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).
