Trigeminal Nerve Anatomy 

The trigeminal nerve is the largest and most complex of the 12 cranial nerves (CNs). It supplies sensations to the face, mucous membranes, and other structures of the head. It is the motor nerve for the muscles of mastication and contains proprioceptive fibers. It exits the brain by a large sensory root and a smaller motor root coming out of the pons at its junction with the middle cerebral peduncle. It passes laterally to join the gasserian (semilunar) ganglion in the Meckel cave. (See the image below.)

Nuclei

The sensory nucleus, located in the pons, is quite extensive. It receives ordinary sensations from the main 3 branches of the trigeminal. The ophthalmic division is in the lower part of the nucleus, and the mandibular branch is in the upper part. The large rostral head is the main sensory nucleus. The caudal tapered part is the spinal tract, which is continuous with substantia gelatinosa of Rolando in the spinal cord. The spinal tract is the sensory nucleus, primarily for pain and temperature. The main sensory nucleus serves mostly for discrimination sense.^{[1, 2, 3, 4, 5]}

The motor nucleus is ventromedial to the sensory nucleus. It lies near the lateral angle of the fourth ventricle in the rostral part of the pons. The mesencephalic nucleus is in the midbrain and receives proprioceptive fibers from all muscles of mastication.

Connections

The main sensory nucleus receives its afferents (as the sensory root) from the semilunar ganglion through the lateral part of the pons ventral surface. Its axons cross to the other side, ascending to the thalamic nuclei to relay in the postcentral cerebral cortex. The descending sensory fibers from the semilunar ganglion course through the pons and medulla in the spinal tract of CN V to end in the nuclei of this tract (as far as the second cervical segment). (See tables 1 and 2, below.)

The axons of these nuclei cross to the opposite side, ascending in the spinothalamic tract, to relay in the thalamic nuclei; from there, they end in the cerebral cortex. The sensory nucleus of CN V is connected to other motor nuclei of the pons and medulla. In addition, the descending sensory spinal tract receives somatic sensory fibers from CNs VII, IX, and X.

The proprioceptive fibers of CN V arise from the muscles of mastication and the extraocular muscles. They terminate in the mesencephalic nucleus. This nucleus has connections to the motor nucleus of CN V.

The motor nucleus of CN V receives cortical fibers for voluntary control of the muscles of mastication. These fibers are mostly crossed. It also receives input from the mesencephalic and sensory nuclei. The axons emerge anterior to the sensory root from the lateral surface of the pons. This motor root joins the semilunar ganglion together with the sensory root.

The semilunar (gasserian or trigeminal) ganglion is the great sensory ganglion of CN V. It contains the sensory cell bodies of the 3 branches of the trigeminal nerve (the ophthalmic, mandibular, and maxillary divisions). The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has sensory and motor functions.

The gasserian ganglion lies in a depression on the petrous apex, within a dural fold called the Meckel cave. The sensory roots of the 3 branches of CN V are received anteriorly. They then pass from the posterior aspect of the ganglion to the pons. The motor root passes under the ganglion to join the sensory division of the mandibular nerve and exits the skull through foramen ovale. The carotid plexus contributes sympathetic fibers to the gasserian ganglion.

Table 1. Summary of the Components, Function, Central Connections, Cell Bodies, and Peripheral Distribution of CN V. (Open Table in a new window)

Table 2. Summary of the Types of Fibers, Function, and Pathways of the Trigeminal Nerve. (Open Table in a new window)

The ophthalmic, maxillary, and mandibular branches of the trigeminal nerve leave the skull through 3 separate foramina: the superior orbital fissure, the foramen rotundum, and the foramen ovale, respectively. (See the image below.)

The ophthalmic nerve

The ophthalmic nerve is the first branch of the trigeminal nerve. It arises from the convex surface of the gasserian ganglion, in the dura of the lateral wall of the cavernous venous sinus under CN IV and above the maxillary nerve, as seen in the image below.

The ophthalmic nerve 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 ophthalmic nerve receives sympathetic filaments from the cavernous sinus and communicating branches from CN III and IV. Just before it exits the skull through the superior orbital fissure, it gives off a dural branch, and then divides into 3 branches: the frontal, lacrimal, and nasociliary. (See the image below.)

