Cranial Nerves: Somatosensation

• Somatosensation from the face is relayed into the central nervous system (CNS) via cranial nerves V, VII, IX, and X.
• The majority of facial somatosensation is relayed by cranial nerve V (trigeminal nerve).

Trigeminal Nerve (CN V) divisions

• The trigeminal nerve (V) has three major divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3).
  • Trigeminal nerve V means '3 born together.
• Ophthalmic (V1) relays somatosensory information from the forehead and eyes.
• Maxillary (V2) relays somatosensory information from the maxilla, upper teeth, nasal cavity, palate, nasopharynx, and meninges of the anterior and middle cranial fossa.
• Mandibular (V3) relays somatosensory information from the mandible, lower teeth, anterior 2/3 of the tongue, and chin.

Peripheral Processes

• Once the peripheral process enters through the skull foramina, the pseudounipolar cell bodies are located in the trigeminal ganglion (lateral to the cavernous sinus).
• The central nerve processes of the primary afferent neuron enter the CNS at the mid-level of the pons.

Other Somatosensory Inputs

• Somatosensory information from the ear is projected centrally via cranial nerve VII.
• The central process enters the brainstem, projecting to and synapsing on the most caudal sections of the spinal trigeminal nucleus.
• Somatosensory information from the palate, posterior 1/3 of the tongue, and pharynx travel with cranial nerve IX.
• The central process enters the brainstem, projecting to and synapsing on the most caudal sections of the spinal trigeminal nucleus.
• Somatosensory information from the larynx above the vocal folds and a small area around the ear project via cranial nerve X.
• The central process enters the brainstem, projecting to and synapsing on the most caudal sections of the spinal trigeminal nucleus.

Nuclei of the Trigeminal Nerve V

• There are four trigeminal nuclei in the CNS: spinal trigeminal nucleus, principal sensory nucleus of V, mesencephalic nucleus of V, and motor nucleus of V.
• The spinal trigeminal nucleus relays pain and temperature from the ipsilateral face.
• The principal sensory nucleus of V relays touch and vibratory sense from the ipsilateral face.
• The mesencephalic nucleus of V is involved in muscle reflex proprioception.
• The motor nucleus of V controls the muscles of mastication.

Synapses

• Nerve processes can synapse upon two nuclei depending on the type of somatosensory modality being relayed.
• If pain and temperature information is being relayed, fibers synapse on the spinal trigeminal nucleus.
• If touch, vibratory, and position sense information is being relayed, fibers synapse on the principal sensory nucleus of the trigeminal nerve.

Proprioception

• Proprioceptive information from the muscle spindles of the jaw travel in a different pathway.
• The peripheral nerve process is part of V3, but it does not pass through the trigeminal ganglion.
• Instead, it passes through the trigeminal ganglion, enters the CNS, and ascends in the trigeminal spinal tract.
• The cell body is located in the mesencephalic nucleus, situated in the rostral pons medial to the superior cerebellar peduncles and lateral to the fourth ventricle, extending into the midbrain lateral to the periaquaductal gray.
• Neuron #1 for chewing muscle proprioception is located in the CNS, derived from neural crest origin and remaining in the neural tube during development.
• Axons from the mesencephalic nucleus will project to the trigeminal motor nucleus or the cerebellum to control the bite reflex.

Pain and Temperature Pathways from face

• Somatosensory information regarding pain and temperature is relayed via A delta or C fibers from the ophthalmic, maxillary, or mandibular trigeminal areas.
• The cell bodies of neuron #1 are located in the trigeminal ganglion.
• The central nerve process of neuron #1 enters the brainstem at the pontine level.
• This first-order neuron travels down the spinal trigeminal tract and synapses on a second-order neuronal cell body located in the spinal trigeminal nucleus.
• The spinal trigeminal nucleus spans from the mid-pontine level to the C2 spinal cord level (overlaps with the pain and temperature path for the body).
• The cell body of the second-order neuron in the spinal trigeminal nucleus projects its axon to the contralateral side.
• These axons form a tract called the ventral trigeminothalamic tract located medial to the ALS.
• The trigeminothalamic tract synapses upon third-order neuronal cell bodies in the ventral posteromedial nucleus of the thalamus.
• The third-order neuronal axons project through the genu of the internal capsule to the lateral postcentral gyrus, which represents the face.
• The area around the lips has the highest peripheral innervation density.

Discriminative Touch Pathways from the face

• Somatosensory information regarding discriminative touch is relayed via large-diameter axons from the ophthalmic, maxillary, or mandibular trigeminal areas.
• The cell bodies are located in the trigeminal ganglion.
• The central nerve process enters the brainstem at the mid-pontine level laterally.
• The first-order neuron synapses upon the cell bodies in the principal (chief) sensory nucleus of V.
• The majority of second-order axons from the principal sensory nucleus of V cross to the contralateral side.
• The second-order axons ascend as the trigeminothalamic tract and synapse onto the VPM of the thalamus.
• The third-order neuronal axons project through the genu of the internal capsule to the lateral postcentral gyrus, representing the face.

