Nervous System Question - EK Bio 1001 #277

[MDJerome]

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Cobra venom (cobra toxin) is a competitive inhibitor of acetylcholine. Which of the following is a likely outcome in an individual bitten by a cobra?

A. excessive stimulation of most parasympathetic neurons
B. CNS neuronal degeneration and death
C. respiratory muscle paralysis
D. rapid heart rate and dehydration





Okay the answer I chose was D and I'll explain why. The Vagus Nerve (of the parasympathetic system) innervates the SA node of the heart reducing the heart rate by releasing acetylcholine. By competiting with acetycholine, the toxin will prevent the neurotransmitter from binding to its receptor and indirectly causes the heart rate to increase (because depolarization of SA node isn't being suppressed - which inturn generates a faster heart beat).

There answer was C - which they explain: "When acetylcholine receptor function is blocked muscle paralysis does result. If respiratory muscles (ie, diaphram) cease to function, death from respiratory failure occurs."

I agree with this answer choice, but isn't answer choice C also acceptable? If I'm wrong, please correct me.

Thanks.

 

[DrYoda]

Cobra venom (cobra toxin) is a competitive inhibitor of acetylcholine. Which of the following is a likely outcome in an individual bitten by a cobra?

A. excessive stimulation of most parasympathetic neurons
B. CNS neuronal degeneration and death
C. respiratory muscle paralysis
D. rapid heart rate and dehydration

.

D has two parts.

 

[MDJerome]

D has two parts.

oopsss lol. thanks, haha

 

[MDJerome]

But wait a minute. Isn't dehydration associated with rapid heart beat? Think about it. If your heart beat is racing, you're most likely sweating.

 

[fizzgig]

isn't sweating a sympathetic response? high HR bc you inhibit parasymp is not the same as high HR due to stimulation of symp when you start looking at associated effects.

inhib of parasymp does not directly cause sweating i dont think, and inhib of musculature will have an immediate and large effect on breathing.

 

[DrYoda]

But wait a minute. Isn't dehydration associated with rapid heart beat? Think about it. If your heart beat is racing, you're most likely sweating.

Dehydration can cause low blood pressure which causes increased heart beat in an attempt to maintain pressure. But I'm not sure that a rapid heartbeat would necessarily cause dehydration.

isn't sweating a sympathetic response? high HR bc you inhibit parasymp is not the same as high HR due to stimulation of symp when you start looking at associated effects.

inhib of parasymp does not directly cause sweating i dont think, and inhib of musculature will have an immediate and large effect on breathing.

Sweat glands are sympathetic and are an exception in that they are activated by acetylcholine (although via a different receptor type). So if anything inhibiting acetylcholine receptors would decrease sweating. (I'm sure this is beyond the MCAT, just background info).

 

[Rabolisk]

Acetylcholine is a neurotransmitter involved in somatic nervous system, as well as both the parasympathetic and the sympathetic autonomic nervous systems. The heart is innervated by both systems, producing opposite effects. Thus inhibition of acetylcholine would simultaneously reduce and increase heart rate, rather than just one.

 

[fizzgig]

eeenteresting yoda.

rabolisk, the end NT for the sympathetic effects on the heart (and almost everywhere except apparently some oddities like sweat glands) is not Ach though, so for the heart specifically blocking Ach receptors should only block parasymp not symp, right? does blocking ach by injecting the agent into the bloodstream block the sympathetic postsynaptic neuron from getting a sympathetic signal from the presynaptic one? i kinda thought it would directly hit and affect the target organ only in any significant way...

 

[Don Draper]

But wait a minute. Isn't dehydration associated with rapid heart beat? Think about it. If your heart beat is racing, you're most likely sweating.

lol. This is called, "justifying my answer".

You can even tell by your language, "most likely" sweating. It doesn't mean you are sweating. Sweating means you need to cool your body temperature, if you were freezing but had an increased heart beat would you be sweating?

Whenever you have to add to the question to get it correct, it is usually wrong.

Plus you are missing the #1 MCAT skill, finding the best answer. If you think only EK will have these answers where "it could also be D" occurs then you will be surprised. You must be able to guess what the test taker is testing. When someone asks you about inhibiting acetylcholine, you should know what they are testing. Increased heart rate alone would not be the most likely answer, with dehydration it is certainly wrong. Typically when you see acetylcholine we are talking somatic nervous system.

The MCAT is basic. Ideas like this:

The heart is innervated by both systems, producing opposite effects. Thus inhibition of acetylcholine would simultaneously reduce and increase heart rate

are rarely if ever tested. Things can get complex but when you do questions they usually end with a simple explanation, not a complex one.