Brainstem 1: Trigeminal Tract

Question 1

sensory and motor branches of CN V: ganglion for sensory and motor branch relationship anatomically

Answer 1

The trigeminal nerve has a large trigeminal (semilunar, Gasserian) ganglion (SVA); the motor root of the trigeminal nerve courses inferior to the ganglion. Both the sensory and motor roots of V pierce the middle cerebellar peduncle (brachium pontis) just posterolateral to the pons. Upon entering the brainstem, the sensory and motor fibers distribute to the appropriate trigeminal nucleus.

Question 2

Primary Axons of V (sensory)

Answer 2

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

Question 3

fibers that will ascend only in the short ascending root (tract) of V

Answer 3

1. terminate in the main sensory nucleus of V.
2. These fibers convey precise discriminative tactile information from the face.

Question 4

fibers in the long ascending root

Answer 4

fibers adjacent to the short ascending root course in the mesencephalic tract of V. Conveys proprioceptive information to the main sensory nucleus.

This tract is composed of both peripheral and central processes of pseudounipolar neurons located in the mesencephalon.

Question 5

descending V fibers

Answer 5

1. half of the incoming primary fibers will descend only in the descending root (tract) of V
2. fibers terminate in the subnucleus caudalis of the descending nucleus of V.
3. convey pain (A delta and C fibers), temperature from face.
4. The descending root of V is continuous with the dorsolateral fasciculus of Lissauer in the spinal cord.

Question 6

the central processes of all GSA components of the facial, glossopharyngeal and vagus nerves course in the ___ and terminate in the ______

Answer 6

descending root of V and terminate in the adjacent descending nucleus of the trigeminal.

Question 7

Main Sensory Nucleus: what is it, what sensations it carries, and what terminates on it

Answer 7

1. the main sensory nucleus (principal trigeminal nucleus) is analogous to the nuclei gracilis and cuneatus
2. This nucleus receives precise tactile, crude tactile and proprioceptive information from the face.
3. The central processes of primary neurons in the trigeminal ganglion and the mesencephalic nucleus terminate in the main sensory nucleus.

Question 8

Descending (Spinal) Nucleus of V

Answer 8

Descending (Spinal) Nucleus of V consists of the subnucleus rostralis, interpolaris, and caudalis.

Subnucleus Rostralis

The subnucleus rostralis extends from the level of the entrance of V to the upper medulla. It receives tactile information from the central region of the face.

Subnucleus Interpolaris

The subnucleus interpolaris extends from the upper medulla to the level of the obex. It receives information from the peripheral region of the face.

Subnucleus Caudalis

The subnucleus caudalis extends from the level of the obex to the second cervical level. It is continuous with the substantia gelatinosa at C2, and serves an analogous function for pain information from the head. The subnucleus caudalis receives pain and temperature information from the anterior one-half of the head.

Question 9

nucleus that receives tactile information from the central region of the face.

Answer 9

The subnucleus rostralis extends from the level of the entrance of V to the upper medulla. It receives tactile information from the central region of the face.

Question 10

receives information from the peripheral region of the face.

Answer 10

The subnucleus interpolaris extends from the upper medulla to the level of the obex. It receives information from the peripheral region of the face.

Question 11

receives pain and temperature information from the anterior one-half of the head.

Answer 11

The subnucleus caudalis extends from the level of the obex to the second cervical level. It is continuous with the substantia gelatinosa at C2, and serves an analogous function for pain information from the head. The subnucleus caudalis receives pain and temperature information from the anterior one-half of the head.

Question 12

Mesencephalic Nucleus of V

Answer 12

1. the only nucleus in the CNS comprised of pseudo- unipolar neurons.
2. conveys unconscious proprioceptive and pressure information from muscles of the head and oral region.
3. The nucleus is located adjacent to the central gray area of the midbrain.

