The 12 Cranial Nerves Anatomy & Function

The Cranial Nerves, typically, are described as comprising 12 pairs, which are referred to by the roman numerals I through XII. The cranial nerves’ roots enter and exit the brain stem to provide sensory and motor innervation to the head and the muscles of the face. Cranial nerve I (olfactory) and cranial nerves II (optic) are not true nerves but rather fiber tracts of the brain.

The following CNs emerge from the mid-brain:

1. Optic (II)
2. Oculomotor (III)
3. Trochlear (IV).

The following CNs emerge from the mid-brain:

1. Optic (II)
2. Oculomotor (III)
3. Trochlear (IV).

The following CNs emerge from the medulla:

1. Glossopharyngeal (IX)
2. Vagus (X)
3. Accessory (XI)
4. Hypoglossal (XII).

Cranial Nerves Mnemonic:

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• CN I Olfactory nerve
• CN II Optic nerve
• CN III Oculomotor nerve
• CN IV Trochlear nerve (Pathetic nerve)
• CN V Trigeminal nerve (Dentist’s nerve)
• CN VI Abducens nerve
• CN VII Facial nerve
• CN VIII Vestibulocochlear nerve (Auditory nerve)
• CN IX Glossopharyngeal nerve
• CN X Vagus nerve
• CN XI Accessory nerve (Spinal accessory nerve)
• CN XII Hypoglossal nerve

Cranial Nerves Anatomy

Cranial Nerve I – Olfactory Nerve

The olfactory tract arises from the olfactory bulb on the inferior aspect of the frontal lobe, just above the cribriform plate. From here it continues posteriorly as the olfactory tract and terminates just lateral to the optic chiasm. The olfactory nerve is responsible for the sense of smell.

Cranial Nerve II – Optic Nerve

The fibers of the optic nerve arise from the inner layer of the retina and proceed posteriorly to enter the cranial cavity via the optic foramen, to form the optic chiasm. The fibers from the nasal half of the retina decussate within the optic chiasm, whereas those from the lateral half do not. The optic nerve is responsible for vision.

Cranial Nerve III – Oculomotor Nerve

The oculomotor nerve arises in the oculomotor nucleus and leaves the brain on the medial aspect of the cerebral peduncle. It then extends from the interpeduncular fossa and runs between the posterior cerebral artery and the superior cerebellar artery, before leaving the cranial cavity and entering the cavernous sinus by way of the superior orbital fissure.

The somatic portion of the oculomotor nerve supplies the levator palpebrae superioris muscle; the superior, medial, and inferior rectus muscles; and the inferior oblique muscles. These muscles are responsible for some eye movements. The visceral efferent portion of this nerve innervates two smooth intraocular muscles: the ciliary and the constrictor pupillae. These muscles are responsible for papillary constriction.

Cranial Nerve IV – Trochlear nerve

The trochlear nerve arises from the trochlear nucleus, just caudal to the oculomotor nucleus at the anterior border of the periaqueductal gray (PAG) matter. The fibers cross within the midbrain and then emerge contralaterally on the posterior surface of the brain stem, before entering the orbit via the superior orbital fissure, to supply the superior oblique muscle.

Cranial Nerve V – Trigeminal Nerve

The trigeminal nerve is the 5th cranial nerve, it’s so named because of its tripartite division into the maxillary, ophthalmic, and mandibular branches. All three of these branches contain sensory cells, but the ophthalmic and the maxillary are exclusively sensory, the latter supplying the soft and hard palate, maxillary sinuses, upper teeth and upper lip, and mucous membrane of the pharynx.

The mandibular branch carries sensory information but also represents the motor component of the nerve, supplying the muscles of mastication, both pterygoids, the anterior belly of digastric, tensor tympani, tensor veli palatini, and mylohyoid.

The spinal nucleus and the tract of the trigeminal nerve cannot be distinguished either histologically or on the basis of afferent reception from the cervical nerves. Consequently, the entire column can be viewed as a single nucleus and, legitimately, may be called the trigeminocervical nucleus.

Cranial Nerve VI – Abducens Nerve

The abducens nerve originates from the abducens nucleus within the inferior aspect of the pons. Its long intracranial course to the superior orbital fissure makes it vulnerable to pathology in the posterior and middle cranial fossa. The nerve innervates the lateral rectus muscle.

Cranial Nerve VII – Facial Nerve

The facial nerve is made up of a sensory (intermediate) root, which conveys taste, and a motor root, the facial nerve proper, which supplies the muscles of facial expression, the platysma muscle, and the stapedius muscle of the inner ear.

The intermediate root, together with the motor nerve and CN VIII, travels through the internal acoustic meatus to enter the facial canal of the temporal bone. From here, the intermediate nerve swells to form the geniculate ganglion and gives off the greater superficial petrosal nerve, which eventually innervates the lacrimal and salivary glands via the pterygopalatine ganglion and the chorda tympani nerve, respectively. The facial nerve proper exits the skull through the stylomastoid foramen.

See Also: Facial Nerve Anatomy

Cranial Nerve VIII – Vestibulocochlear Nerve

The vestibulocochlear nerve subserves two different senses: balance and hearing. The cochlear portion of the nerve arises from spiral ganglia, and the vestibular portion arises from the vestibular ganglia in the labyrinth of the inner ear. The cochlear portion is concerned with the sense of hearing, whereas the vestibular portion is a part of the system of equilibrium, the vestibular system.

Cranial Nerve IX – Glossopharyngeal Nerve

The glossopharyngeal nerve contains somatic motor, visceral efferent, visceral sensory, and somatic sensory fibers. The motor fibers originate in the nucleus ambiguous, leaving the lateral medulla to join the sensory nerve, which arises from cells in the superior and petrous ganglia. The glossopharyngeal nerve exits the skull through the jugular foramen and serves a number of functions, including supplying taste fibers for the posterior third of the tongue.

Cranial Nerve X – Vagus Nerve

The functions of the vagus nerve are numerous and include the motor parasympathetic fibers to all the organs except the suprarenal (adrenal) glands, from its origin down to the second segment of the transverse colon.

The vagus also controls some skeletal muscles, including:

• Cricothyroid muscle
• Levator veli palatini muscle
• Salpingopharyngeus muscle
• Palatoglossus muscle
• Palatopharyngeus muscle
• Superior, middle, and inferior pharyngeal constrictors
• Muscles of the larynx.

The vagus nerve is thus responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, speech, and breathing. It also has some afferent fibers that innervate the inner (canal) portion of the outer ear.

Cranial Nerve XI – Accessory Nerve

The accessory nerve consists of a cranial component and a spinal component. The cranial root originates in the nucleus ambiguous and is often viewed as an aberrant portion of the vagus nerve. The spinal portion of the nerve arises from the lateral parts of the anterior horns of the first five or six cervical cord segments and ascends through the foramen magnum. The spinal portion of the accessory nerve supplies the sternocleidomastoid and the trapezius muscles.

Cranial Nerve XII – Hypoglossal Nerve

The hypoglossal nerve is the motor nerve of the tongue, innervating the ipsilateral side of the tongue as well as forming the descendens hypoglossi, which anastomoses with other cervical branches to form the ansa hypoglossi. The latter, in turn, innervates the infrahyoid muscles.

References & More

1. Dutton’s Orthopaedic Examination, Evaluation, And Intervention 3rd Edition.
2. Sonne J, Lopez-Ojeda W. Neuroanatomy, Cranial Nerve. [Updated 2022 Oct 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: Pubmed
3. Sonne J, Lopez-Ojeda W. Neuroanatomy, Cranial Nerve. 2022 Oct 27. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan–. PMID: 29261885.