Question Video: Describing the Series of Events at a Neuromuscular Junction Biology

The flowchart provided describes the events that occur at a neuromuscular junction, with each step assigned a number. State the number sequence that would give the correct order of events.

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Video Transcript

The flowchart provided describes the events that occur at a neuromuscular junction, with each step assigned a number. State the number sequence that would give the correct order of events. (1) The influx of calcium ions causes vesicles containing the neurotransmitter acetylcholine to fuse with the membrane of the motor neuron. (2) Calcium ion protein channels in the motor neurons are stimulated to open, and calcium ions enter the motor neuron. (3) The sodium ion channels open, and sodium ions diffuse into the muscle fiber. (4) Acetylcholine diffuses across the synaptic cleft and binds to a receptor sites on the membrane of the muscle fiber. (5) The influx of sodium ions depolarizes the membrane of the muscle fiber. (6) An action potential reaches the end of a motor neuron.

Whenever you do any physical activity, whether it be running a marathon, swimming in the sea, or climbing a mountain, electrical impulses, which are also known as action potentials, travel from your brain along motor neurons to your muscles and tell them to start contracting. Where a motor neuron ends and a muscle fiber begins, a special type of synapse is formed called a neuromuscular junction, which is what you can see in this diagram.

Let’s consider the sequence of events that happen here so we can put the statements on the left in the correct order to answer the question. We will write down the correct number sequence at the bottom of the screen as we go along.

First, an action potential reaches the end or terminal of the motor neuron it’s traveling down. This is described by statement six. The action potential causes the terminal of the motor neuron to become depolarized. Depolarization is when the distribution of charge across the membrane changes, causing the internal side to become less negative. This stimulates voltage-gated calcium channels to open, allowing calcium ions to enter the motor neuron. This is described by statement (2).

The influx of calcium ions into the motor neuron causes vesicles containing the neurotransmitter acetylcholine to fuse with the membrane of the motor neuron. Let’s see that in action. This step is described by statement (1). Acetylcholine is released into the synaptic cleft, the space between the motor neuron terminal and the muscle fiber. Now there’s a high concentration of acetylcholine near the motor neuron and a low concentration near the muscle fiber. So the acetylcholine diffuses across the synaptic cleft towards the muscle fiber. Here, the acetylcholine binds to receptor sites on the membrane of the muscle fiber. This is described by statement (4).

The binding of acetylcholine to the receptor sites causes associated ion channels to open, allowing sodium ions to diffuse into the muscle fiber. This step is described by statement (3). The influx of sodium ions, which are positively charged, leads to depolarization of the muscle fiber membrane. This final step is described by statement (5). If the depolarization reaches a certain threshold, an action potential will be generated in the muscle fiber, which will eventually lead to muscle contraction.

Therefore, the number sequence, which gives the correct order of events at the neuromuscular junction is (6), (2), (1), (4), (3), (5).

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