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AAMC FL Chem #8 question on voltage

Discussion in 'MCAT Study Question Q&A' started by medschoolworries, Aug 9, 2015.

  1. medschoolworries

    2+ Year Member

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    The question asks:

    What is the closest distance the electrodes used in an NCV test can be placed in a nerve in order to measure the voltage change as a response to the stimulus?

    a. .01m
    b. .1m
    c. 1m
    d. 10m

    The answer key states that you somehow know the action potential time response is 1 msec...where did this come from? It says figure 1 shows this but figure 1 is just a circuit diagram. I get that once you know the time you can just multiple the speed x time to get the distance but I don't get where the time 1 msec came from.

    Can anybody help shed some light? I have a feeling there's something really obvious in the passage I'm just missing.
     
  2. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer
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    I assume they meant Figure 2. That's what i used.
     
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  3. ElectricNoogie

    ElectricNoogie MCAT enthusiast

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    The AAMC will not always spell things out for you. In fact, most of the time they will not. You are asked for the distance traveled by nerve signal, and all the answers are in meters. This implies some sort of calculation, requiring other numbers to arrive at the answers. The only specific information you are given about the nerve signal is in figure 2 and in the paragraph below, where you are told nerve impulses travel at 100 m/s. With the MCAT when you see calculations are required, if you cannot understand at first how to arrive at the answer, do a quick inventory of 1)what you have and 2) what you want. In this manner you can derive any equation or relationship you might need in a pinch and reason your way to the answer.

    So, to recap we are given velocity (100 m/s) and we want distance (m). Hmmm it sure would be nice to know how long a nerve impulse lasts, so that we could use rate x time = distance and solve for the minimum distance needed to accurately record a nerve impulse. Well well well look at figure 2, it tells you a nerve impulse lasts 1 ms (seen from the time it takes the signal to go from baseline to depolarized max back to baseline).
    upload_2015-8-11_17-0-27.png

    Thus, we would need to place the electrodes at a minimum distance equal to how far the signal can travel in the time it exists. Otherwise we risk missing part of the signal (electrodes too close) or catching multiple signals (electrodes too far apart).

    Even if you weren't sure about the above concept, you can make an educated guess that the only way to arrive @ units of meters is to take the rate and multiply it by the time and we arrive at: 100 m/s x 0.001 sec = 0.1 m

    There is usually more than one way to arrive at the right answer, you will find the way that works best for you but the more tools you have for the MCAT, the better you will do.

    Hope this helps, Good luck!
     
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  4. drechie

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    i have a hard time interpreting the graph with confidence that the whole action potential from start to finish takes 1msec.
     
  5. itsalwayssunny96

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    So the way I solved this one is I paid attention to the voltage change and how long that lasted for. If you look at the question stem, it asks for measuring the "voltage change." The total voltage change (going from resting at -70 to +35 to -90) is encapsulated within approximately 1ms. Then solve using v=d/t.
     
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  6. jazzmetal

    jazzmetal SDN Gold Donor
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    Yeah, I didn't get that from the figure either, until after I saw the answer. The way it's drawn implied (to me) that you were supposed to be taking the refractory period into account too - in other words, the length of time for the voltage to return to where it started from, which looks more like 10ms, so I chose 0.1m. I guess general knowledge might be helpful: wikipedia says that sodium-based action potentials usually last for under 1ms, which calcium-based action potentials can last 100ms or longer. But the figure still seems vague to me.
     
  7. letsgetstarted1234

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    bumping this because i also thought you always had to take the refractory period into account.

    but i guess because it is one single nerve impulse you dont?
     

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