Cranial nerves are the

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

s that emerge directly from the brain (including the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...

), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck, including the special senses of vision, taste,

smell Smell may refer to; * Odor, airborne molecules perceived as a scent or aroma * Sense of smell, the scent also known scientifically as olfaction * "Smells" (''Bottom''), an episode of ''Bottom'' * The Smell, a music venue in Los Angeles, Californ ...

, and hearing. The cranial nerves emerge from the central nervous system above the level of the first vertebra of the vertebral column. Each cranial nerve is paired and is present on both sides. There are conventionally twelve pairs of cranial nerves, which are described with

Roman numerals Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, eac ...

I–XII. Some considered there to be thirteen pairs of cranial nerves, including cranial nerve zero. The numbering of the cranial nerves is based on the order in which they emerge from the brain and brainstem, from front to back. The terminal nerves (0),

olfactory nerve The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell. The afferent nerve fibers of the olfactory receptor neurons tr ...

s (I) and optic nerves (II) emerge from the cerebrum, and the remaining ten pairs arise from the brainstem, which is the lower part of the brain. The cranial nerves are considered components of the peripheral nervous system (PNS), although on a structural level the olfactory (I), optic (II), and trigeminal (V) nerves are more accurately considered part of the central nervous system (CNS). The cranial nerves are in contrast to

spinal nerve A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into th ...

s, which emerge from segments of the spinal cord.

Anatomy

Most typically, humans are considered to have twelve pairs of cranial nerves (I–XII), with the terminal nerve (0) more recently canonized. The nerves are: the

(I), the optic nerve (II),

oculomotor nerve The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of ...

(III), trochlear nerve (IV), trigeminal nerve (V), abducens nerve (VI),

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...

(VII), vestibulocochlear nerve (VIII), glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI), and the hypoglossal nerve (XII).

Terminology

Cranial nerves are generally named according to their structure or function. For example, the olfactory nerve (I) supplies smell, and the facial nerve (VII) supplies the muscles of the face. Because Latin was the ''

lingua franca A lingua franca (; ; for plurals see ), also known as a bridge language, common language, trade language, auxiliary language, vehicular language, or link language, is a language systematically used to make communication possible between groups ...

'' of the study of anatomy when the nerves were first documented, recorded, and discussed, many nerves maintain Latin or Greek names, including the trochlear nerve (IV), named according to its structure, as it supplies a muscle that attaches to a pulley ( el, trochlea). The trigeminal nerve (V) is named in accordance with its three components ( la, trigeminus meaning

triplet A triplet is a set of three items, which may be in a specific order, or unordered. It may refer to: Science * A series of three nucleotide bases forming an element of the Genetic code * J-coupling as part of Nuclear magnetic resonance spectrosc ...

s), and the vagus nerve (X) is named for its wandering course ( la, vagus). Cranial nerves are numbered based on their position from front to back ( rostral-caudal) of their position on the brain, as, when viewing the forebrain and brainstem from below, they are often visible in their numeric order. For example, the olfactory nerves (I) and optic nerves (II) arise from the base of the forebrain, and the other nerves, III to XII, arise from the brainstem. Cranial nerves have paths within and outside the skull. The paths within the skull are called "intracranial" and the paths outside the skull are called "extracranial". There are many holes in the skull called "foramina" by which the nerves can exit the skull. All cranial nerves are ''paired'', which means they occur on both the right and left sides of the body. The muscle, skin, or additional function supplied by a nerve, on the same side of the body as the side it originates from, is an ''ipsilateral'' function. If the function is on the opposite side to the origin of the nerve, this is known as a ''contralateral'' function.

Intracranial course

Nuclei

Grossly, all cranial nerves have a Nucleus. With the exception of the olfactory nerve (I) and optic nerve (II), all the nuclei are present in the brainstem. The midbrain of the brainstem has the nuclei of the oculomotor nerve (III) and trochlear nerve (IV); the pons has the nuclei of the trigeminal nerve (V), abducens nerve (VI), facial nerve (VII) and vestibulocochlear nerve (VIII); and the medulla has the nuclei of the glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI) and hypoglossal nerve (XII). The olfactory nerve (I) emerges from the olfactory bulb, and depending slightly on division the optic nerve (II) is considered to emerge from the lateral geniculate nuclei. Because each nerve may have several functions, the nerve fibres that make up the nerve may collect in more than one nucleus. For example, the trigeminal nerve (V), which has a sensory and a motor role, has at least four nuclei.

