The
trigeminal nerve (the
fifth cranial nerve, also called the
fifth nerve, or simply
CNV or
CN5) contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system. The motor division of the trigeminal nerve is derived from the
basal plateIn the developing nervous system, the basal plate is the region of the neural tube ventral to the sulcus limitans. It extends from the rostral mesencephalon to the end of the spinal cord and contains primarily motor neurons, whereas neurons found in the alar plate are primarily associated with...
of the
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic
ponsThe pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
, while the sensory division originates from the
cranial neural crestThe cranial neural crest is a form of neural crest.The cranial neural crest arises in the anterior and populates the face and the pharyngeal arches giving rise to bones, cartilage, nerves and connective tissue...
.
Function
The sensory function of the trigeminal nerve is to provide the tactile, proprioceptive, and
nociceptiveNociception is defined as "the neural processes of encoding and processing noxious stimuli." It is the afferent activity produced in the peripheral and central nervous system by stimuli that have the potential to damage tissue...
afference of the face and mouth. The motor function activates the
muscles of masticationDuring mastication, four muscles of mastication are responsible for adduction and lateral motion of the jaw. Other muscles, usually associated with the hyoid such as the sternohyomastoid, are responsible for opening the jaw.-Muscles:*The masseter...
, the
tensor tympaniThe tensor tympani, the larger of the two muscles of the tympanic cavity, is contained in the bony canal above the osseous portion of the auditory tube...
, tensor veli palatini,
mylohyoidThe mylohyoid muscle is a muscle running from the mandible to the hyoid bone, forming the floor of the oral cavity. It is named for its two attachments, with the prefix "mylo" coming from the Greek word for "molar". These muscles are mesodermal in origin...
, and anterior belly of the digastric.
Peripheral anatomy
The trigeminal nerve is the largest of the
cranial nervesCranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
. Its name is ("trigeminal" = tri- or three, and -geminus or twin, or thrice twinned) derives from the fact that each nerve, one on each side of the pons, has three major branches: the
ophthalmic nerveThe ophthalmic nerve is one of the three branches of the trigeminal nerve, the fifth cranial nerve. The ophthalmic nerve carries only sensory fibers.-Branches:*Nasociliary nerve**sensory root of ciliary ganglion**posterior ethmoidal nerve...
(V
1), the
maxillary nerve (V
2), and the
mandibular nerveThe mandibular nerve is the largest of the three branches of the trigeminal nerve.-Roots:It is made up of two roots:* a large sensory root proceeding from the inferior angle of the trigeminal ganglion....
(V
3). The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both sensory and motor functions.
The three branches converge on the
trigeminal ganglionThe trigeminal ganglion is a sensory ganglion of the trigeminal nerve that occupies a cavity in the dura mater, covering the trigeminal impression near the apex of the petrous part of the temporal bone.-Relations:It is somewhat crescentic in shape, with its convexity...
(also called the semilunar ganglion or gasserian ganglion), that is located within
Meckel's caveThe trigeminal cave is two layers of dura mater which encase the trigeminal ganglion near the apex of the petrous part of the temporal bone. It is bounded by the dura overlying four structures:...
, and contains the cell bodies of incoming sensory nerve fibers. The trigeminal ganglion is analogous to the
dorsal rootIn anatomy and neurology, the dorsal root is the afferentsensory root of a spinal nerve.At the distal end of the dorsal root is the dorsal root ganglion, which contains the neuron cell bodies of the nerve fibres conveyed by the root.If the dorsal root of a spinal nerve were severed it would lead...
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
ponsThe pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
. 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.
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 show considerable overlap). Injection of local anesthetics such as
lidocaineLidocaine , Xylocaine, or lignocaine is a common local anesthetic and antiarrhythmic drug. Lidocaine is used topically to relieve itching, burning and pain from skin inflammations, injected as a dental anesthetic or as a local anesthetic for minor surgery.- History :Lidocaine, the first amino...
results in the complete loss of sensation from well-defined areas of the face and mouth. For example, the teeth on one side of the jaw can be numbed by injecting the mandibular nerve. However, occasionally injury or disease processes may affect two or all three branches of the trigeminal nerve, and in these cases the involved branches may be termed:
- 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
It is also worth noting that nerves on the left side of the jaw outnumber slightly the number of nerves on the right side of the jaw.
