Question

Rank the sequence of events at the NMJ that initiate an action potential in the muscle fiber, from first to last.

Answer 1

The correct order is:

- Action potential arrives at the axon terminal.

- Calcium ions enter the axon terminal.

- Synaptic vesicles fuse to membrane of axon terminal.

- Acetylcholine is released into the synaptic cleft.

- Acetylcholine binds to its receptors on the junctional folds.

- Junctional folds become depolarized.

- Action potential is initiated on the sarcolemma.

Action potential travels through the membrane of the presynaptic cell causing the channels permeable to calcium ions to open. Ca2+ flow through the presynaptic membrane and increase the Ca concentration in the cell which will activate proteins attached to vesicles that contain a neurotransmitter (e.g. acetylcholine). Vesicles fuse with the membrane of the presynaptic cell, thereby release their contents into the synaptic cleft-space between the membranes of the pre- and postsynaptic cells. Neurotransmitter binds to its receptors on the postsynaptic membrane and its binding causes depolarization of the target cell (muscle cell).