Cranial nerves are nerves that emerge directly from the brain, including the brainstem, in contrast to spinal nerves, which emerge from segments of the spinal cord.

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 I–XII. Some considered there to be thirteen pairs of cranial nerves, including the non-paired 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 cranial nerves are considered components of the peripheral nervous system (PNS),

Anatomy

Most typically, humans are considered to have twelve pairs of cranial nerves (I–XII), with the terminal nerve (0) more recently canonized. and the vagus nerve (X) is named for its wandering course (). 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.

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 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.

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.

Exiting the skull and extracranial course

{| class="wikitable" style="float:right;margin-left:15px"

|+ Exits of cranial nerves from the skull.

|-

! Location !! Nerve

|-

| cribriform plate || Terminal nerve (0)

|-

| cribriform plate || Olfactory nerve (I)

|-

| optic foramen || Optic nerve (II)

|-

| superior orbital fissure || Oculomotor (III)<br/>Trochlear (IV)<br/>Abducens (VI)<br/>Trigeminal V1<br/> (ophthalmic)

|-

| foramen rotundum|| Trigeminal V2 <br/> (maxillary)

|-

| foramen ovale|| Trigeminal V3 <br/> (mandibular)

|-

| stylomastoid foramen|| Facial nerve (VII)

|-

| internal auditory canal|| Vestibulocochlear (VIII)

|-

| jugular foramen || Glossopharyngeal (IX) <br/>Vagus (X)<br/>Accessory (XI)

|-

| hypoglossal canal || Hypoglossal (XII)

|}

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.

Contributions of neural crest cells and placodes to ganglia and cranial nerves

{| class="wikitable"

! Cranial nerve

! Ganglion and type

! Origin of neurons

|-

|CNI – olfactory

(Ensheating glia of olfactory nerves)

|

|Telencephalon/olfactory placode; NCCs at forebrain

|-

|CNIII – oculomotor

(m)

|Ciliary, visceral efferent

|NCCs at forebrain-midbrain junction (caudal diencephalon and the anterior mesencephalon)

|-

|CNV – trigeminal

(mix)

|Trigeminal, general afferent

|NCCs at forebrain-midbrain junction (from r2 into 1st PA), trigeminal placode

|-

|CNVII – facial

(mix)

|<nowiki>-Superior, general and special afferent</nowiki>

-Inferior: geniculate, general and special afferent

-Sphenopalatine, visceral efferent

-Submandibular, visceral efferent

|<nowiki>-Hindbrain NCCs (from r4 into 2</nowiki><sup>nd</sup> PA), 1st epibranchial placode

-1st epibranchial placode (geniculate)

-Hindbrain NCCs (2nd PA)

-Hindbrain NCCs (2nd PA)

|-

|CNVIII – Vestibulocochlear

(s)

|<nowiki>-Acoustic: cochlear, special afferent; and vestibular, special afferent</nowiki>

|<nowiki>-Otic placode and hindbrain (from r4) NCCs</nowiki>

|-

|CNIX – glossopharyngeal

(mix)

|<nowiki>-Superior, general and special afferent</nowiki>

-Inferior, petrosal, general and special afferent

-Otic, visceral efferent

|<nowiki>-Hindbrain NCCs (from r6 into 3</nowiki><sup>rd</sup> PA)

-2nd epibranchial placode (petrosal)

-Hindbrain NCCs (from r6 into 3rd PA)

|-

|CNX – vagus

(mix)

Superior laryngeal branch; and recurrent laryngeal branch

|<nowiki>-Superior, general afferent</nowiki>

-Inferior: nodose, general and special afferent

-Vagal: parasympathetic, visceral efferent

|<nowiki>-Hindbrain NCCs (from r7-r8 to 4</nowiki><sup>th</sup> & 6th PA)

-Hindbrain NCCs (4th& 6th PA); 3rd (nodose) and 4th epibranchial placodes

-Hindbrain NCCs (4th & 6th PA)

|-

|CNXI – accessory

(m)

|No ganglion *

|Hindbrain (from r7-r8 to PA 4); NCCs (4th PA)

|}

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, smell, 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.

Smell (I)

The olfactory nerve (I) conveys information giving rise to the sense of smell.

Vision (II)

The optic nerve (II) transmits visual information. 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.

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, 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.

| align = right

| direction = horizontal

| height1 = 200

The accessory nerve (XI) supplies the sternocleidomastoid and trapezius muscles. Dysfunction of a nerve identified during testing may point to a problem with the nerve or of a part of the brain. 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). 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 oculomotor 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. 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), ophthalmic 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, or can occur spontaneously. Inflammation of the facial nerve (VII) may result in Bell's palsy.

Multiple sclerosis, 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. 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. 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.

<gallery>

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.

</gallery>

See also

  • Cranial nerve mnemonics
  • Spinal nerve
  • Plexus
  • Nerve plexus

:* Brachial plexus

References

Cranial Nerve Exam - by UBC Medicine - Educational Media