1. Muscle Contraction

2. 1.Acetylcholine (Ach) is released from the axon terminal (nerve) into the synaptic cleft and binds to Ach receptors in the sarcolemma (muscle cell)

3. 2. Action potential is generated and muscle fiber becomes excited 3. Calcium (Ca 2+ ) is released from the sarcoplasmic reticulum 4. Ca 2+ binds to troponin, causing tropomyosin on actin to shift and myosin binding sites become exposed.

4. 5. ATP binds to myosin head, causing it to be in the “cocked” position. 6. Actin and myosin bind, forming a cross-bridge

6. 7. Myosin head flexes, causing actin to be pulled along the myosin, shortening the sarcomere (remember, muscles only pull/shorten/contract)

8. Muscle Relaxation

9. Another ATP molecule releases myosin head from actin. Ca2+ is transported back to the sarcoplasmic reticulum, causing tropomyosin to re-cover the myosin binding site Sarcomere “slides back” to its original position.