What is the sequence of structures that action potentials must move through to excite skeletal muscle contraction?

1. Motor neuron: An electrical impulse is generated in the motor neuron, which is a type of nerve cell that controls muscle movement.

2. Synapse: The electrical impulse travels down the motor neuron and reaches the synapse, which is a small gap between the neuron and the muscle fiber.

3. Acetylcholine (ACh) release: When the electrical impulse reaches the synapse, it causes the release of ACh, a chemical messenger, into the synaptic cleft.

4. ACh binding: ACh binds to receptors on the muscle fiber's membrane called nicotinic acetylcholine receptors (nAChRs).

5. Muscle fiber depolarization: The binding of ACh to the nAChRs causes the muscle fiber to depolarize, meaning that the electrical charge of the membrane changes from negative to positive.

6. Action potential generation: The depolarization of the muscle fiber's membrane generates an action potential, an electrical impulse that travels along the muscle fiber's membrane.

7. Sarcoplasmic reticulum (SR) release: The action potential travels down the muscle fiber's membrane and reaches the SR, which is a specialized organelle that stores calcium ions. The action potential causes the SR to release calcium ions, into the muscle fiber's cytoplasm.

8. Calcium binding to troponin: Calcium ions bind to the troponin complex on the actin filaments of the muscle fiber. This binding moves the troponin molecule and exposes a binding site for the myosin head on the myosin filaments allowing contraction to take place.

9. Muscle contraction: Myosin heads bind to the exposed binding sites on the actin filaments, forming crossbridges. The crossbridges use energy from ATP to undergo a power stroke, pulling the actin filaments towards the center of the muscle fiber, causing shortening of the muscle, which is the action of muscle contraction.