Trigeminal nerve (CN V)

Ophthalmic division (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.

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Maxillary division (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).

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Mandibular division (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.

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Mesencephalic nucleus

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.

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