Logical Reasoning

PT152 · S1 · Q22 Although severing a motor nerve

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Although severing a motor nerve kills part of the nerve, it can regenerate, growing about 1 millimeter per day from the point of damage toward the muscle the nerve controlled.

Statements

Causal Comparison Because severed motor nerves grow from the point of damage to the thing that nerve controls, a nerve that controls the hand will grow back more quickly if the nerve is broken near the wrist than near the shoulder.

Conditionals (require = right side) Growing cells guided → Original nerve sheath

(unless = if not) No living nerve tissue → Orig nerve sheath disintegrates

Evaluate

There's an overlapping idea in those two conditionals (the nerve sheath), so they might chain together.

We could probably say, No living orig nerve cells aren't nerve tissue → sheath → guided back for 3 mos disintegrates to damage

We might be able to use that original causal/comparison claim to get into this conversation as well. Since time is of the essence (the longer it takes for the nerve to grow back, the greater risk we run of the the original nerve sheath disintegrating and then having no way to guide the re-growing nerve cells back to the source of damage), it seems like the 1mm / day figure could come into play.

3 months is about 90 days, so if the nerve is severed more than 90mm from the muscle it controls, it might not be able to grow back in time before its sheath-guide disintegrates.

Goal

Stay flexible, but maybe look for something combining the slow re-growth rate with the 3 month expiring time limit.

22.

The statements above, if true, most strongly support which one of the following?

Doubling the speed at which
Too Strong: doubling = doubling
Doubling the speed will certainly increase the likelihood that the motor nerve will reach the muscle it had controlled, but we can't derive such an exactly proportional idea as "double the growth speed, double the probability". (Keep in mind that something with a likelihood of, say, 60% cannot be doubled, or else you'd be saying, nonsensically, that it had a 120% chance of happening).
4% picked this
It is sometimes possible, once
Out of Scope: reverse/slow disintegration
We only hear about the sheath disintegrating in the last sentence, and we only hear that it will begin to disintegrate after about three months. There's no information that suggests that once that occurs, it can be reversed or slowed.
6% picked this
If a severed motor nerve
Too Strong: cannot be restored
After 3 months, we know that the sheath begins to disintegrate. Can we say that once it begins to disintegrate, functioning cannot be restored? It's a little too harsh. There might still be enough sheath to finish off the re-growing process. Or maybe the muscle never lost functioning from the one severed motor nerve.
21% picked this
Correct
If living nerve tissue could
Why this is right
This is very weakly worded: in some cases, if we could do this, the likelihood would increase (in at least one situation, the likelihood goes up at least somewhat). Since we know that ~living tissue → disint. sheath → ~guide we know that not-having living tissue could be a reason that nerves would fail to grow back. Since the re-growth timeline is so slow (1mm / day) and injuries could easily be more than 90 mm from the injury site (i.e. a shoulder is definitely more than 9cm from a hand), there are certainly some cases in which the nerve doesn't have time to grow back before the sheath would disintegrate. So if we could get living tissue in there so that the sheath had a longer timeline before disappearing, we could probably give the re-growing motor nerve a better chance of making it all the way back.
Skill tested: Most Supported · how this choice captures the argument's function is the move to repeat next time.
68% picked this
Without surgical intervention, a muscle
Out of Scope
Out of Scope: surgical intervention Too Strong: unlikely We never talked about "surgical intervention", and we don't have good reason to be so pessimistic that we think the muscle will usually not (unlikely to) regain function.
1% picked this

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