Clinical Aspects of CN V

• Tic Douloureux (trigeminal neuralgia) involves severe burning or stabbing pain within a trigeminal nerve branch. Its cause is unknown, but even light touch can trigger an attack.
• Herpes Zoster (shingles) is caused by a virus that causes chickenpox and then lies dormant in the dorsal root or trigeminal ganglia and can reactivate causing pain and blisters in affected dermatome area.

Pain and Temperature (Comparison)

• Pain and temperature sensation from the body involves a pseudounipolar first neuron with the cell body in the posterior root ganglion, and the pathway enters the posterior spinal cord, travels in the posterolateral fasciculus of Lissauer,and synapses is the substantia gelatinosa, decussates in the spinal cord, traveling to the to postcentral gyrus.
• Pain and temperature sensation from the face involves a pseudounipolar first neuron with the cell body in the trigeminal ganglion. The fibers travel in the spinal trigeminal tract and synapse in the spinal trigeminal nucleus. The axons decussate in the medulla/pons and form the trigeminothalamic tract. They synapse in the VPM of the thalamus and project to the postcentral gyrus.

Touch, Pressure and Vibration (Comparison)

• Touch from the body involves a pseudounipolar first order neuron with the cell body in the posterior root ganglion, and enters the spinal cord, ascends via fasciculus gracilis or cuneatus, synapses on the nucleus gracilis or cuneatus, which decussates, forms medial lemniscus, synapses on the VPL of thalamus, and then projects to postcentral gyrus.
• Touch from the face involves a pseudounipolar first order neuron with the cell body in the trigeminal ganglion, and enters lateral pons, synapses on the principal sensory nucleus of V, which decussates, forms trigeminothalamic tract, synapses on the VPM of thalamus and projects to postcentral gyrus.

Proprioception (Comparison)

• Proprioception from the body is relayed via muscle spindles, travels via dorsal spinocerebellar tract, and travels via superior cerebellar peduncle to ipsilateral cerebellum.
• Proprioception from the face is relayed via muscle spindles, travels via mesencephalic nucleus and superiors cerebellar peduncles to the ipsilateral cerebellum.

Corneal Blink Reflex

• Afferent sensory: Trigeminal nerve, ophthalmic division / cranial nerve V1
• Efferent (motor) Facial nerve / Cranial nerve VII
• Somatosensory information from the eye travels into the CNS via V1. Some axons will synapse upon the facial motor nucleus in the pons which projects its axons to muscles of facial expression and causes the blink reflex.

Innervations of the Tongue

• General sensory components is glossopharyngeal/ IX, trigeminal mandibular / V3
• Special sensory (taste) components are from IX, and Facial / VII
• Movement or the tongue is controlled by hypoglossal / XII
• Somatosensory information is via V3 and IX
• Taste is via VII and IX (special sensory)
• Movement via XII (motor)

Visceral motor control via parasympathetic ANS

• The autonomic nervous system consists of a 2-neuron chain (pre and post ganglionic neurons) and is ultimately controlled by the hypothalamus.
• Cranial Nerve VII (Facial): Tears & Mucous. The cell body is in superior salivatory nucleus in pons (part of cranial nerve VII) axons synapse on the pterygopalatine ganglion whose axons project to the lacrimal gland (tears) or to nasal mucosa (snot).
• Cranial Nerve VII (Facial) and IX (Glossopharyngeal): Saliva. Via Cranial Nerve VII: Cell bodies in the superior salivatory nucleus project out and synapse upon the submandibular ganglion which projects its axons to the submandibular and sublingual glands (saliva). Via Cranial Nerve IX: Cell bodies in the inferior saliviatory nucleus project thier axons and synapse upon the otic ganglion which projects its axons to the parotid gland (more saliva).

Somatic motor control of muscles of facial expression

• Motor facial nucleus (lower motor neuron) whose axons (CN VII) innervate the ipsilateral muscles of facial expression.
• Top half of the facial nucleus innervates the ipsilateral top portion of the face, while the bottom half of the nucleus, the ipsilateral bottom portion of the face.
• Upper motor neurons in the precentral gyrus innervate the contralateral lower motor neurons as well as the top portion of the ipsilateral facial nucleus.
• An upper motor neuron lesion leads to paralysis of voluntary muscles of facial expression in the contralateral lower quadrant, due to little information from the ipsilateral side. An upper motor neuron lesion may cause some partial paralysis in the upper quadrant as it is receives bilateral innervation from both precentral gyri.
• A lower motor neuron lesion leads to paralysis of the muscles in the ipsilateral half of the face because neurons in the facial nuclei and its axons innervate only the ipsilateral muscles.