Wednesday, 2 April 2014


Cranial nerves (sometimes termed cerebral nerves),[1] are nerves that emerge directly from the brain and the brainstem, in contrast to spinal nerves (which emerge from various segments of the spinal cord). Information is exchanged between the brain and various regions, primarily of the head and neck, via the cranial nerves.[2]
Spinal nerves reach as far as the first cervical vertebra, and the cranial nerves fill a corresponding role above this level.[3] Each cranial nerve is paired and is present on both sides. Depending on source there are in humans twelve or thirteen pairs of cranial nerves, which are assigned Roman numerals I-XII,[2] and zero assigned to cranial nerve zero,[4] according to the order in which they originate from the forebrain to the back of the brain and the brainstem.[2]
Cranial nerves 0, I and II emerge from the cerebrum or forebrain;[5] the remaining ten pairs emerge from the brainstem.
The cranial nerves are components of the peripheral nervous system (PNS), with the exception of cranial nerve II (the optic nerve), which is not a true peripheral nerve but a neural tract of the diencephalon connecting the retina with the lateral geniculate nucleus; hence both the optic nerve and the retina are part of the central nervous system (CNS).[6] The axons of the remaining twelve nerves extend beyond the brain and are considered part of the PNS.[7] The central ganglia of the cranial nerves or cranial nerve nuclei originate in the CNS, preferentially from the brainstem.
  Cranial nerves



Inferior schematic view of the brain and brainstem showing the cranial nerves, numbered from olfactory to hypoglossal after the order in which they emerge.
Inferior view of the human brain showing the cranial nerves as visible on an autopsy specimen.
Traditionally, among humans there are considered to be twelve cranial nerves, numbered I-XII, all of which are paired. The cranial nerves arise directly from the central nervous system; the first two pairs, I and II, arise from the base of the forebrain, and the others, nerves III to XII, arise from the brainstem. Their naming scheme is given after their rostral-caudal orientation,[3][8] as, when viewing the brain and brainstem from below, they are often visible in their numeric order.
Unique anatomical terminology is used to describe the course of the cranial nerves. Like all nerves, the nerves have a nucleus, and a course within and outside of the brain. The course within the brain is known as the central course of the nerve, and the course after it has emerged from the brain as the peripheral course. The nerves are paired, which means that they occur on both the right and left sides. Some nerves cross from the right side to the left side, and this is known anatomically as decussating. If a nerve supplies a muscle, skin, or has another function on the same side of the body as where it originates, this is called an ipsilateral course. If the course is opposite to the nucleus of the nerve, this is known as a contralateral course.

Intercranial course

The cranial nerves serve to innervate the head and neck area,[8] including both somatic and autonomic motor innervation as well as sensory innervation. Together the cranial nerves supply sensory innervation of the special senses such as taste, vision; smell; hearing. They also supply afferens of the somatic senses: visceral sensation of the head and neck; balance, and proprioception combining vestibular perception with proprioceptive information from the head and neck.[9]
Distinct from the head the two cranial nerves: IX and X; the glossopharyngeal and vagus nerves innervate both motor and sensory synapses pertaining to abdominal organs (though not pelvic), as well as structures of the neck and chest.[3] Differentiating the cranial nerves from spinal nerves, the cranial nerves are not strictly bound to certain segments of the body (as in dermatomes), but rather organize after function, hence the innervated areas overlap significantly more than those of spinal nerves.[3] The accessory nerve XI, is considered either a cranial nerve or a spinal nerve which emanates level with the brain-stem.[3]
Similar to the dorsal root ganglia of the spinal nerves and parasympathetic ganglia of the sacral parasympathetic system, the sensory cranial nerves have a number of ganglia outside the central nervous system. The sensory ganglia are directly correspondent to dorsal root ganglia and are as known as cranial sensory ganglia, and found along the course of the cranial nerves, outside the brain[8] and skull. Sensory ganglia exist for nerves with sensory function; V, VII, VIII, IX, X.[3] There are also a number of parasympathetic cranial nerve ganglia, while sympathetic ganglia innervating the head and neck reside in the upper regions of the sympathetic trunk, and do not belong to the cranial nerves.[10]