Frontal nerve

This is the largest branch of the ophthalmic nerve (see Table 3, below). It passes in the lateral part of the superior orbital fissure, below the lacrimal nerve and above CN IV, between the periorbita and levator palpebrae superioris. It divides in the middle of the orbit into the supraorbital (larger branch) and supratrochlear nerves.

Table 3. The Ophthalmic Nerve Branches and Distribution. (Open Table in a new window)

The supraorbital nerve exits the skull through the supraorbital notch (or foramen). It supplies the upper lid and then turns superiorly under the frontalis muscle to supply the scalp (via lateral and medial branches) as far posteriorly as the lambdoid suture.

The supratrochlear nerve exits the medial orbit and gives branches to the conjunctiva and the skin of the upper lid, as well as to the lower and medial parts of the forehead. The branch to the frontal sinus pierces it in the supraorbital notch to supply the frontal sinus mucosa.

Lacrimal nerve

The lacrimal nerve arises in the narrow, lateral part of the superior orbital fissure and courses between the lateral rectus and the periorbita. It supplies the lacrimal gland, conjunctiva, and upper lid. In the orbit, it receives a communication from the zygomatic branch of the maxillary nerve. This represents postganglionic parasympathetic secretory fibers from the sphenopalatine ganglion to the lacrimal gland. The preganglionic fibers reach the ganglion via the greater petrosal and vidian nerves from CN VII.

Nasociliary nerve

After passing through the superior orbital fissure, the nasociliary nerve gives origin to the anterior ethmoid nerve that passes to the anterior ethmoid foramen lateral to the crista galli, to supply the fontal and anterior ethmoid sinuses. After dropping in the nose, it supplies the anterior part of the septum and lateral nasal wall. After emerging from the nose as the external nasal nerve, it supplies the skin of the nasal tip.

The nasociliary nerve gives a branch to the ciliary ganglion that passes without synapsing to the cornea, iris, and ciliary body. The posterior ethmoid nerves are given off before the anterior ethmoid and supply the posterior ethmoid and sphenoid sinuses. The nasociliary nerve gives off 2-3 long ciliary nerves that enter the globe with the short ciliary nerves of the ciliary ganglion.

Maxillary nerve

The maxillary nerve 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. (See the image below.) The maxillary nerve is divided into 3 branches: the zygomatic, pterygopalatine (or sphenopalatine), and posterior superior alveolar nerves.

As it leaves the semilunar ganglion, the maxillary nerve passes through the dura of the lateral wall of the cavernous sinus. It exits the skull via the foramen rotundum and crosses the pterygopalatine fossa to enter the orbit through the inferior orbital fissure, where it becomes the infraorbital nerve. Before entering the foramen, it gives off a dural branch (middle meningeal nerve). The zygomatic, pterygopalatine (or sphenopalatine) and posterior superior alveolar branches are given off in the pterygopalatine fossa.

The zygomatic branch divides into the zygomaticotemporal and zygomaticofacial nerves.

In the lateral wall of the orbit, it gives off a branch to the lacrimal nerve, which carries postganglionic fibers from the sphenopalatine ganglion for lacrimation. The zygomaticofacial is inferiorly situated and supplies the skin of the cheek.

The pterygopalatine (or sphenopalatine) nerves are 2 nerves that unite the sphenopalatine ganglion to the maxillary nerve. They transmit afferent sensations from the nose, palate, and pharynx. They also carry parasympathetic fibers to the lacrimal nerve that go to the lacrimal gland. These preganglionic fibers are derived from CN VII via the greater petrosal and vidian nerves. The other branches of the sphenopalatine nerves and their distribution are summarized in Table 4, below.

Table 4. The Maxillary Nerve Branches and Distribution. (Open Table in a new window)

The posterior superior alveolar nerves are usually 2 in number. They supply the mucosa of the posterior cheek and gingiva; Table 4 has their distribution and the other small branches.

The mandibular nerve

The mandibular nerve is the largest branch of the trigeminal nerve, as seen in the image below. It has mixed sensory and motor fibers (see Table 5, below).

