The Trigeminal Nerve
The principal regulator of the sensory modalities of the head is the trigeminal nerve. This is the fifth of twelve pairs of cranial nerves that are responsible for transmitting numerous motor, sensory, and autonomous stimuli to structures of the head and neck . While the trigeminal nerve (CN V) is largely a sensory nerve, it also mingles in the realm of motor supply. Unlike the other cranial nerves, the trigeminal nerve is quite large. It has four nuclei that send fibers to form its tracts and is associated with three separate branches.
The goal of this article will be to discuss the anatomy, pathway, and distribution of the trigeminal nerve. Further discussion surrounding clinical examination to assess the integrity of the trigeminal nerve will also be included.
- Divisions of the Trigeminal Nerve
- Intracranial Course of the Trigeminal Nerve
- Nuclei of the Trigeminal Nerve
- Parasympathetic Association of the Trigeminal Nerve
- Clinical Examination of the Trigeminal Nerve
- Related Atlas Images
Divisions of the Trigeminal Nerve
As the name suggests, the trigeminal nerve is a tripartite entity made up of distinct terminal divisions. Each component of the nerve is responsible for a specific region of the face, and transmits specific impulses. The three divisions of the trigeminal nerve are:
- Ophthalmic division (CN V1 or Va),
- Maxillary division (CN V2 or Vb),
- Mandibular division (CN V3 or Vc).
The acronym MOM can be used to recall the three branches of the trigeminal nerve.
Ophthalmic Branch (CN V1)
The ophthalmic branch is the first division of the trigeminal nerve. It is a purely sensory nerve that carries afferent stimuli of pain, light touch, and temperature from the upper eyelids and supraorbital region of the face, up to the vertex of the head. The nerve also acts as a conduit for sympathetic fibers that require access to the ciliary body, lacrimal glands, cornea, and conjunctiva of the eye. Furthermore, the ophthalmic branch also carries fibers arising from the dura mater of the anterior cranial fossa, the frontal sinus, and the superior aspect of the nasal cavity.
The ophthalmic division also has several tributaries that constitute it. The three main nerves that come together to form CN V1 are the nasociliary, frontal, and lacrimal nerves. The acronym NFL (as in American football) is also useful to recall these three branches. The nerves unite within the superior orbital fissure to form the ophthalmic division. Once formed, the ophthalmic nerve also receives its meningeal tributary from the dura of the anterior cranial fossa.
Additional sympathetic branches from the cavernous sinus also join the ophthalmic nerve as well. CN V1 travels in the lateral wall of the cavernous sinus below CN IV (trochlear nerve) and above CN V2. It continues posteriorly and emerges from the cavernous sinus in Meckel’s cave, where it pierces the meninges to enter the concave surface of the trigeminal ganglion. The branches of the ophthalmic division of the trigeminal nerve are summarized below.
Maxillary Branch (CN V2)
Like the ophthalmic branch, the maxillary division of the trigeminal nerve (CN V2) is a purely sensory entity that carries impulses from the midface. It has a middle meningeal branch that detects stimuli from the dura of the middle cranial fossa. Additionally, the zygomatic, pterygopalatine, and the posterior superior alveolar nerves unite at the opening of the foramen rotundum to form the maxillary branch of the trigeminal nerve.
As this nerve enters the cranial vault, it passes in the lateral wall of the cavernous sinus, below the ophthalmic division of the trigeminal nerve. It maintains a posterior course until it pierces the meninges and joins the trigeminal ganglion within Meckel’s cave. For a short course, the nerve is sandwiched between CN V1 (superiorly) and CN V3 (inferiorly).
Mandibular Branch (CN V3)
The last of the three trigeminal branches is the mandibular division (CN V3). As the largest component of CN V, it carries both sensory and motor stimuli. The motor branches correspond to the muscles that originated from the first pharyngeal arch. The sensory branches supply the lower third of the face, excluding the angle of the mandible (supplied by the second and third cervical segments). Although it carries sensory modalities from the mouth and gingiva, it does not carry special afferent stimuli (i.e. taste). However, the lingual nerve, which is a branch of CN V3 acts as a conduit for the chorda tympani (a branch of CN VII), which carries taste stimuli.
The motor components of CN V3 travel as a single, slender, nerve fibre alongside the larger sensory fibers. Together, they travel through the external opening of the foramen ovale and travel towards Meckel’s cave. The nerve receives the recurrent meningeal nerve that carries afferent stimuli from the dura, prior to penetrating the trigeminal ganglion.
