Curare is a poisonous plant extract. Curare molecules have a chemical structure like the neurotransmitter ACh. Curare can bind to the ACh receptor site on the chemically gated ion channels in the motor end plate. Even though curare will bind to the receptor site it will not open the ion channel and no ions will pass through. What do you think the symptoms of curare poisoning would look like?

Question

BlackTom AI · Accepted Answer

The question describes curare as a plant toxin that mimics acetylcholine (ACh) at the receptor site on the motor end plate, binds to the nicotinic ACh receptors, but does not activate the receptor to open the ion channel. This leads to inhibited depolarization of skeletal muscle fibers and thus paralysis rather than stimulation.

Option A: 'Curare will only affect muscles with ACh receptors, paralyzing them.' This is accurate because curare targets nicotinic ACh receptors at the neuromuscular junctions of skeletal muscles. Since it blocks activation without opening the channel, muscles cannot contract, leading to paralysis. The statement correctly limits the effect to muscles that rely on these receptors, explaining the clinical outcome.

Option B: 'Muscles will respond too quickly and cause a severe tremor.' This is incorrect because curare does not enhance muscle activity or cause tremors. Instead, it prevents normal signaling for contraction, resulting in weakness or paralysis, not hyperactivity.

Option C: 'Smooth muscles will become stimulated causing quick movement of nutrients through the digestive system.' This misstates the receptor distribution. Curare primarily affects skeletal muscle, which is innervated by somatic motor neurons with nicotinic receptors. Smooth muscles are controlled by different autonomic pathways and receptors (such as muscarinic receptors in many cases), and curare does not stimulate them; plus, its effect is generally paralysis, not increased motility.

Option D: 'Curare will only affect cardiac muscle, causing fibrillations of the heart.' This is wrong because the cardiac muscle is not driven by nicotinic ACh receptors at neuromuscular junctions. The heart primarily uses autonomic inputs and other receptor types (including muscarinic receptors). Curare does not selectively target cardiac muscle, and it does not typically cause cardiac fibrillations through this mechanism.

In summary, the key concept is that curare binds to nicotinic ACh receptors at skeletal NMJs without activating them, leading to skeletal muscle paralysis; options B, C, and D mischaracterize the selectivity and the physiological consequences.

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