Acetylcholine is normally broken down by the enzyme acetylcholinesterase. If this enzyme is pharmacologically inhibited, what would be the likely effect on the target cell? A. Additional acetylcholine would be produced. B. One molecule of acetylcholine would bind to each receptor. C. Target cells would overproduce acetylcholinesterase. D. Receptors would be unable to bind acetylcholine. E. Sodium channel receptors would be continuously stimulated.
The correct answer and explanation is:
Correct Answer: E. Sodium channel receptors would be continuously stimulated.
Acetylcholine (ACh) is a neurotransmitter that plays a crucial role in the transmission of nerve impulses across synapses, particularly at neuromuscular junctions and in the autonomic nervous system. Under normal conditions, once acetylcholine is released into the synaptic cleft and binds to receptors on the postsynaptic membrane, its action is quickly terminated by the enzyme acetylcholinesterase. This enzyme hydrolyzes acetylcholine into acetate and choline, which prevents prolonged stimulation of the receptor.
When acetylcholinesterase is pharmacologically inhibited, the breakdown of acetylcholine is prevented. As a result, acetylcholine accumulates in the synaptic cleft and continues to bind to its receptors. One of the major types of acetylcholine receptors found at neuromuscular junctions is the nicotinic receptor, which is directly linked to sodium channels. When acetylcholine binds to these receptors, it opens the sodium channels, allowing sodium ions to enter the cell and initiate an action potential or muscle contraction.
If acetylcholine is not broken down because acetylcholinesterase is inhibited, the sodium channels remain continuously activated due to the persistent presence of acetylcholine. This causes continuous stimulation of the postsynaptic cell. In muscle cells, this could lead to prolonged contractions or muscle spasms. In severe cases, especially with high doses of acetylcholinesterase inhibitors such as certain nerve agents or pesticides, this can result in paralysis or respiratory failure due to the inability of muscles to relax.
The other choices are incorrect because:
- A: More acetylcholine is not produced, it simply accumulates.
- B: Acetylcholine can bind to multiple receptors, not limited to one per receptor.
- C: The cell does not overproduce the enzyme in response to inhibition.
- D: Receptors remain able to bind acetylcholine; they are just overstimulated.