• Head releases, re-extends, and binds a new site.
• Repeated stepping pulls actin toward the myosin center.
• Each cross-bridge operates independently in repeated cycles.
• More attached cross-bridges generate greater contraction force.

ATP Energy and Chemical Events

• Muscle contraction performs work and requires ATP.
• More work increases ATP cleavage (Fenn effect).
• Myosin head cleaves ATP to ADP and phosphate.
• Cleaved products remain bound, cocking the head perpendicularly.
• Uncovered actin sites bind to cocked myosin heads.
• Binding triggers a power stroke using stored spring energy.
• Power stroke releases ADP and phosphate ions.
• New ATP binds, causing head detachment from action.
• New ATP is cleaved to re-cock the head.
• The cycle repeats until full contraction or excessive load.

Filament Overlap and Tension

• Zero overlap yields zero contraction tension (Point D).
• Tension increases as actin overlaps myosin cross-bridges.
• Maximum tension occurs between 2.2 and 2.0 micrometers.
• Actin filaments overlap each other below 2.0 micrometers.
• Tension decreases rapidly from 2.0 to 1.65 micrometers.
• Z discs about myosin ends at 1.65 micrometers.
• Myosin ends crumple at shorter lengths, reducing force.

Muscle Length and Force

• The whole muscle contains connective tissue and varying sarcomeres.
• The whole muscle curve shows the same general form as fiber.
• Resting length (~2 micrometers) generates maximum contraction force.
• Stretching beyond 2.2 micrometers decreases active tension.

Velocity of Contraction vs Load

• Muscle contracts rapidly with zero load (~0.1 seconds).
• Contraction velocity decreases as load increases.
• Velocity reaches zero when load equals maximum muscle force.
• Load acts as a reverse force opposing muscle contraction.
• Net force for shortening velocity is reduced by load.

Work Output During Contraction

• Muscle performs work when contracting against a load.
• Energy transfers to lift objects or overcome movement resistance.
• The work formula is W = L \times D.
• W is work output, L is load, D is distance.
• Energy comes from cellular chemical reactions during contraction.

Energy Uses and ATP Stores

• Muscle contraction depends primarily on ATP energy.
• ATP powers the cross-bridge walk-along mechanism.
• ATP pumps calcium into the sarcoplasmic reticulum.
• ATP pumps sodium and potassium across the membrane.
• Muscle ATP concentration is about 4 millimolar.
• ATP maintains full contraction for 1 to 2 seconds.
• ADP is repository lated to form new ATP rapidly.

Sources of Energy for ATP

• Phosphocreatine: Cleaves instantly to reconstitute ATP from ADP.
• Phosphocreatine stores maintain maximal contraction for 5–8 seconds.
• Glycolysis: Rapidly breaks down stored glycogen without oxygen.
• Glycolysis generates ATP 2.5 times faster than oxidative metabolism.
• Glycolysis sustains maximal contraction for up to 1 minute.
• Oxidative Metabolism: Combines oxygen with foodstuffs to liberate ATP.
• Provides over 95% of sustained long-term contraction energy.
• Consumes carbohydrates, fats, and proteins for long-term activity.
• Fats energy for maximal activity over many hours.

Efficiency of Muscle Contraction

• Efficiency is energy input percentage converted to work.
• Maximum muscle efficiency is less than 25%.
• Remaining input energy is converted into heat.
• Half of foodstuff energy is lost forming ATP.
• Only 40% to 45% of ATP energy becomes work.
• Maximum efficiency occurs at moderate contraction velocity.
• Very slow contractions release maintenance heat with zero efficiency.
• Very rapid contractions lose energy overcoming internal viscous friction.
• Maximum efficiency occurs at 30% of maximum velocity.