Muscle contraction is a tricky bit of A-Level biology. There are lots of new terms to learn and a complex process to get your head around. Before beginning this article, make sure you are happy with the different parts of a muscle fibre cell.
Sarcomeres
Myofibrils are made up of two main protein filaments: actin (the thin filament) and myosin (the thick filament). They can be divided up into sections called sarcomeres. The sarcomeres repeat along the length of the myofibril. Different regions of the sarcomere contain either one or both protein filaments as shown below.
During muscle contraction, actin is pulled over myosin to change the length of the sarcomere. The actin and myosin filaments themselves never change length.
Before contraction
Myosin has a number of projections along it’s length called globular myosin heads. These bind to actin during the muscle contraction process as described below. They are hinged to allow movement, another important part of the contraction process. However, when a muscle fibre cell is not being stimulated, the globular myosin heads are not able to bind to actin because the binding site is blocked by a fibrous protein called tropomyosin. Tropomyosin is bound to another protein called troponin. There is also a molecule of ADP and Pi attached to each myosin head.
The muscle contraction process
Muscle contraction is triggered by the arrival of an action potential from a motor neurone. Acetylcholine is released at neuromuscular junctions as described here. The following process occurs:
- The sarcolemma depolarises. The depolarisation spreads down the T-tubules and reaches the sarcoplasmic reticulum inside the muscle fibre cell.
- When the sarcoplasmic reticulum depolarises, calcium ion (Ca2+) channels in the sarcoplasmic reticulum open and Ca2+ ions diffuse into the sarcoplasm. They diffuse into myofibrils.
- Ca2+ ions bind to troponin which causes troponin to change shape and pull tropomyosin out of the actin-myosin binding site. The globular myosin head is able to bind to actin and form an actin-myosin cross bridge.
- The myosin head changes shape and performs a powerstroke. Actin is pulled over myosin, and the ADP and Pi are released.
- A new molecule of ATP is attached to the myosin head, which breaks the cross bridge between actin and myosin.
- Ca2+ activate ATP hydrolase, which hydrolyses the ATP into ADP and Pi. This provides energy for the myosin head to reset to the original position.
- The myosin head attaches to the next binding site on actin and the process repeats.
This process continues for as along as Ca2+ ions are bound to troponin. The cross bridges between actin and myosin are very quickly formed and broken so that actin is pulled over myosin. The sarcomere shortens.
When the muscle fibre cell is no longer being stimulated by the motor neurone, the Ca2+ are taken back into the sarcoplasmic reticulum by active transport. Therefore they are no longer bound to troponin, and tropomyosin blocks the actin-myosin binding site again. Actin slides back to it’s original position, and the sarcomere lengthens. The muscle is now back in a relaxed state.
Summary
- Myofibrils contain the actin and myosin protein filaments. Sarcomeres are repeating units of myofibrils.
- Ca2+ from the sarcoplasmic reticulum are needed to bind to troponin which pulls tropomyosin out of the actin-myosin binding site. They are also needed to activate ATP hydrolase.
- Energy from ATP is needed for resetting of the globular myosin head and active transport of Ca2+ back into the sarcoplasmic reticulum. Another very important role of ATP is breaking the actin-myosin cross bridges.
- Actin slides over myosin to change the length of the sarcomere. The filaments themselves do not change length.
