VPB 121: Veterinary Physiology - II (2+1)

MUSCLE CONTRACTION

• It is accomplished by a sliding together or telescoping of the interdigtating thick and thin filaments and as a result narrowing and eventual disappearance of the H zone and shortening of I band happen.

Summary of events

• Before contraction ie in the absence of Ca²⁺ in the sarcoplasm, troponin T strongly binds with tropomysion, troponin I and troponin C. At the same time troponin I firmly attached to actin. Here actually the tropomyosin blocks or hides out the binding site of myosin with actin. This is achieved mainly by the association of troponin - I with the actin.
• An action potential travels along a motor nerve to muscle fibre
• At the neuro muscular junction the nerve secretes a neurotransmitter – acetylcholine (Ach).
• Acetylcholine acts on the muscle sarcolemma and opens Ach-gated ion channels
• Flow of Na⁺ ions to the interior of the muscle fibre membrane at the point of the nerve terminal initiates an action potential in the muscle fibre
• The action potential travels along the muscle fibre membrane
• The action potential also travels deeply into the muscle cell through the sarcoplasmic reticulum releases Ca⁺⁺ from the cistern into the myofibrils
• When action potential reaches triad then Ca²⁺ released, Ca²⁺ binds with troponin – C leading to some confirmational change (Structural change in the tt – complex and exposure of “G” actin to myosin head) on the C molenucle leading to troponin – I to loose its affinity towards actin.
• When troponin I looses linkage with actin, troponin – T can no longer hold the tropomyosin strand out of the groove. Only when tropomyosin is placed outside the groove formed by the double stranded actin helix, then only myosin biding site on actin is marked.
• When actin groove occupied by tropomysin, binding site exposed to myosin.

• Myosin heads or cross bridges attach to actin’s binding site and swivel it simultaneously and then dissociate in a sarcomere at a time only 5-20% of the total cross bridge attached to the thin filament at a given instant.
• Now the thin and thick filament slide part each other at a uniform rate without any jerk. Time required for one cycle of cross bridge is 0.1 milliseconds.
• Swiveling or rotating of the cross bridge in an inclined way cause actin filament to move towards the center of the sarcomere.

• Now the thin and thick filament slide past each other at a uniform rate with out any jerk. Time required for one cycle of cross bridge is 0.1 ms.
• Swiveling or rotating of the cross bridge cause actin filament to move towards the center of the sacromere.
• Once swiveling is over ATP is hydrolyzed and energy liberated and this energy used dissociation of cross bridge from the actin. So ATP acts as a dissociator and soon reorientation of the utilization of energy. In other words for swiveling action, no energy is needed but for only dissociation and reorientation of myosin head energy is utilized.
• This series of event continue till Ca²⁺ is rebound to sarcoplasmic reticulum.
• New ATP get bound to the spent cross bridge and hydrolysis of ATP happens only after swiveling.
• After death all myosin cross bridge stop attach with actin at an angled position and results in rigor mortis.