Question about CN12 and the infamous "tongue deviates" question

[rocksmann]

Hi all,

 

I had a question about the tongue and UMN lesions affecting the protrusion of the tongue.

 

A man is asked to protrude his tongue and it deviates to the right which cranial nerve is involved?

Answer is : right CN XII

left CN XII

left CN X

right CN X

left CN IX

 

I know XII is the nerve we test in a neurology exam for tongue protrusion. So I can eliminate 3 answers. Its frustrating because i feel like this question assumes too much about this man. If it is a UMN lesion then it will deviate toward the side of the lesion but if it is an LMN lesion it will deviate toward the contralateral side. But, the question asks about the UMN (cranial nerves) why or how do you guys understand this.

 

I drew a picture in MS paint. I think if the right side is flaccid then when the circumferential and longitudinal muscles of the tongue contract only on the left then the tongue is pushed (if you will) toward the side of the lesion.

 

At the end of the day, this question is irrelevant. If he had jaw surgery on his right side and his tongue deviates toward the right then I would suppose the surgeon nicked his right lingual nerve so send him back to the surgeon with a CT scan before his appointment. If instead, it deviated to the left then I would CT scan head and neck and send him back to the surgeon.

 

Any help would be great, but I would like someone to explain precisely how to remember this for the board exam.

[Guest Samhain_Grim]

 Its frustrating because i feel like this question assumes too much about this man. If it is a UMN lesion then it will deviate toward the side of the lesion but if it is an LMN lesion it will deviate toward the contralateral side. But, the question asks about the UMN (cranial nerves) why or how do you guys understand this.

 

 

By definition, all CNs are LMNs. The upper motor neurons for the tongue are in contralateral motor cortex (MCA territory - think of the motor homunculus); they are cortical neurons, not cranial nerves. The UMNs travel via the corticobulbar tract (analogous to the corticospinal tract, only serving the cranial nerve nuclei instead of the spinal cord). The fibers decussate to the contralateral nucleus of XII in the medulla. The LMN neuron cell bodies are in the nucleus of XII, and the LMN axons (which make up CN XII itself) travel ipsilaterally to the tongue.

 

The corticobulbar tract usually provides bilateral innervation to the cranial nerve nuclei. The exceptions are:

1. The nuclei of CN VII (the upper nucleus gets bilateral corticobulbar innervation, but the lower nuclei only get contralateral corticobulbar innervation; hence why you get sparing of the forehead in an MCA stroke) 

2. The nucleus of CN XII, which is relevant here

 

If I have a stroke on the R cortex that managed to affect only the tongue area, then when I protrude my tongue it will deviate to the L (R UMN is dead; so contralateral (L) tongue muscles are weak). If I have damage to R CN XII or somehow selectively damage the nucleus of XII on the R (LMN lesion), then the ipsilateral tongue muscles ( R ) will be weak and my tongue will deviate to the R.

 

A simple heuristic: Anything in the cortex will control things on the opposite side of the body. The cranial nerves generally control the same side of the body (the exception is CN IV, which really isn't important to know). The cerebellum is unique and controls coordination on the same side of the body (because spinocerebellar tracts either do not decussate, or the fibers cross a second time in the cerebellum).