Sensory branches of the trigeminal nerve
The ophthalmic, maxillary and mandibular branches leave the skull through three separate foramina: the
superior orbital fissure, the
foramen rotundumThe foramen rotundum is a circular hole in the sphenoid bone that connects the middle cranial fossa and the pterygopalatine fossa.-Structure:...
and the
foramen ovaleAt the base of the skull the foramen ovale is one of the larger of the several holes that transmit nerves through the skull. The foramen ovale is situated in the posterior part of the sphenoid bone, posterolateral to the foramen rotundum.-Contents:Several nerves, arteries and veins pass through...
. The
mnemonicA mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...
standing room only can be used to remember that V
1 passes through the
superior orbital fissure, V
2 through the foramen
rotundum, and V
3 through the foramen
ovale.
- 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 meninges is the system of membranes which envelopes the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.-Dura...
(the duraDura may refer to:* Dura , a Palestinian town in the southern West Bank located eleven kilometers southwest of Hebron in the Hebron Governorate* Dura language, a critically endangered language of Nepal...
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 sensation from the mouth. It does not carry taste sensation (chorda tympani is responsible for taste), but one of its branches, the lingual nerve
The lingual nerve is a branch of the mandibular nerve , itself a branch of the trigeminal nerve, which supplies sensory innervation to the tongue...
, carries multiple types of nerve fibers that do not originate in the mandibular nerve.
Motor branches of the trigeminal nerve
Motor branches of the trigeminal nerve are distributed in the
mandibular nerveThe mandibular nerve is the largest of the three branches of the trigeminal nerve.-Roots:It is made up of two roots:* a large sensory root proceeding from the inferior angle of the trigeminal ganglion....
. These fibers originate in the motor nucleus of the fifth nerve, which is located near the main trigeminal nucleus in the pons. Motor nerves are functionally quite different from sensory nerves, and their association in the peripheral branches of the mandibular nerve is more a matter of convenience than of necessity.
In classical anatomy, the trigeminal nerve is said to have general somatic afferent (sensory) components, as well as
special visceral efferentSpecial visceral efferent refers to efferent nerves which supply muscles which derived from the branchial arches.Some sources prefer the term "branchiomotor", or "branchial efferent"....
(motor) components. The motor branches of the trigeminal nerve control the movement of eight muscles, including the four
muscles of masticationDuring mastication, four muscles of mastication are responsible for adduction and lateral motion of the jaw. Other muscles, usually associated with the hyoid such as the sternohyomastoid, are responsible for opening the jaw.-Muscles:*The masseter...
.
- masseter
- temporalis
- medial pterygoid
The medial pterygoid , is a thick, quadrilateral muscle of mastication.The mandibular branch of the fifth cranial nerve, the trigeminal nerve, innervates the medial pterygoid muscle.-Origin and insertion:...
- lateral pterygoid
- tensor veli palatini
- mylohyoid
The mylohyoid muscle is a muscle running from the mandible to the hyoid bone, forming the floor of the oral cavity. It is named for its two attachments, with the prefix "mylo" coming from the Greek word for "molar". These muscles are mesodermal in origin...
- anterior belly of digastric
- tensor tympani
The tensor tympani, the larger of the two muscles of the tympanic cavity, is contained in the bony canal above the osseous portion of the auditory tube...
With the exception of tensor tympani, all of these muscles are involved in biting, chewing and swallowing. All have
bilateral cortical representation. A central lesion (e.g., a
strokeA stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
), no matter how large, is unlikely to produce any observable deficit. Injury to the peripheral nerve can cause paralysis of muscles on one side of the jaw. The jaw deviates to the paralyzed side when it opens. This direction of the mandible is due to the action of normal pterygoids on the opposite side.
Central anatomy
The fifth nerve is primarily a sensory nerve. The anatomy of sensation in the face and mouth is the subject of the remainder of this article. Background information on sensation is reviewed, followed by a summary of central sensory pathways. The central anatomy of the fifth nerve is then discussed in detail.
Sensation
There are two basic types of sensation:
touch/position and
pain/temperature. They are distinguished, roughly speaking, by the fact that touch/position input comes to attention immediately, whereas pain/temperature input reaches the level of consciousness only after a perceptible delay. Think of stepping on a pin. There is immediate awareness of stepping on something, but it takes a moment before it starts to hurt.