Intracranial Course of the Trigeminal Nerve
The three branches of the trigeminal nerve unite within a shallow depression on the posteromedial side of the middle cranial fossa known as Meckel’s cave. In this fossa, the nerves unite to form the semilunar (Gasserian, or trigeminal) ganglion. Medial to these structures is the superior petrosal sinus , which may be superiorly or inferiorly related to the opening of Meckel’s cave. The clivus, basilar venous plexus, ventral aspect of the pons, and brainstem are also medially related to the cave. Laterally, the medial aspect of the temporal lobe is immediately adjacent to Meckel’s cave.
As the fibers of the trigeminal nerve leave the trigeminal ganglion, they travel superomedially toward the pons. Here both sensory and motor divisions of the nerve pierce the lateral surface of the pons near the superior pontine sulcus. Once inside the pons, half of the sensory fibers will divide into ascending and descending groups. The ascending groups will move toward the mesencephalic nucleus, while the descending group will join the spinal trigeminal nucleus. The remaining sensory fibers will travel dorsomedially toward the main sensory nucleus, while the motor fibers will take a similar course to reach the motor nucleus.
Nuclei of the Trigeminal Nerve
Of the twelve cranial nerves within the human body, only the trigeminal nerve is associated with four nuclei. From cranial to caudal, these nuclei are the:
- primary sensory
- spinal nuclei.
The mesencephalic nucleus is a bilaterally paired, thread-like collection of unipolar neurons that extends from the level of the main sensory nucleus in the pons and projects up to the rostral part of the tegmentum (within the lateral periaqueductal grey matter) in the midbrain. Although these nuclei are located within the midbrain, they communicate with the trigeminal nerve and not the basal ganglia. This nucleus is responsible for processing proprioception – which is the body’s ability to detect the spatial orientation of varying body parts with respect to itself and the surrounding structures.
As the myelinated axons leave the mesencephalic nucleus, they coalesce to form the mesencephalic tract. The individual axons then split into central and peripheral branches. The central branches convey impulses from the neuromuscular spindles within the muscles of mastication, and from the bite force reflex arcs, to the motor neuron of the trigeminal nerve. Other central fibers also integrate with the reticular formation and the sensory trigeminal nerve. Others also gain access to the cerebellum by way of the superior cerebellar peduncle. This interplay between the proprioceptive and motor divisions of the trigeminal nerve helps to regulate the activity of the stretch muscles; and by extension, the process of mastication.
On the other hand, the peripheral branches originate from the neuromuscular spindle apparatus within the muscles of mastication, as well as from other proprioceptive points in the teeth of the upper and lower jaws. The fibers of the lower jaw then travel via the mandibular branch of the trigeminal nerve (CN V3), while those arising from the upper jaw gain access to the nucleus via the maxillary division of the trigeminal nerve (CN V2). The fibers of the mandibular division arising from the muscle spindle fibers also travel to the motor nucleus to relay information regarding stretching of the muscles of mastication.
Main Sensory Nucleus
The main sensory nucleus of the trigeminal nerve is also referred to as the pontine, chief, superior, or principal trigeminal nucleus. It is laterally related to the motor nucleus of the trigeminal nerve, within the dorsal aspect of the pontine tegmentum. The cell bodies of the afferent axons that feed the main (as well as the mesencephalic and spinal) sensory nucleus reside in Meckel’s cave as the trigeminal ganglion. Other fibers arising from the mesencephalic nucleus of the trigeminal nerve also send proprioceptive impulses to the main sensory nucleus as well. Other large fibers carrying discriminative touch impulses, as well as other axons carrying light touch terminate in the main sensory nucleus.
Neurons arising from the pontine and spinal trigeminal nuclei decussate at multiple levels. They coalesce to form the ventral trigeminothalamic tract, which moves cranially, adjacent to the medial lemniscus pathway. Fewer fibers (both ipsilateral and contralateral in origin) also continue to the thalamus as the dorsal trigeminothalamic tract. When the dorsal and ventral trigeminothalamic tracts merge in the rostral aspect of the pons (bordering over into the midbrain), they are collectively referred to as the trigeminal lemniscus tract. The fibers access the ventral posteromedial nucleus of the thalamus, after which third order neurons ascend through the internal capsule to gain access to Brodmann area 3, 1, 2 (i.e. the postcentral gyrus) where the sensory input is processed.
Motor Nucleus of the Trigeminal Nerve
The motor nucleus of the trigeminal nerve is an oval collection of cell bodies belonging to a mixture of small to large multipolar neurons, medial to the pontine trigeminal nucleus. Within the confines of the nucleus, the cells are further organized into subnuclei, whose outflow tracts innervate specific muscles of the first pharyngeal arch.