The brainstem, with brainstem cranial nerve nuclei and tracts inside the brainstem shaded red to illustrate the deeper structures.
The nerve fibres in each nerve contain a nucleus either in the brainstem of the mesencephalon. The olfactory nerve (I) emerges from the olfactory bulb, and depending slightly on division the optic nerve (III) is deemed to emerge from the lateral geniculate nuclei. The rest of the nerves have their cell-bodies in the brainstem and thus originate in the brainstem.[10]
Olfactory nerve Olfactory bulb
Optic nerve Lateral geniculate nucleus
Oculomotor nerve Oculomotor nucleus
Edinger-Westphal nucleus
Trochlear nerve Trochlear nucleus
Trigeminal nerve Trigeminal nerve nuclei:
Mesencephalic nucleus
Principle sensory nucleus
Spinal trigeminal nucleus
Trigeminal motor nucleus
Abducens nerve Abducens nucleus
Facial nerve Facial motor nucleus
Superior salivatory nucleus
Vestibulocochlear nerve Vestibular nuclei
w. subnuclei
Cochlear nucleus
w. subnuclei
Glossopharyngeal nerve Solitary nucleus
Spinal nucleus of the trigeminal nerve
Lateral nucleus of vagal trigone.
Nucleus ambiguus
Inferior salivatory nucleus
Vagus nerve Dorsal nucleus of vagus nerve
Nucleus ambiguus
Solitary nucleus
Spinal trigeminal nucleus
Accessory nerve Spinal accessory nucleus
Nucleus ambiguus
Hypoglossal nerve Hypoglossal nucleus

Cranial nerve columns

Postero-lateral or dorso-lateral aspect of the brainstem showing sensory nuclei in blue.
Postero-lateral or dorso-lateral aspect of the brainstem showing motor nuclei in red.
Brainstem showing motor nuclei in red, and sensory nuclei in blue.
Brainstem nuclei with associated functions are often found in similar areas of the brainstem. These are also known as functional columns.[10] Functional columns are a result of the development of the spinal cord. Four columns of gray matter are present in the spinal cord during embryological development. Each column represents a different function, and contributes neurons to different nerves. Each nerve is innervated by neurons from one or more of the columns.[9]
As the spinal cord develops, there are four columns. These are the general somatic efferent column, the general visceral efferent column, the general visceral afferent column and general somatic afferent column. These columns also extend into the brainstem, but are divided into smaller pieces.[9] In the brainstem there are six columns.
Four 'general' columns contain fibres that supply sensation or control muscles:[11]
There are three additional columns which innervate organs and tissues developing from the branchial arches and inner ear. These are the following:

Extracranial course

With the exception of the olfactory nerve (I) and optic nerve (II), the cranial nerves emerge from the brainstem. The oculomotor nerve (III) and trochlear nerve (IV) emerge from the pons, the trigeminal (V), abducens (VI), facial (VII) and vestibulocochlea (VIII) from the midbrain, and the glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII) emerge from the medulla.[12]
The olfactory nerve (I) and optic nerve (II) emerge separately. The olfactory nerves emerge from the olfactory bulbs on either side of the crista galli, a bony projection below the frontal lobe, and the optic nerves (II) emerge from the lateral colliculus, swellings on either side of the temporal lobes of the brain.[12]

Exiting the skull

Exits of cranial nerves from the skull.[11]
Location Nerve
cribiform plate Olfactory nerve (I)
optic foramen Optic nerve (II)
superior orbital fissure Oculomotor (III)
Trochlear (IV)
Abducens (VI)
Trigeminal V1
round foramen Trigeminal V2
oval foramen Trigeminal V3
internal auditory canal Facial (VII)
Vestibulocochlear (VIII)
jugular foramen Glossopharyngeal (IX)
Vagus (X)
Accessory (XI)
hypoglossal canal Hypoglossal (XII)
After emerging from the brainstem, the cranial nerves travel through the skull, but must leave this bony compartment in order to reach their destinations. Some nerves pass through unique holes in the skull, called foramina, as they travel to their destination. Other nerves pass through bony canals, longer canals enclosed by bone. These foramina and canals may contain more than one cranial nerve, and may also contain additional blood vessels.[11]
  • The olfactory nerve (I) passes through the cribiform plate, many small perforations in the ethmoid plate.
  • The optic nerve (II) passes through the optic foramen as it travels to the eye.
  • The oculomotor nerve (III), trochlear nerve (IV), abducens nerve (VI) and the opthalamic branch of the trigeminal nerve (V1) travel through the cavernous sinus into the superior orbital fissure, passing out of the skull into the orbit.
  • The maxillary division of the trigeminal nerve (V2) passes through the round foramen
  • The mandibular division of the trigeminal nerve (V3) passes through the oval foramen
  • The facial nerve (VII) and vestibulocochlear nerve (VIII) both pass through the internal auditory canal
The cranial nerves commonly enter and exit the skull together, for example nerves II, III, IV, V, and VI all pass through foramina near the pituitary fossa.[3]
The facial nerve enters the temporal bone at the internal acoustic meatus but exits the skull via the stylomastoid foramen while the vestibulocochlear nerve never actually exits the skull.


