Trigeminal Complex

Question 1

At what level does the trigeminal nerve join the brainstem? What are its two division as it joins?

Answer 1

Mid pons level - it is the largest cranial nerve

Portio major - all sensory, lateral aspect
Portio minor - all motor axons of CNV, dorso-medial to P. major

Question 2

What muscles does CN5 innervate?

Answer 2

Muscles of mastication, tensor tympani, tensor veli palatini, anterior belly of the digastric

Question 3

What is special about the sensory innervation of the face vs other dermatomes?

Answer 3

There is no peripheral overlap between adjacent divisions of V.

Question 4

What is in the trigeminal ganglion?

Answer 4

Cell bodies of all afferent fibers for pain, temperature, and touch for CN5

Question 5

What is in the mesencephalic nucleus and why is it special? Where is it located?

Answer 5

Cell bodies for all fibers of conscious and unconscious proprioception for the face, located INSIDE the CNS, instead of peripherally.

Travels with mesencephalic tract, starting at mid pons and extends rostrally to midbrain / caudal diencphalic levels, originally posterolateral to motor nucleus of V (ventrolateral periaqueductal gray)

Question 6

What is the chief sensory nucleus of V, and where is it located?

Answer 6

Mid-pons level, receives central processes from pseudounipolar neurons of trigeminal ganglion. Carries fine tactile sense + pressure. This nucleus is responsible for the fine, discriminatory pathway (similar to DC-ML for face).

Question 7

What is the spinal nucleus of V, and where is it located?

Answer 7

Mid-pons level, starts just caudal to the chief sensory nucleus of V and extends all the way down to C2 (mostly in medulla). Receives central processes from trigeminal ganglion carrying pain, temperature, and light touch.

Question 8

What is the motor nucleus of V / where is it located?

Answer 8

Contains all V3 motor cell bodies, located just medial to chief sensory nucleus at mid-pons level.

Question 9

What is the pain, temperature, and crude touch pathway for V?

Answer 9

First order: cell bodies in trigeminal ganglion, central processes enter portio major follow spinal tract of V, giving collateral to spinal nucleus of V.
Second order: Axons from cell bodies in spinal nucleus of 5 CROSS MIDLINE and ascend as VTTT (along with ML) and synapse in VPM thalamus
Third order: Cell body in VPM nucleus of thalamus, axons travel in posterior limb of internal capsule to 3,1,2 forming corona radiata. Face represented in ventral half of postcentral gyrus (sensory homunculus).

Question 10

What is the fine discriminatory, vibratory sense pathway for V?

Answer 10

First order: cell bodies in trigeminal ganglia, axons project to chief sensory nucleus of V
Second order: Cell bodies in chief sensory nucleus 1/2 axons CROSS midline and travel with VTTT to VPM, other half with DTTT to VPM on ipsilateral side. Hard to lesion because bilateral.
Third order: VPM axons thru posterior limb of internal capsule, to 3,1,2 corona radiata, ventral half of postcentral gyrus

Question 11

What is the conscious / unconscious proprioception pathway for V?

Answer 11

First order: cell bodies in mesencephalic nucleus, central processes synapse here. Peripheral processes form part of mesencephalic tract.
Second order: Cell bodies also in mesenephalic nucleus
Conscious - CROSS midline, travel with VTTT to VPM
Unconscious - Travel thru superior cerebellar peduncle, synapse in cerebellum without crossing midline
Third order: Same as other sensory pathways (Except unconscious proprioception, which stays in cerebellum)

Question 12

How does the corneal reflex work?

Answer 12

1. Sensory afferent from V1 (crude touch), synapses in rostral part of spinal nucleus of V (caudal pons).
2. Axons from second order neuron in spinal nucleus of V synapse on ipsilateral + contralateral faicla nucleus
3. Efferent limb: Motor axons from facial nerve innervates orbicularis oculi

Question 13

What does it mean when a cotton swab to the right and left eyes only gives the direct reflex?

Answer 13

Midline lesion of the axons from spinal nucleus of V, preventing contralateral reflex

Question 14

What does it mean when a cotton swab to the cornea causes no response?

Answer 14

V1 problem on that side

Question 15

What does it mean when touching a cotton swap to the left eye triggers only a direct reflex, while touching a cotton swap to the right eye triggers only a consensual reflex (left eye blinks again)

Answer 15

Problem with facial nucleus on the right side

Question 16

How does the jaw reflex work?

Answer 16

1. Sensory afferent from V3 via a jaw tap will trigger stretch receptors in muscles of mastication. Forms the mesencephalic tract as central processes double back and synapse on motor nucleus of V.
2. Motor nucleus of V sends axons to muscles of mastication.

Upper motor cortex injury (giving voluntary contraction signals to motor nucleus of V) will cause pain, as the reflex will be exaggerated.