The trigeminal nerve

The trigeminal nerve (n. trigiinus), being a mixed nerve, innervates the skin of the face, the mucous membrane of the nose and its sinuses, the oral cavity, the anterior 1/3 of the tongue, the teeth, the conjunctiva of the eye, the masticatory muscles, the muscles of the floor of the mouth (mylohyoid, geniohyoid, anterior belly of the digastric muscle), the muscle that tenses the tympanic membrane, and the muscle that tenses the soft palate. The trigeminal nerve has a motor nucleus and three sensory nuclei (midbrain, pontine, and spinal). The trigeminal nerve leaves the brain via two roots - motor and sensory. The sensory root is significantly thicker (5-6 mm) than the motor (1 mm). Both roots exit the brain in the area of the transition of the pons to the middle cerebellar peduncle. The sensory rootlet (radix sensoria) is formed by the central processes of pseudounipolar cells, the bodies of which are located in the trigeminal ganglion. The trigeminal ganglion (ganglion trigeminale; semilunar, Gasserian ganglion) is located in the trigeminal depression on the anterior surface of the pyramid of the temporal bone, in the cleft of the dura mater of the brain (in the trigeminal cavity). The ganglion has a semilunar shape, its length is 1.4-1.8 cm, the width of the ganglion is 3 times less than the length. The sensory rootlet goes to the sensory nuclei of this nerve. The axons of the neurons of the sensory nuclei of the trigeminal nerve, located in the brainstem, cross to the other side (form a decussation) and go to the nerve cells of the thalamus. The peripheral processes of the neurons go as part of the trigeminal nerve and end in receptors in the skin and mucous membranes of the head. The motor root (radix motoria) of the trigeminal nerve is adjacent to the trigeminal ganglion from below (does not enter it) and participates in the formation of the third branch of the trigeminal nerve.

Three large branches extend from the trigeminal nerve:

1. optic nerve;
2. maxillary nerve;
3. mandibular nerve.

The ophthalmic and maxillary nerves contain only sensory fibers, the mandibular nerve contains sensory and motor fibers.

The ophthalmic nerve (n. ophtalmicus) is the first branch of the trigeminal nerve, passing through the thickness of the lateral wall of the cavernous sinus. Together with the oculomotor, trochlear and abducens nerves, it goes to the superior orbital fissure. Before entering the orbit at the level of the sella turcica, the ophthalmic nerve receives connecting branches from the periarterial sympathetic plexus of the internal carotid artery. Here, the ophthalmic nerve gives off a tentorial (meningeal) branch (r. tentorii [meningeus]). This branch goes back and branches in the tentorium cerebelli, in the walls of the straight and transverse sinuses of the dura mater of the brain. At the entrance to the superior orbital fissure, the ophthalmic nerve is located medial to the trochlear nerve, superior and lateral to the oculomotor and lateral to the abducens nerve. Entering the eye socket, the optic nerve divides into the frontal, nasociliary and lacrimal nerves.

The frontal nerve (n. frontalis) is the longest branch of the ophthalmic nerve, it passes under the upper wall of the orbit. On the upper surface of the muscle that raises the eyelid, the frontal nerve divides into the supraorbital and suprapubic nerves. The supraorbital nerve (n. supraorbitalis) exits the orbit through the supraorbital notch and ends in the skin of the forehead. The supratrochlear nerve (n. supratrochlearis) rises above the trochlea of the superior oblique muscle and branches in the skin of the nose, the lower part of the forehead and in the area of the medial angle of the eye, in the skin and conjunctiva of the upper eyelid.

The nasociliary nerve (n. nasociliaris) passes in the eye socket above the optic nerve, between it and the superior rectus muscle of the eye, and then between the oblique and medial rectus muscles of the eye. Here the nasociliary nerve divides into its terminal branches, which go to the conjunctiva of the eye, the skin of the upper eyelid and the mucous membrane of the nasal cavity. Along its course, the nasociliary nerve gives off a number of branches:

1. communicating branch (with the ciliary ganglion) [r. commiinicans (cum gangliociliari)] - a long rootlet to the ciliary ganglion. This rootlet departs from the initial part of the nasociliary nerve, crosses the optic nerve obliquely and from above, and goes to the ciliary ganglion;
2. long ciliary nerves (nn. ciliares longi) in the form of 2-3 branches pass along the upper surface of the nerve to the back of the eyeball;
3. the posterior ethmoidal nerve (n. ethmoidalis posterior) penetrates through the opening of the same name in the medial wall of the orbit into the thickness of the mucous membrane of the posterior cells of the ethmoid bone and the sphenoid sinus;
4. the anterior ethmoid nerve (n. ethmoidalis anterior) penetrates the cranial cavity through the opening of the same name in the medial wall of the orbit, gives off a branch to the dura mater of the brain (in the region of the anterior cranial fossa). Passing forward along the upper surface of the perforated plate, the nerve penetrates through one of its anterior openings into the nasal cavity and branches out in the mucous membrane of the nose, the frontal sinus and in the skin of the tip of the nose;
5. The infratrochlear nerve (n. infratrochlearis) runs along the medial wall of the orbit under the superior oblique muscle of the eye to the lacrimal sac, lacrimal caruncle, skin of the upper eyelid and to the bridge of the nose.