In general, touch/position information is carried by myelinated (fast-conducting) nerve fibers, whereas pain/temperature information is carried by unmyelinated (slow-conducting) nerve fibers. The primary sensory receptors for touch/position (Meissner’s corpuscles, Merkel's receptors, Pacinian corpuscles, Ruffini’s corpuscles, hair receptors, muscle spindle organs, Golgi tendon organs) are structurally more complex than the primitive receptors for pain/temperature, which are bare nerve endings.
The term "sensation", as used in this article, refers to the conscious perception of touch/position and pain/temperature information. It does not refer to the so-called "special senses" (smell, sight, taste, hearing and balance), which are 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 certain nonhuman vertebrates is processed by the equivalent of the fifth cranial nerve in these animals.
The term "touch", as used in this article, 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 grades of sandpaper (coarse, medium and fine). People that lack touch/position perception can still "feel" the surface of their bodies, and can therefore perceive "touch" in a crude, yes-or-no way, but they lack the rich perceptual detail that we normally experience.
The term "position", as used in this article, refers to conscious
proprioceptionProprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
. Proprioceptors (muscle spindle organs and Golgi tendon organs) provide information about joint position and muscle movement. Much of this information is processed at an unconscious level (mainly by the
cerebellumThe cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
and the
vestibularThe vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...
nuclei). However, some of this information is available at a conscious level.
The two types of sensation in humans, touch/position and pain/temperature, are processed by different pathways in the central nervous system. The distinction is hard-wired, and it is maintained all the way to the cerebral cortex. Within the cerebral cortex, sensations are further hard-wired to (associated with) other cortical areas.
Sensory pathways
Sensory pathways from the periphery to the cortex are summarized below. There are separate pathways for touch/position sensation and pain/temperature sensation. All sensory information is sent to specific nuclei in the
thalamusThe thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
. Thalamic nuclei, in turn, send information to specific areas in the
cerebral cortexThe cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
.
Each pathway consists of three bundles of nerve fibers, connected together in series:
It is noteworthy that the
secondary neurons in each pathway decussate (cross to the other side of the spinal cord or brainstem). The reason for this is because initially Spinal Cord forms segmentally. Later on, decussated fibres reach and connect these segments with the Higher Centres.
The main reason for Decussation is that optic chiasma occurs(Nasal fibres of the Optic Nerve cross so each cerebral hemisphere receives the contralateral vision) and to keep interneuronal connections short(responsible for processing of information) all sensory and motor pathways converge and diverge respectively to the contralateral hemisphere.(Courtesy H. Balram Krishna, excerpt from Cunningham's Manual of Practical Anatomy)
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 by input from other areas of the nervous system. For example, cells in the main trigeminal nucleus ("Main V" in the diagram) receive input (not shown) from the reticular formation and from the cerebral cortex. This information contributes to the final output of the cells in Main V to the thalamus.
Touch/position information from the body is carried to the thalamus by the
medial lemniscusThe medial lemniscus, also known as Reil's band or Reil's ribbon, is a pathway in the brainstem that carries sensory information from the gracile and cuneate nuclei to the thalamus.-Path:...
; touch/position information from the face is carried to the thalamus by the
trigeminal lemniscusThe Trigeminal lemniscus is a part of the brain that conveys tactile, pain, and temperature impulses from the skin of the face, the mucous membranes of the nasal and oral cavities, and the eye, as well as proprioceptive information from the facial and masticatory muscles...
. Pain/temperature information from the body is carried to the thalamus by the
spinothalamic tractThe spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch and crude touch...
; pain/temperature information from the face is carried to the thalamus by the
trigeminothalamic tract (also called the quintothalamic tract).
Pathways for touch/position sensation from the face and body merge together in the brainstem. A single touch/position sensory map of the entire body is projected onto the thalamus. Likewise, pathways for pain/temperature sensation from the face and body merge together in the brainstem. A single pain/temperature sensory map of the entire body is projected onto the thalamus.
From the thalamus, touch/position and pain/temperature information is projected onto various areas of the cerebral cortex. Exactly where, when, and how this information becomes conscious is entirely beyond our understanding at the present time. The explanation of consciousness is one of the great unsolved mysteries in science.
