Chapter 14: Cranial Nerves 1, 9, 10, 11, and 12

The olfactory nerves transmit smell information through the cribriform plate to the olfactory bulbs and tracts, which in turn transmit this information to the olfactory cortex in the medial temporal lobes. Smell information is the only sensory modality not transmitted to the thalamus prior to the cortex. In the case of smell, the information first arrives in the olfactory cortex, and is then transmitted to the medial dorsal nucleus of the thalamus.

Loss of smell is referred to as anosmia. Since the olfactory nerve fibers travel through the cribriform plate, and the olfactory tracts lie between the inferior surface of the frontal lobes and the skull base, these structures are susceptible to damage in head trauma and compression by skull base tumors (e.g., olfactory groove meningiomas). More common causes of impaired smell are sinus disease and upper respiratory infections, and rarer causes include Kallman’s syndrome (anosmia and absence of gonadotropin-releasing hormone (GnRH) secreting neurons).

Anosmia may be part of the prodromal phase of neurodegenerative diseases (e.g., Parkinson’s disease and other synucleinopathies (see Ch. 23) and Alzheimer’s disease). In this context, anosmia is noted more commonly in retrospect since it may be attributed to aging when initially present in isolation.

Another scenario in which smell plays a role in neurologic disease is in the olfactory auras associated with temporal lobe seizures: Patients may report a foul odor (e.g., “burning tires”) that precedes temporal lobe seizure onset, presumably due to epileptic activity passing through medial temporal olfactory regions.

CNs 9 and 10 work together to supply the musculature of the pharynx (mostly supplied by CN 10) and transmit visceral afferent information from vascular baroreceptors, and each nerve also has additional individual functions listed below. CN 9 and CN 10 are discussed together since they are difficult to isolate clinically, and are commonly affected together since they both communicate with nuclei in the dorsolateral medulla, both pass through the jugular foramen, and they are adjacent throughout parts of the neck.

CN 9 supplies:

• One pharyngeal muscle: stylopharyngeus

• One gland: parotid

• One region of taste: posterior one third of the tongue

• One region of visceral sensation: carotid body

• Three small regions of somatic sensation: posterior one third of the tongue, pharynx (shared with CN 10), middle ear, and external auditory meatus (shared with CN 7 and CN 10)

The functions of CN 10 include:

• Motor supply to all muscles of the larynx and pharynx except tensor veli palitini (CN 5), mylohyoid (CN 5), stylohyoid (CN 7), stylopharyngeus (CN 9)

• Motor supply to one muscle of the tongue: palatoglossus (all others are innervated by CN 12)

• Somatic sensation from:

  • The dura mater of the posterior fossa aside from the tentorium (the sensory innervation to the rest of the dura including the tentorium is supplied by CN 5)

  • The pharynx (shared with CN 9)

  • The external auditory meatus (shared with CN 7 and CN 9)

• Visceral sensation from the aortic arch

• Visceral parasympathetic efferent supply to and afferent input from all of the viscera of the thorax and abdomen with the exception of the distal third of the GI tract and genitourinary organs (which receive their parasympathetic supply from sacral spinal cord levels S2–S4).

• Taste in the pharynx

There are several functions that are shared across cranial nerves CNs 5, 7, 9, and 10, which are summarized for comparison in Table 14–1. (Mnemonic for some of the miscellaneous muscles: the trigeminal nerve innervates tensor tympani and tensor veli palitini; the seventh nerve innervates stylohoid and stapedius.)

The laryngeal and pharyngeal motor input to CN 9 and CN 10 comes from the nucleus ambiguus in the dorsal medulla. Visceral motor supply that travels in CN 10 originates in the dorsal motor nucleus of the vagus. Afferent visceral information arrives with taste information to the nucleus solitarius (see Table 9–4).

Lesions of CN 10 can cause laryngeal and/or pharyngeal weakness. Laryngeal weakness can lead to softer voice (hypophonia), nasal voice, and guttural dysarthria (difficulty producing the consonants “G” and “K”). Pharyngeal weakness can cause difficulty swallowing (dysphagia).

On examination, CN 9 and CN 10 can be assessed by evaluating palate elevation and gag reflex. When there is unilateral palate weakness, the palate droops on the weak side and is pulled upward toward the stronger side. The gag reflex is mediated predominantly by CN 9 for the afferent limb (palate sensation) and predominantly CN 10 for the efferent limb (palate elevation).

