Human Anatomy and Physiology : Help with Neuromuscular Junction Physiology

Study concepts, example questions & explanations for Human Anatomy and Physiology

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Example Questions

Example Question #1 : Help With Neuromuscular Junction Physiology

What part of the muscle cell allows an action potential to spread uniformly throughout the cell?

Possible Answers:

Sarcoplasmic reticulum

Sarcolemma

Neuromuscular junctions

T-tubules

Correct answer:

T-tubules

Explanation:

T-tubules are small tunnels in the membrane of a muscle cell that allow an action potential to spread evenly. This allows for the whole muscle cell to contract smoothly and in unison.

A neuromuscular junction is the synaptic interface between a neuron and a muscle cell. When an action potential reached the junction, it causes depolarization of the sarcolemma (muscle cell membrane). This depolarization spreads to the T-tubules, which house proteins that directly interface with the sarcoplasmic reticulum. When depolarization of the T-tubules stimulates the sarcoplasmic reticulum, it releases calcium into the cytoplasm. The calcium bind to troponin to initiate the contraction of the sarcomere.

Example Question #2 : Help With Neuromuscular Junction Physiology

A scientist is studying a type of cell and discovers that this cell uses kinesin to transport secretory vesicles down long cytoplasmic extensions. Upon further study, the vesicles are found to contain the substance acetylcholine. What is the most likely classification of this cell?

Possible Answers:

Skeletal muscle cell

Cardiac muscle cell

Smooth muscle cell

Inhibitory interneuron

Efferent (motor) neuron

Correct answer:

Efferent (motor) neuron

Explanation:

Acetylcholine is transported in vesicles, via the protein kinesin, in excitatory motor neurons. Muscle cells typically do not transport secretory vesicles, as they are not active secretors of most proteins. Neurons, in contrast, function to release neurotransmitters from secretory vesicles at synapses.

This logic leaves us with either type of neuron specified in the question. The best answer is motor neuron, as acetylcholine is the primary excitatory neurotransmitter at the neuromuscular junction. Had the question specified that the vesicle was filled with GABA or glycine, inhibitory neuron would have been the better answer.

Example Question #3 : Help With Neuromuscular Junction Physiology

At the neuromuscular junction, receptors respond to neurotransmitters to facilitate the depolarization of muscle cell membranes. This is the first step in that muscle cell's ultimate contraction. Which types of receptors would be most likely found on the muscle cell, directly interacting with the neurotransmitter at the neuromuscular junction?

Possible Answers:

Muscarinic ligand-gated channels

Muscarinic voltage-gated channels

Nicotinic voltage-gated channels

Nicotinic ligand-gated channels

Dopaminergic ligand-gated channels

Correct answer:

Nicotinic ligand-gated channels

Explanation:

Nicotinic ligand-gated channels interact with the acetylcholine released by neurons at the neuromuscular junction. By binding to the neurotransmitter, these channels change shape and allow ions to enter into the muscle cell membrane, thus depolarizing it and driving an ultimate contraction.

Voltage-gated sodium and calcium channels play an important role in propagating the membrane depolarization, but do not interact with the neurotransmitter at the neuromuscular junction. Muscarinic receptors are also stimulated by acetylcholine, but are most commonly found in the parasympathetic nervous system, and do not play a significant role in muscle contraction.

Example Question #4 : Help With Neuromuscular Junction Physiology

Clostridium tetani is a microorganism that causes constant muscle contraction, and results in the characteristic risus sardonicus, or lockjaw. The toxin produced by this organism acts on inhibitory neurons that downregulate the activity of excitatory neurons directly involved in the neuromuscular junction.

The release of which neurotransmitters are most likely to be inhibited during Clostridium tetani infection?

I. Glycine

II. Glutamate

III. GABA 

Possible Answers:

I and II

II, only

II and III

I and III

I, II, and III

Correct answer:

I and III

Explanation:

Glycine and GABA are the main inhibitory neurotransmitters. Thus, they would be used by inhibitory neurons that work to downregulate the excitatory neurons at the neuromuscular junction. Inhibiting the release of these inhibitory signals would result in an involuntary, excitatory response, such as lockjaw.

It is important to note that neurons involved directly in the neuromuscular junction are ALWAYS excitatory. They can be turned off, via the inhibitory neurons discussed in the question above; however, if a neuron is directly part of the neuromuscular junction, it is excitatory.

Example Question #5 : Help With Neuromuscular Junction Physiology

An experimental drug is given to a patient. The drug is known to inhibit the activity of acetylcholinesterase. What effect would most likely be observed in the patient as a result of this drug?

Possible Answers:

Induced muscle contractions

Additional acetylcholine release from the presynaptic neuron of the neuromuscular junction

Weakened muscle contractions

Reduced acetylcholine present in the neuromuscular synapse

Reduced muscle contractions

Correct answer:

Induced muscle contractions

Explanation:

Acetylcholinesterase is an enzyme that is responsible for breaking down excess acetylcholine in the neuromuscular junction. Acetylcholinesterase inhibitors will thus reduce the activity the enzyme that breaks down acetylcholine (ACh), effectively increasing acetylcholine concentrations in the neuromuscular junction. The result of this change will be an exaggeration of the effect of ACh on the postsynaptic muscle at the neuromuscular junction.

Importantly, inhibitors of this enzyme will not increase the amount of ACh produced by the neuron. Instead, it just prolongs the time of synaptic residence for already released ACh.

Example Question #6 : Help With Neuromuscular Junction Physiology

What neurotransmitter is used to signal muscle contraction at the neuromuscular junction?

Possible Answers:

serotonin

acetylcholine

dopamine and acetylcholine

contractin

None of the other answers are correct.

Correct answer:

acetylcholine

Explanation:

A neuron at the neuromuscular junction uses the neurotransmitter acetylcholine to signal the contraction of the muscle via action potential generation, which signals the release of calcium from the muscle sarcoplasmic reticulum.

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