Excitation-Contraction Coupling

Excitation-Contraction (E-C) Coupling refers to the steps in the process of muscular contraction from action potential (excitation) to the power stroke (contraction).

For more on muscle cells, check out these courses:

• Muscle Cell Structure and Function
• Muscle Fiber Types

Steps in the process of Excitation-Contraction (E-C) Coupling:

1. Stimulus (intent, reflex, motor program, etc.) opens sodium ion channels on the membrane of a motor neuron.
2. An action potential is propagated across the motor neuron.
3. The action potential is propagated by the motor neuron axon to the synapse.
4. Acetylcholine (ACh) is released into the neuromuscular junction.
5. ACh binds to receptors on the motor endplate of the muscle cell, opening sodium ion channels.
6. An action potential is propagated across the sarcolemma.
7. The motor nerve stimulates a muscle cell, initiating an action potential across the muscle cell membrane (sarcolemma).
8. The action potential reaches the sarcoplasmic reticulum via t-tubules in the cell membrane, which initiates the release of calcium.
9. Calcium enters the sarcoplasm and binds to troponin, changing the shape of tropomyosin and exposing binding sites on actin.
10. Adenosine triphosphate (ATP) binds to the myosin head.
11. ATP is hydrolyzed, causing the myosin head to extend.
12. Myosin attaches to the binding sites on actin (cross-bridge formation).
13. ADP is released, resulting in the flexing of the myosin head (power stroke).
14. ATP binds again, releasing the myosin head from actin.
15. The cycle repeats as myosin binds to a new site on actin.

Relaxation:

• Muscle relaxation occurs when acetylcholinesterase, an enzyme in the synaptic cleft, breaks down ACh, reducing the concentration of ACh and terminating the signal. Calcium is then pumped back into the terminal cisternae of the sarcoplasmic reticulum, and the troponin-tropomyosin complex returns to its original state, covering the binding sites on actin.