Unilateral palate/larynx dysfunction can be caused by:

• Brainstem pathology: for example, posterior inferior cerebellar artery (PICA) stroke causing lateral medullary syndrome (causing unilateral palate/laryngeal dysfunction due to involvement of the nucleus ambiguus; other symptoms include vertigo, ataxia, Horner’s syndrome, and/or crossed hemisensory loss [decreased pain/temperature sensation in the face ipsilateral to the lesion and body contralateral to the lesion])

• Jugular foramen pathology: for example, glomus jugulare tumor, which can affect CNs 9, 10, and 11

• Local pathology in the neck: for example, lymphadenopathy, carotid dissection

• Complication of neck surgery: for example, thyroid surgery or carotid endarterectomy

Isolated unilateral laryngeal dysfunction can also be caused by pathology in the upper thorax because the recurrent laryngeal nerve branch of the vagus nerve descend into the upper thorax before re-ascending to the larynx. Therefore, mediastinal, aortic, or apical lung pathology or surgery can all cause recurrent laryngeal nerve dysfunction leading to hoarseness of the voice.

Bilateral laryngeal/pharyngeal dysfunction is commonly seen in motor neuron disease (e.g., amyotrophic lateral sclerosis [ALS]; see Ch. 28).

Glossopharyngeal Neuralgia

Glossopharyngeal neuralgia is the CN 9 analogue to trigeminal neuralgia (see Ch. 13). Lancinating neuralgic pain occurs in the throat and/or ear. Pain can be triggered by swallowing. Syncope occurs during attacks of glossophayngeal neuralgia in some cases due to altered visceral afferent transmission in CN 9. As in patients with trigeminal neuralgia, neuroimaging should be obtained to evaluate for the possibility of compression of CN 9 by a vascular loop (although most cases are idiopathic), and antiepileptics such as carbamazepine may be used for treatment.

CN 11 is a nerve derived from upper cervical roots that ascends to the foramen magnum and then leaves the skull through the jugular foramen with CN 9 and CN 10. CN 11 innervates two muscles: trapezius (which raises the shoulder) and sternocleidomastoid (which turns the head to the opposite side). CN 11 can be damaged unilaterally in the neck due to trauma or surgery or at the jugular foramen with CN 9 and CN 10 (e.g., by glomus jugulare tumor).

The upper motor neurons for control of the sternocleidomastoid are believed to be ipsilateral (rather than contralateral as for nearly all other muscles of the body), perhaps since the sternocleidomastoid turns the head to the contralateral side. Support for this idea comes from the observation that patients having seizures tend to turn the head away from the seizing hemisphere and toward the contralateral tonic–clonic motor activity (i.e., activation of the ipsilateral sternocleidomastoid causes the head to turn contralaterally).

CN 12 (hypoglossal nerve) is a pure motor nerve innervating all muscles of the tongue except the palatoglossus (which is innervated by CN 10). The hypoglossal nuclei reside in the medulla, and as with other pure motor cranial nerves that innervate skeletal muscle, these nuclei are dorsal/posterior and midline (see Ch. 9). Just as with CN 7, each CN 12 nucleus receives input from the corticobulbar tracts originating in the contralateral motor cortex (these are the upper motor neurons in the system; the bilateral CN 12s contain the lower motor neurons). Unilateral tongue weakness causes the tongue to deviate to the weak side: when trying to protrude the tongue midline, the stronger side wins, pushing the tongue over to the weaker side. Weakness of the tongue can cause dysarthria, specifically for lingual sounds (e.g., difficulty producing the consonant “L,” “D,” or “T”).

A unilateral hypoglossal nucleus, fascicle, or nerve lesion results in ipsilateral tongue weakness, causing deviation of the tongue toward the side of the lesion. Causes of CN 12 palsy include head and neck tumors, trauma, or surgery, as well as internal carotid artery dissection. In medial medullary syndrome (caused by infarct in the territory of the anterior spinal artery), the CN 12 fascicle (portion of the nerve still in the brainstem) and the descending (not-yet-crossed) corticospinal tract are affected, so there will be ipsilateral tongue weakness and contralateral body weakness. Bilateral tongue weakness with atrophy and fasciculations can be seen in motor neuron disease such as ALS (see Ch. 28).

A lesion of the corticobulbar tract (in the cortex, subcortical white matter, or brainstem superior to the hypoglossal nucleus in the medulla) results in contralateral tongue weakness, causing deviation of the tongue away from the side of the lesion. With a stroke affecting the motor cortex, for example, one will observe hemiparesis contralateral to the side of the hemispheric lesion, and the tongue will also be weak on the side of the hemiparesis. Therefore, the tongue will deviate toward the side of body weakness (which is away from the side of the hemispheric lesion). The reason for tongue deviation toward the weak side contralateral to a cortical lesion is not just simple crossed innervation: All tongue muscles are bilaterally innervated except genioglossus, the main tongue protruder, which is only innervated by the contralateral hemisphere. So the main muscle affected by a hemispheric lesion is the contralateral genioglossus, causing the tongue to protrude toward the weak side (overcome by the strong genioglossus on the normal side).

For summary tables of cranial nerve functions, cranial nerve nuclei functions, and cranial nerve-mediated reflexes, see Tables 9–2, 9–3, and 9–4.