Biology QPack Volume 1: #99

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Biology QPack Volume 1: #99

Post by mayamatsumoto » Sat Jul 06, 2019 4:17 am

Hi! I'm a bit confused about the solution given by the AAMC for question #99.

They say that:
"Addition of a cholinesterase blocker would produce a buildup of acetylcholine in the synapse and prevent receptors from responding to impulses, so option A is incorrect."

This seems counterintuitive to me, especially given how SSRIs work. I thought that by preventing the breakdown of a neurotransmitter like acetylcholine, and thus increasing the amount present in the synapse, would result in more activation of postsynaptic muscle cell. Can someone please explain to me why this is not the case?

Thank you! :)
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Re: Biology QPack Volume 1: #99

Post by NS_Tutor_Mathias » Sat Jul 06, 2019 2:36 pm

This wants you to think about what it means to transmit a signal, and in what way that is NOT synonymous with just activating a receptor. That is also why the question specifies a repeated transmission of an impulse (the kind we need to meaningfully do things). In order to transmit a signal repeatedly, you need activation (conformational change) of the receptor transmembrane protein by binding to a neurotransmitter, but also subsequent unbinding and cessation of graded potentials. If that doesn't happen, you cannot respond to new release of neurotransmitters in any way.

The key points here are to read the question very carefully, and to consider the whole system (what steps are all necessary to repeatedly send a signal?) rather than just a single step (binding of the neurotransmitter). The release, binding and virtually immediate degradation is why the nervous system is so good at transmitting information.

Cholinesterase blockers were first discovered in naturally occurring venoms that induce spasticity, a condition of uninterrupted muscle contraction that rapidly leads to death, in mammals usually due to an inability to relax the diaphragm. They are also potent nerve agents in chemical warfare, and have more recently seen use as therapeutic agents.
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