During MVD surgery to relieve a hemifacial spasm, which CN stimulation elicits a response from the mentalis muscle?

The mentalis muscle, which plays a critical role in the elevation of the chin and the protrusion of the lower lip, is innervated by the facial nerve, also referred to as cranial nerve VII (CN VII). During microvascular decompression (MVD) surgery aimed at alleviating hemifacial spasms, stimulation of CN VII is essential for assessing the functional status of the facial nerve and monitoring the response of muscles innervated by it.

When CN VII is stimulated during the procedure, the mentalis muscle demonstrates a contraction, indicating that the facial nerve is functioning appropriately. This response is crucial for the surgical team, as it provides immediate feedback regarding the integrity of the facial nerve and helps in avoiding damage during the operation.

Other cranial nerves listed—CN III, CN VIII, and CN V—do not directly innervate the mentalis muscle, and thus, stimulation of these nerves would not produce a response from the mentalis. CN III primarily controls ocular movements, CN VIII is responsible for hearing and balance, and CN V is involved in sensory innervation to the face and muscles of mastication, but not specifically to the mentalis. Therefore, stimulation of cranial nerve VII is the correct answer in this context