It is deep to the lateral aspect of the rhomboid fossa (floor of the fourth ventricle), in the upper aspect of the pontine tegmentum. The myelinated motor axons leave the motor nucleus through the superior pontine sulcus, and travels alongside the sensory tracts, before merging with the mandibular division of the trigeminal nerve. As a derivative of the first pharyngeal arch, the fibers of the motor nucleus innervate the muscles that share the same origin. These include the pterygoid muscles, the masseter and temporalis muscles (i.e. the muscles of mastication), as well as the mylohyoid, anterior belly of the digastric, tensor tympani, and the tensor veli palatini muscles.
The motor nucleus receives extensive bilateral corticobulbar (from the cerebral cortices to the cranial nerve nucleus), and rubrobulbar (from the red nucleus to the cranial nerve nucleus) regulation. There are also afferent fibers arising from the main sensory and mesencephalic nuclei that equally contribute to the regulation of the motor nucleus.
Spinal Trigeminal Nucleus
The spinal nucleus of the trigeminal nerve has a pontomedullary distribution, extending along the full length of the medulla oblongata (from the caudal end of the main sensory nucleus of the trigeminal nerve) to the proximal spinal cord (about the second or third cervical segment). The nucleus is divided craniocaudally on a cytoarchitectural basis into three subnuclei:
- The short, proximal third is the pars oralis that extends from the caudal end of the main sensory trigeminal nerve to the rostral third of the inferior olivary nucleus.
- The middle segment, known as the pars interpolaris travels from the end of the pars oralis at the inferior olivary nucleus to the level of the great pyramidal decussation (at the foramen magnum).
- Finally, the caudal portion or the pars caudalis extends from the pyramidal decussation to the second or third cervical segment in the posterolateral tract of Lissauer. Structurally, this layer is more similar to the dorsal horn of the spinal cord (i.e. similar segmentation into Rexed laminae).
The spinal nucleus is fed by afferent fibers from the spinal trigeminal tract; which is formed from both intermediate and fine, unmyelinated central processes arising from the trigeminal ganglion. These fibers take a caudal course after entering the pons via the superior pontine sulcus while carrying sensory information from the ophthalmic, maxillary, and sensory component of the mandibular divisions of the trigeminal nerve. The sensory modalities that are transmitted by these fibers include light touch, pain and temperature sensation from the face up to the vortex of the head. The tract also contains general somatic afferent fibers arising from cranial nerves VII (facial), IX (glossopharyngeal) , and X (vagus) .
As the spinal trigeminal tract descends through the pons and medulla oblongata, the fibers embrace the nucleus along its length. There is a precise somatotopic organization of the spinal trigeminal tract such that the fibers arising from the maxillary division – running centrally – is flanked by fibers of the ophthalmic division ventrolaterally and the mandibular division dorsomedially. The dorsal boundary of the tract is formed by the fibers of CN VII, IX, and X. One school of thought postulates that the spinal trigeminal nucleus also has a somatotopic arrangement such that fibers of the ophthalmic division synapse in the pars caudalis, those of the maxillary division terminate in the pars interpolaris and those of the mandibular division end in the pars oralis. However, other studies suggest that the fibers are distributed throughout the length of the nucleus; with the exception of the ophthalmic division, which would not extend to the cervical segments.
Parasympathetic Association of the Trigeminal Nerve
While the trigeminal nerve does not have parasympathetic innate parasympathetic fibers, it is associated with several parasympathetic ganglia along its course. These ganglia are:
- The ciliary ganglion is formed by presynaptic fibers arising from the Edinger-Westphal nucleus. The fibers are associated with the ophthalmic division of the trigeminal nerve. The postsynaptic fibers leaving the ciliary ganglion supply the ciliary and sphincter pupillae muscles via the short ciliary nerve.
- The otic ganglion receives parasympathetic input from the inferior salivatory nucleus via the glossopharyngeal nerve (CN IX). The fibers are associated with the mandibular division of the trigeminal nerve. The postganglionic fibers go on to supply the parotid gland via the auriculotemporal nerve.
- Like the otic ganglion, the submandibular ganglion is also associated with CN V3. The preganglionic fibers arise from the superior salivatory nucleus and travel through the nervus intermedius branch of CN VII. The fibers eventually join the chorda tympani before entering the lingual nerve. The submandibular and sublingual glands eventually receive postganglionic fibers arising from this ganglion.
- Finally, the pterygopalatine (sphenopalatine) ganglion is associated with CN V2. The parasympathetic fibers travel within the greater petrosal nerve, which is a branch of CN VII. The lacrimal gland, as well as the palatal and nasal mucous glands, are subsequently innervated by the postsynaptic fibers of the ganglion.