Question 13

Trigeminal Motor Nucleus

Answer 13

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

Question 14

Pain Pathways from the Face

Answer 14

Primary Axons from the Trigeminal Nerve: 50% of fibers descend only

Primary Neurons: A delta (δ) pain fibers and temperature from anterior one-half of the head/face descend in the descending (spinal) root (tract) of V —> terminate @ Secondary neurons: subnucleus caudalis of descending nucleus of V –> axons decussate and ascend in the trigeminal lemniscus (TL)–> terminate @ Tertiatry neurons: ventral posterior medial (VPM) nucleus of the dorsal thalamus –> axons from VPM project via the posterior limb of internal capsule and corona
radiata to the inferior portion of the postcentral gyrus (head region of the primary somesthetic cortex).

Question 15

Trigemino-reticulo-thalamic Pathway

Answer 15

1. C type pain fibers w/”slow” pain –> subnucleus caudalis–>project bilaterally to the brainstem reticular formation (RF) as trigeminoreticular fibers–> centromedian and reticular nuclei of the thalamus.
2. analogous to the spino-reticulo- thalamic pathway (“Slow” pain pathway) from the spinal cord.
3. The trigeminal input into the RF facilitates the ascending reticular activating system (ARAS), and plays an important role in arousal and alertness.

Question 16

Crude Tactile Pathways from the Face

Answer 16

1. Primary Axons: primary axons bifurcate into ascending and descending roots carrying general tactile information to –>
   
   • 1) main sensory nucleus: some crude tactile information from the face but mostly precise discriminative and proprioceptive information
   • 2) subnuclei rostralis and interpolaris of the descending (spinal) nucleus of V Secondary Neurons: general tactile information from the face.
2. Secondary axons–> decussate and ascend in two pathways
   
   1. Most: TL PW. ascend in trigeminal lemniscus adjacent to the medial lemniscus in the brainstem–> terminates in the ventral posterior medial (VPM) nucleus of the dorsal thalamus.
   2. Less: DCTT PW. Remain uncrossed, ascend in the ipsilateral dorsal central trigeminal tract (DCTT), also known as the posterior trigeminothalamic tract.
      
      1. In the brainstem, these fibers travel with the medial lemniscus, and terminate in the ipsilateral VPM nucleus of the thalamus.

Most of the secondary fibers from these nuclei
decussate and ascend in the
(TL). Some texts refer to the TL as the
trigeminal lemniscus ascends adjacent to the medial lemniscus in the brainstem, and terminates in the ventral posterior medial (VPM) nucleus of the dorsal thalamus.

1. Some texts refer to the TL as the trigeminal lemniscus ascends adjacent to the medial lemniscus in the brainstem, and terminates in the ventral posterior medial (VPM) nucleus of the dorsal thalamus
2. However, some secondary axons remain uncrossed and ascend in the ipsilateral dorsal central trigeminal tract (DCTT), which is also known as the posterior trigeminothalamic tract. In the brainstem, these fibers travel with the medial lemniscus, and terminate in the ipsilateral VPM nucleus of the thalamus.
3. Crude tactile information from the face has a degree of bilaterality to its ascending pathways, and, therefore, is relatively protected from unilateral lesions.
4. 3. Tertiary axons from the ventral posterior medial nucleus project via the posterior limb of the internal capsule to the inferior portion of the postcentral gyrus (head region of the primary somesthetic cortex).

Question 17

1. primary axons for crude touch fibers from the face–>
2. secondary axons carrying crude touch fibers from the face–>
3. tertiary fibers—>

Answer 17

1. terminate on main sensory nuc of V (precise tactile, crude tactile) and Nuc Rostralis and Nuc Interpolaris (crude tactile)
   
   1. main sen nuc of V, Decussation PW (TL: next to ML) —> Contralateral VPM nuc (Ventral Posterior Medial Nuc of Thalamus)
   2. main sen nuc of V, Undecussated PW (DCTL: Dorsal Central Trigeminal Tract in the ML)–> Ipsilateral VPM nuc
2. VPM nuc—> head region of the primary somesthetic cortex

Question 18

1. Precise Tactile Pathway from the Face
2. Primary Neuron, secondary neuron, tertiary neuron
3. Primary axons, secondary axons, tertiary axons