The details of the pathways connecting the lower body to the cerebral cortex are beyond the scope of this article. The details of the pathways connecting the face and mouth to the cerebral cortex are discussed below.
Trigeminal nucleus
It is not widely appreciated that all sensory information from the face (all touch/position information and all pain/temperature information) is sent to the trigeminal nucleus. In classical anatomy, most sensory information from the face is carried by the fifth nerve, but sensation from certain parts of the mouth, certain parts of the ear and certain parts of the meninges is carried by "general somatic afferent" fibers in cranial nerves VII (the
facial nerveThe facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...
), IX (the
glossopharyngeal nerveThe glossopharyngeal nerve is the ninth of twelve pairs of cranial nerves . It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve...
) and X (the
vagus nerveThe vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
).
Without exception, however, all sensory fibers from these nerves terminate in the trigeminal nucleus. On entering the brainstem, sensory fibers from V, VII, IX, and X are sorted out and sent to the trigeminal nucleus, which thus contains a complete sensory map of the face and mouth. The spinal counterparts of the trigeminal nucleus (cells in the dorsal horn and dorsal column nuclei of the spinal cord) contain a complete sensory map of the rest of the body.
The trigeminal nucleus extends throughout the entire brainstem, from the midbrain to the medulla, and continues into the cervical cord, where it merges with the dorsal horn cells of the spinal cord. The nucleus is divided anatomically into three parts, visible in microscopic sections of the brainstem. From caudal to rostral (i.e., going up from the medulla to the midbrain) they are the
spinal trigeminal nucleus, the
main trigeminal nucleus, and the
mesencephalic trigeminal nucleus.
The three parts of the trigeminal nucleus receive different types of sensory information. The spinal trigeminal nucleus receives pain/temperature fibers. The main trigeminal nucleus receives touch/position fibers. The mesencephalic nucleus receives proprioceptor and mechanoreceptor fibers from the jaws and teeth.
Spinal trigeminal nucleus
The
spinal trigeminal nucleusThe spinal trigeminal nucleus is a nucleus in the medulla that receives information about deep/crude touch, pain, and temperature from the ipsilateral face. The facial, glossopharyngeal, and vagus nerves also convey pain information from their areas to the spinal trigeminal nucleus.This nucleus...
represents pain/temperature sensation from the face. Pain/temperature fibers from peripheral nociceptors are carried in cranial nerves V, VII, IX, and X. On entering the brainstem, sensory fibers are grouped together and sent to the spinal trigeminal nucleus. This bundle of incoming fibers can be identified in cross sections of the pons and medulla as the
spinal tract of the trigeminal nucleus, which parallels the spinal trigeminal nucleus itself. The spinal tract of V is analogous to, and continuous with, Lissauer's tract in the spinal cord.
The spinal trigeminal nucleus contains a pain/temperature sensory map of the face and mouth. From the spinal trigeminal nucleus, secondary fibers cross the midline and ascend in the
trigeminal lemniscusThe Trigeminal lemniscus is a part of the brain that conveys tactile, pain, and temperature impulses from the skin of the face, the mucous membranes of the nasal and oral cavities, and the eye, as well as proprioceptive information from the facial and masticatory muscles...
to the contralateral thalamus. The trigeminal lemniscus runs parallel to the
medial lemniscusThe medial lemniscus, also known as Reil's band or Reil's ribbon, is a pathway in the brainstem that carries sensory information from the gracile and cuneate nuclei to the thalamus.-Path:...
, which carries pain/temperature information to the thalamus from the rest of the body. Pain/temperature fibers are sent to multiple thalamic nuclei. As discussed below, the central processing of pain/temperature information is markedly different from the central processing of touch/position information.
Somatotopic representation
Exactly how pain/temperature fibers from the face are distributed to the spinal trigeminal nucleus has been a subject of considerable controversy. The present understanding is that all pain/temperature information from all areas of the human body is represented (in the spinal cord and brainstem) in an ascending, caudal-to-rostral fashion. Information from the lower extremities is represented in the lumbar cord. Information from the upper extremities is represented in the thoracic cord. Information from the neck and the back of the head is represented in the cervical cord. Information from the face and mouth is represented in the spinal trigeminal nucleus.
