Review Question - QID 108926

The patient in this question presents with difficulty speaking and leftward tongue deviation suggestive of a lesion in the left hypoglossal nerve (CN XII). The hypoglossal nerve innervates the intrinsic musculature of the tongue as well as the genioglossus, hyoglossus, and styloglossus muscles.

The hypoglossal nerve (CN XII) provides motor control to the intrinsic muscles of the tongue as well as the genioglossus, hyoglossus, styloglossus muscles. It has no sensory component. The palatoglossus is the only extrinsic muscle of the tongue that is innervated by a different nerve (the vagus). A unilateral hypoglossal nerve lesion will result in deviation of the tongue to the ipsilateral side due to weakness of the ipsilateral genioglossus, which allows for unopposed pushing action by the contralateral genioglossus.

Figure A demonstrates the characteristic appearance of a unilateral hypoglossal nerve lesion. Note the leftward deviation of the tongue suggestive of a lesion in the left hypoglossal nerve.

Incorrect Answers:
Answer 1: The mylohyoid muscle connects the mandible near the molar teeth (“mylo”) to the hyoid bone (“hyoid”). The mylohyoid muscle functions to elevate the tongue and hyoid bone during swallowing and speaking. It is innervated by a branch of the mandibular nerve (CN V3).

Answer 2: The palatoglossus muscle is the only muscle of the tongue that is not innervated by the hypoglossal nerve. Instead, it is innervated by the vagus nerve (CN X). It functions to elevate the posterior tongue and aid in swallowing.

Answer 4: The stylohyoid originates from the styloid process (“stylo”) and inserts into the hyoid bone (“hyoid”). It functions to elevate the tongue and pull the hyoid bone posteriorly. It is innervated by the facial nerve (CN VII).

Answer 5: The stylopharyngeus originates from the styloid process (“stylo”) and inserts between the pharyngeal constrictors (“pharyngeus”). It functions to elevate the larynx and pharynx. It is the only muscle in the pharynx that is innervated by the glossopharyngeal nerve (CN IX).

Bullet Summary:
The hypoglossal nerve (CN XII) innervates all the extrinsic and intrinsic muscles of the tongue except for the palatoglossus. A unilateral hypoglossal lesion will present with tongue deviation to the affected side.