The trigeminal nerve is the fifth cranial nerve (CN V). Its primary function is to provide sensory and motor innervation to the face. The trigeminal nerve consists of three branches on either side that extend to different territories of the face. These branches join at the trigeminal ganglia which are located within the Meckel cave of the cranial cavity. The different branches are namely the ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves. The ophthalmic nerve is responsible for sensory innervation of the face and skull above the palpebral fissure as well as the eye and portions of the nasal cavity. The maxillary nerve is also a sensory branch and innervates portions of the nasal cavity, sinuses, maxillary teeth, palate, and the middle portion of the face and skull above the mouth and below the forehead. The mandibular nerve is unique in that it contains both sensory and motor fibers. It provides sensory innervation of the buccal mucosa, mandibular teeth, and the skin below the mouth. The motor portion of V3 innervates all the muscles of mastication. Additionally, V3 provides sensory information from the anterior two-thirds of the tongue; this is differentiated from taste which is produced by CN VII.[rx][rx]
The fifth cranial nerve is known as the trigeminal nerve and has 3 branches which are the ophthalmic, maxillary, and mandibular. The third branch is called the mandibular nerve (V3). It is the largest of the 3 branches and carries both afferent and efferent fibers. The first 2 branches of the trigeminal nerve carry only afferent fibers. The mandibular nerve innervates the lower face and the mandible, including the teeth, the temporomandibular joint, and the mucous membrane of the mouth and the anterior two-thirds of the tongue, the mastication muscles, and some smaller muscles.[rx][rx][rx]
Structure and Function
As stated previously, the trigeminal nerve provides both sensory and motor innervation to the face. More specifically, the sensory information conducted by this nerve includes touch, pain, and temperature. This sensory information is relayed through the main trigeminal nucleus and nuclei of the thalamus before it travels to the cerebral cortex and synapses in the post-central gyrus. As with all other sensory information of the body, the sensory information from the face crosses over (decussates) to the contralateral brain hemisphere.
CN V is classified into three different branches that have distinct territories of innervation. These branches are namely the ophthalmic, the maxillary, and the mandibular nerves.[rx][rx][rx][rx][rx]
Ophthalmic Nerve (V1)
The ophthalmic nerve is primarily responsible for the sensory innervation of the face and scalp above the orbits. It also contains sympathetic nerve fibers responsible for pupil dilation and supplies the ciliary body, iris, lacrimal gland, conjunctiva, and cornea. In addition to these superficial sensory functions, the ophthalmic nerve also supplies the superior portion of the nasal cavity, the frontal sinus, and even deeper structures including the dura mater and portions of the anterior cranial fossa.
Maxillary Nerve (V2)
The maxillary nerve is also a sensory branch, and it supplies the territory just below the orbits and above the mouth. This includes the inferior portion of the nasal cavity, the maxillary teeth, and the maxillary sinus.
Mandibular Nerve (V3)
The mandibular nerve is the only branch of the trigeminal nerve that has both sensory and motor components. The motor component innervates all of the muscles of mastication (enumerated below). The sensory portion is responsible for pain and temperature information from the mandibular teeth, buccal mucosa, temporomandibular joint, the anterior two-thirds of the tongue, and the face below the territory of the maxillary nerve.
