NBME 22 Area labeled ‘C’ (Abducens nucleus, right) (NBME Answers)

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Retired NBME 22 Answers

nbme22/Block 1/Question#37 (reveal difficulty score)
A 28-year-old woman is brought to the ...
Area labeled ‘C’ (Abducens nucleus, right) 🔍 / 📺 / 🌳 / 📖
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submitted by ∗imgdoc(183)

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I think its the right MLF (area C), not the right abducens nucleus that is lesioned on the cross section. If the abducens nucleus were lesioned she wouldn't have abduction in her left eye. The MLF mediates cross talk between the abducens nucleus on both sides to the MLF on the opposite side (2 abducens nuclei, 2 MLF one on each side). In her case, her right MLF wasn't functioning hence why she was gazing left but her right middle rectus wasn't contacting to mediate leftward gaze.

This picture helps: http://what-when-how.com/wp-content/uploads/2012/04/tmp15F9_thumb.jpg

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imgdoc *medial rectus +1

kateinwonderland https://en.wikipedia.org/wiki/Medial_longitudinal_fasciculus#/media/File:Gray711.png +2

quip13 This shows the MLF pathway in a saggital cut: https://ars.els-cdn.com/content/image/3-s2.0-B9781437719260100128-f12-08-9781437719260.jpg +2

Quick review of gaze control; using leftward gaze as example

horizontal gaze control stems from left frontal gaze center
signal crosses to right pontine gaze center
signal goes to right MLF that yokes together right eye CN III to adduct and left eye CN VI to abduct and look to the left

+1/- jackie_chan(34)

Correction to myself sorry, wish could edit/delete I got the crossing mixed

For left gaze + horizontal gaze stems from RIGHT frontal gaze center + crosses to LEFT pontine gaze center + left pontine gaze center links to left CN VI to abduct left eye; sends crossing to link with the RIGHT MLF + RIGHT MLF then adducts the right eye via CN III

+3/- jackie_chan(34)

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submitted by ∗lnsetick(107)

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This is internuclear ophthalmoplegia. In INO you would see impaired adduction during horizontal gaze due to a lesion of the ipsilateral MLF. However, since the MLF is not involved in convergence, the affected eye is still able to converge normally.

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submitted by ahd_ve(7)

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This imagen is a transverse section through the pons at the level of trigeminal nuclei: A and B: Superior Cerebellar peduncle. C and D: Medial longinutidinal fasciculus. E and F: Bundles of corticospinal and cortinuclear fibers.

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submitted by ∗drdoom(1206)

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Here’s another very nice one that superimposes the pathway onto a simplified brainstem drawing (nice for the anatomical relations):

https://webeye.ophth.uiowa.edu/eyeforum/cases-i/case252/Fig2-INO-LRG.png

Source article:

https://webeye.ophth.uiowa.edu/eyeforum/cases/252-internuclear-ophthalmoplegia.htm

To see even more, try google image search on “medial longitudinal fasciculus”:

https://www.google.com/search?q=medial+longitudinal+fasciculus&tbm=isch

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endochondral1 what is A and B in this pic? i knew it was dorsal pons ipsilateral but i just didn't know what part that was on the pic? +1

nwinkelmann A and B are the superior cerebellar peduncles.http://what-when-how.com/wp-content/uploads/2012/04/tmp15F2.jpg +1

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submitted by ∗hyperfukus(111)

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so the lesion is in the Right MLF right? If so I'm just about to memorize the eye see SAME MiLF lol its the MLF on the same side of the eye keep it simple i hope that's what yall are saying lol

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submitted by ∗drdoom(1206)

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Nice schematic of how horizontal gaze is coordinated through the abducens/MLF/oculomotor pathway:

https://n.neurology.org/content/neurology/70/17/e57/F1.large.jpg

In the diagram, the system is coordinating gaze toward pt’s left, which (conveniently) is the same as in the stem.

Source article: https://n.neurology.org/content/70/17/e57

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hello https://media.springernature.com/full/springer-static/image/art%3A10.1038%2Fs41433-018-0216-y/MediaObjects/41433_2018_216_Fig1_HTML.png?as=webp +1

hello Oops meant https://imgur.com/pD4amBJ +2

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submitted by ∗pppro(25)

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The lesion is in the medial longitudinal fasiculus.

http://what-when-how.com/neuroscience/brainstem-iii-the-midbrain-organization-of-the-central-nervous-system-part-1/

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submitted by ∗hello(429)

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If you need a different orientation for a diagram of the pathways:

https://media.springernature.com/full/springer-static/image/art%3A10.1038%2Fs41433-018-0216-y/MediaObjects/41433_2018_216_Fig1_HTML.png?as=webp

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hello Oops meant https://imgur.com/pD4amBJ +1

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submitted by ∗okokok1(38)

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The question stem is describing a pt with Multiple Sclerosis. INO is HIGHLY associated with that. Then if you follow the rule of 4's, you know that the MLF is medial and ipsilateral to the affected side. Thus, it must be the medial right structure.

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