The Trigeminal Sensory System

Question 1

What are the 3 divisions of the trigeminal nerve?

Answer 1

1. opthalamic (V1)
2. maxillary (V2)
3. mandibular (V3)

Question 2

What does CN V innervate?

Answer 2

• face
• head and also nasal sinuses
• inside of nose
• mouth (including teeth and gums)
• anterior 2/3 of tongue

Question 3

Cell bodies of sensory neurons innervating head/face are found in the __________.

Answer 3

trigeminal ganglion (one on each side)

Question 4

Trigeminal sensory systems for face are homologous to _________ and ___________ for the body.

Answer 4

• dorsal column medial lemniscus
• anterolateral system for body

Question 5

Trigeminal nuclei:

Answer 5

• **Main (chief or principal) sensory nucleus **
• Spinal trigeminal nucleus
  
  • extending from pons down to cervical spinal cord
  • homologous to spinal dorsal horn for body
  • Caudal nucleus: extends from obex to spinal cord. Important nucleus for conveying pain and temp info from face, head

Question 6

Main (chief or principal) sensory nucleus:

Answer 6

• Fine touch:
• located in pons
• homologous to dorsal column nuclei for body

Question 7

Spinal trigeminal nucleus:

Answer 7

• **Pain and temp: **
  
  • extending from pons down to cervical spinal cord
  • homologous to spinal dorsal horn for body
• Caudal nucleus: extends from obex to spinal cord.
  
  • Important nucleus for conveying pain and temp info from face, head

Question 8

Thalamic nuclei:

Answer 8

• Ventral Posterior Medial nucleus
  
  • both fine touch info, and pain and temp info go through VPM
  • stay segregated

Question 9

What is the primary afferent input for the trigeminal system?

Answer 9

Mechanoreceptor fibers enter from 3 branches of the Trigeminal nerve V

Question 10

Describe the major pathway:

Answer 10

1. Most fine touch mechanoreceptors synapse in the Chief (or Main) sensory nucleus of V
   
   • on ipsilateral side
2. 2nd order neurons then send off fibers that cross to the other side of the pons and form the Trigeminal Lemniscus
   
   • joins the Medial Lemniscus
3. Trigeminal lemniscus ascends to the Ventral Posterior Medial (VPM) nucleus
   
   • terminates on to 3rd order thalamic neurons

Question 11

Somatotopy of head and face in VPM:

Answer 11

• Tongue: medial
• Face: lateral

Question 12

Minor pathway for some crude touch:

Answer 12

• A few mechanoreceptors:
  
  • enter the pons but descend directly in the spinal tract of V to the spinal nucleus of V
  • Then synapse
• 2nd order neurons:
  
  • send fibers that cross the midline
  • ascend in the trigeminothalamic tract to VPM

Question 13

Minor pathway for oral cavity:

Answer 13

Dorsal trigeminal tract:

• A few mechanoreceptors:
  
  • enter the pons
  • synapse in Chief sensory nucleus
  • 2nd order neurons don’t cross but ascend on the ipsilateral side to the ipsilateral VPM
• Conveys touch and pressure sensation from the oral cavity
  
  • including teeth, mouth and gums

Question 14

Function of 3rd order VPM thalamic neurons:

Answer 14

• send projections through the posterior limb of the internal capsule and corona radiata
• terminate in the lateral aspect of SI cortex
  
  • near lateral fissure

Question 15

Fibers carrying fine touch from the body and face enter tracts at __________ of the brainstem , but rostrally they _______ in ______________.

Answer 15

• different levels
• run together
• Medial/Trigeminal lemniscus tracts

Question 16

• Body fibers enter at _______.
• Face fibers enter at _______.

Answer 16

• medulla
• pons

Question 17

Trigeminothalamic tract: Primary Afferent Input:

Answer 17

Nociceptors from the face (3 branches of trigeminal nerve) enter in nerve V at pons

Question 18

Nociceptors don’t synapse immediately but rather descend ___________ in the __________ and terminate in the ____________.

Answer 18

• ipsilaterally
• Spinal Trigeminal Tract
• spinal trigeminal nucleus of V

Question 19

• Why does dirt under the contacts always hurt?
• Why is dental work painful?

Answer 19

• Some head tissues like tooth pulp and cornea are innervated only by nociceptors

Question 20

Trigeminothalamic Tract: 2nd Order Neurons

Answer 20

• send axons that cross to other side of medulla
• ascend as the trigeminothalamic tract along side the anterolateral tracts ⇒ Ventral Posterior Medial (VPM) nucleus of the thalamus

Question 21

Trigeminothalmaic Tract: 3rd order neurons

Answer 21

• In VPM project through the posterior limb of the internal capsule and corona radiata up to the lateral SI cortex
• Some others project to the reticular formation

Question 22

What is trigeminal neuralgia?

Answer 22

• Neuralgia = pain along course of a nerve
• lancinating, severe pain that last seconds to minutes
• episodes brought on by chewing or shaving or touching trigger points in face
• Unknown etiology
• Fine touch, vibration facial sensation is normal
• Occurs in any or many: ear, eye, lips, nose, scalp, forehead, teeth, jaw
• Region of V2/V3 territory
• Treatment: carbamazepine (anticonvulsant; acts on Na+ channels,
  reduces excitability of neurons); sometimes antidepressants

Question 23

Temperomandibular joint disorder (TMD):

Answer 23

• chronic pain localized at the temperomandibular joint or in muscles of mastication
• recurrent headaches
• ear pain
• Associated with teeth grinding, clenching at night, gum chewing, fingernail biting; trauma to jaw; stress
• Treatment: mouth guard, medications (NSAIDS); stress management