Trigeminal nerve

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^[3] slightly rostrally and medially to the sensory root.^[4]

Motor fibers pass through the trigeminal ganglion without synapsing on their way to peripheral muscles, their cell bodies being located in the nucleus of the fifth nerve, deep within the pons.

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.

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.

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.

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.

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.

Con excepción del músculo tensor del tímpano, todos estos músculos participan en la mordida, la masticación y la deglución, y todos poseen representación cortical bilateral . Es improbable que una lesión central unilateral (por ejemplo, un accidente cerebrovascular ), por muy extensa que sea, produzca un déficit observable. Una lesión en un nervio periférico puede causar parálisis de los músculos de un lado de la mandíbula, lo que provoca que esta se desvíe hacia el lado paralizado al abrirse. Esta desviación se debe a la acción de los músculos pterigoideos funcionales del lado opuesto.

Los dos tipos básicos de sensación son la de tacto-posición y la de dolor-temperatura. La información táctil-posicional llega a la atención de inmediato, pero la información de dolor-temperatura alcanza el nivel de conciencia después de un retraso; cuando una persona pisa un alfiler, la conciencia de haber pisado algo es inmediata, pero el dolor asociado se manifiesta con retraso.

La información de posición táctil generalmente se transmite mediante fibras nerviosas mielinizadas (de conducción rápida), y la información de dolor y temperatura mediante fibras no mielinizadas (de conducción lenta). Los receptores sensoriales primarios para la posición táctil ( corpúsculos de Meissner , receptores de Merkel , corpúsculos de Pacini , corpúsculos de Ruffini , receptores pilosos , órganos del huso muscular y órganos tendinosos de Golgi ) son estructuralmente más complejos que los de dolor y temperatura, que son terminaciones nerviosas.

En este contexto, la sensación se refiere a la percepción consciente de la información táctil, posicional y dolorosa, y no a los sentidos especiales (olfato, vista, gusto, oído y equilibrio) procesados ​​por distintos nervios craneales y transmitidos a la corteza cerebral a través de diferentes vías. La percepción de campos magnéticos, campos eléctricos, vibraciones de baja frecuencia y radiación infrarroja en algunos vertebrados no humanos es procesada por el equivalente del quinto nervio craneal.

En este contexto, el tacto se refiere a la percepción de información táctil detallada y localizada, como la discriminación de dos puntos (la diferencia entre tocar un punto y dos puntos muy próximos) o la diferencia entre papel de lija grueso, medio o fino. Las personas sin percepción de la posición táctil pueden sentir la superficie de su cuerpo y percibir el tacto en un sentido amplio, pero carecen de detalles perceptivos.

En este contexto, la posición se refiere a la propiocepción consciente . Los propioceptores (husos musculares y órganos tendinosos de Golgi) proporcionan información sobre la posición de las articulaciones y el movimiento muscular. Si bien gran parte de esta información se procesa a nivel inconsciente (principalmente por el cerebelo y los núcleos vestibulares ), una parte está disponible a nivel consciente.

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 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:

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.^[5]

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 cerebellar 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, 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, 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.

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.

El núcleo trigeminal espinal representa la sensación de dolor y temperatura en la cara. Las fibras nerviosas que transmiten estas sensaciones, provenientes de los nociceptores periféricos , se transportan a través de los nervios craneales V, VII, IX y X. Al entrar en el tronco encefálico, las fibras sensitivas se agrupan y se dirigen al núcleo trigeminal espinal. Este haz de fibras aferentes puede identificarse en cortes transversales del puente y el bulbo raquídeo como el tracto espinal del núcleo trigeminal, que discurre paralelo a dicho núcleo. El tracto espinal del nervio trigémino es análogo y continuo con el tracto de Lissauer en la médula espinal.

El núcleo trigeminal espinal contiene un mapa sensorial de dolor y temperatura de la cara y la boca. Desde este núcleo, fibras secundarias cruzan la línea media y ascienden por el tracto trigeminotalámico (quintotalámico) hasta el tálamo contralateral. Las fibras de dolor y temperatura se envían a múltiples núcleos talámicos. El procesamiento central de la información de dolor y temperatura difiere del procesamiento de la información de tacto y posición.

Representación somatotópica

Actualmente, se entiende que la información sobre dolor y temperatura proveniente de todas las áreas del cuerpo humano se representa en la médula espinal y el tronco encefálico de forma ascendente, desde la región caudal a la rostral . La información de las extremidades inferiores se representa en la médula lumbar, y la de las extremidades superiores en la médula torácica. La información del cuello y la parte posterior de la cabeza se representa en la médula cervical, y la de la cara y la boca en el núcleo trigeminal espinal.

