17 - Cranial Nerves III

Question 1

CN V

Answer 1

Trigeminal nerve

Question 2

CN V function

Answer 2

The trigeminal nerve provides:

• sensory innervation to the face and underlying mucous membranes
• motor innervation to the muscles of mastication

Question 3

Describe the general sensation component of CN V

Answer 3

GSA:

General sensation from the scalp, cranial dura, major intracranial blood
vessels, face, cornea, conjuctiva, lacrimal gland, mucous membrane of the: paranasal sinuses, nasal cavity, and oral cavity; teeth, gums, and hard palate

Question 4

Describe the general proprioception component of CN V

Answer 4

GP:

General proprioception
extraocular muscles, TMJ, muscles of mastication, and periodontal ligament (PDL) surrounding roots of teeth.

Proprioceptive sense helps us so that we can chew better and not bite our tongue when we are eating or when we are speaking

Question 5

Describe the skeletal motor component of CN V

Answer 5

SVE:

Skeletal motor (BranchioMOTOR) to the muscles of mastication and a
few other muscles (the trigeminal nerve is the nerve of the 1st branchial / pharyngeal arch). The muscles of mastication include the temporalis, masseter, and the medial and lateral pterygoids), also innervates the mylohyoid, anterior belly of the digastric, tensor tympani, and tensor veli palatini

Question 6

Describe the funciton of the tensor tympani

Answer 6

The tensor tympani attaches to the malleus and the malleus attaches to the inner surface of the tympanic membrane. When the TT contracts,
it tenses the tympanic membrane (becomes stiffer), preventing it from vibrating, therefore does not move the malleus.

This tiny muscle protects the eardrum from loud sounds and diminishes the transmission of sound vibrations from the eardrum to the inner ear (via the 3 ossicles: malleus – incus, stapes).

When chewing is initiated (by motor fibers of V), TT contracts.

Question 7

What is UNIQUE about CN V?

Answer 7

in addition to carrying its own fibers, the trigeminal nerve is the only cranial nerve that carries parasympathetic “hitch-hickers” of other cranial nerves and sympathetic fibers from the internal carotic plexus, in its own branches.

Question 8

Which three nerves does it carry parasympathetics for?

Answer 8

Oculomotor
Facial
Glosopharyngeal

Question 9

Parasympathetic “hitch-hikers” of the oculomotor nerve

Answer 9

oculomotor nerve: parasympathetic fibers leave the ciliary ganglion via the ophthalmic division (V1) to terminate in the sphincter pupillae and ciliary muscles

Question 10

Parasympathetic “hitch-hikers” of the facial

Answer 10

There are TWO sets of fibers

1 - facial nerve: parasympathetic fibers leave the pterygopalatine ganglion via the maxillary division (V2) to terminate in the lacrimal, nasal and palatine glands

2 - facial nerve: parasympathetic fibers leave the submandibular ganglion via the mandibular division (V3) to terminate in the sublingual and submandibular glands

Question 11

Parasympathetic “hitch-hikers” of the glossopharyngeal nerve

Answer 11

glossopharyngeal nerve:

parasympathetic fibers leave the otic ganglion via the mandibular division (V3) to terminate in the parotid gland

Question 12

What other “hitch-hikers” does CN V carry?

Answer 12

Sympathetic “hitch-hikers” from the internal carotid plexus are carried by the ophthalmic division (V1) to the dilator pupillae muscle

Question 13

Three divisions of CN V

Answer 13

1 - ophthalmic division (sensory)
2 - maxillary division (sensory)
3 - mandibular division (mixed: sensory and motor)

Question 14

Three sensory nuclei of CN V

Answer 14

1 - Mesencephalic nucleus
2 - Principal (main, chief) sensory nucleus
3 - Spinal nucleus

Question 15

Mesencephalic sensory nucleus of CN V

Answer 15

mesencephalic nucleus of the trigeminal

Location: in midbrain

Function: general proprioception from muscles innervated by the trigeminal nerve, and the periodontal ligament (PDL).

