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OPINION
Year : 2021  |  Volume : 19  |  Issue : 4  |  Page : 282-283

Paradox of the cranial nerves: Does the description of the facial nerve need a reinterpretation?


Department of Medicine, M.O.S.C Medical College and Hospital, Ernakulam, Kerala, India

Correspondence Address:
Prof. Abraham M Ittyachen
Department of Medicine, M.O.S.C Medical College and Hospital, Kolenchery, Ernakulam - 682 311, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/cmi.cmi_26_21

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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 ?.


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