Arrange the following in the order in which they occur at the postsynaptic side of a neuromuscular junction: 1. An action potential is propagated over the muscle cell membrane 2. Depolarization of the postsynaptic membrane 3. Sodium ions move into muscle cell

The correct order is: 3 (Sodium ions move into muscle cell), 2 (Depolarization of the postsynaptic membrane), 1 (An action potential is propagated over the muscle cell membrane).

Explanation:

The somatic motor neurons produce a stimulation through their axons that get in contact with muscle fibers through a neuromuscular synapse. When an action potential is originated in the central nervous system, it travels to the somatic motor neuron membrane: the muscle fiber, and activates the calcium channels releasing it in the neuron. Calcium makes vesicles to fuse with the membrane and release the neurotransmitter acetylcholine (Ach) into the synaptic space in the juncture. Ach binds to its receptors on the skeletal muscle fiber, causing the ion channels to open. Positively charged sodium ions get into the muscle fiber (sarcoplasm) and potassium gets out. The difference in charges caused by the migration of sodium and potassium makes the muscle fiber membrane to become more positively charged (depolarized). The action potential caused by this depolarization enters the t-tubules depolarizing the inner portion of the muscle fiber.