MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A patient is prescribed a medication that acts as an antagonist to acetylcholine receptors at the neuromuscular junction. Which of the following physiological effects would most likely be observed in this patient?
- AIncreased salivary gland secretion
- BReduced skeletal muscle contraction
- CEnhanced skeletal muscle contraction
- DIncreased heart rate
Show answer & explanationAnswer & explanation
Correct answer: B. Reduced skeletal muscle contraction
At the neuromuscular junction, acetylcholine (ACh) is released and binds to nicotinic acetylcholine receptors on the muscle fiber, leading to muscle contraction. An antagonist to these receptors would block ACh binding, thereby inhibiting the transmission of the nerve signal to the muscle and resulting in reduced skeletal muscle contraction.
Why the other options are wrong
- A. Increased salivary gland secretion is mediated by muscarinic acetylcholine receptors, primarily in the parasympathetic nervous system, not the nicotinic receptors at the neuromuscular junction.
- C. An antagonist would block, not enhance, muscle contraction.
- D. Increased heart rate is typically mediated by sympathetic nervous system activity (norepinephrine), not by blocking ACh receptors at the neuromuscular junction.
Neuromuscular Junction
The neuromuscular junction (NMJ) is the synapse between a motor neuron and a skeletal muscle fiber. Acetylcholine (ACh) is the neurotransmitter released at the NMJ, binding to nicotinic ACh receptors on the muscle fiber to initiate muscle contraction.
- Synapse between motor neuron and muscle fiber.
- Neurotransmitter is acetylcholine (ACh).
- ACh binds to nicotinic receptors on muscle.
- Leads to muscle fiber depolarization and contraction.
Memory trick: NMJ: 'Nerve' tells 'Muscle' to 'Jump' with ACh.