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neuroanatomy Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...
, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the
face The face is the front of an animal's head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may aff ...
and motor functions such as biting and
chewing Chewing or mastication is the process by which food is crushed and ground by teeth. It is the first step of digestion, and it increases the surface area of foods to allow a more efficient break down by enzymes. During the mastication process, th ...
; it is the most complex of the
cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
. Its name ("trigeminal", ) derives from each of the two nerves (one on each side of the
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
) having three major branches: the
ophthalmic nerve The ophthalmic nerve (V1) is a sensory nerve of the face. It is one of three divisions of the trigeminal nerve (CN V). It has three branches that provide sensory innervation to the eye, the skin of the upper face, and the skin of the anterior sc ...
(V), the maxillary nerve (V), and the mandibular nerve (V). The ophthalmic and maxillary nerves are purely sensory, whereas the mandibular nerve supplies motor as well as sensory (or "
cutaneous Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different d ...
") functions. Adding to the complexity of this nerve is that autonomic nerve fibers as well as special
sensory fibers A sensory nerve, or afferent nerve, is a general anatomic term for a nerve which contains predominantly somatic afferent nerve fibers. Afferent nerve fibers in a sensory nerve carry sensory information toward the central nervous system (CNS) from ...
( taste) are contained within it. The motor division of the trigeminal nerve derives from the basal plate of the embryonic
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
, 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 The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
.


Structure

The three major branches of the trigeminal nerve—the
ophthalmic nerve The ophthalmic nerve (V1) is a sensory nerve of the face. It is one of three divisions of the trigeminal nerve (CN V). It has three branches that provide sensory innervation to the eye, the skin of the upper face, and the skin of the anterior sc ...
(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 The dorsal root of spinal nerve (or posterior root of spinal nerve or sensory root) is one of two "roots" which emerge from the spinal cord. It emerges directly from the spinal cord, and travels to the dorsal root ganglion. Nerve fibres with the ve ...
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 (portio major) enters the brainstem at the level of the
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
. Immediately adjacent to the sensory root, a smaller motor root (portio minor) emerges from the pons at the same level. Motor fibers pass through the trigeminal ganglion without synapsing 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 sensory 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: * 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

The ophthalmic, maxillary and mandibular branches leave the skull through three separate
foramina In anatomy and osteology, a foramen (;Entry "foramen"
in
: 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 The chorda tympani is a branch of the facial nerve that originates from the taste buds in the front of the tongue, runs through the middle ear, and carries taste messages to the brain. It joins the facial nerve (cranial nerve VII) inside the fa ...
is responsible for taste), one of its branches—the
lingual nerve The lingual nerve carries sensory innervation from the anterior two-thirds of the tongue. It contains fibres from both the mandibular division of the trigeminal nerve (CN V3 ) and from the facial nerve (CN VII). The fibres from the trigeminal nerv ...
—carries sensation from the tongue. The peripheral processes of mesencephalic nucleus of V neurons run in the motor root of the trigeminal nerve and terminate in the muscle spindles in the muscles of mastication. They are proprioceptive fibers, conveying information regarding the location of the masticatory muscles. The central processes of mesencephalic V neurons synapse in the motor nucleus V.


Function

The sensory function of the trigeminal nerve is to provide tactile,
proprioceptive Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...
, and
nociceptive Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, c ...
afference to the face and mouth. Its motor function activates the muscles of mastication, the
tensor tympani The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or th ...
,
tensor veli palatini The tensor veli palatini muscle (tensor palati or tensor muscle of the velum palatinum) is a broad, thin, ribbon-like muscle in the head that tenses the soft palate. Structure The tensor veli palatini is found anterior-lateral to the levator ve ...
, mylohyoid and the anterior belly of the digastric. The trigeminal nerve carries
general somatic afferent fibers The general somatic afferent fibers (GSA, or somatic sensory fibers) afferent fibers arise from neurons in sensory ganglia and are found in all the spinal nerves, except occasionally the first cervical, and conduct impulses of pain, touch and tempe ...
(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 Special visceral efferent fibers (SVE) are the efferent nerve fibers that provide motor innervation to the muscles of the pharyngeal arches in humans, and the branchial arches in fish. Some sources prefer the term "branchiomotor" or "branchial eff ...
(SVE)
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...
s, which innervate the muscles of mastication via the mandibular (V3) division.