Mesencephalic Tract and Nucleus
This tract and nucleus are in the caudal midbrain and rostral pons near the periaqueductal gray. While not yet fully understood, the belief is that it is a representation of the primary sensory ganglion— much like dorsal root ganglion— that became incorporated into the brainstem during embryonic development. It is responsible for unconscious proprioception that stems from muscle spindles in the muscles of mastication and other muscles of the head and neck.[rx] It is thus essential in the process of receiving sensory information about tooth pain, helps prevent excessive biting that may break a tooth, receives information about stretch sensation from the muscles of mastication, and serves as the afferent limb of the jaw jerk reflex.[rx][rx] In this reflex, there is an interplay between the mesencephalic nucleus and motor V nucleus. This nucleus is unique in that the fibers do not have a cell body in the trigeminal ganglion. Instead, pseudounipolar afferent fibers from stretch receptors relay information directly to the cell body in the nucleus, which is then relayed bilaterally to the trigeminal motor nuclei, which project to cause contraction of the masseter.[rx]
Chief/Principal Sensory Nucleus
This nucleus is in the mid-pons lateral to the trigeminal motor nucleus and the fibers of the trigeminal nerve. It contains second-order cell bodies that synapse with primary order neuronal fibers from the trigeminal ganglion. There are two divisions of this nucleus: the dorsomedial and ventrolateral divisions. The dorsomedial division receives input only from the oral cavity, whereas the ventrolateral division receives input from all three divisions of the trigeminal nerve. This fact is important because second-order neuronal fibers conveying information from the dorsomedial division form the dorsal trigeminothalamic tract (DTTT) and the second-order neuronal fibers from the ventrolateral division decussate and form the ventral trigeminothalamic tract (VTTT). It is responsible for 2-point discrimination, conscious proprioception, vibration, and fine touch.[rx]
Spinal Trigeminal Nucleus
This nucleus is the largest trigeminal nucleus and is in the lateral tegmentum of the medulla and caudal pons. The spinal trigeminal nucleus travels adjacent to the spinal trigeminal tract. The spinal trigeminal nucleus is continuous with the substantial gelatinosa, while the track is continuous with Lissauer’s tract. Pseudounipolar neurons located in the trigeminal ganglion receive sensory information from the face and send that information down to the spinal trigeminal nucleus where it synapses with a second-order neuron, which will project to the thalamus as the ventral trigeminothalamic tract (see below). This nucleus divides into three subnuclei: pars oralis (most rostral), pars interpolar, and pars caudalis (most caudal). It is important to note that the somatotopy of the three trigeminal sensory divisions that supply the face is maintained in the spinal trigeminal nucleus. Therefore, sensory information from the lateral face projects more caudally to the pars caudalis, the middle (cheek and eye) to the pars interpolates, and the central face (mouth and nose) to the pars oralis. The remaining sensory fibers that do not travel to these two nuclei will instead travel to the mesencephalic nucleus (previously described).[rx][rx][rx]
It is responsible for pain, temperature, and crude touch. Unique to this nucleus is that it receives sensory information from cranial nerves VII, IX, and X (ear, tongue, pharynx, and larynx).[rx]
Motor V Nucleus
This nucleus is in the dorsolateral pontine tegmentum at the mid-pons, medial to the trigeminal nerve fibers, and the chief sensory nucleus and lateral to the mesencephalic nucleus. Its fibers are only found in the mandibular division of the trigeminal nerve and receive input from the cortex bilaterally.[rx][rx][rx]
Tracts of the Trigeminal System
There are three main tracts of the trigeminal system; the spinal trigeminal tract (discussed above), the ventral trigeminothalamic tract, and the dorsal trigeminothalamic tract. These tracts ultimately synapse with third-order neurons in the VPM and continue to the primary sensory cortex.
Ventral Trigeminothalamic Tract (VTTT)
This tract conveys information from both the spinal trigeminal nucleus and the chief sensory nucleus. General somatic afferents are picked up by Merkel tactile disks and free nerve endings and travel to one of these nuclei where they synapse with second-order neurons. As stated above, the second-order neuronal fibers then form this tract. When the fibers originate in the spinal trigeminal nucleus, they first decussate and then form the VTTT. This specific tract conveys fast pain and temperature from one side of the face to the contralateral ventral posteromedial nucleus of the thalamus (VPM) and eventually to the primary sensory cortex. In contrast, fibers that synapse in the chief sensory nucleus (i.e., 2-point discrimination, conscious proprioception, vibration, and fine touch) can decussate and form the VTTT to the contralateral ventral posteromedial nucleus of the thalamus, or they can ascend as the DTTT.[rx]
Ventral Posteromedial Nucleus of the Thalamus (VPM)
This nucleus contains third-order neuronal cell bodies and sends that information somatotopically via third-order neuronal fibers to the postcentral gyrus (Brodmann areas 3, 1, and 2) of the primary sensory cortex. Additionally, the VPM conveys slow, dull pain to the contralateral reticular formation, which is then sent to the intralaminar thalamic nuclei and finally to the widespread cortex. Since the course of the fibers all over the cortex, pain that is not acute is often diffuse and difficult to pinpoint. Lastly, information related to reflexive head and neck movements transmits to the contralateral tectum (superior colliculus and periductal gray).[rx][rx]
Dorsal Trigeminothalamic Tract (DTTT)
This tract conveys information only from the ipsilateral chief sensory nucleus (2-point discrimination, conscious proprioception, vibration, and fine touch) to the ipsilateral VPM.[rx]
Muscles
The only branch of the trigeminal nerve that has a motor component in the mandibular nerve (V3). This branch supplies motor innervation to the facial muscles involved in mastication which include the masseter, temporalis muscle, and the lateral and medial pterygoids. Additionally, V3 gives off branches that innervate the tensor veli palatine, the mylohyoid, the tensor tympani, and the anterior portion of the digastric muscle.
