MCAT® ExamBiological and Biochemical Foundations of Living SystemsEasy
A patient is prescribed a medication that acts as an antagonist to acetylcholine receptors at the neuromuscular junction. This drug effectively blocks the binding of acetylcholine to its receptors. Which of the following physiological effects would be the most direct consequence of this medication?
- AStimulation of glandular secretions in the digestive system
- BIncreased heart rate due to sympathetic nervous system activation
- CEnhanced skeletal muscle contraction due to prolonged depolarization
- DReduced ability of skeletal muscles to contract, leading to paralysis
Show answer & explanationAnswer & explanation
Correct answer: D. Reduced ability of skeletal muscles to contract, leading to paralysis
Acetylcholine binding to receptors at the neuromuscular junction is essential for initiating skeletal muscle contraction. Blocking these receptors directly prevents the muscle cell from depolarizing and initiating an action potential, thus impairing contraction and potentially causing paralysis.
Why the other options are wrong
- A. Acetylcholine stimulates many glandular secretions, but the question specifies the neuromuscular junction, which primarily controls skeletal muscle.
- B. Acetylcholine generally slows heart rate (parasympathetic); blocking its receptors here wouldn't directly increase heart rate in this context.
- C. Blocking acetylcholine receptors would prevent, not enhance, muscle contraction.
Neuromuscular Junction
The specialized synapse between a motor neuron and a skeletal muscle fiber, where neurotransmitter release initiates muscle contraction.
- Motor neuron releases acetylcholine (ACh).
- ACh binds to receptors on the muscle fiber (motor end plate).
- Binding causes depolarization and muscle action potential.
- Leads to excitation-contraction coupling.
Memory trick: ACh Antagonist: Muscle Action Arrested