12 - Motor III Flashcards
The following cranial nerves provide motor innervation to muscles in the head and neck region
III, IV, V, VI, VII, IX, X, XI and XII
The nerve MOTOR NUCLEI of which cranial nerves receive corticonuclear (UMN) axon terminals, relaying motor commands from the motor cortex? There are 6
- trigeminal
- facial
- glossopharyngeal
- vagus
- accessory
- hypoglossal
These nuclei include…
- The trigeminal motor nucleus (CN V)
- The facial nucleus (CN VII)
- The nucleus ambiguus (CN’s IX, X and XI)
- The hypoglossal nucleus (CN XII)
Where does the trigeminal motor nucleus receive input from?
The trigeminal motor nucleus (CN V) – receives bilateral UMN projections
Where does the facial nucleus receive input from?
The facial nucleus (CN VII) – receives bilateral UMN projections in its upper half and contralateral projections in its lower half
Where does the nucleus ambiguus receive input from?
The nucleus ambiguus (CN’s IX, X and XI), receives bilateral UMN projections
Where does the hypoglossal nucleus receive input from?
The hypoglossal nucleus (CN XII) – receives bilateral UMN projections, except to the neurons that innervate the genioglossus muscle which receive only contralateral projections
Where do the other three CNs receive motor input from?
The oculomotor, trochlear and abducens motor nuclei do not receive corticonuclear fibers, but instead receive motor messages from the frontal and parietal motor eye fields via a different group of axons that make a stop in the reticular formation, which in turn projects to the III, IV, and VI nuclei.
Corticonuclear Fibers to the Trigeminal (V) Motor Nucleus
Note: corticonuclear fibers are axons of upper motoneurons (UMN’s)
Corticonuclear fibers descend along with the corticospinal fibers to the brainstem.
At the pontine level, corticonuclear fibers terminate bilaterally on interneurons in
the reticular formation next to the CN V motor nucleus
What is the motor function of CN V?
The V motor nucleus innervates the muscles of mastication (which move the jaw up
and down, and from side to side – during chewing and speaking), and a few other
muscles.
Corticonuclear Fibers to the Facial Nucleus
Corticonuclear fibers terminate in the facial nucleus (a motor nucleus) as follows:
- The motor cortex sends bilateral (~ an equal number of ipsilateral and contralateral) upper motoneuron (UMN) projections to the superior half of the facial nucleus
- The motor cortex sends predominantly contralateral UMN projections to
the inferior half of the facial nucleus.
What is the function of the corticonuclear fibers of the facial nucleus (superior half and inferior half)?
- The superior half of the facial nucleus contains lower motoneurons (LMN’s) that innervate the muscles of the upper half of the face
- The inferior half of the facial nucleus contains LMN’s that innervate the muscles of the lower half of the face.
Symptoms of a unilateral upper motor neuron (UMN) lesion involving the corticonuclear fibers terminating in the facial nucleus
The upper half of the face is spared
The lower half of the face will show weakness/paralysis contralateral to the side of the lesion
Why is the upper half of the face spared?
Because the upper half of the facial nucleus (that innervates the muscles of the upper half of the face) receives bilateral corticonuclear (UMN) projections. So if UMN fibers from one side are damaged, there is a “backup” of corticonuclear projections to the facial nucleus from the opposite side.
Why will the lower half of the face show contralateral weakness?
Because the lower half of the facial nucleus (that innervates the muscles of
the lower half of the face) receives predominantly contralateral corticonuclear (UMN) projections. There are few if any backup UMN fibers from the opposite side.
The muscles of the lower half of the face are more vulnerable to weakness / paralysis following an UMN lesion (anywhere along the path of the corticonuclear tract).
Will more deficits will be apparent in the body on the same side or opposite side as the facial weakness.
Same side as the facial weakness
Because the primary motor cortex area innervating the face is next to that of the hand and tongue. Thus an UMN lesion in the motor cortex, may also result in weakness of the thumb, fingers, and tongue on the same side as the facial weakness.
What will also likely be damaged in a unilateral upper motor neuron (UMN) lesion involving the corticonuclear fibers terminating in the facial nucleus?