Nerve Supply
CN V is classified into three different branches that have distinct territories of innervation. These branches are namely the ophthalmic, the maxillary, and the mandibular nerves.[rx][rx][rx][rx][rx]
- Ophthalmic Nerve (V1) – The ophthalmic nerve is primarily responsible for the sensory innervation of the face and scalp above the orbits. It also contains sympathetic nerve fibers responsible for pupil dilation and supplies the ciliary body, iris, lacrimal gland, conjunctiva, and cornea. In addition to these superficial sensory functions, the ophthalmic nerve also supplies the superior portion of the nasal cavity, the frontal sinus, and even deeper structures including the dura mater and portions of the anterior cranial fossa.
- Maxillary Nerve (V2) – The maxillary nerve is also a sensory branch, and it supplies the territory just below the orbits and above the mouth. This includes the inferior portion of the nasal cavity, the maxillary teeth, and maxillary sinus.
- Mandibular Nerve (V3) – The mandibular nerve is the only branch of the trigeminal nerve that has both sensory and motor components. The motor component innervates all of the muscles of mastication (enumerated below). The sensory portion is responsible for pain and temperature information from the mandibular teeth, buccal mucosa, temporomandibular joint, the anterior two-thirds of the tongue, and the face below the territory of the maxillary nerve.
Divisions
Ophthalmic Nerve
Ophthalmic nerve gives rise to 3 terminal branches: frontal, lacrimal, and nasociliary, which innervate the skin and mucous membrane of derivatives of the frontonasal prominence derivatives:
- Forehead and scalp
- Frontal and ethmoidal sinus
- Upper eyelid and its conjunctiva
- Cornea (see clinical relevance)
- Dorsum of the nose
Parasympathetic Supply
- Lacrimal gland: Postganglionic fibers from the pterygopalatine ganglion (derived from the facial nerve), travel with the zygomatic branch of V2 and then join the lacrimal branch of V1. The fibers supply parasympathetic innervation to the lacrimal gland.
Clinical Relevance: Corneal Reflex
- The corneal reflex is the involuntary blinking of the eyelids – stimulated by tactile, thermal or painful stimulation of the cornea.
- In the corneal reflex, the ophthalmic nerve acts as the afferent limb – detecting the stimuli. The facial nerve is the efferent limb, causing contraction of the orbicularis oculi muscle.
- If the corneal reflex is absent, it is a sign of damage to the trigeminal/ophthalmic nerve, or the facial nerve.
Maxillary Nerve
Maxillary nerve gives rise to 14 terminal branches, which innervate the skin, mucous membranes and sinuses of derivatives of the maxillary prominence of the 1st pharyngeal arch:
- Lower eyelid and its conjunctiva
- Cheeks and maxillary sinus
- Nasal cavity and lateral nose
- Upper lip
- Upper molar, incisor and canine teeth and the associated gingiva
- Superior palate
Parasympathetic Supply
- Lacrimal gland – Postganglionic fibers from the pterygopalatine ganglion (derived from the facial nerve), travel with the zygomatic branch of V2 and then join the lacrimal branch of V1. The fibers supply parasympathetic innervation to the lacrimal gland.
- Nasal glands – Parasympathetic fibers are also carried to the mucous glands of the nasal mucosa. Post-ganglionic fibers travel with the nasopalatine and greater palatine nerves (branches of V2)
Mandibular Nerve
Mandibular nerve gives rise to four terminal branches in the infra-temporal fossa: buccal nerve, inferior alveolar nerve, auriculotemporal nerve and lingual nerve.
