Define sliding filament theory of muscle contraction.

Explanation

Rolf Niedergerke Andrew F. Huxley proposed the sliding-filament theory to explain muscle contraction. According to this theory overlapping actin and myosin filaments of fixed length slide past one another is an energy requiring process resulting in muscle contraction.
1. Contraction is the creation of tension in the muscle and relaxation is the release of tension created by contraction.
Muscle contraction is initiated by a nerve impulse sent by the central nervous system (CNS) through a motor neuron.
3. The junction between the motor neuron and sarcolemma of the muscle fibre is called the neuromuscular junction or motor end plate. When nerve impulse reaches this junction acetylcholine is released.
4. An action potential is generated which initiates opening of multiple gated channels of sarcolemma.
5. This causes the flow of large quantities of calcium ions from sarcoplasmic reticulum. The `Ca^+` ions bind to the troponin of thin filaments. The active sites are exposed to the heads of myosin to form a cross bridge and phosphateion is released.
6. Hydrolysis of ATP occurs and energy released helps the myosin head to rotate `(90^(@)` angle with long axis of filament). In this position myosin binds to an actin and activates contraction relaxation cycle which is followed by a power stroke.
7. Power stroke (cross- bridge tilting ) begins after rest of myosin binding sites are uncovered. Myosin head and hinge region tilt form `90^(@)` angle to `45^(@)` angle. Cross bridges are transformed into high- force bouds as myosin releases phosphate ions allowing myosin head to swivel.
8. At the end of power stroke , the myosin head releases actin and swivels back to bind to a new actin molecule to start another contraction cycle. The power stroke repeats many times until a muscle fibre contracts. The process continues as long as muscle receives stimuli and there is steady flow calcium ions.
Myosin returns back to relaxed state and releases ADP. A new ATP then binds to the head of mysoin and cross-bridge is broken. The cycle of cross-bridge formation and breakage repeatedly causes sliding of the filaments. Similar to motion of oar on a boat.
10. Relaxation
i) Motor impulses stop.
ii) Calcium ions are pumped back into sarcoplasm.
iii) Masking of active sites of actin filament.
iv) Failure of binding of myosin head with active sites of actin.
v) The thin filaments assume their normal position and muscle is released.