Muscles

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 In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...
, the temporal muscle, and the medial and lateral pterygoids. The other four muscles are the
tensor veli palatini The tensor veli palatini muscle (tensor palati or tensor muscle of the velum palatinum) is a broad, thin, ribbon-like muscle in the head that tenses the soft palate. Structure The tensor veli palatini is found anterior-lateral to the levator ve ...
, the mylohyoid, the anterior belly of the
digastric The digastric muscle (also digastricus) (named ''digastric'' as it has two 'bellies') is a small muscle located under the jaw. The term "digastric muscle" refers to this specific muscle. However, other muscles that have two separate muscle belli ...
and the
tensor tympani The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or th ...
. 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

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 The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a musc ...
) 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 In medicine and anatomy, the special senses are the senses that have specialized organs devoted to them: * vision (the eye) * hearing and balance (the ear, which includes the auditory system and vestibular system) * smell (the nose) * ta ...
(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 Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...
. 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 The Vestibular (from pt, vestíbulo, "entrance hall") is a competitive examination and is the primary and widespread entrance system used by Brazilian universities to select the students admitted. The Vestibular usually takes place from Nove ...
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 The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
. Each pathway consists of three bundles of nerve fibers connected in series: 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 In neuroanatomy, the optic chiasm, or optic chiasma (; , ), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrat ...
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. Touch-position information from the body is carried to the thalamus by the
medial lemniscus In neuroanatomy, the medial lemniscus, also known as Reil's band or Reil's ribbon (for German anatomist Johann Christian Reil), is a large ascending bundle of heavily myelinated axons that decussate (cross) in the brainstem, specifically in the ...
, and from the face by the trigeminal lemniscus (both the anterior and posterior trigeminothalamic tracts). Pain-temperature information from the body is carried to the thalamus by the
spinothalamic tract The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory co ...
, and from the face by the anterior division of the trigeminal lemniscus (also called the anterior trigeminothalamic tract). Pathways for touch-position and pain-temperature sensations from the face and body merge in the brainstem, and touch-position and pain-temperature sensory maps of the entire body are projected onto the thalamus. From the thalamus, touch-position and pain-temperature information is projected onto the cerebral cortex.


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 detailed information received from peripheral touch-position receptors is superimposed on a background of awareness, memory and emotions partially set by peripheral pain-temperature receptors. Although thresholds for touch-position perception are relatively easy to measure, those for pain-temperature perception are difficult to define and measure. "Touch" is an objective sensation, but "pain" is an individualized sensation which varies among different people and is conditioned by memory and emotion. Anatomical differences between the pathways for touch-position perception and pain-temperature sensation help explain why pain, especially chronic pain, is difficult to manage.


Trigeminal nuclei

All sensory information from the face, both touch-position and pain-temperature, is sent to the trigeminal nucleus. In classical anatomy most sensory information from the face is carried by the fifth nerve, but sensation from parts of the mouth, parts of the ear and parts of the meninges is carried by
general somatic afferent fibers The general somatic afferent fibers (GSA, or somatic sensory fibers) afferent fibers arise from neurons in sensory ganglia and are found in all the spinal nerves, except occasionally the first cervical, and conduct impulses of pain, touch and tempe ...
in cranial nerves VII (the
facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste ...
), IX (the glossopharyngeal nerve) and X (the vagus nerve). 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 and sent to the trigeminal nucleus (which contains a sensory map of the face and mouth). The spinal counterparts of the trigeminal nucleus (cells in the dorsal horn and
dorsal column Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...
nuclei of the spinal cord) contain a sensory map of the rest of the body. The trigeminal nucleus extends throughout the brainstem, from the midbrain to the medulla, continuing into the cervical cord (where it merges with the dorsal horn cells of the spinal cord). The nucleus is divided into three parts, visible in microscopic sections of the brainstem. From caudal to rostral (ascending from the medulla to the midbrain), they are the spinal trigeminal, the principal sensory and the mesencephalic nuclei. The parts of the trigeminal nucleus receive different types of sensory information; the spinal trigeminal nucleus receives pain-temperature fibers, the principal sensory nucleus receives touch-position fibers and the mesencephalic nucleus receives proprioceptor and mechanoreceptor fibers from the jaws and teeth.


Spinal trigeminal nucleus

The spinal trigeminal 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 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. The spinal tract of V is analogous to, and continuous with,
Lissauer's tract The posterolateral tract (fasciculus of Lissauer, Lissauer's tract, tract of Lissauer, dorsolateral fasciculus, dorsolateral tract, zone of Lissauer) is a small strand situated in relation to the tip of the posterior column close to the entrance o ...
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 trigeminothalamic (quintothalamic) tract to the contralateral thalamus. Pain-temperature fibers are sent to multiple thalamic nuclei. The central processing of pain-temperature information differs from the processing of touch-position information.


