Trigeminal Pathways

Question 1

Describe primary axons of trigeminal

Answer 1

After entering brainstem, many of the primary axons bifurcate into ascending and descending roots
These fibers convey general tactile information to both main sensory nucleus, subnuclei rostralis and interpolaris of descending (spinal) nerves of V

Question 2

What does the short ascending root of V do?

Answer 2

Terminate in main sensory nucleus of V

Convey precise discriminative tactile information from face

Question 3

What does the mesencephalic tract of V do?

Answer 3

Composed of both peripheral and central processes of pseudounipolar neurons located in mesencephalon
Conveys proprioceptive information to main sensory nucleus

Question 4

What does the descending root of V do?

Answer 4

Teminate subnucleus caudalis of descending nucleus of V

Fibers convey pain (A delta and C fibers) and temperature information from the face

Question 5

What are the four central nuclei associated with the trigeminal nerve?

Answer 5

Main sensory nucleus
Descending (spinal) nucleus of V
Messencephalic nucleus of V
Trigeminal motor nucleus

Question 6

Describe the main sensory nucleus

Answer 6

Analagous to nuclei gracilis and cuneatus
Located at level of trigeminal nerve
Central processes of primary neurons in trigeminal ganglion and mesencephalic nucleus terminate in main sensory nucleus
Receives precise tactile, crude tactile, and proprioceptive information from the face

Question 7

Describe the descending (spinal) nucleus of V

Answer 7

Continuous with and directly analogous to dorsolatearl fasciculus of Lissauer in spinal cord
All pain and temperature sensations as well as general tactile information from face terminate in one or more of its three subnuclei

Question 8

Describe subnucleus of rostralis

Answer 8

Extends from level of entrance of V to upper medulla

Receives tactile information from central region of face

Question 9

Describe subnucleus interpolaris

Answer 9

Extends from upper medulla to level of obex

Receives information from peripheral region of face

Question 10

Describe subnucleus caudalis

Answer 10

Extends from level of obex to second cervical level
Continuous with substantia gelatinosa at C2
Receives pain and temperature information from anterior one-half of head

Question 11

Describe mesencephalic nucleus of V

Answer 11

Only nucleus in CNS comprised of pseudounipolar neurons
May be considered as displaced trigeminal ganglion cells, although origin from neural crest is questionable
Conveys unconscious proprioceptive and pressure information from muscles of head and oral region

Question 12

Describe trigeminal motor nucleus

Answer 12

Innervate muscles derived form mesenchymal cell in first branchial arch
It is located at same level of brainstem as the main sensory nucleus

Question 13

Summarize the fast pain pathway from face

Answer 13

Fibers descend in descending root of V and terminate in subnucleus caudalis of descending nucleus of V (convey A delta)
Secondary axons decussate and ascend in trigeminal lemnisucs (TL)
This terminates in ventral posterior medial nucleus of dorsal thalamus
Axons form VPM project to posterior limb of internal capsule and corona radiata to inferior portion of postcentral gyrus (head region of primary somesthetic cortex)

Question 14

Describe the trigemino-reticulo-thalamic pathway

Answer 14

C type fibers convey slow pain to subnucleus caudalis
Neurons project to brainstem reticular formation (RF) as trigeminoreticular fibers
This will eventually reach centromedian and reticular nuclei of thalamus
Analogous to spino-reticulo-thalamic pathway (slow pain) from spinal cord
Trigeminal input into RF facilitates ascending reticular activating system (ARAS) and plays an important role in arousal and alertness

