Cranial Nerves

The 12 pairs of cranial nerves bring information to the brain. Most cranial nerves innervate structures of the head & neck. Ten of the twelve cranial nerves originate from the brainstem, anywhere on the midbrain, pons, or medulla oblongata. While two of the twelve cranial nerves originate from the cerebrum which travels via tracts in the cerebral cortex. Cranial nerves can be categorized as either a sensory, motor, or mixed (both sensory & motor) nerves. Note that:

  • 3 cranial nerves are sensory nerves whose axons have purely sensory functions only (I, II, VIII)

  • 4 cranial nerves are mixed nerves that contain both sensory & motor axons (V, VII, IX, X)

  • 5 are motor nerves that contain primarily motor axons only (III, IV, VI, XI, XII)

Each nerve are given two names: 

  • A sequential Roman numeral in order of its attachment to the brain

  • A name that describes the nerve’s location or function

For example: Cranial nerve III is the third cranial nerve that attaches to the brain. It is also called the oculomotor nerve because one of its functions is to provide motor axons to some of the muscles that move the eyeball. 

When naming cranial nerves it needs to have these 4 parts to be considered a complete answer: 

  • 1. The term "CN" at the beginning; CN stands for Cranial Nerve

  • 2. Its Roman Numeral value

  • 3. The name of the nerve itself

  • 4. The term "nerve" at the end

For example: CN IV Trochlear nerve

Fig 4.38.PNG

Figure 4.36 Cranial nerves, shown from the inferior view of the brain

 The 12 Pairs of Cranial Nerves

Remember, there are 12 pairs of cranial nerves & not 12 cranial nerves only. The following is an overview of facts, main functions, origin, & passage in & out of the skull of each cranial nerve:

Note: One way to remember whether a cranial nerve originates from the cerebrum or brainstem, is to remember the order of cranial nerves starting from the  superior to inferior portion of the brainstem.

Note: One way to remember where the cranial nerve enter/exit out of the skull (passage), is to track the different openings from the anterior side of the of the skull to its posterior side.

Originates from the Cerebrum:

CN I Olfactory Nerve

  • It is a purely sensory nerve that innervates the olfactory mucosa in the superior nasal cavity, where it provides for the special sense of smell

  • Once the signal travels from the olfactory nerve it then synapses with the olfactory bulb, which then sends the impulses down the axons of the olfactory tract to the olfactory cortex

  • Origin: It is located superior to the midbrain & CN II Optic Nerve. It originates from the cerebrum, not the brainstem

  • Passage: It passes through the cribriform foramina of the ethmoid bone

CN II Optic Nerve

  • This is also a purely sensory nerve that provides for the sense of vision

  • Its axons emerge from the retina of the eye & meet at the optic chiasma, where the nerves partially exchange axons before diverging to form the optic tracts

  • Origin: It is located superior to the midbrain. It originates from the cerebrum, not the brainstem

  • Passage: It passes through the optic canal of the sphenoid bone

Originates from the Midbrain:

CN III Oculomotor Nerve

  • This is a motor cranial nerve, that innervates:

    •  4 out of the 6 extrinsic (outside) eye muscles that move the eyeball

      • Inferior oblique, medial rectus, inferior rectus, & superior rectus muscles

    • The muscle that opens the eyelid 

      • Levator palpebrae superioris muscle

    • The muscle that constricts the pupil

      • Iris sphincter muscles

    • The muscle that changes the shape of the lens for near vision, an adjustment called accommodation

      • Ciliary muscle

  • Origin: This nerve originates from the medial portion of the midbrain, just above the pons

  • Passage: It's the 1st of 4 cranial nerves to pass through the superior orbital fissure

CN IV Trochlear Nerve

  • This is a small motor nerve that innervates:

    • 1 out of 6 extrinsic eye muscles that moves the eyeball called superior oblique muscle, which is responsible for rotating the eyeball towards the nose

      • This muscle is located superior, anterior & medial in the orbit

      • This muscle is held in place by a fibrous loop called the trochlea, hence its name

  • Origin: It originates from the posterolateral portion of the midbrain, just above the pons; take note that this is the only cranial nerve that originates posteriorly 

  • Passage: It's the 2nd of 4 cranial nerves to pass through the superior orbital fissure

Originates from the Pons:

CN V Trigeminal Nerve

  • This is a large mixed (both) nerve named for its three divisions:

