Put the following events of the neuromuscular junction in the order in which they occur. 1. Action potential is propagated in the sarcolemma. 2. Acetylcholine binds to ligand gated sodium channels. 3. Action potential arrives at the neuromuscular junction. 4. Vesicles full of acetylcholine are stored at the axon terminal. 5. Acetylcholine is released into the synaptic cleft.

Muscle contraction occurs when somatic neurons and muscle fibers connect through neuromuscular synapse. The order of events is 4, 3, 5, 2, 1.

How does muscle contaction occur?

Muscle fiber contractions are due to stimulation produced by somatic motor neurons, when their axons get in contact with muscle fibers through a neuromuscular synapse.

The sequence of events during muscle contraction is as follows,

1) At rest, attraction strengths between myosin and actin filaments are inhibited by the tropomyosin.

2) An action potential originates in the central nervous system, and travels to the somatic motor neuron membrane, where it activates the calcium channels.

3) Calcium channels open and the ions get into the neuron.

4) Calcium makes vesicles fuse with the membrane.

5) Vesicles release the neurotransmitter acetylcholine (Ach) into the synaptic space at the juncture.

6) Ach moves through the synaptic cleft and binds to its receptors on the skeletal muscle fiber.

7) This recognition event causes the ion channels to open,

sodium ions cross the membrane to get into the muscle fiber (sarcoplasm),
potassium gets out.

8) The difference in charges caused by the migration of sodium and potassium makes the muscle fiber membrane turns more positively charged (depolarized).

9) The action potential caused by the depolarization enters the t-tubules. Consequently, it depolarizes the inner portion of the muscle fiber.

10) Depolarization activates the T tubules membrane calcium channels and releases the ion into the sarcolemma.

At this point, tropomyosin is obstructing binding sites for myosin on the thin filament.

11) When calcium binds the troponin C, the troponin T alters the structure of tropomyosin by moving it and unblocks the binding sites.

12) Myosin binds to the uncovered actin-binding sites, and while doing it, ATP is transformed into ADP and inorganic phosphate.

13) Z-bands are then pulled toward each other, thus shortening the sarcomere and the I-band and producing muscle fiber contraction.

According to this framework, the correct order of events is,

4. Vesicles full of acetylcholine are stored at the axon terminal.

3. Action potential arrives at the neuromuscular junction.

5. Acetylcholine is released into the synaptic cleft.

2. Acetylcholine binds to ligand gated sodium channels.

1. Action potential is propagated in the sarcolemma.

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