Skeletal Muscle Contraction: Excitation-Contraction Coupling

1

Motor nerve impulse stimulates the release of acetylcholine at the neuromuscular junction (each muscle fiber/cell is innervated by a motor neuron)

2

Acetylcholine causes depolarization of the sarcolemma

3

The action potential travels along the sarcolemma through the T-tubules

4

The terminal cisternae of the sarcoplasmic reticulum releases calcium into the cytoplasm

5

Calcium binds to troponin, lifting tropomyosin off of the myosin binding sites on the actin

6

The myosin heads bind to the myosin binding sites on the actin (cross-bridging)

7

The myosin then tilts on its hinges and pulls actin (thin filament) over itself, shortening the sarcomeres (power stroke)

8

ATP attaches to the myosin head to allow it to detach from the myosin binding site on the actin

9

ATP is hydrolyzed to ADP and Pi to reposition the myosin so it is ready to bind to another myosin binding site on actin

10

The cycle (cross-bridge cycling) continues as long as calcium and ATP are present in the cytosol

11

The contraction cycle is stopped by calcium uptake back into the SR (via Ca⁺⁺ ATPase in the SR membrane; called SERCA)