1. muscle

2. bundle of muscle fibres

3. muscle fibre

4. myofibrils

5. myofilaments (actin + myosin)

actin = thin

myosin = thick

each myofibril can be divided into units called sarcomeres

• sarcomere = functional units. seperated by Z-lines

• I band = distance between myosin strands

• A band = length of myosin

• H zone = distance between actin strands

• Z line = end of the sarcomere

• Myosin = thick filament

• Actin = thin filament

1. sarcoplasmic reticulum (stores Ca) releases calcium ions (keys) once a nervous impulse has been received

2. the calcium ions bind to troponin (lock) which causes the troponin-tropomysin complexes move away from the myosin active site on actin

3. once this happens contraction can begin

4. myosin heads contain a site for ATP binding

5. ATP is hyrolyzed into ADP freeing up a phosphate molecule, this energizes the myosin head (power stroke)

6. myosin attaches to the myosin active site on actin and release the free phosphate group

7. phosphate release by myosin triggers the actual power contraction stroke

8. myosin releases the ADP and energy

9. as the head rotates it moves in a ratchet motion and pulls the thin filaments closer to the centre of the M long of the sarcomere

10. when the ratchet motion is over the myosin awaits another ATP so it can be released from the actin active site

11. is Ca or ATP is available the myosin detaches and begins another stroke of the cycle - contraction continues