Sliding Filament Model of Muscle Contraction
The mechanism of muscle contraction is explained by sliding filament model. The arrangement of actin and myosin myofilament within a sarcomere is crucial in the mechanism of muscle contraction.
Actin: An actin myofilament is made up of actin molecule, tropomyosin and troponin complex. Troponin is composed of three sub-units (troponin I, T and C). Tropomyosin form two helical strand which are wrapped around actin molecule longitudinally in thin twisted stranded form.
Myosin: A myosin filament consists of a long rod-shaped tail called myosin rod and two globular intertwined myosin head. The globular head appear at interval along the myosin myofilament.
Figure: Actin and Myosin myofilament
- When the nerve impulse from brain and spinal cord are carried along motor neuron to muscle fibre Ca++ ions are released in the terminal axon.
- Increases calcium ion concentration stimulates the release of neurotransmitter (Acetylcholine) in the synaptic cleft.
- The neurotransmitter binds to the receptor on the sarcolemma and depolarization and generate action potential across muscle fibre for muscle contraction.
- The action potential propagates over entire muscle fibre and move to the adjacent fibres along transverse tubules.
- The action potential in transverse tubules causes the release of calcium ion from sarcoplasmic reticulum, which stimulate for muscle contraction.
- The sequences of muscle contraction explained by sliding filament model are as follows
Figure: diagrammatic representation of muscle contraction mechanism
Steps of muscle contraction
1. Blocking of myosin head:
- Actin and myosin overlaps each other forming cross bridge. The cross bridge is active only when myosin head attached like hook to the actin filament. When muscle is at rest, the overlapping of actin filament to the myosin head is blocked by tropomyosin. The actin myofilament is in OFF position.
2. Release of calcium ion:
- Nerve impulse causing depolarization and action potential in the sarcolemma trigger the release of calcium ions.
- The calcium ion then binds with the troponin complex on the actin myofilament causing displacement of tropomyosin from its blocking site.
- As soon as the actin binding site is exposed, myosin cross bridge with actin.
- The actin myofilament is in ON position.
3. Cross bridge formation:
- The cross bridge between actin and myosin acts as an enzyme (Myosin ATPase), which hydrolyses ATP stored in myosin head into ADP and inorganic phosphate and release energy.
- This released energy is used for movement of myosin head toward actin filament. The myosin head tilts and pull actin filament along so that myosin and actin filament slide each other. The opposite end of actin myofilament within a sarcomere move toward each other, resulting in muscle contraction.
- After sliding the cross bridge detached and the actin and myosin filament come back to original position.