Answer 18

Precise Tactile Pathway from the Face

1. precise discriminative tactile information from the face via primary neuro–> ascend in the short ascending root of V–> terminate in the main sensory nucleus of V–> secondary axons conveying precise (two-point tactile discrimination) information from the main sensory nucleus ascend in the ipsilateral dorsal central trigeminal tract (posterior trigeminothalamic tract) that travel with the medial lemniscus, and terminate in the ipsilateral VPM nucleus of the thalamus
2. Some fibers may decussate and ascend in the trigeminal lemniscus (anterior trigeminothalamic tract): ascends adjacent to the medial lemniscus in the brainstem, terminates in the ventral posterior medial (VPM) nucleus of the dorsal thalamus.
3. Tertiary axons from the VPM nucleus project via the posterior limb of the internal capsule and corona radiata to the inferior portion of the postcentral gyrus (primary somesthetic cortex).

Question 19

mesencephalic nucleus

Answer 19

1. receives unconscious proprioceptive and pressure information from muscles of the head and oral region.
2. peripheral processes of these pseudounipolar neurons (primary neurons) course with the mandibular nerve,
3. pass through the trigeminal ganglion and enter the brainstem with the trigeminal nerve.
4. process ascends towards the mesencephalic nucleus in the mesencephalic root of V.
5. Cell body is in the mesencephalic nucleus.
6. mesencephalic nucleus is the only nucleus in the CNS comprised of pseudounipolar neurons.
7. central process terminates in the main sensory nucleus.
8. This connection permits the conscious awareness of facial and oral proprioception via the trigeminal lemniscus and the dorsal central trigeminal tract
9. Some central processes terminate directly upon the alpha motor neurons of the trigeminal motor nucleus. This monosynaptic circuit forms the jaw-jerk reflex.

Question 20

Conscious Proprioceptive Pathway

Answer 20

Conscious Proprioceptive Pathway

Facial and oral proprioceptive information entering the main sensory nucleus from the mesencephalic nucleus may ascend in the trigeminal lemniscus and dorsal central trigeminal tract to the ventral posterior medial nucleus of the thalamus.

The VPM nucleus projects to the head region of the primary somesthetic cortex. We may be consciously aware of this information at either the thalamic or cortical levels.

Question 21

Jaw-jerk (Masseteric) Reflex

Answer 21

1. monosynaptic reflex pathway
   
   1. Rapid depression of chin–>
   2. neuromuscular spindles in masseter (receptor)–>
   3. masseteric nerve (afferent fiber)–>
   4. mesencephalic nucleus (sensory neuron)–>
   5. motor nucleus of V (efferent neuron)—>
   6. masseteric nerve (efferent fiber)—>
   7. motor end-plate (effector).

Question 22

Trigeminocerebellar Pathways: crude touch and precise touch

Answer 22

1. Crude (light) touch from head–> subnuclei rostralis and interpolaris project to the cerebellum via trigeminocerebellar fibers –> anterior vermis via ipsilateral inferior cerebellar peduncle.
2. precise tactile and proprioceptive from head—> main sensory nucleus—> secondary fibers superior cerebellar peduncle –> anterior vermis of cerebellum

Question 24

1. Crude tactile PWs
2. Precise tactile PWs
3. Proprioception PWs
4. Pain and Temp PWs

Answer 24

1. Crude Tactile: Main S. Nuc of V+ Nuc Inter/Rostralist: TL (primary)/DCTT (lesser) –> VPM nuc Thal
2. Precise Tactile: Main Sens Nuc –> DCTT (primary)/TG (lesser) –> VPM nuc Thal
3. Proprioception (general): Main Sens Nuc–> DCTT –> VPM nuc Thal
4. Proprioception (unconscious): Mesencephalic Nuc –> Main S. Nuc –> DCTT–> VPM nuc Thal
5. Pain/Temp–> A delta–> Nuc Caudalis–> TL –> VPM nuc Thal
6. Pain/Temp –> C type fibers –> Nuc Caudalis –> ARAS–> VPM nuc of Thal