The mandibular nerve carries sensory information from the lower lip, the lower teeth, gums, the chin and jaw (except the angle of the mandible, 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. It does not carry taste sensation (the chorda tympani is responsible for taste), but one of its branches, the lingual nerve, carries multiple types of nerve fibers that do not originate in the mandibular nerve.

Motor branches of the trigeminal nerve are distributed in the mandibular nerve. These fibers originate in the motor nucleus of the fifth nerve, which is located near the main trigeminal nucleus in the pons. (See the image below.)

The mandibular nerve has the following 9 branches:

• Recurrent meningeal nerve - This nerve enters the skull via the foramen spinosum with the meningeal artery
• Medial pterygoid nerve - After passing through the otic ganglion without synapsing, this nerve supplies the medial pterygoid, tensor veli palatini, and tensor tympani muscles
• Masseteric nerve - This nerve passes through the mandibular notch to innervate the masseter muscle and temporomandibular joint (TMJ)
• Deep temporal nerves - The anterior and posterior branches supply the temporal muscle
• Lateral pterygoid nerve
• Buccal nerve - This nerve divides into the temporal and buccinator branches
• Auriculotemporal nerve - This nerve begins as 2 roots that encircle the middle meningeal artery, then forms a single trunk medial to the neck of the mandible; it emerges superficially between the ear and the mandibular condyle deep to the parotid gland and ends in 2 superficial temporal branches (for autonomic supply to the parotid gland, see below)
• Lingual nerve - This nerve runs parallel to the inferior alveolar nerve, is joined by the chorda tympani nerve of the facial nerve (CN VII) near the internal maxillary artery, courses forward between the hyoglossus muscle and the deep part of the submandibular gland, and, as it passes forward, crosses the submandibular (Wharton) duct; the lingual nerve could be injured in this location during surgery on the floor of mouth or during excision of the submandibular gland (for more details regarding the nerve supply of the salivary glands, see below)
• Inferior alveolar nerve - This nerve accompanies the inferior alveolar artery in the mandibular foramen and courses into the mandibular canal to exit through the mental foramen; the different branches are listed in Table 5, below

Table 5. Mandibular Nerve Branches and Distribution. (Open Table in a new window)

Sensory nerve endings that respond to stimuli and convert them to nervous energy toward the central nervous system are called receptors or central transducers. Sensory receptors are classified into the following 3 main groups: exteroreceptors, interoreceptors, and proprioceptors.^{[6, 7, 8]}

Exteroreceptors

These are stimulated by the external environment. Examples of these types of receptors include the following:

• Merkel corpuscles - Located in submucosa of the tongue and oral cavity (see the image below)(2) Merkel disc ending. Horseradish peroxidase (HRP) has diffused into the hair shaft and surrounded the disc-shaped nerve terminal. Key: Merkel cell (M), nerve terminal (nt). Inset: Incorporated HRP in the nerve terminal, x8, 750. Inset: x32, 4003 (3 and 4). Detail of a Merkel disc ending. HRP is seen in various vacuoles in the nerve terminal. x 39,000.
• Meissner corpuscles - Tactile receptors in the skin
• Ruffini corpuscles - Pressure and warmth receptors
• Krause corpuscles or end bulbs - Cold receptors
• Free nerve endings - Perceive superficial pain and tactile sensations

Interoreceptors

These are located in and transmit sensations from body cavities. Most of the sensations for these structures deal with body functions and are below the conscious level. Examples include the following:

• Pacinian corpuscles - Detect pressure sense
• Free nerve endings - Perceive visceral or other sensations

Proprioceptors

The sensations associated with proprioceptors are also below conscious level; examples include the following:

• Muscle spindles - Respond to passive stretch of the muscle
• Golgi tendon organs - Located in tendons and respond to muscle tension (contraction and stretching)
• Pacinian corpuscles - Respond to pressure
• Proprioceptors - Respond to periodontal sensation
• Sensory nerve endings - Perceive deep somatic pain

