Trigeminal System | Anatomy2Medicine
Trigeminal System

Trigeminal System

 

  • Trigeminal System

 

      • Trigeminal Nerve (CN V)

 

  • is the largest cranial nerve.
  • exits the brainstem from the pons.

 

        • is the nerve of the first branchial arch (mandibular nerve).

 

  • Contains sensory (general somatic afferent [GSA] ) and motor (special visceral efferent [SVE] ) fibers.
  • provides sensory innervation to the face and oral cavity.

 

        • innervates the dura of the anterior and middle cranial fossae.
        • innervates the muscles of mastication.

 

  • consists of a large ganglion that gives rise to three major divisions: ophthalmic, maxillary, and mandibular.
  • Trigeminal (or semilunar or gasserian) ganglion

 

        • is located in the trigeminal fossa of the petrous bone in the middle cranial fossa.

 

  • is enclosed by a duplication of the dura (Meckel cave).
  • contains pseudounipolar ganglion cells, which are first-order neurons for the trigeminothalamic tracts.
  • Ophthalmic nerve (CN V-1)
  • lies in the lateral wall of the cavernous sinus.

 

          • enters the orbit via the superior orbital fissure.

 

  • innervates the forehead, dorsum of the nose, upper eyelid, orbit (cornea and conjunctiva), mucous membranes of the nasal vestibule and frontal sinus, and the cranial dura.

 

          • mediates the afferent limb of the corneal reflex.

 

  • Maxillary nerve (CN V-2)

 

          • lies in the lateral wall of the cavernous sinus.
          • exits the skull via the foramen rotundum.

 

  • innervates the upper lip and cheek, lower eyelid, anterior portion of the temple,
  • paranasal sinuses, oral mucosa of the upper mouth, nose, pharynx, gums, teeth, hard palate, soft palate, and cranial dura.
  • Mandibular nerve (CN V-3)

 

          • exits the skull via the foramen ovale.
          • consists of a motor component that innervates the

 

  • muscles of mastication (i.e., the temporalis, masseter, and lateral and medial pterygoids)

 

            • two suprahyoid muscles, the mylohyoid and the anterior belly of the digastric
            • the tensores tympani and veli palatini.

 

  • consists of a sensory component that innervates the lower lip and chin, posterior por- tion of the temple, external auditory meatus and tympanic membrane, external ear, teeth of the lower jaw, oral mucosa of the cheeks and the floor of the mouth, anterior two-thirds of the tongue, temporomandibular joint, and cranial dura.

 

      • Cranial nerves VII, IX, and X
        • contribute GSA fibers from the external ear to the trigeminal system.
        • use the spinal trigeminal tract and nucleus.
      • Spinal trigeminal tract
        • extends from C3 to the level of the trigeminal nerve in the mid pons.
        • is a homolog of the dorsolateral tract of Lissauer.

 

  • receives pain, temperature, and light touch input from CN V, CN VII, CN IX, and CN X.

 

        • transection (tractotomy) results in ipsilateral facial anesthesia.
        • projects to the spinal trigeminal nucleus as follows:
          • Pain fibers terminate in the caudal third of the spinal trigeminal nucleus.

 

  • Corneal reflex fibers terminate in the rostral two-thirds of the spinal trigeminal nucleus.
  • Ascending Trigeminothalamic Tracts

 

        • convey GSA information from the face, oral cavity, and dura mater to the thalamus.
        • consist of chains of three neurons.
        • first-order neurons
        • are pseudounipolar ganglion cells

 

  • located  in the trigeminal ganglion and in the sensory ganglia of CN VII, CN IX, and CN X.

 

      • Ventral trigeminothalamic tract
        • serves as a pain, temperature, and light touch pathway from the face and oral cavity.
        • contains GSA fibers from CN VII, CN IX, and CN X (ear and external auditory meatus).
        • receives input from free nerve endings and Merkel tactile disks.

 

  • Receives discriminative tactile and pressure input from the contra lateral principal sensory nucleus of CN V, which terminates in the ventral posteromedial (VPM) nucleus of the thalamus.

 

        • ascends to the contralateral sensory cortex via three neurons:
          • First-order neurons
            • are located in the trigeminal ganglion.
            • mediate pain and temperature sensation and give rise to axons that descend in the spinal trigeminal tract.
            • mediate light touch sensation and give rise to bifurcating axons that ascend and descend in the spinal trigeminal tract.
            • synapse with second-order neurons in the spinal trigeminal nucleus.
          • Second-order neurons
            • are located in the spinal trigeminal nucleus.

 

  • give rise to decussating axons that terminate in the contralateral VPM nucleus of the thalamus.

 

            • project axons to the reticular formation and to motor cranial nerve nuclei to mediate reflexes (e.g., tearing and corneal reflexes).

 

  • mediate painful stimuli and are found in the caudal third of the spinal trigeminal nucleus.