The cranial nerves give rise to a number of ganglia, collections of the cell bodies of neurons in the nerves that are outside of the brain. These ganglia are both parasympathetic and sensory ganglia.
The ganglion of the sensory nerves, which are similar in structure to the dorsal root ganglion of the spinal cord, include:[13]
Additional ganglia for nerves with parasympathetic function exist, and include the Ciliary ganglion of the oculomotor nerve (III), the pterygopalatine ganglion of the maxillary nerve (V2), the submandibular ganglion of the lingual nerve, a branch of the facial nerve (VII), and the otic ganglion of the glossopharyngeal nerve (IX).[14]


The following images show the cranial nerves schematically showing their respective exits from the CNS or brain-stem (not including the optic nerve which does not leave the CNS), and their path, as well as conceptual innervation targets.


No. Name Sensory,
or both
Origin/Target Function
0 Terminal Purely sensory Lamina terminalis Animal research indicates that the terminal nerve is involved in the detection of pheromones.[15][unreliable medical source?][16]
I Olfactory Purely sensory Telencephalon Transmits the sense of smell from the nasal cavity.[17] Located in the olfactory foramina in the cribriform plate of the ethmoid bone.
II Optic Sensory Retinal ganglion cells Transmits visual signals from the retina of the eye to the brain.[18] Located in the optic canal.
III Oculomotor Mainly motor Anterior aspect of Midbrain Innervates the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique, which collectively perform most eye movements. Also innervates the sphincter pupillae and the muscles of the ciliary body. Located in the superior orbital fissure.
IV Trochlear motor Dorsal aspect of Midbrain Innervates the superior oblique muscle, which depresses, rotates laterally, and intorts the eyeball. Located in the superior orbital fissure.
V Trigeminal Both sensory and motor Pons Receives sensation from the face and innervates the muscles of mastication.
Located in the;
superior orbital fissure (ophthalmic nerve - V1),
foramen rotundum (maxillary nerve - V2),
foramen ovale (mandibular nerve - V3).
VI Abducens Mainly motor Nuclei lying under the floor of the fourth ventricle
Innervates the lateral rectus, which abducts the eye. Located in the superior orbital fissure.
VII Facial Both sensory and motor Pons (cerebellopontine angle) above olive Provides motor innervation to the muscles of facial expression, posterior belly of the digastric muscle, stylohyoid muscle, and stapedius muscle. Also receives the special sense of taste from the anterior 2/3 of the tongue and provides secretomotorinnervation to the salivary glands (except parotid) and the lacrimal gland. Located in and runs through the internal acoustic canal to the facial canal and exits at the stylomastoid foramen.
VIII Vestibulocochlear
(also auditory,[19] acoustic,[19] or auditory-vestibular)
Mostly sensory Lateral to CN VII (cerebellopontine angle) Mediates sensation of sound, rotation, and gravity (essential for balance and movement). More specifically, the vestibular branch carries impulses for equilibrium and the cochlear branch carries impulses for hearing. Located in the internal acoustic canal.
IX Glossopharyngeal Both sensory and motor Medulla Receives taste from the posterior 1/3 of the tongue, provides secretomotor innervation to the parotid gland, and provides motor innervation to the stylopharyngeus. Some sensation is also relayed to the brain from the palatine tonsils. Located in the jugular foramen. This nerve is involved together with the vagus nerve in the gag reflex.
X Vagus Both sensory and motor Posterolateral sulcus of Medulla Supplies branchiomotorinnervation to most laryngeal and pharyngeal muscles (except the stylopharyngeus, which is innervated by the glossopharyngeal). Also provides parasympathetic fibers to nearly all thoracic and abdominal viscera down to the splenic flexure. Receives the special sense of taste from the epiglottis. A major function: controls muscles for voice and resonance and the soft palate. Symptoms of damage:dysphagia (swallowing problems), velopharyngeal insufficiency. Located in the jugular foramen. This nerve is involved (together with nerve IX) in the pharyngeal reflex or gag reflex.
XI Accessory
cranial accessory
spinal accessory
Mainly motor Cranial and Spinal Roots Controls the sternocleidomastoid and trapezius muscles, and overlaps with functions of the vagus nerve (CN X). Symptoms of damage: inability to shrug, weak head movement. Located in the jugular foramen.
XII Hypoglossal Mainly motor Medulla Provides motor innervation to the muscles of the tongue (except for the palatoglossal muscle, which is innervated by the vagus nerve) and other glossal muscles. Important for swallowing (bolus formation) and speech articulation. Passes through the hypoglossal canal.