The muscles of mastication, and PDL contain nerve receptors that convey information to the brain about the position of the jaw

Question 16

Principal (main, cheif) sensory nucleus of CN V

Answer 16

principal (main, chief) sensory nucleus of the trigeminal

Location: in midpons

Function: discriminatory (fine, detailed) touch and pressure sensation from orofacial structures, GP from extraocular muscles and TMJ.

Sensory input is relayed to the principal nucleus by the central processes of sensory neurons located in the V ganglion

Question 17

What do the mesencephalic and principal nuclei have in common?

Answer 17

The above 2 nuclei are similar to / are the head counterpart of the dorsal column nuclei (“nucleus gracilis” and “nucleus cuneatus”)

Question 18

Spinal nucleus of the trigeminal

Answer 18

Spinal nucleus of the trigeminal

Location: in pons, medulla, extends to spinal cord levels of C1 – C2

It is sitting on top of the substantia gelatinosa of the spinal cord; it resembles the substantia gelatinosa structurally and functionally

Function: nociceptive and thermal sensation and some tactile sense from orofacial structures

Note: the trigeminal nerve is the only cranial nerve that has a “pain nucleus”.

Question 19

Describe the one motor nucleus of the trigeminal

Answer 19

Motor nucleus of the trigeminal

Location: in midpons
Function: contains the cell bodies of motor neurons whose axons gather to form the motor root of the trigeminal nerve.

These neurons innervate the muscles of mastication and a few other muscles

Question 20

Four tracts of trigeminal nerve

Answer 20

spinal tract of trigeminal
ventral trigeminal lemniscus
dorsal trigeminal lemniscus
trigeminoreticular fibers

Question 21

Spinal tract of trigeminal (origin, course, termination, function)

Answer 21

origin:
pons, near sensory root of V

course:
descends in pons, through medulla, to C1 or C2

termination:
spinal nucleus of trigeminal (in pons, medulla, C1-C2)

function:
pain, temperature, some tactile sense

• merges with Lissauer’s tract at upper cervical levels

Question 22

Ventral trigeminal lemniscus (origin, course, termination, function)

Answer 22

AKA ventral trigeminothalamic tract

origin:
spinal nucleus of V

course:
ascends to the thalamus

termination:
VPM nucleus of the thalamus

function:
pain, temperature, tactile sensation

• is similar to the “spinothalamic tract” of the anterolateral system (ALS)

Question 23

(origin, course, termination, function)

Answer 23

AKA dorsal trigeminothalamic tract

origin:
main sensory nucleus of V

course:
ascends to the thalamus

termination:
VPM nucleus of the thalamus

function:
discriminative (fine, detailed) touch, proprioception

*Is similar to the “medial lemniscus” (of the DC-ML pathway)

Question 24

(origin, course, termination, function)

Answer 24

trigeminoreticular fibers

origin:
main sensory nucleus
spinal nucleus of V

course:
project to nearby brainstem reticular formation, bilaterally

termination:
reticular formation

function:
arousal, produced by stroking or slapping the face, or “smelling salts” (containing ammonia)

• similar to / counterparts of the “spinoreticular tract”

Question 25

Four pathways of trigeminal nerve

Answer 25

1 - discriminatory (fine) touch and pressure sensation 2 - pain and temperature sensation 3 - motor pathway 4 - general proprioception pathway

Question 26

Discriminatory (fine) touch and pressure sensation from the orofacial region

Answer 26

a. 1st order neuron cell bodies are located in the trigeminal ganglion b. their central processes terminate in the main sensory nucleus of V c. 2nd order neurons are located in the main sensory nucleus of V d. 2nd order neuron axons join the dorsal trigeminal lemniscus to ascend to the VPM nucleus of the thalamus e. 3rd order neurons in VPM project to the postcentral gyrus (representing the orofacial region)

Question 27

Pain and temperature sensation from the orofacial region

Answer 27

a. 1st order neuron cell bodies are located in the trigeminal ganglion b. their central processes descend in the spinal tract of V c. they terminate in spinal nucleus of V d. 2nd order neurons located in the spinal nucleus of V join the ventral trigeminal lemniscus to ascend to the VPM e. 3rd order neurons in the VPM project to the postcentral gyrus, as well as the anterior cingulate and anterior insular cortices. The cingulate and insular cortices process the affective (emotional) component of pain