Sin embargo, existen opiniones contradictorias sobre el patrón de terminación de las fibras de dolor-temperatura de la cara en el núcleo espinal. ^{[ 6 ]}

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.

Some sources state that, in clinical practice, the progression of anesthesia on the face commonly has a dermatomal rather than an onion-skin distribution.^[6] According to this view, the sensory fibers from the ophthalmic division terminate in the inferior part of the spinal nucleus; fibers from the maxillary division terminate in the middle part of the spinal nucleus; and fibers from the mandibular division end in the superior part of the spinal nucleus.^[6][7]

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 family stimulate receptors found in trigeminal ganglia, bypassing the olfactory system.^[8]

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 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, 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 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.

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 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.

Two types of sensory fibers have cell bodies in the mesencephalic nucleus: proprioceptor 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.

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 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 (VPM) of the thalamus. From the VPL and VPM, information is projected to the primary somatosensory cortex (SI) in the parietal lobe.

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 [ref?], with the toes and genitals on the mesial surface of the cortex when they are actually represented on the convexity.^[9] 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 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.

A diferencia de la información de posición táctil, la información de dolor y temperatura también se envía a otros núcleos talámicos y se proyecta a áreas adicionales de la corteza cerebral. Algunas fibras de dolor y temperatura se envían al núcleo talámico dorsomedial (MD), que proyecta a la corteza cingulada anterior . Otras fibras se envían al núcleo ventromedial (VM) del tálamo, que proyecta a la corteza insular . Finalmente, algunas fibras se envían al núcleo intralaminar (IL) del tálamo a través de la formación reticular . El IL proyecta difusamente a todas las partes de la corteza cerebral.

Las cortezas insular y cingulada son partes del cerebro que representan la posición táctil y la temperatura dolorosa en el contexto de otras percepciones simultáneas (vista, olfato, gusto, oído y equilibrio) dentro del contexto de la memoria y el estado emocional. La información periférica sobre la temperatura dolorosa se canaliza directamente al cerebro a un nivel profundo, sin procesamiento previo. La información sobre la posición táctil se procesa de manera diferente. Las proyecciones talámicas difusas del IL y otros núcleos talámicos son responsables de un determinado nivel de conciencia, con el tálamo y la formación reticular "activando" el cerebro; la información periférica sobre la temperatura dolorosa también se integra directamente en este sistema.

El síndrome medular lateral (síndrome de Wallenberg) es una demostración clínica de la anatomía del nervio trigémino, que resume cómo procesa la información sensorial. Un accidente cerebrovascular generalmente afecta solo un lado del cuerpo; la pérdida de sensibilidad debida a un accidente cerebrovascular se lateraliza hacia el lado derecho o izquierdo del cuerpo. Las únicas excepciones a esta regla son ciertas lesiones de la médula espinal y los síndromes medulares, de los cuales el síndrome de Wallenberg es el ejemplo más conocido. En este síndrome, un accidente cerebrovascular causa una pérdida de la sensibilidad al dolor y la temperatura en un lado de la cara y en el otro lado del cuerpo.

Esto se explica por la anatomía del tronco encefálico. En la médula oblongada, el tracto espinotalámico ascendente (que transmite información sobre dolor y temperatura del lado opuesto del cuerpo) es adyacente al tracto espinal ascendente del nervio trigémino (que transmite información sobre dolor y temperatura del mismo lado de la cara). Un accidente cerebrovascular que interrumpe el suministro de sangre a esta zona (por ejemplo, un coágulo en la arteria cerebelosa posteroinferior) destruye ambos tractos simultáneamente. El resultado es una pérdida de la sensibilidad al dolor y la temperatura (pero no a la posición táctil) con un patrón en forma de tablero de ajedrez (cara ipsilateral, cuerpo contralateral), lo que facilita el diagnóstico.

Sensory neuronopathy (also known as sensory ganglionopathy) is a type of peripheral neuropathy in which sensory nerve cell bodies in the dorsal root ganglia, commonly including the trigeminal ganglion of the trigeminal nerve, are damaged due to a variety of mechanisms leading to sensory symptoms such as parasthesias, dysesthesias, or hyperalgesia in the affected nerve distribution including the distribution of the trigeminal nerve.^[10]