Within the spinal trigeminal nucleus, information is represented in an onion skin fashion. The lowest levels of the nucleus (in the upper cervical cord and lower medulla) represent peripheral areas of the face (the scalp, ears and chin). Higher levels (in the upper medulla) represent more central areas (nose, cheeks, lips). The highest levels (in the pons) represent the mouth, teeth, and pharyngeal cavity.
The onion skin distribution is entirely different from the dermatome distribution of the peripheral branches of the fifth nerve. Lesions that destroy lower areas of the spinal trigeminal nucleus (but which spare higher areas) preserve pain/temperature sensation in the nose (V
1), upper lip (V
2) and mouth (V
3) while removing pain/temperature sensation from the forehead (V
1), cheeks (V
2) and chin (V
3). Analgesia in this distribution is "nonphysiologic" in the traditional sense, because it crosses over several dermatomes. Nevertheless, analgesia in exactly this distribution is found in humans after surgical sectioning of the spinal tract of the trigeminal nucleus.
The spinal trigeminal nucleus sends pain/temperature information to the
thalamusThe thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
. It also sends information to the
mesencephalonThe midbrain or mesencephalon is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal , and temperature regulation....
and the
reticular formationThe reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...
of the brainstem. The latter pathways are analogous to the spinomesencephalic and spinoreticular tracts of spinal cord, which send pain/temperature information from the rest of the body to the same areas. The mesencephalon modulates painful input before it reaches the level of consciousness. The reticular formation is responsible for the automatic (unconscious) orientation of the body to painful stimuli.
Main trigeminal nucleus
The main trigeminal nucleus represents touch/position sensation from the face. It is located in the pons, close to the entry site of the fifth nerve. Fibers carrying carry touch/position information from the face and mouth (via cranial nerves V, VII, IX, and X) are sent to the main trigeminal nucleus when they enter the brainstem.
The main trigeminal nucleus contains a touch/position sensory map of the face and mouth, just as the spinal trigeminal nucleus contains a complete pain/temperature map. The main nucleus is analogous to the dorsal column nuclei (the
gracileThe English word "gracile" means slender. It derives from the Latin adjective gracilis , or gracile which in either form means slender, and when transferred for example to discourse, takes the sense of "without ornament", "simple", or various similar connotations.In his famous "Glossary of Botanic...
and
cuneateCuneate means "wedge-shaped," it can apply to:*cuneate leaf, a leaf shape*the cuneate nucleus, a part of the brainstem*the cuneate fasciculus, a tract from the spinal cord into the brainstem...
nuclei) of the spinal cord, which contain a touch/position map of the rest of the body.
From the main trigeminal nucleus, secondary fibers cross the midline and ascend in the
trigeminal lemniscusThe Trigeminal lemniscus is a part of the brain that conveys tactile, pain, and temperature impulses from the skin of the face, the mucous membranes of the nasal and oral cavities, and the eye, as well as proprioceptive information from the facial and masticatory muscles...
to the contralateral
thalamusThe thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
. The trigeminal lemniscus runs parallel to the medial leminscus, which carries touch/position information from the rest of the body to the thalamus.
Some sensory information from the teeth and jaws is sent from the main trigeminal nucleus to the
ipsilateral thalamus, via the small
dorsal trigeminal tractThe dorsal trigeminal tract is a tract which receives signals from Meissner's corpuscles and Pacinian corpuscles.It mediates pressure and tactile discrimination sensation...
. Thus touch/position information from the teeth and jaws is represented
bilaterally in the thalamus (and hence in the cortex). The reason for this special processing is discussed below.
Mesencephalic trigeminal nucleus
The mesencephalic trigeminal nucleus is not really a "nucleus." Rather, it is a sensory ganglion (like the trigeminal ganglion) that happens to be embedded in the brainstem. The mesencephalic "nucleus" is the sole exception to the general rule that sensory information passes through peripheral sensory ganglia before entering the central nervous system.
