The document describes the process of muscle contraction initiated by a motor neuron signal. It explains that the signal causes vesicles to release acetylcholine into the synaptic cleft. The acetylcholine binds to receptors on the muscle fiber, changing the fiber's permeability to sodium ions. Sodium ions then rush into the fiber, depolarizing it. The fiber then repolarizes as potassium ions diffuse out and the sodium-potassium pump restores ion balances.

1. 1. A “ signal ” is sent from the brain down the axon of a motor neuron . Action Potential!!! Part 1 - Depolarization

2. 2. The signal hits the terminal bud (button) of the axon.

3. 3. The vesicles break open releasing the neurotransmitter , acetylcholine , in to the synaptic cleft .

4. 4. The acetylcholine crosses the synaptic cleft and binds to receptor sites on the sarcolemma .

5. 5. The sarcolemma undergoes a permeability shift from Na + impermeable to Na + permeable .

6. 6. Na + ions rush in to the muscle fiber through channels in the sarcolemma, but they “ overshoot ” equilibrium .

7. 7. Cholinesterase , an enzyme that “ destroys ” acetylcholine, deactivates and removes the acetylcholine from the receptor sites on the sarcolemma.

8. 8. The permeability of the sarcolemma shifts back to Na + impermeable thus trapping the Na + ions inside the muscle fiber.

9. 9. The neuromuscular junction is “ depolarized ” with respect to Na + .

10. 1. A “ signal ” is sent from the brain down the axon of a motor neuron . 2. The signal hits the terminal bud (button) of the axon. 3. The vesicles break open releasing the neurotransmitter , acetylcholine , into the synaptic cleft . 4. The acetylcholine crosses the synaptic cleft and binds to receptor sites on the sarcolemma . 5. The sarcolemma undergoes a permeability shift from Na + impermeable to Na + permeable . 6. Na + ions rush in to the muscle fiber through channels in the sarcolemma, but they “ overshoot ” equilibrium . 7. Cholinesterase , an enzyme that “ destroys ” acetylcholine, deactivates and removes the acetylcholine from the receptor sites on the sarcolemma. 8. The permeability of the sarcolemma shifts back to Na + impermeable thus trapping the Na + ions inside the muscle fiber. 9. The neuromuscular junction is “ depolarized ” with respect to Na + .

11. Action Potential (Part 2): Repolarization

12. 1. K + ions diffuse out of the muscle fiber to help balance the positive charge “imbalance” caused by the Na + being trapped inside the fiber.

13. 2. Na + ions are “pumped” out of the muscle fiber and K + ions are “pumped” back in by the active transport mechanism known as the Na + /K + pump in a ratio of 3 Na + out to 2 K + in .

14. 3. Once the original balance of Na + and K + is restored, the NMJ is said to be repolarized .

15. 1. K + ions diffuse out of the muscle fiber to help balance the positive charge “imbalance” caused by the Na + being trapped inside the fiber. 2. Na + ions are “pumped” out of the muscle fiber and K + ions are “pumped” back in by the active transport mechanism known as the Na + /K + pump in a ratio of 3 Na + out to 2 K + in . 3. Once the original balance of Na + and K + is restored, the NMJ is said to be repolarized .