1. Muscle Contraction

2. Muscle Movement Muscle fiber must be stimulated: – By an electrical signal called muscle action potential (AP) – Delivered by motor neuron Motor neuron + muscle fibers = motor unit Simulation of 1 motor neuron causes all the muscle fibers in that motor unit to act at the same time. –Precise movements = many motor units controlling few muscles fibers –Powerful movements = few motor units controlling many muscle fibers.

3. Axon of Motor Neuron Action Potential Nucleus Axon Terminal of NMJ Sarcolemma of Muscle Fiber Myofibril NMJ Vesicle Mitochondrion Synaptic Cleft Sarcolemma #1 Nerve impulse (AP) arrives at axon terminal #2 Calcium ions (Ca 2+ ) released into axon terminal Ca 2+ #3 Ca 2+ causes synaptic vesicles to release Acetylcholine (ACh) via exocytosis

4. #4 ACh diffuses across synaptic cleft and binds to receptors in the sarcolemma #5 Binding of ACh to its receptors opens ion channels. - Na + moves into muscle fiber (more positive) - K + moves out of muscle fiber (more negative) Creates a new AP #6 ACh effects are stopped by the enzyme acetylcholinesterase (AChE) -- Breaks down remaining ACh in the synaptic cleft Na + K+K+

5. #5 from NMJ Binding of ACh to its receptors opens ion channels. - Na + moves into muscle fiber (more positive) - K + moves out of muscle fiber (more negative) Creates a new AP

6. Sarcoplasmic Reticulum (SR) Network of tubules that surround each myofibril T (Transverse) Tubules Tunnel like extensions of the sarcolemma, run between terminal cisternae of the SR, and allow for AP to reach deep regions of muscle cells. Terminal Cisternae of SR Network of tubules that surround each myofibril and store calcium ions

7. #1 Action potential is spread along the sarcolemma and down the T tubules #2 Causes the terminal cisternae of the SR to release stored calcium ions into the sarcoplasm. -- within 1 millisecond!

8. #2 Causes the terminal cisternae of the SR to release stored calcium ions into the sarcoplasm. -- within 1 millisecond! #3 Calcium binds to troponin and removes the blocking action of tropomyosin. When Ca 2 + binds, troponin changes shape, exposing binding sites for myosin on the actin myofilaments. #4 Myosin binding to actin forms cross bridges and results in the sliding of the actin filaments towards the center of the sarcomere (power stroke) = muscle contraction.