Different anatomic variations have been described regarding the trigeminal nerve, its branches, and its subdivisions. Examples include the very rare occurrence of unilateral trigeminal nerve hypoplasia, in which no corneal sensitivity exists on the affected side and facial sensitivity is reduced in all branches of the trigeminal nerve. Anomalies may coexist also in association with craniofacial anomalies, such as hypoplasia of the trigeminal nerve in Goldenhar syndrome (oculo-auriculo-vertebral dysplasia). A few other examples affecting the different divisions are described below.^{[9, 10, 11, 12, 13, 14]}

Frontal nerve

A variation has been reported in which the frontal nerve divides at a variable point before leaving the orbit to form the supratrochlear and supraorbital branches. In such cases, the supraorbital branch passes through the supraorbital foramen, through which the undivided nerve ordinarily passes. When the foramen is absent, it may have a special groove, the frontal notch (Henle notch).

The frontal nerve runs, at first forward, in a sagittal direction. In approximately 90% of subjects, it divides during its course within the orbit, but in 10% of persons it remains undivided. It divides into the larger lateral supraorbital nerve and smaller supratrochlear nerve, which runs medially. In 60% of subjects, the supraorbital nerve does not divide, but in 30% it divides into the medial branch, which leaves the orbit through the frontal foramen or notch, and the lateral branch passes out through the frontal foramen. In about 90% of subjects, the supratrochlear nerve runs along the surface of the superior oblique muscle. In 4% of subjects, 2 supratrochlear nerves exist.

Ethmoidal nerve

This nerve may be limited to the nasal cavity. It may also traverse the posterior ethmoidal foramen to gain entrance to the cranial cavity.

Lacrimal nerve

This nerve may appear to be derived from the trochlear nerve. However, the probable source in such cases is the ophthalmic nerve, through its communicating branch to the trochlear nerve (CN IV) in the cavernous sinus.

The lacrimal nerve may be small at its origin, increasing in size later in its course by the addition of fibers derived from the temporal branch of the maxillary division of the trigeminal nerve. The lacrimal nerve may be absent and replaced by the temporal branch of the maxillary division of the trigeminal nerve.

The lacrimal nerve occasionally gives rise to a ciliary nerve, or it receives a branch from a long ciliary nerve of the ciliary ganglion or a branch from the ganglion directly. It may receive accessory roots from the supraorbital or nasociliary nerves.

The bifurcation of the lacrimal into its terminal branches may occur on the posterior wall of the orbital cavity. A branch of the lacrimal has been noted to pierce the sclera.

The lacrimal nerve may exchange fibers with the ciliary ganglion.

Nasociliary (nasal) nerve

Several variations in the branches of this nerve have been reported. The nasociliary nerve may send branches to the superior rectus, medial rectus, and levator palpebral superioris muscles. Branches emanating from a small ganglion connected to the nasal nerve have been followed to the oculomotor (CN III) and abducens (CN VI) nerves.

The infratrochlear branch of the nasal (nasociliary) nerve may be missing, in which case the areas normally supplied by this branch (skin of the upper eyelid, root of nose, conjunctiva, lacrimal sac) receive their supply from the supratrochlear branch of the frontal nerve.

Branches of the nasal nerve have been described passing to the frontal, ethmoid, and sphenoid sinuses. The branches to the frontal and anterior ethmoid sinuses arise in the anterior ethmoid foramen; branches to the sphenoid and posterior ethmoid sinuses arise in the posterior ethmoid foramen. The branches to the sphenoid sinuses are known as sphenoid branches, whereas the branches to the posterior ethmoid sinuses are known as sphenoethmoid or posterior ethmoid branches. An anastomosis between the nasal and lacrimal nerves has been reported.

Maxillary division (V2)

The maxillary nerve may split into 2 trunks, each entering the skull through a separate foramen

Zygomatic nerve

The following variations have been reported in this nerve or its 2 branches (the temporal or facial or malar). The nerve may pass through the zygomatic bone before it divides into 2 branches, or the 2 branches may pass separately through foramina in the zygomatic bone instead of passing through a common foramen (sphenozygomatic foramen). The temporal branch in some cases passes through the sphenomaxillary fissure into the temporal fossa.

