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, and hearing.

[1] The cranial nerves emerge from the central nervous system above the level of the first vertebra of the vertebral column.

The numbering of the cranial nerves is based on the order in which they emerge from the brain and brainstem, from front to back.

[3] The cranial nerves are considered components of the peripheral nervous system (PNS),[3] 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 oculomotor nerve (III) and trochlear nerve (IV) emerge from the midbrain, the trigeminal (V), abducens (VI), facial (VII) and vestibulocochlear (VIII) from the pons, and the glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII) emerge from the medulla.

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.

[14] After emerging from the brain, the cranial nerves travel within the skull, and some must leave it in order to reach their destinations.

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:[15] Contributions of neural crest cells and placodes to ganglia and cranial nerves (Ensheating glia of olfactory nerves) (m) (mix) (mix) -Inferior: geniculate, general and special afferent -Sphenopalatine, visceral efferent -Submandibular, visceral efferent -1st epibranchial placode (geniculate) -Hindbrain NCCs (2nd PA) -Hindbrain NCCs (2nd PA) (s) (mix) -Inferior, petrosal, general and special afferent -Otic, visceral efferent -2nd epibranchial placode (petrosal) -Hindbrain NCCs (from r6 into 3rd PA) (mix) Superior laryngeal branch; and recurrent laryngeal branch -Inferior: nodose, general and special afferent -Vagal: parasympathetic, visceral efferent -Hindbrain NCCs (4th& 6th PA); 3rd (nodose) and 4th epibranchial placodes -Hindbrain NCCs (4th & 6th PA) (m)

The sensory supply includes both "general" sensation such as temperature and touch, and "special" senses such as taste, vision, smell, balance and hearing.

[11] 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.

[17] Damage to the oculomotor nerve (III) can cause double vision and inability to coordinate the movements of both eyes (strabismus), also eyelid drooping (ptosis) and pupil dilation (mydriasis).

[18] 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.

[17] Damage to the trochlear nerve (IV) can also cause double vision with the eye adducted and elevated.

[17] 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.

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.

Function of the vestibular nerve may be tested by putting cold and warm water in the ears and watching eye movements caloric stimulation.

[17] The glossopharyngeal nerve (IX) supplies the stylopharyngeus muscle and provides sensation to the oropharynx and back of the tongue.

[16] 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.

Major effects of damage to the vagus nerve may include a rise in blood pressure and heart rate.

[3] 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.

The sensation of the face is tested, and patients are asked to perform different facial movements, such as puffing out of the cheeks.

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.

[23] 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).

[22][25] Occlusion of blood vessels that supply the nerves or their nuclei, an ischemic stroke, 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.

[26] 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).

[27] Other rarer inflammatory causes affecting the function of multiple cranial nerves include sarcoidosis, miliary tuberculosis, and inflammation of arteries, such as granulomatosis with polyangiitis.

Finally, in 1778, German anatomist Samuel Soemmering named the 12 pairs of nerves that are generally accepted today.