Cranial nerves
Cranial nerves | |
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skull base with foramina through which many nerves exit the skull. | |
Details | |
Identifiers | |
Latin | nervus cranialis (pl: nervi craniales) |
MeSH | D003391 |
TA98 | A14.2.01.001 A14.2.00.038 |
TA2 | 6142, 6178 |
FMA | 5865 |
Anatomical terms of neuroanatomy] |
Cranial nerves |
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Cranial nerves are the
The cranial nerves emerge from the
The terminal nerves (0), olfactory nerves (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.[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).[4]
The cranial nerves are in contrast to spinal nerves, which emerge from segments of the spinal cord.[3]
Anatomy
Most typically, humans are considered to have twelve pairs of cranial nerves (I–XII), with the terminal nerve (0) more recently canonized.[2][5] The nerves are: the olfactory nerve (I), the optic nerve (II), oculomotor nerve (III), trochlear nerve (IV), trigeminal nerve (V), abducens nerve (VI), facial nerve (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
Cranial nerves are numbered based on their position from front to back (rostral-caudal) of their position on the brain,[8] 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.[8]
Cranial nerves have paths within and outside the
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.[2]
The midbrain 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).[10] 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.[10]
Because each nerve may have several functions, the
Exiting the brainstem
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 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.[12]
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.[12]
Ganglia
The cranial nerves give rise to a number of
The sensory ganglia of the cranial nerves, directly correspond to the
The ganglion of the sensory nerves, which are similar in structure to the dorsal root ganglion of the spinal cord, include:[13]
- The 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
Exiting the skull and extracranial course
Location | Nerve |
---|---|
cribriform plate | Terminal nerve (0) |
cribriform plate | Olfactory nerve (I) |
optic foramen |
Optic nerve (II) |
superior orbital fissure | Oculomotor (III) Trochlear (IV) Abducens (VI) Trigeminal V1 (ophthalmic) |
foramen rotundum | Trigeminal V2 (maxillary) |
foramen ovale | Trigeminal V3 (mandibular) |
stylomastoid foramen | Facial nerve (VII) |
internal auditory canal |
Vestibulocochlear (VIII) |
jugular foramen | Glossopharyngeal (IX) Vagus (X) 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.[13]
- 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 boneas 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, 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 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 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.
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:[15]
Contributions of neural crest cells and placodes to ganglia and cranial nerves
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) |
-Superior, general and special afferent
-Inferior: geniculate, general and special afferent -Sphenopalatine, visceral efferent -Submandibular, visceral efferent |
-Hindbrain NCCs (from r4 into 2nd PA), 1st epibranchial placode
-1st epibranchial placode (geniculate) -Hindbrain NCCs (2nd PA) -Hindbrain NCCs (2nd PA) |
CNVIII – Vestibulocochlear
(s) |
-Acoustic: cochlear, special afferent; and vestibular, special afferent | -Otic placode and hindbrain (from r4) NCCs |
CNIX – glossopharyngeal
(mix) |
-Superior, general and special afferent
-Inferior, petrosal, general and special afferent -Otic, visceral efferent |
-Hindbrain NCCs (from r6 into 3rd PA)
-2nd epibranchial placode (petrosal) -Hindbrain NCCs (from r6 into 3rd PA) |
CNX – vagus
(mix) Superior laryngeal branch; and recurrent laryngeal branch |
-Superior, general afferent
-Inferior: nodose, general and special afferent -Vagal: parasympathetic, visceral efferent |
-Hindbrain NCCs (from r7-r8 to 4th & 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
Terminal nerve (0)
The terminal nerve (0) may not have a role in humans,[3] although it has been implicated in hormonal responses to smell, sexual response and mate selection.[5]
Smell (I)
The olfactory nerve (I) conveys information giving rise to the sense of smell.[16]
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.[16][17]
Vision (II)
The optic nerve (II) transmits visual information.[2]
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), or may have difficulty seeing objects from their outer visual fields (bitemporal hemianopsia) if the optic chiasm is involved. Inflammation (optic neuritis) may impact the sharpness of vision or colour detection[16]
Eye movement (III, IV, VI)
The oculomotor nerve (III), trochlear nerve (IV) and abducens nerve (VI) coordinate eye movement. The oculomotor nerve (III) 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)[16]
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).[17]
Damage to the oculomotor nerve (III) can cause double vision and inability to coordinate the movements of both eyes (
Damage to the trochlear nerve (IV) can also cause double vision with the eye adducted and elevated.[18] 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.[17]
Damage to the abducens nerve (VI) can also result in double vision.[18] This is due to impairment in the lateral rectus muscle, supplied by the abducens nerve.[17]
Trigeminal nerve (V)
The
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.[19][16]
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.[16] 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.[16]
Damage to the facial nerve (VII) may cause
Hearing and balance (VIII)
The
When damaged, the vestibular nerve may give rise to the sensation of spinning and dizziness (
Oral sensation, taste, and salivation (IX)
The
Damage to the nerve may cause failure of the
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.[2]
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.[10]
Damage to this nerve may result in difficulties swallowing.[17]
Shoulder elevation and head-turning (XI)
The
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 position.[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.[16]
Tongue movement (XII)
The hypoglossal nerve (XII) supplies the intrinsic muscles of the tongue, controlling tongue movement.[16] The hypoglossal nerve (XII) is unique in that it is supplied by the motor cortices of both hemispheres of the brain.[17]
Damage to the nerve may lead to fasciculations or wasting (atrophy) 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.[17] 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.[17]
Clinical significance
Examination
Doctors,
A cranial nerve exam starts with observation of the patient, as some cranial nerve lesions may affect the symmetry of the eyes or face.
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.[16][17]
Damage
Compression
Nerves may be compressed because of increased
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 (
The cause of
Stroke
Occlusion of blood vessels that supply the nerves or their nuclei, an
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.[27]
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.[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.[25]
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.[25]
History
The Graeco-Roman anatomist Galen (AD 129–210) named seven pairs of cranial nerves.[7] 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.[7] 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.[7] For example, there is reason to consider both the olfactory (I) and optic (II) nerves to be brain tracts, rather than cranial nerves.[7]
Other animals
top; ventral bottom; lateral
The accessory nerve (XI) and hypoglossal nerve (XII) cannot be seen, as they are not always present in all vertebrates.
Cranial nerves are also present in other
-
The cranial nerves in the horse
-
Ventral view of a sheep's brain. The exits of the various cranial nerves are marked with red.
See also
References
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