Either branch of the zygomatic may be absent or smaller than normal, in which case the other branch compensates by carrying the additional nerve fibers. The area usually supplied by the zygomatic branch (skin of the zygomatic region) may be supplied instead by the infraorbital nerve. The area usually supplied by the temporal branch (skin of the anterior temporal region) may be supplied solely or additionally by the lacrimal nerve.

Posterior superior alveolar nerve

In the absence of the buccal nerve, the posterior superior alveolar nerve distributes branches to the areas normally supplied by this nerve (mucous membrane and skin of the cheek).

Inferior alveolar nerve

The inferior alveolar nerve may form a single trunk with the lingual nerve, extending as far as the mandibular foramen. The inferior alveolar nerve is sometimes perforated by the internal (medial) maxillary artery. It may have accessory roots from other divisions of the mandibular nerve. In some cases, the mylohyoid branch of the inferior alveolar gives rise to a branch that pierces the mylohyoid muscle and joins the lingual nerve.

Branches have been described arising from the mylohyoid branch and supplying the depressor anguli oris muscle and parts of the platysma (that are usually supplied by the facial nerve), the skin below the chin, and the submandibular (submaxillary) gland (which is usually supplied by the facial nerve). The inferior alveolar may form connections with the auriculotemporal nerve. In one case, the roots of the third lower molar tooth were found to be surrounding the inferior alveolar nerve.

Auriculotemporal nerve

This nerve carries the otic ganglion, which is derived from glossopharyngeal neurons. The nerve usually arises by 2 roots from the posterior division of the mandibular nerve. The 2 roots usually surround the middle meningeal nerve before joining to form a single trunk. A variation in this relationship has been described in which the middle meningeal artery pierces the anterior root instead of passing between the 2 roots.

According to Baumel et al, the auriculotemporal nerve is commonly misrepresented in illustrations and textbooks.^[11] Their 85 dissections of the nerve demonstrated that the roots of the "typical" auriculotemporal nerve do not form a tight buttonhole around the middle meningeal artery. Instead, the roots outline an elongated, V-shaped interval, with the roots widely separated from one another. At their junction, the roots form a short trunk that immediately breaks up in line with the posterior border of the mandible into a spray of branches.

The superficial temporal ramus of the auriculotemporal nerve should not be considered as the main continuation of the nerve but merely as its largest branch. A substantial portion of the nerve makes up its 2 communicating rami with the facial nerve; these are the strongest and most consistent of the many peripheral communications between trigeminal and facial nerves. Common variations in configuration, branching, and relationships of the nerve are discussed in the report by Baumel et al.

Lingual nerve

A minute sublingual ganglion has been described arising from the lingual nerve or submandibular ganglion (a ganglion of the facial nerve carried by the lingual nerve), supplying the sublingual gland. This nerve may pierce the lateral pterygoid muscle rather than pass between the 2 pterygoid muscles. It occasionally provides motor branches to the medial and lateral pterygoids and to the palatoglossus muscle.

A few diseases should be mentioned when we study the trigeminal nerve. With regard to trigeminal neuralgia, for example, the differential diagnoses are as follows (also, see Table 6, below)^{[6, 15, 16, 17, 18, 19]} :

• Arteriovenous malformations
• Low-grade astrocytoma
• Brainstem gliomas
• Meningioma
• Cavernous sinus syndromes
• Migraine headache
• Trigeminal neuropathy
• Trigeminal neuritis
• Chronic paroxysmal hemicrania
• Migraine variants
• Cluster headache
• Multiple sclerosis
• Craniopharyngioma
• Persistent idiopathic facial pain
• Glioblastoma multiforme
• Polyarteritis nodosa
• Hemifacial spasm
• Postherpetic neuralgia
• Hydrocephalus
• Subarachnoid hemorrhage
• Atypical facial pain
• Ramsay Hunt syndrome
• Glossopharyngeal neuralgia
• Malignant and nonmalignant pain syndromes
• Occipital neuralgia
• Tic convulsif
• Cerebral aneurysms
• Brainstem syndrome

Table 6. The Difference Between Atypical Facial Pain and Trigeminal Neuralgia. (Open Table in a new window)

Marcus Gunn phenomenon

Marcus Gunn phenomenon (also known as Marcus-Gunn jaw-winking or trigemino-oculomotor synkineses) is an autosomal-dominant condition with incomplete penetrance, in which nursing infants have rhythmic upward jerking of their upper eyelid. This condition has been associated with amblyopia (in 54% of cases), anisometropia (26%), and strabismus (56%).