    • Mnemonic: MOM (not in same order as below)

    • Ophthalmic, Maxillary, & Mandibular

  • They provide sensory innervation from the face & motor innervation to the muscles of mastication

  • It is also known as the largest cranial nerve

  • Origin: It originates from the lateral side of the pons

  • Passage: It enters through the:

    • Mnemonic:Super-RiO

    • Superior Orbital Fissure for its ophthalmic division

    • Foramen Rotundum for its maxillary division

      • From the foramen rotundum, the maxillary division exits through the infraorbital foramen of the maxilla bone

    • Foramen Ovale for its mandibular division 

    • Also, it is the 3rd of 4 cranial nerves to pass through the superior orbital fissure

CN VI Abducens Nerve

  • This is a small motor nerve that innervates:

    • The final extrinsic eye muscle called lateral rectus muscle

      • This muscle is involved in the abduction movement of the eye, hence it's name

  • Origin: It originates from the inferior & medial side of the pons, just above the medulla oblongata

  • Passage: It is the 4th of 4 cranial nerves to pass through the superior orbital fissure

CN VII Facial Nerve

  • The mixed (both) fibers of this cranial nerve innervate many structures & provide for the following: 

    • Motor to the muscles of facial expression

    • Taste from the anterior two-thirds of the tongue

    • Motor to the glands that produce tears (the lacrimal glands), mucus, & saliva

    • Sensory from part of the face & mouth

    • It has 5 branches: temporal, zygomatic, buccal, mandibular, & cervical 

      • (Mnemonic:The Zit By My Chin) 

  • Origin: It originates from the inferior & lateral side of the pons; next to CN VIII

  • Passage: It is 1st of the 2 cranial nerves that enters through the internal acoustic meatus, but it is the only one that exits through the stylomastoid foramen of the temporal bone

CN VIII Vestibulocochlear Nerve

  • The final sensory cranial nerve. It innervates the structures of the inner ear & provides for the senses of hearing & equilibrium. It will split into two nerves: 

    • Vestibular Nerve that targets the vestibule & semicircular canals for equilibrium

    • Cochlear Nerve that targets the cochlea for hearing

  • Origin: It originates from the inferior & lateral side of the pons; next to CN VII

  • Passage: It is 2nd of the 2 cranial nerves that enter through the internal acoustic meatus

Originates from the Medulla Oblongata:

CN IX Glossopharyngeal Nerve

  • The small mixed (both) cranial nerve provides:

    • Glosso function: Sensory fibers from the posterior one-third of the tongue for taste sensation

    • Pharyngeal function: Motor axons to the muscles of the pharynx (throat) involved in swallowing

  • Origin: It originates from the superior & lateral side of the medulla oblongata, superior to CN X

  • Passage: It's 1st of the 3 cranial nerves that passes through the jugular foramen

CN X Vagus Nerve

  • This mixed (both) nerve is the only cranial nerve that “wanders” outside of the head & neck (“vagus” means “wanderer”)

    • In the head & the neck, it provides:

      • Some sensory innervation from the skin of the head & the pharynx

      • Motor axons to muscles involved in speech & swallowing

      • Motor axons to certain salivary glands

    • Outside the head & neck it:

      • Innervates most of the thoracic & abdominal viscera as the main nerve of the parasympathetic (rest & digest) nervous system

  • It is also known as the longest cranial nerve

  • Origin: It originates from the lateral portion of the medulla oblongata & inferior to the CN IX 

  • Passage: It's 2nd of the 3 cranial nerves that passes through the jugular foramen

CN XI Accessory Nerve

  • It is a motor nerve the only cranial nerve that has both: 

    • A cranial component originating from the brainstem 

    • A spinal component originating from the spinal cord

    • Its motor axons innervate the muscles that move the head & the neck, such as the trapezius & sternocleidomastoid muscles

  • Origin: It originates from the lateral portion of medulla oblongata (cranial component) & from C1-C6 of the spinal cord (spinal component)

  • Passage: It's 3rd of the 3 cranial nerves that passes through the jugular foramen

CN XII Hypoglossal Nerve

  • It is a small motor nerve that innervates:

    • The muscles that move the tongue

    • It does not provide any taste sensation to the tongue

  • Origin: It originates from the anterior & inferior portion of the medulla oblongata

  • Passage: It enters through the hypoglossal canal found in the occipital bone