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Joints & Movement
Muscles bring about movement at a joint. They can only pull; they cannot push, so at least 2 muscles are needed to move a bone to and fro. A
pair of muscles that work in this way are described as antagonistic.
An extensor is a muscle that contracts to cause the extension of a joint.
The flexor is the corresponding muscle that contracts to reverse the movement.
Muscles are made up of bundles of muscle fibres. Each fibre is a single muscle cell. Each cell has several nuclei (it is multinucleate) which
occurs because a single nucleus could not effectively control the metabolism of such a long cell. Muscle fibres are bound together by
Within each muscle fibre there are numerous myofibrils; each is composed OF repeated contractile units called sarcomeres.
Sarcomeres are made up of 2 protein molecules thin actin and thicker myosin. The proteins overlap which give the muscle fibre its
characteristic striped appearance. Actin moves over the myosin shortening the length of the sarcomere, and hence the length of the muscle.
SLIDING FILAMENT THEORY
When a nerve impulse arrives at a neuromuscular junction, calcium ions are released from the sarcoplasmic reticulum. The calcium ions
diffuse through the sarcoplasm (muscle cell cytoplasm) which initiates the movement of protein filaments as follows:
1. Calcium ions attach to troponin molecules, causing it to move.
2. Therefore the tropomyosin on the actin shifts position, exposing myosin binding sites.
3. Myosin heads bind with the binding sites forming cross-bridges.
4. When they bind, ADP and P on the myosin head are released.
5. Myosin changes shape, causing the head to nod forwards, which results in the relative movement of the filaments; the attached actin
moves over the myosin.
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ATP molecules bind to the myosin head which causes the head to detach.
7. ATPase on the myosin head hydrolyses the ATP, forming ADP and P.
8. The hydrolysis causes a change in shape of the myosin head it returns to its upright position enabling the cycle to start again.
When a muscle relaxes, it is no longer stimulated by nerve impulses. Calcium ions are pumped out of the sarcoplasm, using ATP.…read more