Marcus Gunn phenomenon is an exaggeration of a very weak physiologic cocontraction that has been disinhibited secondary to a congenital brainstem lesion. The stimulation of the trigeminal nerve by contraction of the pterygoid muscles results in the excitation of the branch of the oculomotor nerve (CN III) that innervates the levator palpebrae superioris ipsilaterally.

Inverse Marcus Gunn phenomenon

Inverse Marcus Gunn phenomenon is a rare condition that causes the eyelid to fall upon opening of the mouth. In this case, trigeminal innervation to the pterygoid muscles is associated with an inhibition of the branch of the oculomotor nerve to the levator palpebrae superioris, as opposed to stimulation in Marcus Gunn jaw-winking.

Tolosa-Hunt syndrome

Tolosa-Hunt syndrome (THS) is a painful ophthalmoplegia caused by nonspecific inflammation of the cavernous sinus or superior orbital fissure. Ophthalmoparesis or disordered eye movements occur when CNs III, IV, and VI are damaged by granulomatous inflammation. Pupillary dysfunction may be present and is related to injury to the sympathetic fibers or oculomotor nerve. Trigeminal nerve involvement (primarily V1) may cause paresthesias of the forehead.

Lateral medullary syndrome

This condition is also called Wallenberg syndrome or posterior inferior cerebellar artery (PICA) syndrome. The PICA supplies the lower cerebellum, the lateral medulla, and the choroid plexus of the fourth ventricle. In lateral medullary syndrome, the patient has dysphagia and/or difficulty speaking owing to 1 or more patches of infarction caused by interrupted blood supply to parts of the brainstem. For features of lateral medullary syndrome, see Table 7, below.

Table 7. Features of Lateral Medullary Syndrome. (Open Table in a new window)

Four small parasympathetic (accessory) ganglia are associated anatomically (but not functionally) with the branches of the trigeminal nerve.^{[2, 4]} They are as follows:

• Ciliary ganglion
• Sphenopalatine (or pterygopalatine) ganglion
• Otic ganglion
• Submandibular ganglion

The ciliary ganglion is associated with the ophthalmic nerve. It is the size of a pinhead and has the following 3 roots:

• The parasympathetic root from the nerve to inferior oblique (CN III) from Edinger Westphal nucleus and caudal central nucleus to supply the sphincter papillae and ciliary muscles
• Sympathetic root from the nasociliary nerve to dilator papillae muscle of the eye
• Sensory root from the nasociliary nerve to the cornea

The sphenopalatine ganglion is associated with the maxillary nerve. It receives its parasympathetic fibers from CN VII (as seen in the image below). The otic and submandibular ganglia are associated with the mandibular nerve. They receive parasympathetic fibers from CNs IX and VII, respectively.

Autonomic supply to the salivary glands

Submandibular gland

Parasympathetic fibers arise from the superior salivary nucleus in the pons. Fibers pass through the facial nerve to the chorda tympani and then to the lingual nerve. Synapsing occurs in the submandibular ganglion and from there to the submandibular salivary gland. Sympathetic supply is from the plexus around the facial artery.

Parotid gland

Parasympathetic fibers arise from the inferior salivary nucleus in the medulla oblongata, pass through the glossopharyngeal nerve (CN IX), and then travel through its tympanic branch to the tympanic plexus (Jacobson nerve). They emerge from the middle ear through a hiatus on the anterior surface of the petrous temporal bone, as the lesser superficial petrosal nerve. This nerve passes via the foramen ovale to the otic ganglion (which hangs from the medial side of the mandibular nerve).

Relay occurs in the otic ganglion, and from there it is distributed to the parotid gland via the auriculotemporal nerve. Sympathetic fibers are from the superior cervical ganglion; they go to the plexus around the meningeal artery and from there to the auriculotemporal nerve, which distributes them to the parotid salivary gland.