Cranial Nerves VII-XII

1. Cranial Nerve VII - Facial
2. Cranial Nerve VIII - Vestibulocochlear
3. Cranial Nerve IX - Glossopharyngeal
4. Cranial Nerve X - Vagus
5. Cranial Nerve XI - Accessory
6. Cranial Nerve XII - Hypoglossal
7. Quiz

• The facial nerve supplies motor innervation (the nerve supply that makes muscles move) to the muscles of facial expression and supplies the special sensation in the form of taste to the anterior 2/3 of the tongue.
• The vestibulocochlear nerve supplies special sensation to the ear in the form of hearing and contributes to balance maintenance.
• The glossopharyngeal nerve supplies general sensation to the tonsils and the oropharynx (the part of the throat behind the mouth). It also supplies special sensation (taste) to the posterior 1/3 of the tongue, and motor innervation to the stylopharyngeus muscle of the pharynx.
• The vagus nerve supplies general sensation to the larynx. It supplies motor innervation to most of the muscles of the soft palate (the muscular back part of the roof of the mouth), the larynx and the pharynx.
• The accessory nerve supplies motor innervation to the sternocleidomastoid (the muscle at the side of the neck that turns the head) and trapezius muscles.
• The hypoglossal nerve supplies motor innervation to most of the muscles of the tongue.

For an introduction to cranial nerves, including tips for remembering their names and functions, and their anatomical location, please see the first article on Cranial Nerves I-VI found here.

Cranial Nerve VII - Facial

Pathway and Function 

This nerve is responsible for both motor and sensory function. It originates from the pons (part of the brainstem) and enters the petrous part (the dense, rock-like part) of the temporal bone via the internal acoustic meatus (a bony canal that carries the nerve to the inner ear).

From there, it runs through the facial canal (a bony tunnel in the temporal bone), where it gives off the first three branches below. It then leaves the skull through the stylomastoid foramen (a small opening behind the ear) and passes through the parotid gland (the large salivary gland in front of the ear), where it splits into the terminal branches that supply the muscles of facial expression. The main branches are:

• Greater Petrosal - carries parasympathetic secretomotor fibres (nerve fibres that make glands produce secretions) to the lacrimal (tear) glands and to glands of the nasal cavity and palate.
• Chorda tympani - provides special sensation (taste) to the anterior 2/3 of the tongue and supplies parasympathetic innervation to the submandibular (under the jaw) and sublingual (under the tongue) salivary glands.
• Nerve to Stapedius - innervates the stapedius muscle, a tiny muscle of the middle ear which adjusts the movement of the ossicles (the three tiny ear bones), reducing how much loud sound reaches the inner ear and protecting it from damage.
• Nerves to the Muscles of Facial Expression (five branches remembered by the phrase ‘to Zanzibar by motor car’):
  
  • Temporal
  • Zygomatic
  • Buccal
  • Mandibular
  • Cervical

Pathology

Depending on which branch is affected, patients can present differently. Patients may present with a facial palsy (weakness or paralysis of the muscles of facial expression). They may also have other symptoms, such as hyperacusis (increased sensitivity to sound), dry eyes and altered taste. A nerve (peripheral) cause such as Bell’s palsy weakens the forehead as well as the lower face, so if forehead strength is preserved, a central cause (a problem in the brain, such as a stroke) should be considered. A face that droops on one side can be a sign of stroke, which is a medical emergency. Middle ear pathology affecting the petrous bone can involve the facial nerve.

Image - Multiple pictures illustrating a facial palsy on the left side. Notice the lack of facial expression on the left

Creative commons source by Nervous and mental diseases by Church, Archibald, b. 1861 Peterson, Frederick, 1859-1938 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Testing

Test the facial nerve by examining the muscles of facial expression - this can be done by encouraging the patient to pull faces as seen in the photograph above.

It can also be tested by eliciting the corneal reflex (blinking when the cornea, the clear front of the eye, is touched), as the temporal and zygomatic branches of the facial nerve supply the efferent (motor) part of this reflex.

Cranial Nerve VIII - Vestibulocochlear

Function

This nerve has two parts. The vestibular part supplies the vestibule (the part of the inner ear responsible for the special sense of balance). The cochlear part supplies the cochlea (the spiral-shaped part of the inner ear responsible for the special sense of hearing).

Pathway

Stemming from the inner ear, the vestibulocochlear nerve passes through the internal acoustic meatus (a bony canal in the temporal bone), into the pons (part of the brainstem).

Diagram - The location of the vestibulocochlear nerve within the inner ear

Creative commons source by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Pathology

Pathology of the vestibulocochlear nerve depends on which part of the nerve is affected.

If only the cochlear portion is affected (which is rare) hearing loss will be the main symptom.

If only the vestibular portion is affected, for example in vestibular neuritis (inflammation of the vestibular nerve, thought to follow a viral infection), balance issues such as vertigo (a spinning sensation), nausea and unsteadiness will be the main symptoms.

If both are affected together in the vestibulocochlear nerve, such as in a vestibular schwannoma (also called an acoustic neuroma, a slow-growing, non-cancerous tumour on the nerve), hearing and balance may be affected.

Testing

Test the vestibulocochlear nerve by testing hearing: start with a simple bedside check of hearing in each ear, then use Weber’s and Rinne’s tuning fork tests, which help to tell conductive hearing loss (a problem in the outer or middle ear) from sensorineural hearing loss (a problem in the inner ear or the nerve). These tests do not replace formal hearing tests (audiometry). Also assess balance by asking about it.

Cranial Nerve IX - Glossopharyngeal

Function

This nerve performs multiple functions:

• Provides general sensation to the palatine tonsils, oropharynx (the part of the throat behind the mouth), middle ear and tympanic membrane (eardrum). It also carries sensation from the carotid body and carotid sinus (sensors in the neck that detect blood oxygen levels and blood pressure).
• Provides the sense of taste (special sensation) and general sensation to the posterior 1/3 of the tongue.
• Supplies the stylopharyngeus muscle, which assists in swallowing.
• Provides parasympathetic innervation to the parotid gland (the large salivary gland in front of the ear), making it produce saliva.

Pathway

This nerve extends from the medulla (the lowest part of the brainstem) and exits the skull through the jugular foramen (an opening at the base of the skull) before supplying innervation.

Pathology

Damage to this nerve can be caused by compression of the nerve by other structures, for example an artery pressing on the nerve where it leaves the brainstem, or a tumour. It can also be injured during neck surgery, such as carotid endarterectomy (an operation to clear a narrowed carotid artery). Compression can cause glossopharyngeal neuralgia, a rare condition of sudden, severe, stabbing pain on one side of the throat and ear, often triggered by swallowing, talking or coughing.

Testing

Assessment of this nerve is usually based on history and examination of swallowing and voice. The gag reflex can be elicited to demonstrate the afferent (sensory) limb, but it is often absent in healthy individuals and is not routinely required.

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