The lacrimal nerve (n. lacrimalis) initially passes between the lateral and superior rectus muscles of the eye, then is located near the superolateral angle of the orbit. It gives off branches to the lacrimal gland, the conjunctiva of the upper eyelid and the skin in the area of the outer angle of the eye. A communicating branch from the zygomatic nerve - a branch of the maxillary nerve [r. communicans (cum n. zygomatici)], which carries secretory parasympathetic fibers for the lacrimal gland, approaches the lacrimal nerve.

The maxillary nerve (n. maxillaris) enters the orbit through the inferior orbital fissure, lies in the infraorbital groove, which passes into the infraorbital canal. At the level of the infraorbital groove and canal, the superior alveolar nerves (nn. alveolares superiores), as well as the anterior, middle and posterior alveolar branches (rr. alveolares anteriores, medius et posteriores) branch off from the infraorbital nerve. They form the superior dental plexus (plexus dentalis superior), located in the maxillary bone and in the mucous membrane of the maxillary sinus. The superior dental branches (rr. dentales superiores) to the teeth and the superior gingival branches (rr. gingivales superiores) to the gums of the upper jaw emerge from the plexus. The internal nasal branches (rr. nasales interni) also extend from the maxillary nerve to the mucous membrane of the anterior parts of the nasal cavity.

The infraorbital nerve (n. infraorbitalis) at the exit from the infraorbital foramen gives off fan-shaped lower branches of the eyelids (rr. palpebrales inferiores), external nasal branches (rr. nasales externi), and superior labial branches (rr. labiales superiores; "small goose foot"). Two or three external nasal branches pass through the nasal muscle into the skin of the ala of the nose. Three or four superior labial branches are directed downwards to the mucous membrane of the upper lip.

The zygomatic nerve (n. zygomaticus) departs from the maxillary nerve in the pterygopalatine fossa and enters the orbit through the superior orbital fissure. In the orbit, it gives off a parasympathetic branch (from the pterygopalatine ganglion) to the lacrimal nerve, intended for the secretory innervation of the lacrimal gland. In the orbit, the zygomatic nerve passes near its lateral wall, enters the zygomaticoorbital foramen, where it divides into the zygomaticotemporal and zygomaticofacial branches. The zygomaticotemporal branch (r. zygomaticotiporalis) exits the zygomatic bone through the zygomaticotemporal foramen and divides into 2 branches that innervate the skin of the anterior temporal region and the lateral forehead.

The zygomaticofacial branch (r. zygomaticofacialis) usually emerges with two or three trunks through the opening of the same name onto the face and innervates the skin of the upper part of the cheek and the lateral part of the lower eyelid.

In the pterygopalatine fossa, the maxillary nerve gives off two or three thin nodal branches (rr. ganglionares, s. ganglionici) to the pterygopalatine ganglion, containing sensory nerve fibers. A smaller portion of the nodal fibers enter directly into the pterygopalatine ganglion. The greater number of these fibers go near the lateral surface of the ganglion and pass into its branches.

The pterygopalatine ganglion (ganglion pterygopalatinum) belongs to the parasympathetic part of the autonomic nervous system. It is located in the pterygopalatine fossa, medial and inferior to the maxillary nerve. In addition to the sensory, transit branches, preganglionic parasympathetic fibers approach the ganglion. They enter the pterygopalatine ganglion in the form of the large petrosal nerve (from the facial nerve) and end on the neurons that are part of the ganglion. The axons of the neurons of the ganglion in the form of postganglionic parasympathetic fibers exit the ganglion as part of its branches. Postganglionic sympathetic fibers from the nerve of the pterygoid canal also approach the pterygopalatine ganglion. These fibers pass through the pterygopalatine ganglion in transit and are part of the branches of this ganglion [see "Autonomic Nervous System"].

The following branches extend from the pterygopalatine ganglion:

1. The medial and lateral superior posterior nasal branches (rr. nasales posteriores superiores mediales et laterales) penetrate through the sphenopalatine opening into the nasal cavity, where they innervate its mucous membrane. The nasopalatine nerve (n. nasopalatine) branches off from the superior medial branches. It innervates the mucous membrane of the nasal septum, and after exiting through the incisive canal into the oral cavity, the mucous membrane of the anterior part of the hard palate. The lateral and medial superior posterior nasal branches also go to the vault of the pharynx, the walls of the choanae and the sphenoid sinus;
2. the greater palatine nerve (n. palatinus major) penetrates through the greater palatine opening onto the lower surface of the hard palate, innervating the mucous membrane of the gums, hard palate, including the palatine glands. The nerve also gives off posterior nasal branches (rr. nasales posteriores inferiores) to the mucous membrane in the area of the inferior nasal concha, middle and inferior nasal passages, and the maxillary sinus;
3. The lesser palatine nerves (nn. palatini minores) pass through the lesser palatine openings to the mucous membrane of the soft palate and to the palatine tonsil.

