Muscles are responsible for nearly all animal movement and within the body you’ll find three different types:

• skeletal muscle: this is attached to the skeleton and controlled by the somatic nervous system’s motor neurones
• cardiac muscle: this is what the heart’s composed of and is not under voluntary control
• smooth muscle: this is located in internal body organs, like the arteries, gut wall and the uterus, and is under the involuntary control of the automatic nervous system

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Muscle Contraction Control

Muscle contractions are controlled by the troponin and tropomyosin in the thin filament. Tropomyosin, a long, thin molecule, is able to move around on the thin filament. When a muscle’s relaxed, it covers the actin molecules on the outer side of the filament thereby preventing myosin cross-bridges from attaching. When the muscles contracts, is moves into a space within the double helix, exposing the actin molecules to the cross-bridges.

The contraction of skeletal muscles relies on nerve impulses and the whole process, from impulse to contraction, is known as excitation-contraction coupling.

• First of all an action potential in the muscle cell membrane is initiated when the neurotransmitter acetykcholine is released from the motor neurone.
• The action potential travels rapidly through the muscle cell due to T-tubules which are invaginations located in the cell membrane.
• A large vesicle, called the sarcoplasmic reticulum, is stimulated by the action potential into releasing its calcium ions.
• The calcium ions enter into the myofibrils and bind themselves to the troponin located on the thin filament.
• The thin filament changes its shape which moves the tropomyosin into the gaps of the double helix.
• This allows the myosin cross-bridges to attach to the actin and the cross-bridge cycle can begin. The cycling continues with the presence of the calcium ions.

In order for the muscle to relax, the reverse of this process occurs. Although complicated, all these steps happen very rapidly.

Hopefully this will have helped your As and A level biology revision.