Muscle Contraction – Muscles Ep 2

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. Make sure to review it regularly so that it sticks in your brain. 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 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 slides over myosin to change the length of the sarcomere. The actin and myosin filaments themselves never change length.

Muscles at rest

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 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.

The muscle contraction process

Muscle contraction is triggered by the arrival of an action potential from a motor neurone. The following process occurs:

1. The sarcolemma depolarises. The depolarisation spreads down the T-tubules and reaches the sarcoplasmic reticulum inside the muscle fibre cell.
2. Calcium ion (Ca²⁺) channels in the sarcoplasmic reticulum open and Ca²⁺ ions are released into the sarcoplasm.
3. 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.
4. Ca²⁺ ions activate the ATP hydrolase enzyme. This enzyme hydrolyses ATP into ADP and Pi which releases energy.
5. The energy allows movement of the hinged globular myosin head, which pulls the actin filament along. Energy is also required to break the actin-myosin cross bridge so that the myosin head can attach to the next binding site on actin.

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 slides 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 the movement of the globular myosin head, breaking the actin-myosin cross bridge, and active transport of Ca²⁺ back into the sarcoplasmic reticulum.
• Actin slides over myosin to change the length of the sarcomere. The filaments themselves do not change length.