Clinical Examination of the Trigeminal Nerve
Since the trigeminal nerve supplies motor and carries sensory stimuli, clinical examination of this nerve should evaluate the integrity of these modalities with regards to these stimuli. However, based on the territories supplied by the trigeminal nerve, it is also possible to test its integrity by evaluating the corneal and jaw jerk reflexes. Recall that prior to every clinical examination; the clinician should obtain informed consent from the patient. This is done by explaining the steps involved in the test, the expected results and what they would suggest, and where to go from there. This should be done using non-medical jargon to ensure that the patient understands exactly what will be done and what the expected outcome will be.
The sensory modalities being tested are pain perception and light touch. Temperature is not usually checked during clinical exams. For light touch, show the patient the wisp of cotton that will be used to touch their face. This may help to alleviate any unwarranted anxiety about the exam. Then, touch the patient with the cotton wisp on an exposed area of skin, in the same manner, you would during the evaluation. This is used as a reference point (assuming that there are no sensory deficits in that area) for the patient to compare the other stimuli to. The patient is given clear instructions to close their eyes for the duration of this part of the exam and say ‘yes’ every time they feel a touch. Proceed to examine each trigeminal division by lightly tapping the cotton wisp on the territory supplied by each branch of the trigeminal nerve. Avoid applying too much pressure as the patient may misinterpret the pressure sensation for the light touch. Always compare left to right in order to determine whether a unilateral or bilateral (if any at all) deficit exists.
The same steps are then repeated using a neurotip to assess for superficial pain. Record the findings, and interpret any deficits found. Another part of the sensory test that is not frequently performed is the nasal tickle test. It includes gently gliding a cotton wisp inside each nostril. The sensation is rather unpleasant and the patient readily recognizes it. It would be wise for the clinician not to stand directly in front of the patient while performing this test, as they may burst into a fit of sneezing; which is a normal physiological response.
The corneal reflex test is also an uncomfortable experience for the patient. Spend enough time to ease any anxiety or concerns the patient may have regarding this test. With both eyes open, the patient is asked to look up and away to the left. Approach the right eye with a clean cotton wisp inferiorly, from the right side. This is to ensure that the patient doesn’t see the object coming towards their eye and blink prematurely. While approaching the eye with the cotton wisp, the free hand should be used to gently depress the lower lid. Lightly touch the edge of the cornea with the cotton and observe for bilateral blinking. This test evaluates the nasociliary branch of the ophthalmic nerve and its interaction with CN VII, which causes blinking to occur.
Jaw Jerk Reflex
The jaw jerk test is rather straightforward and can be quickly performed. The patient is asked to hang the lower jaw loosely. Place the forefinger of the free hand between the lower lip and the chin. With a patellar hammer, tap the examining finger (not the patient’s chin) in an inferior direction. It is normal to have a slight or absent jerking reflex of the jaw. This test evaluates the mesencephalic nucleus, which is responsible for transmitting muscle distension from the local proprioceptors of the muscles to the motor nucleus of the trigeminal nerve.
Considering that temporalis and masseter are the only two superficial muscles of mastication, the test is usually limited. Additionally, motor signs are rare with trigeminal disease. Nevertheless, the examination requires inspection of the muscles. Temporalis is the most superficial of the group; and wasting of this muscle is readily apparent as Bitemporal guttering. Palpate the masseter and temporalis while the patient clenches their teeth. This helps in assessing the muscle mass. Power can be assessed by placing the fingers of the examining hand under the chin and asking the patient to open their mouth and resist closure; which should be effortlessly achieved.
- The trigeminal nerve is a mixed cranial nerve that has both sensory and motor functions.
- There are three divisions of the trigeminal nerve:
- Ophthalmic division (CN V1 or Va),
- Maxillary division (CN V2 or Vb), Mandibular division (CN V3 or Vc).
- The acronym MOM can be used to recall the three branches of the trigeminal nerve.
- There are four nuclei associated with the trigeminal nerve:
- Main sensory nucleus
- Mesencephalic nucleus
- Spinal trigeminal nucleus
- Motor nucleus
The intracranial course of the trigeminal nerve is as follows:
- Both sensory and motor fibers emerge from the superior pontine sulcus.
- The sensory fibers form synapse at the semilunar (Gasserian, or trigeminal) ganglion in Meckel’s cave.
- The three divisions emerge from the sensory ganglion
- CN V1 and CN V2 enters the cavernous sinus, while CN V3 (along with the motor division) leave the skull through foramen ovale .
- CN V1 enters the skull through the superior orbital fissure, while CN V2 enters the skull through foramen rotundum.
- There are four parasympathetic ganglia that are anatomically associated with the trigeminal nerve. These are the:
- Otic ganglia,
- Pterygopalatine (sphenopalatine) ganglia,
- Ciliary ganglia, and
- Submandibular ganglia.