These branches innervate the skin, mucous membrane, and striated muscle derivatives of the mandibular prominence of the 1st pharyngeal arch.
Sensory supply
- Mucous membranes and floor of the oral cavity
- External ear
- Lower lip
- Chin
- Anterior 2/3 of the tongue (only general sensation; special taste sensation supplied by the chorda tympani, a branch of the facial nerve)
- Lower molar, incisor and canine teeth and the associated gingiva
Motor Supply
- Muscles of mastication; medial pterygoid, lateral pterygoid, masseter, temporalis
- Anterior belly of the digastric muscle and the mylohyoid muscle (these are suprahyoid muscles)
- Tensor veli palatini
- Tensor tympani
Parasympathetic Supply
- Submandibular and Sublingual glands – Post-ganglionic fibres from the submandibular ganglion (derived from the facial nerve), travel with the lingual nerve to innervate these glands.
- Parotid gland – Post-ganglionic fibres from the otic ganglion (derived from the glossopharyngeal nerve, CN IX), travel with the auriculotemporal branch of the V3 to innervate the parotid gland.
Nuclei
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:
- mesencephalic
- primary sensory
- motor
- spinal nuclei.
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 interpolates 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) .
Function
The sensory function of the trigeminal nerve is to provide tactile, proprioceptive, and nociceptive afference to the face and mouth. Its motor function activates the muscles of mastication, the tensor tympani, tensor veli palatine, mylohyoid, and the anterior belly of the digastric.
The trigeminal nerve carries general somatic afferent fibers (GSA), which innervate the skin of the face via ophthalmic (V1), maxillary (V2), and mandibular (V3), divisions. The trigeminal nerve also carries special visceral efferent (SVE) axons, which innervate the muscles of mastication via the mandibular (V3) division.
Muscles of mastication
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. A useful mnemonic for remembering these muscles is “My Tensors Dig Ants 4 MoM” (Mylohyoid—Tensor Tympani + Tensor Veli Palatini—Digastric (Anterior) – 4 Muscles of Mastication (Temporalis, Masseter, Medial and Lateral Pterygoids))
With the exception of the tensor tympani, all these muscles are involved in biting, chewing and swallowing and all have bilateral cortical representation. A unilateral central lesion (for example, a stroke), no matter how large, is unlikely to produce an observable deficit. Injury to a peripheral nerve can cause paralysis of muscles on one side of the jaw, with the jaw deviating towards the paralyzed side when it opens. This direction of the mandible is due to the action of the functioning pterygoids on the opposite side.
Sensation
The two basic types of sensation are touch-position and pain-temperature. Touch-position input comes to attention immediately, but pain-temperature input reaches the level of consciousness after a delay; when a person steps on a pin, the awareness of stepping on something is immediate but the pain associated with it is delayed.
Touch-position information is generally carried by myelinated (fast-conducting) nerve fibers and pain-temperature information by unmyelinated (slow-conducting) fibers. The primary sensory receptors for touch-position (Meissner’s corpuscles, Merkel’s receptors, Pacinian corpuscles, Ruffini’s corpuscles, hair receptors, muscle spindle organs, and Golgi tendon organs) are structurally more complex than those for pain-temperature, which are nerve endings.
Sensation in this context refers to the conscious perception of touch-position and pain-temperature information, rather than the special senses (smell, sight, taste, hearing, and balance) processed by different cranial nerves and sent to the cerebral cortex through different pathways. The perception of magnetic fields, electrical fields, low-frequency vibrations, and infrared radiation by some nonhuman vertebrates is processed by their equivalent of the fifth cranial nerve.
Touch in this context refers to the perception of detailed, localized tactile information, such as two-point discrimination (the difference between touching one point and two closely spaced points) or the difference between coarse, medium or fine sandpaper. People without touch-position perception can feel the surface of their bodies and perceive touch in a broad sense, but they lack perceptual detail.
Position, in this context, refers to conscious proprioception. Proprioceptors (muscle spindle and Golgi tendon organs) provide information about joint position and muscle movement. Although much of this information is processed at an unconscious level (primarily by the cerebellum and the vestibular nuclei), some is available at a conscious level.
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.
References
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