|
 
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| OPINION |
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| Year : 2021 | Volume
: 19
| Issue : 4 | Page : 282-283 |
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Paradox of the cranial nerves: Does the description of the facial nerve need a reinterpretation?
Abraham M Ittyachen
Department of Medicine, M.O.S.C Medical College and Hospital, Ernakulam, Kerala, India
| Date of Submission | 28-Mar-2021 |
| Date of Decision | 01-May-2021 |
| Date of Acceptance | 29-May-2021 |
| Date of Web Publication | 07-Dec-2021 |
Correspondence Address:
Prof. Abraham M Ittyachen
Department of Medicine, M.O.S.C Medical College and Hospital, Kolenchery, Ernakulam - 682 311, Kerala
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/cmi.cmi_26_21
For years, generations of medical students were taught that in stroke, “;In a upper motor neuron (UMN) lesion, the upper part of the face is spared because this part of the face has bilateral representation while in a lower motor neuron (LMN) lesion, both parts of the face are involved.” Is this explanation apt ? Most of the fibers of the corticospinal tract decussate to the opposite side. But only 50% of the corticobulbar fibers decussate, meaning the muscles they supply receive fibers from both motor cortex (bilateral representation). Bilateral representation in the cortex is a perfect example of symmetry in the human body. An exception to this rule would be the lower part of the face which receives fibers from the contralateral cortex alone (unilateral representation). This goes against the general rule and so this represents the first paradox of the cranial nerves. The second paradox is entirely man made and is reversible. Should it not be, “;In a UMN lesion of the facial nerve, only the contralateral lower half of the face is affected, because this part of the face has unilateral representation only” and not “;In a UMN lesion of the facial nerve, the contralateral upper part of the face is spared because this part of the face has bilateral representation.” After all, is not bilateral representation the rule and unilateral representation the exception ?.
Keywords: Cranial nerves, facial nerve, decussation of cranial nerves
How to cite this article:
Ittyachen AM. Paradox of the cranial nerves: Does the description of the facial nerve need a reinterpretation?. Curr Med Issues 2021;19:282-3 |
For years, generations of medical students were taught (and continue to be taught) that in stroke, “;In a upper motor neuron (UMN) lesion, the upper part of the face is spared because this part of the face has bilateral representation while in a lower motor neuron (LMN) lesion, both parts of the face are involved.” Is this explanation apt?
Any student of clinical medicine knows that knowledge of neuroanatomy is the key to localization of a nerve lesion. The cranial nerve nuclei in the brainstem along with the corresponding nerve comprise the LMN. The LMN in turn receives fibers from the contralateral motor cortex through the descending corticobulbar tracts (also called corticonuclear tracts). The neurons from which corticobulbar tracts arise are known as UMN, and their cell bodies lie in the primary motor cortex (Brodmann area 4) in the frontal lobe.
From the time of Hippocrates, in ancient Greece, it was known that injury to the left part of the brain resulted in weakness of the right side of the body. As science progressed, so did our knowledge about neuroanatomy. Most of the fibers of the corticospinal tract decussate (cross over) to the opposite side. This explains the weakness on the opposite side. But why decussation occurs in the nervous system has always been a mystery.[1] The first person to develop a conceptual framework to explain contralateral representation and decussation in a functional manner was Cajal.[2] Although Cajal's theory is still popular, it has got several problems. Hence, other theories have been put forward.[3],[4] A currently accepted theory is that decussation evolved as a byproduct of a genetically determined partial inversion of the body plan, which resulted in a 180° rotation posterior to the brain and oropharynx.[5]
But what about the corticobulbar tracts? Unlike the corticospinal tract, only 50% of the corticobulbar fibers decussate, meaning the muscles they supply receive fibers from both motor cortexes (bilateral representation). This includes corticobulbar fibers to the motor nuclei of the trigeminal (Cranial nerve V), part of facial (Cranial nerve VII), glossopharyngeal (Cranial nerve IX), vagus (Cranial nerve X), accessory (Cranial nerve XI), and hypoglossal (Cranial nerve XII) nerves.[6],[7] Hence, in a one-sided lesion involving the corticobulbar tract, there would be relative sparing of the orofacial and neck musculature (with the exception of muscles of the lower half of the face supplied by the facial nerve) because of bilateral innervation.
There are also other lesser-known pathways in the central nervous system where the decussation is not symmetric and one such pathway is the dentatorubrothalamic tract.[8] Bilateral influence on limb movement by the individual cerebellar hemispheres is yet another example and has been shown in several functional magnetic resonance imaging studies under physiological conditions.[9] Then, there is the question of emotional movements which are considered involuntary and are clinically dissociated from voluntary facial movements. This can be explained only by a separate supranuclear pathway. The current thinking is that the cingulate motor area and/or supplementary motor areas are the areas considered vital for emotional facial movement.[10] However, it is not known with certain pathways which mediate the innervation of facial muscles for this movement.[11],[12]
Symmetry is well known in physics. The laws of physics are symmetric under translation in space and time; they do not change.[13] Bilateral representation in the cortex that was explained above is a perfect example of symmetry in the human body. One well-known exception to this rule as explained above would be the lower part of the face which receives fibers from the contralateral cortex alone (unilateral representation only). This goes against the general rule of distribution of motor fibers through the cranial nerves, and so, this represents the first paradox of the cranial nerves. This paradox is probably one of creation; apart from that we can only theorize. This paradox is permanent and cannot be changed.
Now, the second paradox is entirely man made and is reversible. Should it not be, “;In a UMN lesion of the facial nerve, only the contralateral lower half of the face is affected, because this part of the face has unilateral representation only” and not “;In a UMN lesion of the facial nerve, the contralateral upper part of the face is spared because this part of the face has bilateral representation.” After all, is not bilateral representation the rule and unilateral representation the exception?
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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