Question 15

Summarize crude tactile pathways from face

Answer 15

Primary axons bifurcate and ascend and descend
Subnuclei rostralis and interpolaris receive general tactile info from face
Main sensory nucleus receives some crude tactile info from face, but most info is concerned with precise discriminative and propriorceptive info from face
Most secondary fibers decussate and ascend in trigeminal lemniscus (anterior trigeminothalamic tract), which is adjacent to ML in brainstem. These terminate in ventral posterior medial VPM nucleus of dorsal thalamus
Some secondary axons remain uncrossed and ascend in ipsilateral dorsal central trigeminal tract (DCTT) (posterior trigeminothalamic tract). These travel with medial lemniscus and terminate in ipsilateral VPM nucleus of thalamus
Tertiary axons from VPM project via posterior limb of internal capusle to inferior portion of postcentral gyrus (primary somesthetic cortex)

Question 16

Summarize precise tactile pathway from face

Answer 16

Primary neuron fibers ascend in short ascending root of V and terminate in main sensory nucleus of V. These convey precise discriminative tactile information from face
Most secondary axons ascend in ipsilateral dorsal central trigeminal tract, which travel with ML and terminate in VPM
Some fibers decussate and ascend in TL, which ascends adjacent to ML and terminates in VPM
Tertiary axons from VPM project to posterior limb of internal capsule and corona radiata to inferior portion of postcentral gyrus (primary somesthetic cortex)

Question 17

Describe proprioceptive pathways from face

Answer 17

Mesencephalic nucleus receives unconscious proprioceptive and pressure info from muscles of head and oral region
Peripheral processes of these pseudounipolar neurons (primary neurons) course with mandibular nerve, pass through trigeminal ganglion and enter brainstem with V.
Process ascends towards mesencephalic nucleus of mesencephalic root of V
Central process terminates in main sensory nucleus. This connection permits conscious awareness of facial and oral proprioception via TL and dorsal central trigeminal tract
Some terminate on alpha motor neurons of trigeminal motor nucleus

Question 18

Describe jaw-jerk (masseteric) reflex

Answer 18

Rapid depression of chin results in reflex clenching of teeth
This monosynaptic reflex pathway involves neuromuscular spindles in masseter (receptor), masseteric nerve (afferent fiber), mesencephalic nucleus (sensory neuron), motor nucleus of V (efferent neuron), masseteric nerve (efferent fiber), and motor endplate (effector)

Question 19

Subnuclei rostralis and interpolaris project trigeminocerebellar fibers to anterior vermis via ipsilateral inferior cerebellar peduncle. What do the fibers convey?

Answer 19

Crude tactile (light touch) info from head

Question 20

A moderate number of secondary neurons in main sensory nucleus project to anterior vermis of cerebellum via superior cerebellar peduncle. What do these fibers convey?

Answer 20

Precise tactile and proprioceptive information from head to cerebellum

Question 21

What do unilateral lesions of the trigeminal nerve result in?

Answer 21

Anesthesia and loss of general sensations in trigeminal dermatomes
Loss of jaw jerk reflex
Atrophy of muscles of mastication
Loss of ipsilateral and consensual corneal reflex

Question 22

What is alternating analgesia?

Answer 22

Brainstem lesions in upper medulla may destroy primary fibers in descending tract of V and secondary fibers in spinal lemniscus
Patients demonstrate ipsilateral hemianalgesia of face and contralateral hemianalgesia of body

Question 23

What is alternating trigeminal hemiplegia?

Answer 23

A result of unilateral destruction of trigeminal nerve and corticospinal tract in pons
Results in ipsilateral trigeminal anesthesia and paralysis and contralateral spastic hemiplegia

Question 24

What is trigeminal neuralgia (Tic Douloureux)?

Answer 24

Cutaneous region on head (trigger zone) may initiate abnormal epileptic-like discharges from subnucleus caudalis
An intractable, lacerating facail pain, which follows distribution of affected division of trigeminal nerve
May be treated with anticonvulsant medication, cryosurgery of trigeminal ganglion or trigeminal/medullary tractotomy

Question 25

Describe what the trigeminal nerve innervates

Answer 25

Ophthalmic division V1: sensory receptors in skin of scalp, forehead, and orbital regions
Maxillary division V2: malar and infraorbital regions
Mandibular division V3: cheek/lower jaw regions. Also muscles of mastication