Contraction of Skeletal Muscle From Stiumulus to Relaxation The Events at the Neuromuscular Junction Step 1: An action potential travels the length of the axon of a motor neuron to an axon terminal. Step 2: Voltage-gated calcium channels open and calcium ions diffuse into the terminal. Step 3: Calcium entry causes synaptic vesicles to release acetylcholine via exocytosis. Step 4: Acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors, which contain ligand-gated cation channels. Step 5: These ligand-gated cation channels open. Step 6: Sodium ions enter the muscle fiber and potassium ions exit the muscle fiber. The greater inward flux of sodium ions relative to the outward flux of potassium ions causes the membrane potential to become less negative. Step 7: Once the membrane potential reaches a threshold value, an action potential propagates along the sarcolemma. (motor end plate becomes depolarized) [Neural transmission to a muscle fiber ceases when acetylcholine is removed from the synaptic cleft by the enzyme acetylcholinesterase] Excitation-Contraction Coupling Step 8: The action potential travels across the entire sarcolemma and is rapidly conducted into the interior of the muscle fiber by structures called transverse tubules. Step 9: As an action potential travels down the T tubule, it causes a voltage sensitive protein to change shape. This shape change opens a calcium release channel in the SR, allowing calcium ions to flood the sarcoplasm. Cross Bridge Cycle Step 10: The formation of a cross bridge is initiated when calcium ions, released from the sarcoplasmic reticulum, bind to troponin. This binding causes troponin to change shape. Step 11: As troponin changes shape, tropomyosin moves away from the myosin binding sites on actin, allowing the myosin head to bind actin, and form a cross bridge. Step 12: Cross Bridge Formation. The activated myosin head binds to actin forming a cross bridge. Inorganic phosphate is released. And the bond between myosin and actin becomes stronger. Step 13: The Power Stroke. ADP is released and the activated myosin head pivots, sliding the thin myofilament toward the center of the sarcomere. Step 14: Cross Bridge Detachment. When another ATP binds to the myosin head, the link between the myosin head and actin weakens, and the myosin head detaches. (“cocking” of myosin head) Step 15: Reactivation of the Myosin Head. ATP is hydrolyzed to ADP and inorganic phosphate. The energy released during hydrolysis reactivates the myosin head, returning it to the cocked position. [As long as the binding sites on actin remain exposed, the cross bridge cycle will repeat. And as the cycle repeats, the thin myofilaments are pulled toward each other, and the sarcomere shortens.]