cranial nerves

Cranial nerves are exceptionally important for speech and hearing sciences because of their direct

implications for both of these processes. Cranial nerves serve and arise from the brainstem, but also

“serve at the pleasure” of the cerebrum. Remember to always refer to cranial nerve number and

name, and use the Roman numeral. There are 6 general classifications of cranial nerves:

1. General somatic afferent (GSA): pain, temperature, mechanical stimulation of skin, muscle or

joints

2. Special somatic afferent (SSA): special body senses: vision & hearing

3. General visceral afferent (GVA): sensation within the viscera (e.g., abdominal organs, glands)

4. Special visceral afferent (SVA): special visceral senses of taste and smell

5. General visceral efferent (GVE): autonomic efferent fibers serving viscera and glands

6. General somatic efferent (GSE): innervate skeletal muscle

7. Special visceral efferent (SVE): innervate striated muscle of branchial arch origin, including

muscles of larynx, pharynx, soft palate, face, and mastication

Summary, Cont’d

• I Olfactory: This is SVA (special visceral afferent) but is not a true cranial nerve since it doesn’t go

through thalamus. It mediates the sense of smell (olfaction) to the limbic system, and has olfactory

sensors in the nasal mucosa terminate in olfactory bulbs

• II Ophthalmic (often called “optic”): This mediates the visual sense via retinal receptors. The

ophthalmic nerve synapses with the lateral geniculate body, and also continues as the optic

radiation to the occipital lobe. The optic nerve becomes the optic tract at the optic chiasm.

Information from the right visual field goes to the left hemisphere. Hemianopia (or “hemianopsia”)

refers to blindness of half of visual field (note that this is different from blindness in one eye).

Homonymous refers to “one field” or “same side” field, while heteronymous refers to different fields

(i.e., two fields).

• III Oculomotor nerve: Motor innervation of all muscles of eye movement except for abduction and

rotation. Motor innervation for light accommodation (pupil reflex)

• IV Trochlear nerve: turns eye down and in

• VI Abducens: Abducts eyeball (moves eye laterally)

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Summary, cont’d

• V trigeminal: This is very important for speech-language pathology, and

has three branches.

• The ophthalmic branch provides sensory innervation for the upper face and scalp

(skin of the upper face, forehead, scalp, Cornea & iris, upper eyelid, conjunctiva, and

nasal cavity mucous membrane).

• The maxillary branch provides sensory innervation for the mid-face (lower eyelid,

sides of the nose, maxilla, upper teeth, upper lip, buccal mucosa, maxillary sinuses,

and nasopharynx).

• The sensory component of the mandibular branch innervates the mandible, lower

teeth, lower gingiva, lower lip, somatic sense for the anterior 2/3 of the tongue and

floor of mouth, the external auditory meatus and pinna, the temporomandibular joint,

temporal bone, and proprioceptive sense for the muscles of mastication

• The motor component of the mandibular branch serves the muscles of mastication

(masseter, temporalis, internal and external pterygoids) and the tensor veli palatini

(auditory tube dilator) and the tensor tympani of the middle ear.

Summary, cont’d

• VII facial nerve: This is another very important nerve for SLP. The motor component innervates all

facial muscles, remembering that the lower face is contralaterally innervated (so unilateral lesion on

the left side of the brain or efferent pathway will cause right side paralysis), but the upper half of the

face is bilaterally innervated (so unilateral lesion will not cause paralysis of the area above the

eyes). The sensory component is very important, as it mediates the sense of taste for the anterior

2/3 of the tongue. The VII facial nerve also innervates lacrimal glands (for tears) and the anterior

salivary glands (sublingual and submandibular glands).

• VIII vestibulocochlear nerve: This is also very important for both speech-language pathology and

audiology. The auditory branch of the VIII vestibulocochlear nerve conveys auditory information to

the brainstem, while the vestibular branch conveys information about the position of the body in

space.

• The spiral ganglion of the cochlea is the collection of cells bodies for the VIII nerve. The VIII

nerve fibers terminate in the cochlear nerve, which projects to the superior olivary complex.

The next way station is the lateral lemniscus, followed by the inferior colliculus and medial

geniculate body of the thalamus.

• The crossed olivocochlear bundle (COCB) and uncrossed olivocochlear bundle (UOCB)

provide efferent innervation of the hair cells, which attenuates the output of specific cells to

assist in listening in background of noise

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Summary, cont’d

• IX glossopharyngeal nerve: The IX glossopharyngeal nerve is very important for swallowing. It

mediates the sense of taste from the posterior 1/3 of the tongue, and mediates the sensory

activation of the pharyngeal stage of swallowing. It also mediates somatic sense (pain,

temperature) for the posterior tongue. It provides motor innervation for the parotid gland and

superior constrictor and stylopharyngeus muscles.

• X vagus nerve: This is also very important. It mediates the sense of taste from the epiglottis and

laryngeal area (surprisingly!), as well as the somatic sense for those regions as well. It is involved

in regulation of the abdominal viscera (pancreas, intestines, etc.).

• The recurrent laryngeal nerve (RLN) innervates all of the muscles of adduction and abduction.

• The superior laryngeal nerve (SLN) innervates the muscles associated with pitch change.

• The pharyngeal branch (nerve) innervates works with XI to innervate the velum and with IX to

innervate pharyngeal muscles

• XI accessory nerve: This nerve provides motor innervation to the sternocleidomastoid and

trapezius muscles

• XII hypoglossal: This provides motor innervation to all tongue muscles except the palatoglossus.