 

          • Third-order neurons
            • are located in the VPM nucleus.
            • project via the posterior limb of the internal capsule to the face area of the postcentral gyrus (areas 3, 1, and 2).

 

  • Dorsal trigeminothalamic tract
  • is the rostral equivalent of the dorsal column–medial lemniscus system

 

          • subserves discriminative tactile and pressure sensation from the face and oral cavity via the GSA fibers of CN V.
          • contains some discriminative GSA fibers from CN VII, CN IX, and CN X, which innervate the ear.
          • receives input from Meissner and Pacini corpuscles.
          • is an uncrossed tract.
          • ascends to the sensory cortex via three neurons.

 

  • First-order neurons

 

            • are located in the trigeminal ganglion.
            • synapse in the principal sensory nucleus of CN V.

 

  • Second-order neurons

 

            • are located in the principal sensory nucleus of CN V.

 

  • project to the ipsilateral VPM nucleus of the thalamus.
  • Third-order neurons

 

            • are located in the VPM nucleus.
            • project via the posterior limb of the internal capsule to the face area of the postcentral gyrus (areas 3, 1, and 2).

 

  • Trigeminal Sensory Nuclei
  • Principal (or chief) sensory nucleus
  • is a homolog of the dorsal column nuclei of the medulla.

 

          • is located in the rostral pontine tegmentum at the level of the trigeminal motor nucleus of CN V.
          • receives discriminative tactile input from the face.

 

  • projects via the uncrossed dorsal trigeminothalamic tract to the VPM nucleus of the thalamus.
  • projects via the crossed ventral trigeminothalamic tract to the VPM nucleus of the thalamus.
  • Spinal trigeminal nucleus

 

          • is located in the spinal cord (C1–C3), medulla, and pons.
          • receives pain and temperature input from the face and oral cavity.
          • projects via the crossed ventral trigeminothalamic tract to the VPM nucleus of the thalamus.

 

  • Mesencephalic nucleus

 

          • subserves GSA proprioception from the head.
          • consists of large pseudounipolar neurons
          • receives input from muscle spindles and pressure and joint receptors.
          • receives input from the muscles of mastication and extraocular muscles, the teeth and hard palate, and the temporomandibular joint.

 

  • projects to the trigeminal motor nucleus to mediate the muscle stretch (jaw jerk) reflex and regulate the force of bite.

 

        • Trigeminal motor nucleus (SVE)

 

  • is located in the rostral pontine tegmentum at the level of the principal sensory nucleus of CN V.

 

          • innervates the muscles of mastication.
          • receives bilateral corticobulbar input.
          • receives input from the mesencephalic nucleus.

 

  • Trigeminocerebellar Fibers
  • project from the mesencephalic nucleus of CN V via the superior cerebellar peduncle to the dentate nucleus.
  • project from the principal sensory and spinal trigeminal nuclei via the inferior cerebellar peduncle to the cerebellar vermis.

 

      • Trigeminal Reflexes

 

  • Jaw jerk (masseter) reflex

 

          • is a monosynaptic myotatic reflex.
          • The afferent limb is the mandibular nerve (CN V-3)
          • The efferent limb is the mandibular nerve (CN V-3).

 

  • Corneal reflex

 

          • is a consensual and disynaptic reflex.
          • has its first-order neuron (afferent limb) in the trigeminal ganglion.
          • has its second-order neuron in the rostral two-thirds of the spinal trigeminal nucleus.
          • has its third-order neuron (efferent limb) in the facial nucleus.
          • The afferent limb is the ophthalmic nerve (CN V-1).
          • The efferent limb is the facial nerve (CN VII).

 

  • Lacrimal (tearing) reflex
  • The afferent limb is the ophthalmic nerve (CN V-1)

 

        • it receives impulses from the cornea and conjunctiva.
      • The efferent limb is the facial nerve (CN VII).
      • It transmits impulses via the superior salivatory nucleus, greater petrosal nerve, pterygopalatine ganglion, and the zygomatic (CN V-2) and lacrimal (CN V-1) nerves to the lacrimal gland

 

 

  • Trigeminal neuralgia (tic douloureux)

 

        • is characterized by recurrent paroxysms of sharp, stabbing pain in one or more branches of the trigeminal nerve on one side of the face.
        • can result from a redundant loop of the superior cerebellar artery that impinges on the trigeminal root.

 

  • Herpes zoster ophthalmicus

 

        • is a viral infection affecting the ophthalmic nerve (CN V-1).
        • corneal ulceration with infection may result in blindness.

 

  • Paratrigeminal (Raeder) syndrome

 

        • is due to lesions of the trigeminal ganglion and sympathetic fibers
        • the lesion is usually in the parasellar region.

 

  • results in miosis, ptosis, facial pain (similar to trigeminal neuralgia), and trigeminal palsy.

 

may involve CN III, CN IV, and CN VI