12.3: Cranial Nerves

By the end of this section, you will be able to:

• Name the twelve cranial nerves and explain the functions associated with each

Cranial Nerves

The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (with the exception of one that targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CN I through CN XII for “Cranial Nerve,” using Roman numerals for 1 through 12, based on the anatomical location on the inferior view of the brain, from rostral to caudal (Figure \(\PageIndex{1}\)). They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brainstem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck or to autonomic ganglia. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves.

Figure \(\PageIndex{1}\): Cranial Nerves. The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain. Cranial nerves are numbered with Roman numbers from I to XII based on their exit from the brain, from rostral to caudal (CN I is not seen). (Image credit: "Cranial Nerves" by Justin Greene and Jennifer Lange is licensed under  CC BY-NC-SA 4.0.)

Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is “Oh Oh Oh To Touch And Feel Very Good Velvet Ah Heaven,” in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the cranial nerves are listed in Table \(\PageIndex{1}\) along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle).

• The olfactory nerve is responsible for the sense of smell.  It is usually not seen on the brain as its short fibers pass through the cribriform plate and thus are torn when the brain is extracted.  Instead, we see the afferent nerves: the olfactory bulb and the olfactory tract.
• The optic nerves are responsible for the sense of vision.  They enter the cranial vault through the optic canals.
• The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, for pupillary constriction and focusing light by adjusting the lens of the eye.  It passes through the superior orbital fissure.
• The trochlear nerve and the abducens nerve are both responsible for eye movement, each controlling a singular extraocular muscle.  They also pass through the superior orbital fissure.
• The trigeminal nerve is responsible for cutaneous sensations of the face and mouth and controlling the muscles of mastication.  It has three branches, hence "tri", that pass through the superior orbital fissure, foramen rotundum, and foramen ovale.
• The facial nerve is responsible for the muscles involved in facial expressions as well as controlling the production and secretion of tears and of saliva.  Its sensory component includes part of the sense of taste.  It travels with CN VIII through the internal acoustic meatus.
• The vestibulocochlear nerve is responsible for the senses of hearing and balance.
• The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper throat and the production of saliva.  Its sensory components include taste from the posterior tongue, blood pressure, and blood pH, pO2, and pCO2.  It passes through the jugular foramen along with nerves X, and XI.
• The vagus nerve is responsible for both sensory and motor from/to the organs of the thoracic and abdominal cavities.  It also carries motor signals to the muscles of the larynx, pharynx and soft palate.  Additional sensory functions include taste and general sensory from the larynx.  In innervating this broad area, the vagus nerve is responsible for the regulation of internal organ functions, such as digestion, heart rate, and respiratory rate, as well as vasomotor activity, and certain reflex actions, such as coughing, sneezing, swallowing, and vomiting.
• The accessory nerve's spinal component is responsible for controlling two muscles of the neck, the sternocleidomastoid and the superior portion of the trapezius.
• The hypoglossal nerve is responsible for controlling the muscles of the tongue.  It exits the cranium through the hypoglossal canal.

Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see Table \(\PageIndex{1}\)). The sentence, “Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the type(s) of information carried by each nerve: 

• Sensory only: the first, second, and eighth nerves are purely sensory: the olfactory (CN I), optic (CN II), and vestibulocochlear (CN VIII) nerves.
• Motor only: The three eye-movement nerves are all only motor: the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI). The accessory (CN XI) and hypoglossal (CNXII) nerves are also strictly motor.
• Both: The remainder of the nerves contain both sensory and motor fibers, these are mixed nerves: the trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves. 

Table \(\PageIndex{1}\): Cranial Nerves

Mnemonic	#	Name	Function (S/M/B)	Central connection (nuclei)	Peripheral connection (ganglion or muscle)
Oh	I	Olfactory	Smell (S)	Olfactory bulb	Olfactory epithelium
Oh	II	Optic	Vision (S)	Hypothalamus/thalamus/midbrain	Retina (retinal ganglion cells)
Oh	III	Oculomotor	Eye movements (M)	Oculomotor nucleus	Extraocular muscles (other 4), levator palpebrae superioris, ciliary ganglion (autonomic)
To	IV	Trochlear	Eye movements (M)	Trochlear nucleus	Superior oblique muscle
Touch	V	Trigeminal	Sensory – face and mouth; Motor - mastication (B)	Trigeminal nuclei in the midbrain, pons, and medulla	Trigeminal ganglion; Muscles of mastication
And	VI	Abducens	Eye movements (M)	Abducens nucleus	Lateral rectus muscle
Feel	VII	Facial	Motor – facial; Sensory - taste (B)	Facial nucleus, solitary nucleus, superior salivatory nucleus	Facial muscles, Geniculate ganglion, Pterygopalatine ganglion (autonomic)
Very	VIII	Vestibulocochlear	Hearing/balance (S)	Cochlear nucleus, Vestibular nucleus/cerebellum	Spiral ganglion (hearing), Vestibular ganglion (balance)
Good	IX	Glossopharyngeal	Motor – throat; Sensory - taste (B)	Solitary nucleus, inferior salivatory nucleus, nucleus ambiguus	Pharyngeal muscles, Geniculate ganglion, Otic ganglion (autonomic)
Velvet	X	Vagus	Motor/sensory – viscera (autonomic) (B); taste (S)	Medulla	Terminal ganglia serving thoracic and upper abdominal organs (heart and small intestines)
Ah	XI	Accessory	Motor – head and neck (M)	Spinal accessory nucleus	Neck muscles
Heaven	XII	Hypoglossal	Motor – tongue (M)	Hypoglossal nucleus	Muscles of the tongue