Only certain types of sensory fibers have cell bodies in the mesencephalic nucleus:
proprioceptorProprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
fibers from the jaw and mechanoreceptor fibers from the teeth. Some of these incoming fibers go to the motor nucleus of V, thus entirely bypassing the pathways for conscious perception. The
jaw jerk reflexThe jaw jerk reflex or the masseter reflex is a stretch reflex used to test the status of a patient's trigeminal nerve . The mandible—or lower jaw—is tapped at a downward angle just below the lips at the chin while the mouth is held slightly open. In response, the masseter muscles will jerk the...
is an example. Tapping the jaw elicits a reflex closure of the jaw, in exactly the same way that tapping the knee elicits a reflex kick of the lower leg. Other incoming fibers from the teeth and jaws go to the main nucleus of V. As noted above, this information is projected
bilaterally to the thalamus. It is available for conscious perception.
Activities like biting, chewing and swallowing require symmetrical, simultaneous coordination of both sides of the body. They are essentially automatic activities, to which we pay little conscious attention. They involve a sensory component (feedback about touch/position) that is processed at a largely unconscious level.
The unusual anatomy of "mesencephalic V" has been found in all vertebrates, with the exception of lampreys and hagfishes. Lampreys and hagfishes are the only vertebrates without jaws. It is evident, therefore, that information about biting, chewing and swallowing is singled out for special processing in the vertebrate brainstem, specifically in the mesencephalic nucleus.
Lampreys and hagfishes have cells in their brainstems that can be identified as the evoutionary precursors of the mesencephalic nucleus. These "internal ganglion" cells were discovered in the latter part of the 19th century by a young medical student named
Sigmund FreudSigmund Freud , born Sigismund Schlomo Freud , was an Austrian neurologist who founded the discipline of psychoanalysis...
.
Pathways to the thalamus and the cortex
We have defined sensation as the conscious perception of touch/proprioception and pain/temperature information. With the sole exception of smell, all sensory input (touch/position, pain/temperature, sight, taste, hearing, and balance) is sent to the thalamus before being sent to the cortex.
The thalamus is anatomically subdivided into a number of separate nuclei. The thalamic nuclei involved in sensation, and their cortical projections, are discussed below.
Touch/position sensation
Touch/position information from the body is sent to the
ventral posterolateral nucleusThe ventral posterolateral nucleus is a nucleus of the thalamus. Together with the ventral posteromedial nucleus , VPI and VMpo, it constitutes the ventral posterior nucleus.-Input and output:...
(VPL) of the thalamus. Touch/position information from the face is sent to the
ventral posteromedial nucleus-Inputs and outputs:It conveys information of the trigeminothalamic tract, from the solitary tract and the trigeminal nerve and projects to the postcentral gyrus....
(VPM) of the thalamus. From the VPL and VPM, information is projected to the primary sensory cortex (SI) in the postcentral gyrus of the
parietal lobeThe parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...
.
The representation of sensory information in SI is organized somatotopically. Adjacent areas in the body are represented by adjacent areas in the cortex. When body parts are drawn in proportion to the density of their innervation, however, the result is a strangely distorted "little man," the sensory
homunculusA cortical homunculus is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex , i.e., the portion of the human brain directly responsible for the movement and exchange of sense and motor information of the rest of the body.There...
.
Many textbooks reproduce the classic
Penfield-People:*Daniel Penfield , founder of Penfield, New York*Edward Penfield , an American illustrator and poster artist*Thomas Penfield Jackson , former US District Court Judge*Wilder Penfield , Canadian neurosurgeon...
-Rasmussen diagram, which is now outdated. For example, the toes and genitals are shown in the classic diagram on the mesial surface of the cortex, when in fact they are represented on the convexity. What is more important, the classic diagram implies a single primary sensory map of the body, when in fact there are multiple primary maps. At least four separate, anatomically distinct sensory homunculi have been identified in SI. They represent different blends of input from surface receptors, deep receptors, rapidly adapting receptors, and slowly adapting peripheral receptors. For example, smooth objects will activate certain cells, whereas edged objects will activate other cells.
Information from all four maps in the primary sensory cortex (SI) is sent to the secondary sensory cortex (SII) in the parietal lobe. SII contains two more sensory homunculi.
In general, information from one side of the body is represented on the
opposite side in SI, but on
both sides in SII. Functional MRI imaging of a defined stimulus (e.g., stroking the skin with a toothbrush) "lights up" a
single focus in SI and
two foci in SII.
Pain/temperature sensation
PainPain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...
/temperature information is sent to the VPL (body) and VPM (face) of the thalamus (the same nuclei that receive touch/position information). From the thalamus, pain/temperature and touch/position information is projected onto SI.