The mandibular nerve (n. mandibularis) is the third and largest branch of the trigeminal nerve, containing both motor and sensory fibers. The mandibular nerve exits the cranial cavity through the foramen ovale and immediately divides into motor and sensory branches.

Motor branches of the mandibular nerve:

1. masseteric nerve (n. massetericus);
2. deep temporal nerves (nn. temporales profundi);
3. lateral and medial pterygoid nerves (nn. pterygoidei lateralis et medialis). These nerves go to the masticatory muscles.

The motor branches also include the nerve of the muscle that tenses the eardrum (n. musculi tensoris tympani) and the nerve of the muscle that tenses the soft palate (n. musculi tensoris veli palatini).

Sensory branches of the trigeminal nerve:

1. the meningeal branch (r. meningeus), or spinous nerve, departs just below the oval opening, enters the cranial cavity through the spinous opening together with the middle meningeal artery and divides into anterior and posterior branches. The anterior branch innervates the dura mater of the brain. The posterior branch exits through the petrosquamous fissure, innervates the mucous membrane of the mastoid process cells of the temporal bone;
2. the buccal nerve (n. buccalis) runs between the lateral and medial pterygoid muscles, pierces the buccal muscle, branches in the mucous membrane of the cheek, and gives off branches to the skin in the area of the corner of the mouth;
3. The auriculotemporal nerve (n. auriculotiporalis) with two roots embraces the middle meningeal artery. Then, as a single trunk, the nerve goes up, passes through the parotid salivary gland and gives off a number of branches:
   • articular branches (rr. articulares) are directed to the capsule of the temporomandibular joint;
   • parotid branches (rr. parotidei) go to the parotid salivary gland. These branches contain postganglionic parasympathetic (secretory) fibers to the parotid gland;
   • anterior auricular branches (nn. auriculares anteriores) go to the anterior part of the auricle;
   • the nerves of the external auditory canal (nn. meatus acustici externi) innervate the walls of the external auditory canal at the junction of its cartilaginous and bony parts and the eardrum;
   • branches of the eardrum (rr. mebranae tympani) go to the eardrum;
   • superficial temporal branches (rr. temporales superficiales) go to the skin of the temporal region.

Under the oval opening on the medial side of the temporomandibular joint is the vegetative otic ganglion (ganglion oticum), oval in shape and 3-4 mm long. Preganglionic parasympathetic fibers to the otic ganglion come as part of the lesser petrosal nerve (from the facial nerve);

4. The lingual nerve (n. lingualis) passes between the lateral and medial pterygoid muscles, then the nerve turns sharply forward, passes along the inner surface of the body of the lower jaw between the submandibular salivary gland and the hyoglossus muscle upward. Numerous sensory branches of the lingual nerve end in the mucous membrane of the anterior Vl of the tongue and in the sublingual region.

The lingual nerve also sends out nodal branches to the submandibular and sublingual parasympathetic ganglia [see "Parasympathetic part of the autonomic nervous system"]. Fibers that join the lingual nerve as part of the chorda tympani, one of the branches of the facial nerve, approach these ganglia. The chorda tympani approaches the lingual nerve at an acute angle in its initial part (between the medial and lateral pterygoid muscles). It carries taste fibers that innervate the mucous membrane of the anterior 2/3 of the tongue;

5. The inferior alveolar nerve (n. alveolaris inferior) contains sensory and motor fibers and is the largest branch of the mandibular nerve. This nerve initially passes between the medial and lateral pterygoid muscles, then enters the mandibular canal through its entrance on the inner surface of the lower jaw. At the point of entry into the canal, motor branches extend from the inferior alveolar nerve to the mylohyoid and geniohyoid muscles, and to the anterior belly of the digastric muscle - the mylohyoid branch (r. mylohyoideus). In the mandibular canal, the inferior alveolar nerve (passes together with the artery and vein of the same name) gives off branches that form the inferior dental plexus (plexus dentalis inferior). From the plexus, the lower dental branches (rr. dentales inferiores) extend to the teeth of the lower jaw, and the lower gingival branches (rr. gingivales inferiores) extend to the gums.
6. After exiting through the mental foramen, the inferior alveolar nerve passes into the mental nerve (n. mentalis), which ends in the skin of the chin and lower lip. It gives off mental branches (rr. mentales), lower labial branches (rr. labiales inferiores), and also branches to the gums (rr. gingivales).