The corticonuclear fibers travel close to the corticospinal fibers in the internal capsule, the basis pedunculi of the midbrain, the basilar pons and the pyramid of the medulla. Thus a lesion that damages the corticonuclear fibers, probably also damages axons of the corticospinal tract
If a patient experiences an UMN lesion of BOTH the corticonuclear and corticospinal tracts, the following symptoms will be present…
The patient would have symptoms involving the muscles of the upper and /or lower limbs on the same side as the facial weakness
Are the corticonuclear fibers to the nucleus ambiguus bilateral or unilateral?
Bilateral
What cranial nerves are involved and are the cell bodies UMNs or LMNs?
The nucleus ambiguus contains the cell bodies of LMN’s that run in the branches of the glossopharyngeal, vagus and accessory nerves.
What do the branches of the nucleus ambiguus CNs innervate?
The branches of the glossopharyngeal
and vagus innervate the muscles of the soft palate (swallowing) larynx (speaking),
and pharynx (swallowing).
Are the corticonuclear fibers to the accessory nucleus of the spinal portion of the accessory nerve bilateral or unilateral?
Unilateral - they project ipsilaterally (SAME side)
What cranial nerves are involved and are the cell bodies UMNs or LMNs?
The accessory nerve is involved
UMN’s synapse with LMN’s in the accessory nucleus of the cervical spinal cord
What muscles does the accessory nucleus send fibers to?
sternocleidomastoid and trapezius muscles
Are the corticonuclear fibers to the hypoglossal nucleus bilateral or unilateral?
Bilateral
Although some neurons project contralateral input
What muscles do the bilateral projections of the hypoglossal nucleus innervate?
all of the muscles of the tongue (except the genioglossus muscle)
What muscles do the unilateral projections of the hypoglossal nucleus innervate? Are they ipsilateral or contralateral?
Contralateral
The neurons of the hypoglossal nucleus that innervate the genioglossus muscle (which protrudes the tongue), receive mainly contralateral input
What happens when there is a lesion that damages the corticonuclear (UMN) fibers to the hypoglossal nucleus?
There are no deficits in most of the tongue muscles
HOWEVER, since the genioglossus muscle only receives contralateral UMN projections, if they are damaged (and there are no “back up” fibers from the other side to stimulate the LMN’s to the genioglossus) the genioglossus contralateral to the lesion will be weak, causing the tongue to deviate to the weak side upon protrusion.
How are the projections to CN III, CN IV and CN VI different than the past corticonuclear fibers?
The motor cortex influences eye movements not by projecting corticonuclear fibers directly to the oculomotor, trochlear and abducens motor nuclei, but by projecting via a different system to the reticular formation first
VIA THE RETICULAR FORMATION
Describe the sequence of signaling for CN III, CN IV and CN VI
- The frontal and parietal motor eye fields give bilateral projection
- These projections lead to the midbrain RETICULAR FORMATION and paramedian pontine reticular formation (PPRF)
- The projections then reach the motor nuclei of oculomotor (CN III), trochlear (IV) and abducent (VI) cranial nerves
- This produces conjugate eye movements
Are the eye movements produced contralateral or ipsilateral to the side of origin of cortical input?
Contralateral
What is the blood supply to the lateral surface of the precentral gyrus (head area)?
Middle cerebral artery
What is the blood supply to the GENU and posterior limb of the internal capsule
Branches of the middle cerebral artery called the lenticulostriate arteries
What is the blood supply to the midbrain?
Posterior cerebral artery and basilar artery
What is the blood supply to the pons?
Pontine arteries from the basilar artery
What is the blood supply to the rostral medulla?
Anterior spinal artery and vertebral artery
What is the blood supply to the caudal medulla?
Anterior spinal artery
Why does a lesion that affects the corticospinal tract usually involve the corticonuclear tract?
The reason for this is because the corticonuclear tract descends close to the corticospinal tract
Describe the path of descent for both the corticonuclear tract and the corticospinal tract
The corticonuclear tract descends in the genu of the internal capsule and then shifts posteriorly descending in the anterior part of the posterior limb of the internal capsule.
The corticospinal tract descends in the anterior half of the posterior limb of the
internal capsule.
The corticonuclear tract descends next to (medial to) the corticospinal tract in the basis pedunculi
The corticonuclear tract fibers mingle with the corticospinal tract fibers in the basis pontis
and the pyramid in the medulla
