|Brain: Cranial nerves|
Cranial nerves as they pass through the skull base to the brain.
(pl: nervi craniales)
Cranial nerves (sometimes termed cerebral nerves), are nerves that emerge directly from the brain and the brainstem. This is 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.
Spinal nerves reach as far as the first cervical vertebra, and the cranial nerves fill a corresponding role above this level. 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, and zero assigned to cranial nerve zero, according to the order in which they originate from the forebrain to the back of the brain and the brainstem.
Cranial nerves 0, I and II emerge from the cerebrum or forebrain; 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). The axons of the remaining twelve nerves extend beyond the brain and are considered part of the PNS. The central ganglia of the cranial nerves or cranial nerve nuclei originate in the CNS, preferentially from the brainstem.
- 1 Anatomy
- 2 Clinical significance
- 3 History
- 4 Society and culture
- 5 Other animals
- 6 References
- 7 External links
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, as, when viewing the brain and brainstem from an inferior view, they present themselves in their numeric order.
The cranial nerves serve to innervate the head and neck area, 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. 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. 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. The accessory nerve XI, is considered either a cranial nerve or a spinal nerve which emanates level with the brain-stem.
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 and skull. Sensory ganglia exist for nerves with sensory function; V, VII, VIII, IX, X. 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.
Cranial nerve 0 controversy
Cranial nerve zero which is also called the terminal nerve (or cranial nerve nulla since there is no Roman numeral for zero) has been largely neglected from textbooks, even though it was first clearly identified over a century ago. It was first shown to be present in the shark but its presence in humans (and other mammals) remained somewhat controversial. More recent studies have shown the nerve to be quite distinct in human fetuses and infants, and has also regularly been seen in the adult brain. The nerve axons are unmyelinated and arise from ganglia. Cranial nerve zero has also been shown to release luteinising hormone. Another study has shown the cranial nerve zero to be a microscopic plexus of unmyelinated fibres in the frontal lobes. It was concluded in the study and it confirmed earlier findings by light microscope, that this nerve is a common finding in the human brain.
|0||Terminal||Purely sensory||Lamina terminalis||Animal research indicates that the terminal nerve is involved in the detection of pheromones.[unreliable medical source?]|
|I||Olfactory||Purely sensory||Telencephalon||Transmits the sense of smell from the nasal cavity. 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. 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.
|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.|
(also auditory, acoustic, 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.|
|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.|
The cranial nerves give rise to a number of peripheral ganglia, both parasympathetic and sensory autonomic ganglia.
- Pterygopalatine ganglion
- Ciliary ganglion
- Otic ganglion
- Submandibular ganglion
- Peripheral autonomic ganglion - arise from the vagus nerve are found in gut, around respiratory organs and the heart.
- Trigeminal ganglion
- Spiral ganglion
- Vestibular ganglion
- Geniculate ganglion
- Superior (jugular) ganglia of glossopharyngeal
- Inferior (petrosal) ganglia of glossopharyngeal
- Superior (rostral) ganglion of vagus nerve
- Inferior (nodose) ganglion of vagus nerve
Each nerve emerges from either a brainstem or mesencephalic nuclei. The first nerve; the olfactory emerges from the olfactory bulb, and depending slightly on division the optic nerve 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.
|Olfactory nerve||Olfactory bulb|
|Optic nerve||Lateral geniculate nucleus|
|Oculomotor nerve||Oculomotor nucleus
|Trochlear nerve||Trochlear nucleus|
|Trigeminal nerve||Trigeminal nerve nuclei:
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
|Glossopharyngeal nerve||Solitary nucleus
Spinal nucleus of the trigeminal nerve
Lateral nucleus of vagal trigone.
Inferior salivatory nucleus
|Vagus nerve||Dorsal nucleus of vagus nerve
Spinal trigeminal nucleus
|Accessory nerve||Spinal accessory nucleus
|Hypoglossal nerve||Hypoglossal nucleus|
Cranial nerve columns
Brainstem nuclei with associated functions are often found in correspondingly similar areas of the brainstem, also known as functional columns of the brainstem. As the spinal cord develops, four columns of gray matter bilaterally may be found. These can be divided into 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 and contribute in a varied fashion to the different cranial nerves. That is to say, cranial nerves do not receive innervation from all columns.
In the brainstem the columns are divided into:
- General somatic efferent column (GSE)
- Innervating through nerves III, IV, VI the striate orbital musculature, and through nerve XII the tongue.
- General visceral efferent column (GVE)
- Innervates chief parasympathetic structures emanating from the cranium - cranial parasympathetic system.
- General visceral afferent column
- Reaches visceral structures innervated by nerves; IX, and X.
- General somatic afferent column
- Reaches mucous membranes and skin chiefly from nerve V such as the dura mater, face, mouth, and nose.
- Innervating striated musculature of the branchial arches in; pharynx, larynx, roof of the mouth, jaw, and face; via nerves, V, VII, IX, X, XI.
- Special visceral afferent column
- Innervating taste-buds developed from the endodermal regions of the branchial arches.
- Innervating the vestibulocochlear system.
Exiting the skull
- Their individual exits are as follows:
- Note: 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.
|superior orbital fissure
fissura orbitalis superior
|internal auditory canal
meatus acusticus internus
canalis nervi hypoglossi
|Olfactory nerve (I)||Optic nerve (II)||Oculomotor (III)
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.