Exiting the brainstem

With the exception of the olfactory nerve (I) and optic nerve (II), the cranial nerves emerge from the

. The oculomotor nerve (III) and trochlear nerve (IV) emerge from the midbrain, the trigeminal (V), abducens (VI), facial (VII) and vestibulocochlea (VIII) from the pons, and the glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII) emerge from the medulla. 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.

Ganglia

The cranial nerves give rise to a number of

ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...

, 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 sensory ganglia of the cranial nerves, directly correspond to the dorsal root ganglia of

s and are known as

cranial nerve ganglia In neuroanatomy, the cranial nerve ganglia are ganglia of certain cranial nerves. They can be parasympathetic or sensory. All cranial nerve ganglia are bilateral. Parasympathetic The four cranial parasympathetic ganglia are: *ciliary ganglion ...

. Sensory ganglia exist for nerves with sensory function: V, VII, VIII, IX, X. There are also a number of

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of t ...

. Sympathetic ganglia supplying the head and neck reside in the upper regions of the sympathetic trunk, and do not belong to the cranial nerves. The ganglion of the sensory nerves, which are similar in structure to the dorsal root ganglion of the spinal cord, include: * The trigeminal ganglia of the trigeminal nerve (V), which occupies a space in the dura mater called Meckel's cave. This ganglion contains only the sensory fibres of the trigeminal nerve. * The geniculate ganglion of the facial nerve (VII), which occurs just after the nerve enters the facial canal. * A superior and inferior ganglia of the glossopharyngeal nerve (IX), which occurs just after it passes through the jugular foramen. Additional ganglia for nerves with

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).

Exiting the skull and extracranial course

After emerging from the brain, the cranial nerves travel within the skull, and some must leave it in order to reach their destinations. Often the nerves pass through holes in the skull, called foramina, as they travel to their destinations. Other nerves pass through bony canals, longer pathways enclosed by bone. These foramina and canals may contain more than one cranial nerve and may also contain blood vessels. *The terminal nerve (0), is a thin network of fibers associated with the dura and lamina terminalis running rostral to the olfactory nerve, with projections through the cribriform plate. * The olfactory nerve (I), passes through perforations in the cribriform plate part of the ethmoid bone. The nerve fibres end in the upper nasal cavity. * The optic nerve (II) passes through the optic foramen in the

sphenoid bone The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of occipital bone, basilar part of the occipital bone. The sphenoid bone is one of the seven bones ...

as it travels to the eye. * The oculomotor nerve (III), trochlear nerve (IV), abducens nerve (VI) and the ophthalmic branch of the trigeminal nerve (V1) travel through the cavernous sinus into the

superior orbital fissure The superior orbital fissure is a foramen or cleft of the skull between the lesser and greater wings of the sphenoid bone. It gives passage to multiple structures, including the oculomotor nerve, trochlear nerve, ophthalmic nerve, abducens ner ...

, passing out of the skull into the orbit. * The maxillary division of the trigeminal nerve (V2) passes through foramen rotundum in the sphenoid bone. * The mandibular division of the trigeminal nerve (V3) passes through

foramen ovale There are multiple structures in the human body with the name foramen ovale (plural: ''foramina ovalia''; Latin for "oval hole"): * Foramen ovale (heart), in the fetal heart, a shunt from the right atrium to left atrium * Foramen ovale (skull), at ...

of the sphenoid bone. * The facial nerve (VII) and vestibulocochlear nerve (VIII) both enter the internal auditory canal in the temporal bone. The facial nerve then reaches the side of the face by using the stylomastoid foramen, also in the temporal bone. Its fibers then spread out to reach and control all of the muscles of facial expression. The vestibulocochlear nerve reaches the organs that control balance and hearing in the temporal bone and therefore does not reach the external surface of the skull. * The glossopharyngeal (IX), vagus (X) and accessory nerve (XI) all leave the skull via the jugular foramen to enter the neck. The glossopharyngeal nerve provides sensation to the upper throat and the back of the tongue, the vagus supplies the muscles in the

larynx The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...

and continues downward to supply parasympathetic supply to the chest and abdomen. The accessory nerve controls the trapezius and sternocleidomastoid muscles in the neck and shoulder. *The hypoglossal nerve (XII) exits the skull using the hypoglossal canal in the

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobe ...

Development

The cranial nerves are formed from the contribution of two specialized embryonic cell populations, cranial neural crest and ectodermal placodes. The components of the sensory nervous system of the head are derived from the neural crest and from an embryonic cell population developing in close proximity, the cranial sensory placodes (the olfactory, lens, otic, trigeminal, epibranchial and paratympanic placodes). The dual origin cranial nerves are summarized in the following Table: Contributions of neural crest cells and placodes to ganglia and cranial nerves Abbreviations: CN, cranial nerve; m, purely motor nerve; mix, mixed nerve (sensory and motor); NC, neural crest; PA, pharyngeal (branchial) arch; r, rhombomere; s, purely sensory nerve. * There is no known ganglion of the accessory nerve. The cranial part of the accessory nerve sends occasional branches to the superior ganglion of the vagus nerve.