Somatotopic representation

Exactly how pain-temperature fibers from the face are distributed to the spinal trigeminal nucleus is disputed. The present general understanding is that 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, and that from the upper extremities in the thoracic cord. Information from the neck and the back of the head is represented in the cervical cord, and that from the face and mouth in the spinal trigeminal nucleus. Within the spinal trigeminal nucleus, information is represented in a layered, or "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 central areas (nose, cheeks and lips). The highest levels (in the pons) represent the mouth, teeth and pharyngeal cavity. The onion skin distribution differs from the dermatome distribution of the peripheral branches of the fifth nerve. Lesions which destroy lower areas of the spinal trigeminal nucleus (but spare higher areas) preserve pain-temperature sensation in the nose (V1), upper lip (V2) and mouth (V3) and remove pain-temperature sensation from the forehead (V1), cheeks (V2) and chin (V3). Although analgesia in this distribution is "nonphysiologic" in the traditional sense (because it crosses several dermatomes), this analgesia 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 thalamus and sends information to the mesencephalon and the reticular formation of the brainstem. The latter pathways are analogous to the spinomesencephalic and spinoreticular tracts of the 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. Incidentally, Sulfur-containing compounds found in plants in the
onion An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onio ...
family stimulate receptors found in trigeminal ganglia, bypassing the
olfactory system The olfactory system, or sense of smell, is the sensory system used for smelling ( olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an ...
.


Principal nucleus

The principal nucleus represents touch-pressure sensation from the face. It is located in the pons, near the entrance for the fifth nerve. Fibers carrying touch-position information from the face and mouth via cranial nerves V, VII, IX, and X are sent to this nucleus when they enter the brainstem. The principal nucleus contains a touch-position sensory map of the face and mouth, just as the spinal trigeminal nucleus contains a complete pain-temperature map. This nucleus is analogous to the
dorsal column Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...
nuclei (the gracile and cuneate nuclei) of the spinal cord, which contain a touch-position map of the rest of the body. From the principal nucleus, secondary fibers cross the midline and ascend in the ventral trigeminothalamic tract to the contralateral thalamus. The ventral trigeminothalamic tract runs parallel to the
medial lemniscus In neuroanatomy, the medial lemniscus, also known as Reil's band or Reil's ribbon (for German anatomist Johann Christian Reil), is a large ascending bundle of heavily myelinated axons that decussate (cross) in the brainstem, specifically in the ...
, 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 principal nucleus to the
ipsilateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
thalamus via the small dorsal trigeminal tract. Touch-position information from the teeth and jaws of one side of the face is represented bilaterally in the thalamus and cortex.


Mesencephalic nucleus

The mesencephalic nucleus is not a true nucleus; it is a sensory ganglion (like the trigeminal ganglion) embedded in the brainstem and the sole exception to the rule that sensory information passes through peripheral sensory ganglia before entering the central nervous system. It has been found in all vertebrates except
lampreys Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like su ...
and hagfishes. They are the only vertebrates without jaws and have specific cells in their brainstems. These "internal ganglion" cells were discovered in the late 19th century by medical student
Sigmund Freud Sigmund Freud ( , ; born Sigismund Schlomo Freud; 6 May 1856 – 23 September 1939) was an Austrian neurologist and the founder of psychoanalysis, a clinical method for evaluating and treating pathologies explained as originating in conflicts ...
. Two types of sensory fibers have cell bodies in the mesencephalic nucleus:
proprioceptor Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...
fibers from the jaw and mechanoreceptor fibers from the teeth. Some of these incoming fibers go to the motor nucleus of the trigeminal nerve (V), bypassing the pathways for conscious perception. The jaw jerk reflex is an example; tapping the jaw elicits a reflex closure of the jaw in 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. This information is projected bilaterally to the thalamus and available for conscious perception. Activities such as biting, chewing and swallowing require symmetrical, simultaneous coordination of both sides of the body. They are automatic activities, requiring little conscious attention and involving a sensory component (feedback about touch-position) processed at the unconscious level in the mesencephalic nucleus.


Pathways to the thalamus and cortex

Sensation has been defined as the conscious perception of touch-position and pain-temperature information. With the exception of smell, all sensory input (touch-position, pain-temperature, sight, taste, hearing and balance) is sent to the thalamus and then the cortex. The thalamus is anatomically subdivided into nuclei.


