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
