The trigeminal nerve
The trigeminal nerve
The trigeminal nerve (the fifth
cranial nerve, or simply CN V) is a nerve responsible for sensation in the face
and motor functions such as biting and chewing; it is the largest of the
cranial nerves. Its name ("trigeminal" = tri-, or three, and -
geminus, or twin: thrice-twinned) derives from the fact that each of the two
nerves (one on each side of the pons) has three major branches: the ophthalmic
nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The
ophthalmic and maxillary nerves are purely sensory, whereas the mandibular
nerve supplies motor as well as sensory (or "cutaneous") functions.
The motor division of the trigeminal
nerve derives from the basal plate of the embryonic pons, and the sensory
division originates in the cranial neural crest. Sensory information from the
face and body is processed by parallel pathways in the central nervous system.
Structure
The three major branches of the
trigeminal nerve—the ophthalmic nerve (V1), the maxillary nerve (V2) and the
mandibular nerve (V3)—converge on the trigeminal ganglion (also called the
semilunar ganglion or gasserian ganglion), located within Meckel's cave and
containing the cell bodies of incoming sensory-nerve fibers. The trigeminal
ganglion is analogous to the dorsal root ganglia of the spinal cord, which
contain the cell bodies of incoming sensory fibers from the rest of the body.
From the trigeminal ganglion a single,
large sensory root enters the brainstem at the level of the pons. Immediately
adjacent to the sensory root, a smaller motor root emerges from the pons at the
same level. Motor fibers pass through the trigeminal ganglion on their way to
peripheral muscles, but their cell bodies are located in the nucleus of the
fifth nerve, deep within the pons.
Drawing of the head, with areas served
by specific nerves color-coded
Dermatome distribution of the
trigeminal nerve
The areas of cutaneous distribution
(dermatomes) of the three branches of the trigeminal nerve have sharp borders
with relatively little overlap (unlike dermatomes in the rest of the body,
which have considerable overlap). The injection of a local anesthetic, such as
lidocaine, results in the complete loss of sensation from well-defined areas of
the face and mouth. For example, teeth on one side of the jaw can be numbed by
injecting the mandibular nerve. Occasionally, injury or disease processes may
affect two (or all three) branches of the trigeminal nerve; in these cases, the
involved branches may be termed:
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| Dermatome distribution of the trigeminal nerve |
V1/V2 distribution – Referring to the
ophthalmic and maxillary branches
V2/V3 distribution – Referring to the
maxillary and mandibular branches
V1-V3 distribution – Referring to all
three branches
Nerves on the left side of the jaw
slightly outnumber the nerves on the right side of the jaw.
Sensory branches
Profile of the head, with the three
sub-nerves color-coded
Dermatome distribution of the
trigeminal nerve
The ophthalmic, maxillary and
mandibular branches leave the skull through three separate foramina: the
superior orbital fissure, the foramen rotundum and the foramen ovale,
respectively. The ophthalmic nerve (V1) carries sensory information from the
scalp and forehead, the upper eyelid, the conjunctiva and cornea of the eye,
the nose (including the tip of the nose, except alae nasi), the nasal mucosa,
the frontal sinuses and parts of the meninges (the dura and blood vessels). The
maxillary nerve (V2) carries sensory information from the lower eyelid and
cheek, the nares and upper lip, the upper teeth and gums, the nasal mucosa, the
palate and roof of the pharynx, the maxillary, ethmoid and sphenoid sinuses and
parts of the meninges. The mandibular nerve (V3) carries sensory information
from the lower lip, the lower teeth and gums, the chin and jaw (except the
angle of the jaw, which is supplied by C2-C3), parts of the external ear and
parts of the meninges. The mandibular nerve carries touch-position and pain-temperature
sensations from the mouth. Although it does not carry taste sensation (the
chorda tympani is responsible for taste), one of its branches—the lingual
nerve—carries sensation from the tongue.
Function
The sensory function of the trigeminal
nerve is to provide tactile, proprioceptive, and nociceptive afference to the
face and mouth. Its motor function activates the muscles of mastication, the
tensor tympani, tensor veli palatini, mylohyoid and the anterior belly of the
digastric.
The trigeminal nerve carries general
somatic afferent fibers (GSA), which innervate the skin of the face via
ophthalmic (V1), maxillary (V2) and mandibular (V3) divisions. The trigeminal
nerve also carries special visceral efferent (SVE) axons, which innervate the
muscles of mastication via the mandibular (V3) division.
Muscles of mastication
The motor component of the mandibular
division (V3) of the trigeminal nerve controls the movement of eight muscles,
including the four muscles of mastication: the masseter, the temporal muscle,
and the medial and lateral pterygoids. The other four muscles are the tensor
veli palatini, the mylohyoid, the anterior belly of the digastric and the
tensor tympani. A useful mnemonic for remembering these muscles is "My
Tensors Dig Ants 4 MoM" (Mylohyoid—Tensor Tympani + Tensor Veli
Palatini—Digastric (Anterior) – 4 Muscles of Mastication (Temporalis, Masseter,
Medial and Lateral Pterygoids))
With the exception of the tensor
tympani, all these muscles are involved in biting, chewing and swallowing and
all have bilateral cortical representation. A unilateral central lesion (for
example, a stroke), no matter how large, is unlikely to produce an observable
deficit. Injury to a peripheral nerve can cause paralysis of muscles on one
side of the jaw, with the jaw deviating towards the paralyzed side when it
opens. This direction of the mandible is due to the action of the functioning
pterygoids on the opposite side.
Sensation
Main article: Somatosensory system
The two basic types of sensation are
touch-position and pain-temperature. Touch-position input comes to attention
immediately, but pain-temperature input reaches the level of consciousness
after a delay; when a person steps on a pin, the awareness of stepping on
something is immediate but the pain associated with it is delayed.