Touch-position sensation

Touch-position information from the body is sent to the ventral posterolateral nucleus (VPL) of the thalamus. Touch-position information from the face is sent to the
ventral posteromedial nucleus The ventral posteromedial nucleus (VPM) is a nucleus of the thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve f ...
(VPM) of the thalamus. From the VPL and VPM, information is projected to the primary somatosensory cortex (SI) in the
parietal lobe The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus. The parietal lobe integrates sensory informa ...
. The representation of sensory information in the postcentral gyrus is organized somatotopically. Adjacent areas of the body are represented by adjacent areas in the cortex. When body parts are drawn in proportion to the density of their innervation, the result is a "little man": the cortical homunculus. Many textbooks have reproduced the outdated Penfield-Rasmussen diagram ef? with the toes and genitals on the
mesial This is a list of definitions of commonly used terms of location and direction in dentistry. This set of terms provides orientation within the oral cavity, much as anatomical terms of location provide orientation throughout the body. Terms ...
surface of the cortex when they are actually represented on the convexity. The classic diagram implies a single primary sensory map of the body, when there are multiple primary maps. At least four separate, anatomically distinct sensory homunculi have been identified in the postcentral gyrus. They represent combinations of input from surface and deep receptors and rapidly and slowly adapting peripheral receptors; smooth objects will activate certain cells, and rough objects will activate other cells. Information from all four maps in SI is sent to the secondary sensory cortex (SII) in the parietal lobe. SII contains two more sensory homunculi. Information from one side of the body is generally represented on the opposite side in SI, but on both sides in SII. Functional MRI imaging of a defined stimulus (for example, stroking the skin with a toothbrush) "lights up" a single focus in SI and two foci in SII.


Pain-temperature sensation

Pain-temperature information is sent to the VPL (body) and VPM (face) of the thalamus (the same nuclei which receive touch-position information). From the thalamus, pain-temperature and touch-position information is projected onto SI. Unlike touch-position information, however, pain-temperature information is also sent to other thalamic nuclei and projected onto additional areas of the cerebral cortex. Some pain-temperature fibers are sent to the
medial dorsal thalamic nucleus The medial dorsal nucleus (or dorsomedial nucleus of thalamus) is a large nucleus in the thalamus. It is believed to play a role in memory. Structure It relays inputs from the amygdala and olfactory cortex and projects to the prefrontal corte ...
(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 The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) within each hemisphere of the mammalian b ...
. Finally, some fibers are sent to the intralaminar nucleus (IL) of the thalamus via the reticular formation. The IL projects diffusely to all parts of the cerebral cortex. The insular and cingulate cortices are parts of the brain which represent touch-position and pain-temperature in the context of other simultaneous perceptions (sight, smell, taste, hearing and balance) in the context of memory and emotional state. Peripheral pain-temperature information is channeled directly to the brain at a deep level, without prior processing. Touch-position information is handled differently. Diffuse thalamic projections from the IL and other thalamic nuclei are responsible for a given level of consciousness, with the thalamus and reticular formation "activating" the brain; peripheral pain-temperature information also feeds directly into this system.


Clinical significance

*
Trigeminal neuralgia Trigeminal neuralgia (TN or TGN), also called Fothergill disease, tic douloureux, or trifacial neuralgia is a long-term pain disorder that affects the trigeminal nerve, the nerve responsible for sensation in the face and motor functions such as ...
*
Cluster headache Cluster headache (CH) is a neurological disorder characterized by recurrent severe headaches on one side of the head, typically around the eye(s). There is often accompanying eye watering, nasal congestion, or swelling around the eye on the af ...
* Migraine


Lateral medullary syndrome

Lateral medullary syndrome Lateral medullary syndrome is a neurological disorder causing a range of symptoms due to ischemia in the lateral part of the medulla oblongata in the brainstem. The ischemia is a result of a blockage most commonly in the vertebral artery or the p ...
(Wallenberg 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 The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory co ...
(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.


Additional images


See also

* List of mnemonics#Anatomy * Trigeminovascular system


References


Sources

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External links


Pigeons Detect Magnetic Fields
An experiment indicating that the trigeminal nerve in ''Columba livia'' may be the mechanism through which "homing pigeons" detect magnetic fields * ()
Trigeminal nerve anatomy, part 1
an
part 2
on
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Trigeminal neuralgia
{{Authority control Cranial nerves Innervation of the face Medical mnemonics