3 The oculomotor controls a number of motor function of the eye, both somatic and autonomic.
5 The trigeminal innervates a large number of structures, both motor and sensory.
7 The facial nerve stands for innervation of a large number of structures both motor and sensory.
9 The glossopharyngeal innervates a number of motor and sensory structures of the tongue and pharynx.
12 The hypoglossal nerve innervates the musculature of the tongue and pharynx.
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. 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. Mononeuropathy of a cranial nerve may sometimes be the first symptom of an intracranial or skull base cancer.
Various forms of damage or lesion involving the olfactory nerve can cause anosmia or parosmia, i.e. absence or distortion of the sense of smell. Testing is performed when an individual perceives lack of taste or affected taste. Nostrils are tested individually, and with consideration of airflow. Substances which may be used to test function include coffee or soap. Using stronger smelling substances, for example ammonia, may lead to the activation of nociceptors of the trigeminal nerve.
Lesions involving the optic nerve will affect vision. E.g. bitemporal hemianopsia if the optic chiasm is involved, or homonymous hemianopsia if the optic tract is involved.:82 This 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.
Oculomotor, Trochlear, Abducens nerves
Damage or lesion of nerves III, IV, or VI may manifest through various degrees of abnormal movement of the eye or pupill.
Such symptoms may be found by looking for limits of how far the eyes can follow an object in certain directions. Either both or one eye may be affected, and if both eyes are affected no diplopia will be found. This can be assessed by asking the subject to follow an object, such as a finger in different directions, or while moving slowly to assess if the subject is capable of achieving normal pursuit velocity.
If the eyes work disconjugately the most likely cause is damage to a specific cranial nerve or nuclei.
- Lesions of the oculomotor nerve can cause diplopia (double vision) with lateral strabismus, and also ptosis and mydriasis.: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.
- Lesions of the trochlear nerve can also cause diplopia with the eye adducted and elevated.: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.
- Lesions of the abducens nerve can also result in diplopia with medial strabismus.:84 This is due to impairment in the lateral rectus muscle innervated by the nerve.
Damage to these nerves may also give rise to involuntary nystagmus, or repetitive eye movements, which may affect only one or both eyes. Various different forms of nystagmus may give in indication as to what is the cause of the symptoms.
- Pendular nystagmus - often associated with grave visual impairment in early childhood.
- Rythmical nystagmus - slow movement in one direction followed be a faster movement in the other while the eyes are resting may be due to ocular deviation.
- Horizontal nystagmus (or rotatory nystagmus) - may be associated with lesion of the vestibular nerve rather than other nerves, which may be either peripheral or central in origin.
- Vertical nystagmus - tends to be associated with brainstem lesions.
- Horizontally directed nystagmus occurring upon viewing things in a lateral field of view may be due to usage of certain drugs, mainly anticonvulsants or sedatives.
Conditions affecting the trigeminal nerve include trigeminal neuralgia, cluster headache, and trigeminal zoster. 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. 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.In Parkinson's disease the trigeminal nerve is involved in the glabellar reflex which causes involuntary eye-blinking.
Lesions of the facial nerve may manifest as facial palsy. The cause may be Bell's palsy, where only the peripheral nerve is affected. This leads to different degrees of unilateral, ipsilateral paralysis of the entire facial musculature. Central facial palsy will manifest in a similar fashion, but as the innervation of the frontalis arrises from a different area than that of the remaining facial muscles it will still be possible to raise the eyebrows. The effect is most often unilateral, and indicates contralateral damage or engagement of the cerebrum.
- The vestibular nerve may when damaged give rise to the senstation of spinning and dizziness, and may cause rotatory nystagmus. Function of the vestibular nerve may be tested through caloric stimulation.
- Damage to the cochlear nerve will give rise to hearing problems in the affected ear.
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. Damage may result in difficulties swallowing.
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
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. 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.
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. Weakness or an inability to elevate the scapula may be present, since the levator scapulae is alone in providing this function.
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.
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.
Society and culture
Many mnemonics exist to help students memorize the names and order of the cranial nerves.
An example mnemonic sentence for the initial letters "OOOTTAFAGVSH" is "On old Olympus's towering tops, a Finn and German viewed some hops," and for the initial letters "OOOTTAFVGVAH" is "Oh, oh, oh, to touch and feel very good velvet ... ah, heaven." The differences between these depend on "acoustic" versus "vestibulocochlear" and "spinal-accessory" versus "accessory".
A common example mnemonic for remembering which nerves are motor (M), sensory (S), or both (B), "Some Say Marry Money But My Brother Says Big Brains Matter More". There are a very large number of additional mnemonics.
Cranial nerves in non-human vertebrates
Other amniotes (non-amphibian tetrapods) have cranial nerves similar to those of humans. In anamniotes (fishes and amphibians), in contrast, cranial nerves XI and XII do not exist as such, nerve XI being an integral part of nerve X and nerve XII being represented by a variable number of spinal nerves emerging from vertebral segments fused into the occiput; nerves XI and XII only became discrete nerves in the ancestors of amniotes (non-amphibian tetrapods).
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