Touch-position information is
generally carried by myelinated (fast-conducting) nerve fibers, and
pain-temperature information by unmyelinated (slow-conducting) fibers. The
primary sensory receptors for touch-position (Meissner’s corpuscles, Merkel's
receptors, Pacinian corpuscles, Ruffini’s corpuscles, hair receptors, muscle
spindle organs and Golgi tendon organs) are structurally more complex than
those for pain-temperature, which are nerve endings.
Sensation in this context refers to
the conscious perception of touch-position and pain-temperature information,
rather than the special senses (smell, sight, taste, hearing and balance)
processed by different cranial nerves and sent to the cerebral cortex through
different pathways. The perception of magnetic fields, electrical fields,
low-frequency vibrations and infrared radiation by some nonhuman vertebrates is
processed by their equivalent of the fifth cranial nerve.
Touch in this context refers to the
perception of detailed, localized tactile information, such as two-point
discrimination (the difference between touching one point and two closely
spaced points) or the difference between coarse, medium or fine sandpaper.
People without touch-position perception can feel the surface of their bodies
and perceive touch in a broad sense, but they lack perceptual detail.
Position, in this context, refers to
conscious proprioception. Proprioceptors (muscle spindle and Golgi tendon
organs) provide information about joint position and muscle movement. Although
much of this information is processed at an unconscious level (primarily by the
cerebellum and the vestibular nuclei), some is available at a conscious level.
Touch-position and pain-temperature
sensations are processed by different pathways in the central nervous system.
This hard-wired distinction is maintained up to the cerebral cortex. Within the
cerebral cortex, sensations are linked with other cortical areas.
Sensory pathways
Sensory pathways from the periphery to
the cortex are separate for touch-position and pain-temperature sensations. All
sensory information is sent to specific nuclei in the thalamus. Thalamic
nuclei, in turn, send information to specific areas in the cerebral cortex.
Each pathway consists of three bundles of nerve fibers connected in series:
Flow chart from sensory receptors to
the cerebral cortex
The secondary neurons in each pathway
decussate (cross the spinal cord or brainstem), because the spinal cord develops
in segments. Decussated fibers later reach and connect these segments with the
higher centers. The optic chiasm is the primary cause of decussation; nasal
fibers of the optic nerve cross (so each cerebral hemisphere receives
contralateral—opposite—vision) to keep the interneuronal connections
responsible for processing information short. All sensory and motor pathways
converge and diverge to the contralateral hemisphere.
Although sensory pathways are often
depicted as chains of individual neurons connected in series, this is an
oversimplification. Sensory information is processed and modified at each level
in the chain by interneurons and input from other areas of the nervous system.
For example, cells in the main trigeminal nucleus (Main V in the diagram below)
receive input from the reticular formation and cerebral cortex. This
information contributes to the final output of the cells in Main V to the
thalamus.
Clinical significance
Trigeminal neuralgia
rigeminal neuralgia (TN or TGN) is a
chronic pain disorder that affects the trigeminal nerve. There are two main
types: typical and atypical trigeminal neuralgia. The typical form results
in episodes of severe, sudden, shock-like pain in one side of the face that
lasts for seconds to a few minutes. Groups of these episodes can occur over
a few hours. The atypical form results in a constant burning pain that is
less severe. Episodes may be triggered by any touch to the face.Both
forms may occur in the same person. It is one of the most painful
conditions, and can result in depression.
The exact cause is unclear, but
believed to involve loss of the myelin around the trigeminal nerve. This
may occur due to compression from a blood vessel as the nerve exits the brain
stem, multiple sclerosis, stroke, or trauma. Less common causes include a
tumor or arteriovenous malformation.It is a type of nerve pain. Diagnosis is typically based on the symptoms, after ruling out other possible
causes such as postherpetic neuralgia.
Treatment includes medication or
surgery. The anticonvulsant carbamazepine or oxcarbazepine is usually the
initial treatment, and is effective in about 80% of people. Other options
include lamotrigine, baclofen, gabapentin, and pimozide. Amitriptyline may
help with the pain, but opioids are not usually effective in the typical
form. In those who do not improve or become resistant to other measures, a
number of types of surgery may be tried.
It is estimated that 1 in 8,000 people
per year develop trigeminal neuralgia. It usually begins in people over 50
years old, but can occur at any age. Women are more commonly affected than
men. The condition was first described in detail in 1773 by John Fothergill.
Cluster headache
Wallenberg syndrome
Wallenberg syndrome (lateral medullary
syndrome) is a clinical demonstration of the anatomy of the trigeminal nerve,
summarizing how it processes sensory information. A stroke usually affects only
one side of the body; loss of sensation due to a stroke will be lateralized to
the right or the left side of the body. The only exceptions to this rule are
certain spinal-cord lesions and the medullary syndromes, of which Wallenberg
syndrome is the best-known example. In this syndrome, a stroke causes a loss of
pain-temperature sensation from one side of the face and the other side of the
body.
This is explained by the anatomy of
the brainstem. In the medulla, the ascending spinothalamic tract (which carries
pain-temperature information from the opposite side of the body) is adjacent to
the ascending spinal tract of the trigeminal nerve (which carries
pain-temperature information from the same side of the face). A stroke which
cuts off the blood supply to this area (for example, a clot in the posterior
inferior cerebellar artery) destroys both tracts simultaneously. The result is
a loss of pain-temperature (but not touch-position) sensation in a
"checkerboard" pattern (ipsilateral face, contralateral body),
facilitating diagnosis.
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| Distribution schemes of the trigeminal nerve |
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