 

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Vision Loss

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AGING AND THE...

Nervous System: Anosmia

Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them.

Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity. Anosmia results in a loss of the enjoyment of food.

As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure.

Autonomic Functions

Four of these cranial nerves make up the cranial component of the autonomic nervous system, as they innervate smooth muscle, cardiac muscle, or glands. 

• The oculomotor nerves are responsible for innervating the intra-ocular muscles responsible for pupillary diameter and adjusting the eyes' focal point.
• The facial and glossopharyngeal nerves control salivation and lacrimation, which are glandular secretions.
• The regulation of the organs of the thoracic and abdominal cavities such as the heart, bronchial tree, stomach, pancreas, adrenal glands, and kidneys by the vagus nerve.

Anatomical Variation

Is CN XI really a cranial nerve? Maybe.

The spinal root of the accessory nerve ascends from the superior region of the cervical spinal cord and enters the cranium through the foramen magnum.  The cranial root emerges from the medulla oblongata.  Reasons for the debate:

• the spinal root has numerous anastomoses with the spinal dorsal and ventral rootlets
• 20% of specimens in one study did not have a cranial root
• variation exists in axonic pathways before the jugular foramen - in some individuals the cranial root and the spinal root travel separately through the jugular foramen
• variation exists in axonic pathways after the jugular foramen - in some individuals the axons from both roots are found in the nerve's two branches, whereas in others the branches contained axons from only one root
• the cranial root has considerable functional and termination overlap with the vagus nerve, and multiple patterns of interconnections between the two have been found
• the vagus nerve and the accessory nerve develop from the same embryonic ganglion, with variable patterns of separation during development

Studies on this topic have come to opposing findings, so the real debate may end up being "what is the definition of a cranial nerve?".

For further reading:

• Johal, J., Iwanaga, J., Tubbs, K., Loukas, M., Oskouian, R.J. and Tubbs, R.S. (2019) The Accessory Nerve: A Comprehensive Review of its Anatomy, Development, Variations, Landmarks and Clinical Considerations. Anat. Rec., 302: 620-629. https://doi.org/10.1002/ar.23823.

• Liu HF, Won HS, Chung IH, Kim IB, Han SH (2014a) Morphological characteristics of the cranial root of the accessory nerve. Clin Anat 27(8): 1167–1173. https://doi.org/10.1002/ca.22451.

• Liu HF, Won HS, Chung IH, Oh CS, Kim IB (2014b) Variable composition of the internal and external branches of the accessory nerve. Clin Anat 27(1): 97–101. https://doi.org/10.1002/ca.22308.

• Ong CK, Chong VFH. (2010) The glossopharyngeal, vagus and spinal accessory nerves. Eur J Radiol 74(2): 359–367. https://doi.org/10.1016/j.ejrad.2009.05.064.

• Ryan S, Blyth P, Duggan N, Wild M, Al-Ali S. Is the cranial accessory nerve really a portion of the accessory nerve? Anatomy of the cranial nerves in the jugular foramen. Anatomical Science International. 2007, 82(1):1-7. https://doi.org/10.1111/j.1447-073X.2006.00154.x.

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Concept Review

The twelve cranial nerves can be strictly sensory in function, strictly motor in function, or a combination of the two functions. The olfactory nerve (CN I) and optic nerve (CN II) are responsible for the sense of smell and vision, respectively. The oculomotor nerve (CN III) is responsible for eye movements, lifting the upper eyelid and size of the pupil. The trochlear nerve (CN IV) and the abducens nerve (CN VI) are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve (CN V) is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve (VII) is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve (VIII) is responsible for the senses of hearing and balance. The glossopharyngeal nerve (IX) is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve (CN X) is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The accessory nerve (CN XI) is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve (CN XII) is responsible for controlling the muscles of the lower throat and tongue.

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