Question 28

Motor pathway of the trigeminal nerve

Answer 28

a. lower motor neuron cell bodies are located in the motor nucleus of V b. their axons exit the brainstem at midpontine level as the motor root of V c. axons join the mandibular division of the trigeminal to distribute to all the muscles innervated by the trigeminal

Question 29

4. general proprioception (GP) pathway of the trigeminal nerve

Answer 29

a. 1st order neuron cell bodies are located in the mesencephalic nucleus Note that this is an unusual nucleus because it contains pseudounipolar neurons instead of multipolar neurons enclosed in other nuclei. Pseudounipolar neurons are typically found in the peripheral nervous system and are enclosed in sensory ganglia such as the trigeminal and DRG and not in nuclei. These pseudounipolar neurons though, are the only primary afferents whose cell bodies are located within the CNS b. their peripheral processes distribute with branches of the trigeminal nerve c. their central processes terminate in the main sensory nucleus of V (for sensation) and the motor nucleus of V (for reflex movement of the jaw)

Question 30

What is the role of the trigeminal nerve in the corneal blink reflex?

Answer 30

1. the ophthalmic division of the trigeminal nerve forms the afferent (sensory) limb of the corneal blink reflex arc 2. the facial nerve forms the efferent (motor) limb of the reflex arc

Question 31

Damage to ophthalmic division, will:

Answer 31

1. reduce sensation over the V1 zone | 2. inhibit the corneal blink reflex

Question 32

Damage to the maxillary division, will:

Answer 32

reduce or inhibit sensation over the V2 zone

Question 33

Damage to the mandibular division, will:

Answer 33

1. reduce or inhibit sensation over the V3 zone 2. may cause paralysis of the muscles of mastication, resulting in the jaw to deviate to the side of the lesion

Question 34

Trigeminal neuralgia characteristics

Answer 34

Is characterized by sharp, excruciating, intermittent pain on one side of the face. Spontaneous onset. Each attack lasts for only a few seconds, but individual feels fine between attacks. The excruciating pain is triggered by touching of the affected side of the face, brushing teeth, or moving the mouth while talking, or chewing.

Question 35

Two possible causes

Answer 35

1. demyelination of the trigeminal nerve (resulting from demyelinating disease) as it enters the pons of the brain stem. 2. vascular structures or "loops" may develop next to, or surround (compressing) one of the divisions of the trigeminal nerve (ophthalmic or maxillary). The compression results in demyelination

Question 36

Ephaptic transmission

Answer 36

Demyelination results in ephaptic transmission of nerve impulses ("short circuiting" or “cross-talk” between adjacent fibers).

Question 37

Severity of pain

Answer 37

The pain may be so excruciating, that it may cause the patient to contemplate suicide. Thus this condition should be taken seriously by the physician.

Question 38

SUMMARY - the trigeminal nerve is the only cranial nerve that...

Answer 38

1 - carries "hitch-hikers" 2 - has 4 parasympathetic ganglia suspended from its three divisions 3 - has a pain nucleus

Question 39

"Hitch-hikers" review

Answer 39

Parasympathetic fibers belonging to other cranial nerves i. oculomotor ii. facial iii. glossopharyngeal Sympathetic fibers from the internal carotid plexus

Question 40

4 parasympathetic ganglia in three divisions review

Answer 40

1 - ophthalmic division --- suspends ciliary ganglion (of CN III) 2 - maxillary division --- suspends pterygopalatine ganglion (of CN VII) 3 - mandibular division --- suspends otic and submandibular ganglia (of CN IX and CN VII, respectively)

Question 41

Pain nucleus review

Answer 41

Since the following nerves do not have a pain nucleus of their own, they pick up nociception from their territories and then “dump” nociception in the trigeminal system for further processing a. facial nerve b. glossopharyngeal nerve c. vagus nerve

Question 42

Where else does the trigeminal nerve process nociception from?

Answer 42

Note that the trigeminal nerve also processes nociception from the sensory territory of the dorsal rami of the upper (1-2) cervical nerves.

Question 43

Lesion in trigeminal nerve review

Answer 43

A lesion to the trigeminal motor root will result in deviation of the jaw to the side of the lesion (upon protrusion of the jaw)