In marked contrast to touch/position information, however, pain/temperature information is also sent to other thalamic nuclei, and is projected onto additional areas of the cerebral cortex. Some pain/temperature fibers are sent to the medial dorsal thalamic nucleus (MD), which projects to the anterior cingulate cortex. Other fibers are sent to the ventromedial (VM) nucleus of the thalamus, which projects to the insular cortex. Finally, some fibers are sent to the
intralaminarThe intralaminar nucleus is a nucleus of the thalamus that contains the following nuclei:* central lateral* centromedian * paracentral* parafascicular.Some sources also include a "central dorsal" nucleus....
(IL) nuclei of the thalamus via the
reticular formationThe reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...
. The IL project diffusely to all parts of the cerebral cortex.
The
insulaIn each hemisphere of the mammalian brain the insular cortex is a portion of the cerebral cortex folded deep within the lateral sulcus between the temporal lobe and the frontal lobe. The cortical area overlying it towards the lateral surface of the brain is the operculum...
and
cingulate cortexThe cingulate cortex is a part of the brain situated in the medial aspect of the cortex. It includes the cortex of the cingulate gyrus, which lies immediately above the corpus callosum, and the continuation of this in the cingulate sulcus...
are areas of the brain that represent our perception of touch/position and pain/temperature in the context of other simultaneous perceptions (sight, smell, taste, hearing and balance), and in the context of our memories and present emotional state. It is noteworthy that peripheral pain/temperature information is channeled directly into the brain at these deep levels, without prior processing. This contrasts markedly with the way that touch/position information is handled.
Diffuse thalamic projections from the IL and other thalamic nuclei are responsible for one’s overall level of consciousness. The thalamus and reticular formation "activate" the entire brain. It is noteworthy that peripheral pain/temperature information feeds directly into this system as well.
Summary
The complex processing of pain/temperature information in the thalamus and cerebral cortex (as opposed to the relatively simple, straightforward processing of touch/position information) reflects a phylogenetically older, more primitive sensory system. The rich, detailed information we receive from peripheral touch/position receptors is superimposed on a background of awareness, memory and emotions that is set, in part, by peripheral pain/temperature receptors.
The thresholds for touch/position perception are relatively easy to measure, and are similar in all humans. The thresholds for pain/temperature perception are difficult to define and even more difficult to measure. "Touch" is an objective sensation. "Pain" is a highly individualized, personal sensation that varies markedly among different people. It is conditioned by their memories and by their emotions. The fundamental anatomical differences between the pathways for touch/position perception and pain/temperature sensation help to explain why pain, especially chronic pain, is so difficult to manage.
Wallenberg syndrome
Wallenberg syndrome (also called the lateral medullary syndrome) is a classic clinical demonstration of the anatomy of the fifth nerve. It provides a useful summary of essential points about the processing of sensory information by the trigeminal nerve.
A
strokeA stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
usually affects only one side of the body. If a stroke causes loss of sensation, the deficit will be lateralized to the right side 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 most famous example. In Wallenberg syndrome, a stroke causes loss of pain/temperature sensation from
one side of the face and the
other side of the body.
The explanation involves 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 descending spinal tract of the fifth nerve (which carries pain/temperature information from the
same side of the face). A stroke that cuts off the blood supply to this area (e.g., a clot in the posterior inferior cerebellar artery) destroys
both tracts simultaneously. The result is loss of pain/temperature sensation (but
not touch/position sensation) in a unique "checkerboard" pattern (ipsilateral face, contralateral body) that is entirely diagnostic.
See also
- Trigeminal neuralgia
Trigeminal neuralgia , tic douloureux is a neuropathic disorder characterized by episodes of intense pain in the face, originating from the trigeminal nerve. It has been described as among the most painful conditions known...
- Trigeminovascular system
The trigeminovascular system consists of neurons in the trigeminal nerve that innervate cerebral blood vessels. It has been hypothesized that the trigeminovascular system may be involved in some types of headaches.- External links :* * *...
- Cluster headache
Cluster headache, nicknamed "suicide headache", is a neurological disease that involves, as its most prominent feature, an immense degree of pain in the head. Cluster headaches occur periodically: spontaneous remissions interrupt active periods of pain. The cause of the disease is currently unknown...
- List of mnemonics for the cranial nerves
External links