Function

The cranial nerves provide motor and sensory supply mainly to the structures within the head and neck. The sensory supply includes both "general" sensation such as temperature and touch, and "special" senses such as taste, vision,

, balance and hearing. The vagus nerve (X) provides sensory and autonomic (parasympathetic) supply to structures in the neck and also to most of the organs in the chest and abdomen.

Terminal nerve (0)

The terminal nerve (0) may not have a role in humans, although it has been implicated in hormonal responses to smell, sexual response and mate selection.

Smell (I)

The

(I) conveys information giving rise to the sense of smell. Damage to the olfactory nerve (I) can cause an inability to smell ( anosmia), a distortion in the sense of smell ( parosmia), or a distortion or lack of taste.

Vision (II)

The optic nerve (II) transmits visual information. Damage to the optic nerve (II) affects specific aspects of vision that depend on the location of the damage. A person may not be able to see objects on their left or right sides (

homonymous hemianopsia Hemianopsia, or hemianopia, is a visual field loss on the left or right side of the vertical midline. It can affect one eye but usually affects both eyes. Homonymous hemianopsia (or homonymous hemianopia) is hemianopic visual field loss on the sa ...

), or may have difficulty seeing objects from their outer visual fields (

bitemporal hemianopsia Bitemporal hemianopsia, is the medical description of a type of partial blindness where vision is missing in the outer half of both the right and left visual field. It is usually associated with lesions of the optic chiasm, the area where the opt ...

) if the optic chiasm is involved. Inflammation ( optic neuritis) may impact the sharpness of vision or colour detection

Eye movement (III, IV, VI)

The

(III), trochlear nerve (IV) and abducens nerve (VI) coordinate eye movement. The oculomotor nerve controls all muscles of the eye except for the superior oblique muscle controlled by the trochlear nerve (IV), and the lateral rectus muscle controlled by the abducens nerve (VI). This means the ability of the eye to look down and inwards is controlled by the trochlear nerve (IV), the ability to look outwards is controlled by the abducens nerve (VI), and all other movements are controlled by the oculomotor nerve (III) Damage to these nerves may affect the movement of the eye. Damage may result in double vision ( diplopia) because the movements of the eyes are not synchronized. Abnormalities of visual movement may also be seen on examination, such as jittering ( nystagmus). Damage to the oculomotor nerve (III) can cause double vision and inability to coordinate the movements of both eyes ( strabismus), also eyelid drooping (

ptosis Ptosis (from the Greek: πτῶσις 'falling', 'a fall', 'dropped') refers to droopiness or abnormal downward displacement of a body part or organ. Particular cases include: * Ptosis (eyelid) * Ptosis (chin) * Ptosis (breasts) * Visceroptosis, ...

) and pupil dilation (

mydriasis Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drugs. Normally, as ...

). Lesions may also lead to inability to open the eye due to paralysis of the levator palpebrae muscle. Individuals suffering from a lesion to the oculomotor nerve may compensate by tilting their heads to alleviate symptoms due to paralysis of one or more of the eye muscles it controls. Damage to the trochlear nerve (IV) can also cause double vision with the eye adducted and elevated. The result will be an eye which can not move downwards properly (especially downwards when in an inward position). This is due to impairment in the superior oblique muscle. Damage to the abducens nerve (VI) can also result in double vision. This is due to impairment in the lateral rectus muscle, supplied by the abducens nerve.

Trigeminal nerve (V)

The trigeminal nerve (V) and its three main branches the ophthalmic (V1), maxillary (V2), and mandibular (V3) provide sensation to the skin of the face and also controls the muscles of chewing. Damage to the trigeminal nerve leads to loss of sensation in an affected area. Other conditions affecting the trigeminal nerve (V) include trigeminal neuralgia, herpes zoster, sinusitis pain, presence of a dental abscess, and cluster headaches. Bellspalsy

Facial expression (VII)

The facial nerve (VII) controls most muscles of facial expression, supplies the sensation of taste from the front two-thirds of the tongue, and controls the stapedius muscle. Most muscles are supplied by the cortex on the opposite side of the brain; the exception is the frontalis muscle of the forehead, in which the left and the right side of the muscle both receive inputs from both sides of the brain. Damage to the

(VII) may cause facial palsy. This is where a person is unable to move the muscles on one or both sides of their face. The most common cause of this is

Bell's palsy Bell's palsy is a type of facial paralysis that results in a temporary inability to control the facial muscles on the affected side of the face. In most cases, the weakness is temporary and significantly improves over weeks. Symptoms can vary fr ...

