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NBME 24 Answers

nbme24/Block 4/Question#10

A 60-year-old woman is receiving cisplatin therapy ...

Hyporeflexia

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 +4  upvote downvote
submitted by consistentwrongdoer3(4),

The answer is hyporeflexia because the afferent arc of the muscle stretch reflex has to go through the dorsal rami and dorsal root ganglia. Dumb question, I know, but it’s the only answer that made sense. If you hurt the DRG, you not only lose afferent somatic sensory fibers, you also lose the sensory bodies involved in the various reflexes.

You can also get hyporeflexia from damaging the efferent neurons that innervate the muscle (like a LMN), but as you know these are in the anterior horn and ventral rami.

ankistruggles  Thanks! I agree with you. +  
brethren_md  Great explanation. +  
gonyyong  Agreed - I think I got this by thinking about tabes dorsalis (syphillis) and why it has hyporeflexia is due to dorsal root damage +  
duat98  I'm confused about why it wouldn't cause muscle atrophy. Isn't that a fever of LMN damage? +1  
charcot_bouchard  Muscle atrophy wont occur because alpha motor neuron is intact. Motor control of Corticospinal tract on this is intact. so no atrophy. u can move shiti/ But remeber muscle spindle that is responsible for INITIATING stretach reflex send Ia fibre to DRG from where it synapse with Alpha motor neuron. if DRG is damage ur muscle is fine but u cant initiate strech reflex. areflexia +  




 +0  upvote downvote
submitted by enbeemee(3),

i get why it's hyporeflexia, but why not fibrillations? it's also an LMN sign

et-tu-bromocriptine  Imagine a simple reflex arc: you have an afferent neuron, some interneuron shenanigans, and an efferent neuron (aka LMN neuron). If you damage the LMN, you will get hyporeflexia (due to damaged reflex arc) and fibrillations (because your LMN won't be able to effectively contract muscle on command). However, if you damage the afferent part of the arc, you will still get a damaged reflex arc (hyporeflexia), but your motor neuron will still be able to do its stimulating effectively, so your muscles won't show weak contractions when stimulated by a higher pathway. Kinda confusing but I hope I made it a tad simpler! +1