Steps to Muscle Contraction
1. A nerve impulse travels to the neuromuscular junction on a
muscle cell. The neuromuscular junction is the point where
the axons of the nerve meet with the muscle cell.
2. Ach is released from the axon to receptors located on the
sarcolemma
3. The binding Ach causes depolarization of the sarcolemma by
opening ion channels and allowing Na+ ions into the muscle
cell.
4. Na+ ions diffuse into the muscle fiber and depolarization
occurs.
5. Depolarization creates a wave of action potential (electrical
current) across the sarcolemma
6. Action potential travels across the sarcolemma and down
the T-tubules which triggers the sarcoplasmic reticulum
SR to release Ca+
7. As Ca+ levels rise, Ca+ ions bind with Troponin which
removes the blocking action of Tropomyosin from the Actin
binding sites.
8. Myosin is now ready to bind with the actin and form cross
bridges which begins the contraction process
9. In order to contract, ATP binds to the Myosin
10. ATP is then hydrolyzed (broken down) to ADP and Pi,
which gives the myosin the energy to “cock” its head to the
high-energy position.
11. Actin and myosin bind together to form a cross bridge
12. The myosin heads then pull the actin filaments inward and
release the ADP and Pi and return to a low energy position.
The myosin is now ready for more ATP to bind and repeat the
cycle. This process will continue for as long as there are Ca+ ions
and ATP available.
RED = the E-C coupling stage