First 282 words of the document:
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 occur because a single nucleus
could not effectively control the metabolism of such a long cell. Muscle fibres are bound together by connective tissue.
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.
6. 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.