, the ultimate cause of which is unknown. Patients with Bell's palsy often have a drooping mouth on the affected side and often have trouble chewing because the buccinator muscle is affected. The facial nerve is also the most commonly affected cranial nerve in

blunt trauma Blunt trauma, also known as blunt force trauma or non-penetrating trauma, is physical traumas, and particularly in the elderly who fall. It is contrasted with penetrating trauma which occurs when an object pierces the skin and enters a tissue ...

Hearing and balance (VIII)

The vestibulocochlear nerve (VIII) supplies information relating to balance and hearing via its two branches, the vestibular and cochlear nerves. The vestibular part is responsible for supplying sensation from the vestibules and semicircular canal of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

, including 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 ( vertigo). Function of the vestibular nerve may be tested by putting cold and warm water in the ears and watching eye movements

caloric stimulation In medicine, the caloric reflex test (sometimes termed vestibular caloric stimulation) is a test of the vestibulo-ocular reflex that involves irrigating cold or warm water or air into the external auditory canal. This method was developed by Ro ...

. Damage to the vestibulocochlear nerve can also present as repetitive and involuntary eye movements ( nystagmus), particularly when the eye is moving horizontally. Damage to the cochlear nerve will cause partial or complete deafness in the affected ear.

Oral sensation, taste, and salivation (IX)

The glossopharyngeal nerve (IX) supplies the stylopharyngeus muscle and provides sensation to the oropharynx and back of the tongue. The glossopharyngeal nerve also provides parasympathetic input to the parotid gland. Damage to the nerve may cause failure of the gag reflex; a failure may also be seen in damage to the vagus nerve (X).

Vagus nerve (X)

The vagus nerve (X) provides sensory and parasympathetic supply to structures in the neck and also to most of the organs in the chest and abdomen. Loss of function of the vagus nerve (X) will lead to a loss of parasympathetic supply to a very large number of structures. Major effects of damage to the vagus nerve may include a rise in blood pressure and heart rate. Isolated dysfunction of only the vagus nerve is rare, but – if the lesion is located above the point at which the vagus first branches off – can be indicated by a hoarse voice, due to dysfunction of one of its branches, the recurrent laryngeal nerve. Damage to this nerve may result in difficulties swallowing.

Shoulder elevation and head-turning (XI)

The accessory nerve (XI) supplies the

sternocleidomastoid The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the access ...

and trapezius muscles. Damage to the accessory nerve (XI) will lead to weakness in the trapezius muscle on the same side as the damage. The trapezius lifts the shoulder when shrugging, so the affected shoulder will not be able to shrug and the shoulder blade ( scapula) will protrude into a

winged A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expresse ...

position. Depending on the location of the lesion there may also be weakness present in the sternocleidomastoid muscle, which acts to turn the head so that the face points to the opposite side.

Tongue movement (XII)

The hypoglossal nerve (XII) supplies the intrinsic muscles of the tongue, controlling tongue movement. The hypoglossal nerve (XII) is unique in that it is supplied by the motor cortices of both hemispheres of the brain. Damage to the nerve may lead to fasciculations or wasting (

atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...

) of the muscles of the tongue. This will lead to weakness of tongue movement on that side. When damaged and extended, the tongue will move towards the weaker or damaged side, as shown in the image. The fasciculations of the tongue are sometimes said to look like a "bag of worms". Damage to the nerve tract or nucleus will not lead to atrophy or fasciculations, but only weakness of the muscles on the same side as the damage.

Clinical significance

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. Dysfunction of a nerve identified during testing may point to a problem with the nerve or of a part of the brain. A cranial nerve exam starts with observation of the patient, as some cranial nerve lesions may affect the symmetry of the eyes or face. Vision may be tested by examining the visual fields, or by examining the retina with an ophthalmoscope, using a process known as

funduscopy Ophthalmoscopy, also called funduscopy, is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination and may be done as part ...

. Visual field testing may be used to pin-point structural lesions in the optic nerve, or further along the visual pathways. Eye movement is tested and abnormalities such as nystagmus are observed for. The sensation of the face is tested, and patients are asked to perform different facial movements, such as puffing out of the cheeks. Hearing is checked by voice and tuning forks. The patient's uvula is examined. After performing a shrug and head turn, the patient's tongue function is assessed by various tongue movements. Smell is not routinely tested, but if there is suspicion of a change in the sense of smell, each nostril is tested with substances of known odors such as coffee or soap. Intensely smelling substances, for example ammonia, may lead to the activation of pain receptors of the trigeminal nerve (V) located in the nasal cavity and this can confound olfactory testing.

