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|Brain: Cranial nerves|
Cranial nerves as they pass through the skull base to the brain.
(pl: nervi craniales)
|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, 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.
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.
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 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.
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.
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.
|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|
Brainstem nuclei with associated functions are often found in similar areas of the brainstem. These are also known as functional columns. 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.
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. In the brainstem there are six columns.
Four 'general' columns contain fibres that supply sensation or control muscles:
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.
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.
|cribiform plate||Olfactory nerve (I)|
|optic foramen||Optic nerve (II)|
|superior orbital fissure||Oculomotor (III)|
|round foramen||Trigeminal V2|
|oval foramen||Trigeminal V3|
|internal auditory canal||Facial (VII)|
|jugular foramen||Glossopharyngeal (IX)|
|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.
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.
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).
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.
|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.|
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.
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.
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.: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.
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.
If the eyes do not work together, the most likely cause is damage to a specific cranial nerve or nuclei.
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. 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.
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.
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.
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, testing the function of the olfactory nerve (I) first, and progressing sequentially for each nerve. 
A cranial nerve exam starts with observation of the patient, as some cranial nerve lesions may affect the symmetry of the eyes or face. The eyes are examined and the visual acuity is tested through reading a Snellen chart. The visual fields are tested, and specific movements of the eyes requested to test for nerve lesions or nystagmus. The sensation of the face is tested, and patients are asked to perform different movements with their face, such as puffing out of the cheeks. Hearing is checked, by voice and tuning forks. The patient's uvula is examined, and finally the patient is asked to shrug, turn their head, and tongue function assessed by various tongue movements. 
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.
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.
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.
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 are also present in other 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).
Ventral view of the brain of a sheep, showing cranial nerves which are very similar in structure and origin to ours.
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