Question

Refer to the figure above showing the neuromuscular junction. Each arrow is labeled with a number and represents either passive or active flow of an ion. The ion fluxes that directly determine the calcium transient are...

1) 5 and 6
2) 1 and 8
3) 3 and 4
4) 7 and 8
5) 2, 5 and 6

Answer 1

The calcium transient at a neuromuscular junction is determined by the entry of calcium ions into the axon terminal when voltage-gated Ca2+ channels open, which then triggers neurotransmitter release.

Step-by-step explanation:

The ion fluxes that directly determine the calcium transient at a neuromuscular junction are the influx of calcium ions into the axon terminal and their subsequent actions within the cell. When an action potential reaches the axon terminal, it depolarizes the membrane, resulting in the opening of voltage-gated Na+ channels, then voltage-gated Ca2+ channels. This causes calcium ions to flow into the cell, which then initiate a series of events that lead to the fusion of synaptic vesicles with the presynaptic membrane and the release of neurotransmitter into the synaptic cleft. Once the neurotransmitter is released, it binds to receptors on the muscle cell, leading to depolarization and eventually, muscle contraction. The steps involving calcium ions, specifically their entry into the cell and their role in vesicle fusion, are the ones that account for the calcium transient.