Damage

Compression

Nerves may be compressed because of increased intracranial pressure, a mass effect of an intracerebral haemorrhage, or tumour that presses against the nerves and interferes with the transmission of impulses along the nerve. Loss of function of a cranial nerve may sometimes be the first symptom of an intracranial or skull base cancer. An increase in intracranial pressure may lead to impairment of the optic nerves (II) due to compression of the surrounding veins and capillaries, causing swelling of the eyeball (

papilloedema Papilledema or papilloedema is optic disc swelling that is caused by increased intracranial pressure due to any cause. The swelling is usually bilateral and can occur over a period of hours to weeks. Unilateral presentation is extremely rare. I ...

). A cancer, such as an optic nerve glioma, may also impact the optic nerve (II). A pituitary tumour may compress the optic tracts or the optic chiasm of the optic nerve (II), leading to visual field loss. A pituitary tumour may also extend into the cavernous sinus, compressing the oculuomotor nerve (III), trochlear nerve (IV) and abducens nerve (VI), leading to double-vision and strabismus. These nerves may also be affected by herniation of the temporal lobes of the brain through the falx cerebri. The cause of trigeminal neuralgia, in which one side of the face is exquisitely painful, is thought to be compression of the nerve by an artery as the nerve emerges from the brain stem. An acoustic neuroma, particularly at the junction between the pons and medulla, may compress the facial nerve (VII) and vestibulocochlear nerve (VIII), leading to hearing and sensory loss on the affected side.

Stroke

Occlusion of blood vessels that supply the nerves or their nuclei, an ischemic

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

, may cause specific signs and symptoms relating to the damaged area. 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. Thrombosis, such as a cavernous sinus thrombosis, refers to a clot ( thrombus) affecting the venous drainage from the cavernous sinus, affects the optic (II), oculomotor (III), trochlear (IV), opthalamic branch of the trigeminal nerve (V1) and the abducens nerve (VI).

Inflammation

Inflammation of a cranial nerve can occur as a result of infection, such as viral causes like reactivated

herpes simplex virus Herpes simplex virus 1 and 2 (HSV-1 and HSV-2), also known by their taxonomical names ''Human alphaherpesvirus 1'' and '' Human alphaherpesvirus 2'', are two members of the human ''Herpesviridae'' family, a set of viruses that produce viral inf ...

, or can occur spontaneously. Inflammation of the facial nerve (VII) may result in

Multiple sclerosis Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...

, an inflammatory process resulting in a loss of the myelin sheathes which surround the cranial nerves, may cause a variety of shifting symptoms affecting multiple cranial nerves. Inflammation may also affect other cranial nerves. Other rarer inflammatory causes affecting the function of multiple cranial nerves include sarcoidosis, miliary tuberculosis, and inflammation of arteries, such as granulomatosis with polyangiitis.

Other

Trauma to the skull, disease of bone, such as Paget's disease, and injury to nerves during surgery are other causes of nerve damage.

History

The Graeco-Roman anatomist Galen (AD 129–210) named seven pairs of cranial nerves. Much later, in 1664, English anatomist Sir Thomas Willis suggested that there were actually 9 pairs of nerves. Finally, in 1778, German anatomist Samuel Soemmering named the 12 pairs of nerves that are generally accepted today. However, because many of the nerves emerge from the brain stem as rootlets, there is continual debate as to how many nerves there actually are, and how they should be grouped. For example, there is reason to consider both the olfactory (I) and optic (II) nerves to be brain tracts, rather than cranial nerves.

Other animals

Cranial nerves are also present in other vertebrates. Other

amniote Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disti ...

s (non-

amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, ar ...

tetrapods) have cranial nerves similar to those of humans. In anamniotes (fishes and amphibians), the accessory nerve (XI) and hypoglossal nerve (XII) do not exist, with the accessory nerve (XI) being an integral part of the vagus nerve (X); the hypoglossal nerve (XII) is represented by a variable number of spinal nerves emerging from vertebral segments fused into the occiput. These two nerves only became discrete nerves in the ancestors of amniotes. The very small terminal nerve (nerve N or O) exists in humans but may not be functional. In other animals, it appears to be important to sexual receptivity based on perceptions of pheromones. File:Quiring 1950 146.png, The cranial nerves in the horse File:Cranial nerve sheep ventral.png, Ventral view of a sheep's brain. The exits of the various cranial nerves are marked with red.

References

External links

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