Cranial nerve function is an important element in neurological examination, as specific dysfunction may indicate as to which portion of the brainstem is damaged. It is of clinical importance to know the path and origin of the cranial nerves, both intracranially as well as extracranially.[3]
The cranial nerves are often the first structures to be affected by different forms of brain injury such as hemorrhaging or tumors, partly because they are sensitive to compression.[10] Mononeuropathy of a cranial nerve may sometimes be the first symptom of an intracranial or skull base cancer.[20]

Smell (I)

Damage to the olfactory nerve can cause an inability to smell (anosmia), a distortion in the sense of smell (parosmia), or a distortion or lack of taste. Specific testing is performed when an individual perceives lack of taste or affected taste. The smell from each nostril is tested individually, and with consideration of airflow. Different substances are used, and these include coffee or soap. Using stronger smelling substances, for example ammonia, may lead to the activation of nociceptors of the trigeminal nerve.[21]

Vision (II)

Damage to the optic nerve affects vision. Vision is affected depending on the location of the lesion. A person may not be able to see things on their left or right side (homonymous hemianopsia), or may have difficulty seeing things on their outer visual fields (bitemporal hemianopsia) if the optic chiasm is involved.[1]:82 Vision may be tested using a number of different tests, examining the visual field, or by examining the cornea with a ophthalmoscope, using a process known as funduscopy. Visual field testing may be used to pin-point structural lesions in optic nerve, or further along the visual pathways.[21]

Eye movement (III, IV, VI)

Various deviations of the eyes due to abnormal function of the targets of the cranial nerves
Damage or lesion of nerves III, IV, or VI may affect the movement of the eye or pupil. Either both or one eye may be affected, and if both eyes are affected no double vision (diplopia) will occur. These nerves might be examined by observing how the eye follows an object in different directions. This object may be a finger or a pin, and may be moved at different directions to test for pursuit velocity.[21]
If the eyes do not work together, the most likely cause is damage to a specific cranial nerve or nuclei.[21]
    • Damage to the oculomotor nerve can cause double vision (diplopia) with lateral strabismus, and also ptosis and mydriasis.[1]:84 All but specific deviations may be due to damage in this nerve or any of the muscles it innervated, (though not internuclear ophthalmoplegia). Lesion may also lead to inability to open the eye, due to disrupted innervation of the levator palpebrae (unlike in Horner syndrome, which only results in a droopy eyelid. Individuals suffering from lesion or damage to the oculomotor nerve may compensate by tilting their heads to alleviate symptoms due to lack of control from oblique muscles when the eye is not adducted.[21]
    • Damage to the trochlear nerve can also cause diplopia with the eye adducted and elevated.[1]:84 The result will be an eye which can not move downwards or inwards properly (especially downwards when in an inward position). This is due to impairment in the superior oblique muscle innervated by the trochlear nerve.[21]
    • Damage to the abducens nerve can also result in diplopia with medial strabismus.[1]:84 This is due to impairment in the lateral rectus muscle innervated by the nerve.[21]

Facial sensation (V)

Conditions affecting the trigeminal nerve include trigeminal neuralgia,[10] cluster headache,[22] and trigeminal zoster.[10] Trigeminal neuraliga occurs later in life, from middle age onwards, most often after an age of 60; and is a condition associated with very strong pain distributed over the area innervated by the trigeminal nerve. Often the pain follows the distribution of the maxillary or mandibular nerve, (branches V2 and V3.[23] The trigeminal nerve is also present in the tendon reflexive jaw jerk. A reflex involving an induced twitch in muscles involved in closing the jaw when upon tapping on the jaw. A stronger reflex may be present if there is a supranuclear lesion of the trigeminal nerves motor nucleus, for example in pseudobulbar palsy.[23] In Parkinson's disease the trigeminal nerve is involved in the glabellar reflex which causes involuntary eye-blinking.
The facial nerve passes through the petrous temporal bone and internal auditory meatus after which it enters the facial canal to finally exit the stylomastoid foramen before passing through the parotid gland on its way to innervate the face.

