Understanding:
• Calcium ions and the proteins tropomyosin and troponin control muscle contractions
The process of muscular contraction occurs over a number of key steps, including:
- Depolarisation and calcium ion release
- Actin and myosin cross-bridge formation
- Sliding mechanism of actin and myosin filaments
- Sarcomere shortening (muscle contraction)
1. Depolarisation and Calcium Ion Release
- An action potential from a motor neuron triggers the release of acetylcholine into the motor end plate
- Acetylcholine initiates depolarisation within the sarcolemma, which is spread through the muscle fibre via T tubules
- Depolarisation causes the sarcoplasmic reticulum to release stores of calcium ions (Ca2+)
- Calcium ions play a pivotal role in initiating muscular contractions
Muscle Innervation
2. Actin and Myosin Cross-Bridge Formation
- On actin, the binding sites for the myosin heads are covered by a blocking complex (troponin and tropomyosin)
- Calcium ions bind to troponin and reconfigure the complex, exposing the binding sites for the myosin heads
- The myosin heads then form a cross-bridge with the actin filaments
The Role of Calcium in Cross-Bridge Formation
Understanding:
• The contraction of the skeletal muscle is achieved by the sliding of actin and myosin filaments
• ATP hydrolysis and cross bridge formation are necessary for the filaments to slide
3. Sliding Mechanism of Actin and Myosin
- ATP binds to the myosin head, breaking the cross-bridge between actin and myosin
- ATP hydrolysis causes the myosin heads to change position and swivel, moving them towards the next actin binding site
- The myosin heads bind to the new actin sites and return to their original conformation
- This reorientation drags the actin along the myosin in a sliding mechanism
- The myosin heads move the actin filaments in a similar fashion to the way in which an oar propels a row boat
Sliding Filaments Mechanism
4. Sarcomere Shortening
- The repeated reorientation of the myosin heads drags the actin filaments along the length of the myosin
- As actin filaments are anchored to Z lines, the dragging of actin pulls the Z lines closer together, shortening the sarcomere
- As the individual sarcomeres become shorter in length, the muscle fibres as a whole contracts
Diagrams of Sarcomere Shortening
Summary of Muscle Contractions
- Action potential in a motor neuron triggers the release of Ca2+ ions from the sarcoplasmic reticulum
- Calcium ions bind to troponin (on actin) and cause tropomyosin to move, exposing binding sites for the myosin heads
- The actin filaments and myosin heads form a cross-bridge that is broken by ATP
- ATP hydrolysis causes the myosin heads to swivel and change orientation
- Swiveled myosin heads bind to the actin filament before returning to their original conformation (releasing ADP + Pi)
- The repositioning of the myosin heads move the actin filaments towards the centre of the sarcomere
- The sliding of actin along myosin therefore shortens the sarcomere, causing muscle contraction
Muscle Contraction Summary