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How Do Muscles Work? Chapter 7: Lesson 3 Initiation of Contraction 1. A nerve impulse reaches the neuromuscular junction Arrival of an action potential at the synaptic terminal Step Two 2. The Motor Neuron releases acetylcholine (neurotransmitter) into synapse Step Three 3. Acetylcholine opens channels in the plasma membrane (sarcolemma) of the muscle fiber allowing Na+ ions into the cell and activate an action potential. Step Four 4. The action potential sweeps across the membrane and through the ttubules Action potential Axon Arriving action potential Synaptic terminal Sarcolemma Vesicles ACh AChE molecules Synaptic cleft Sarcolemma of motor end plate Release of acetylcholine Release of acetylcholine Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft. ACh receptor site Muscle fiber ACh binding the AChatbinding at the motor andmotor plate and plate The binding of ACh to the receptors increases the membrane permeability to sodium ions. Sodium ions then rush into the cell. Appearance of an action potential in the sarcolemma An action potential spreads across the surface of the sarcolemma. While this occurs, AChE removes the ACh. Step Five 5. When the t-tubules depolarize they trigger the adjacent sarcoplasmic reticulum to release Ca+ ions which spill into the cytoplasm (sarcoplasm). Step Six 6. The calcium ions cause the movement of troponin and tropomyosin on their thin (actin) filaments, which then enables the myosin molecule heads to "grab and swivel" their way along the thin filament. •When muscle contracts the actin filaments slide into the A band, overlapping with myosin When Muscle Contracts Protein Filaments Slide Together