Facial expression (VII)

Lesions of the facial nerve may manifest as facial palsy. This is where a person is unable to move the muscles on one or both sites of their face. If only the peripheral nerve itself is affected, this may cause Bell's palsy. Palsy that occurs is on the same side of the affected nerve. Central facial palsy will manifest in a similar fashion. If the nerve is damaged only on one side, a person will still be able to raise the eyebrows and crease the forehead on that side. That is because the frontalis muscle is innervated by both the left and the right cranial nerve. The effect is most often unilateral, and indicates contralateral damage or engagement of the cerebrum.[10]

Hearing and balance (VIII)

The vestibulocochlear nerve splits into the vestibular and cochlear nerve. The vestibular part is responsible for innervating the vestibules and semicircular canal of the inner ear, which transmits information about balance, and is an important component of the vestibuloocular reflex, which keeps the head stable and allows the eyes to track moving objects. The cochlear nerve transmits information from the cochlea, allowing sound to be heard.
When damaged:
  • The vestibular nerve may give rise to the sensation of spinning and dizziness, and may cause rotatory nystagmus. Function of the vestibular nerve may be tested through caloric stimulation.[21] Damage to the vestibulocochlear nerve can also present as repetitive and involuntary eye movements (nystagmus), particularly when looking in a horizontal plane.[21]
  • The cochlear nerve will cause partial or complete deafness in the affected ear.[21]

Oral sensation and taste (IX)

Deviating uvula due to cranial nerve X lesion.
The glossopharyngeal nerve is almost exclusively sensory in supplying five afferent nuclei of the brainstem, covering the oropharynx and back of the tongue with innervation.[24] Damage may result in difficulties swallowing.[21]

Vagus nerve (X)

Loss of function of the vagal nerve will lead to a loss of parasympathetic innervation to a very large number of structures. Of the major effects a rise in blood pressure and heart rate may occur. Isolated dysfunction of only the vagus nerve is rare, but can be diagnosed by a hoarse voice, due to dysfunction of the superior laryngeal nerve[10]
Testing of function may be performed by assessing ability to drink liquids. Choking on either saliva or liquids may indicated neurological damage to the vagal nerve.[21] Damage to the glossopharyngeal can be assessed by asking the subject to say "Ah" during phonation inspect to see if the uvula deviates. Positive sign indicative of unilateral damage occurs with finding of asymmetrically deviating uvula, towards the side with an intact or healthy nerve.[21]

Shoulder elevation and head-turning (XI)

Winged scapula may occur due to lesion of the spinal accessory.
Damage to the accessory nerve may lead to contralateral weakness in the trapezius, which can be tested by asking the subject to raise their shoulders or shrug, upon which the scapula will move out into a winged position if the nerve is damaged.[21] Weakness or an inability to elevate the scapula may be present, since the levator scapulae is alone in providing this function.[23]
There may also be weakness present of the sternocleidomastoid muscle, but as it received cortical innervation from the ipilateral side any damage will give rise to ipsilateral weakness.[21]

Tongue movement (XII)

A damaged hypoglossal nerve will result in an inability to stick the tongue out straight.
The hypoglossal nerve is unique in that it is innervated bilaterally from both hemispheres motor cortex. Damage to the nerve at lower motor neuron level may lead to fascinations of atrophy of the musculature of the tongue. The fasciculations of the tongue are sometimes said to look like a "bag of worms". Upper motor neuron damage will not lead to atrophy or fasciculations, but only weakness of the innervated muscles.[21]
When the nerve is damaged it will lead to unilateral weakness, and the tongue will upon being stuck out move towards the weaker or damaged side, as shown in the image.[21]

Clinical significance

Use of a Snellen chart to examine the optic nerve (II) may constitute one part of the cranial nerve examination.


Doctors, neurologists and other medical professionals may conduct a cranial nerve examination as part of a neurological examination to examine the cranial nerves. This is a highly formalised series of steps involving specific tests for each nerve.



Stroke may damage the blood supply to areas of the nerves. It may also damage the areas of the brain that control the nerves. If there is a stroke of the midbrain, pons or medulla, various cranial nerves may be damaged, resulting in dysfunction and symptoms of a number of different syndromes.
Cavernous sinus thrombosis refers to a thrombus affecting the venous drainage from the cavernous sinus, which several cranial nerves pass through.


Nerves may be compressed because of increased intercranial pressure or tumour masses that presses against the nerves. For example, an optic glioma may impact on the optic nerve (II), and an acoustic neuroma may compress the facial nerve (VII) and vestibulocochlear nerve (VIII)


Inflammation can be a result of infection, such as viral causes, or can occur spontaneously. Inflammation is more common in some nerves than others. Spontaneous inflammation may result in a palsy of a nerve that self-resolves, such as Trochlea palsy. Inflammation of the facial nerve (VII) may result in Bell's palsy. Inflammation of the trigeminal nerve (V) may result in Trigeminal neuralgia, a phenominon in which the face is exquisitely tender.


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