Motor CNs- V, VII, IX, X, XI, and XII Flashcards
This slide is presented simply as an overview of the brainstem motor nuclei and their cranial nerves. These are highlighted by the red and purple boxed legends. This slide presents Netter’s rendition of the brainstem with the location of the motor nuclei and nerves in red and the parasympathetic nuclei and nerves in purple.
I will be showing you a repeated sequence of slides for each of the cranial nerves. The sequence begins with a slide of the location of the motor nuclei in the brain stem, then a slide showing the emergence of the cranial nerves from the brain stem, next a slide of the skull base showing the foramina through which the cranial nerves pass, then an anatomical slide showing the target organs for each cranial nerve, and finally the clinically relevant features for each cranial nerve.
Describe cranial nerve V, the trigeminal nerve.
This is a mixed motor and sensory nerve.
The motor nucleus of the trigeminal nuclei lies where?
at the mid pontine level.
Where do the motor fibers of CN V travel?
Along the floor of Meckel’s cave to join to the mandibular or V3 division of the nerve, as shown by the red arrow, and exits the calvarium via the foramen ovale.
What does Motor V innervate?
the muscles of mastication including the masseter, temporalis, and pterygoid muscles. The first two muscles clench the jaw and the pterygoid muscles grind the teeth and open the jaw.
also innervates the tensor tympani (that dampens sound), the tensor veli palatini (that opens the eustachian tube),
CN V exits the brainstem at the mid pontine level. Note that unlike the picture on the right, the actual specimen shows the motor fascicle, labeled 7, to be considerably smaller in caliber when compared to the sensory nerve labeled 1.
Below: ventral brainstem
The numbers on the left identify 1 the sensory root of trigeminal nerve, 2 Pons, 3 Vestibulocochlear nerve, 4 Facial nerve, 5 Abducent nerve, 6 Medulla oblongata, 7 Motor root of trigeminal nerve, 8 Basilar sulcus.
Motor CN V travels with the V3 or mandibular branch of the trigeminal nerve and exits the calvarium via ________
the foramen ovale.
What is the jaw reflex, also known as the jaw jerk?
Tapping gently on the lower jaw triggers muscle spindles in the masseter muscle to send an impulse through the sensory fibers of cranial nerve V whose cell bodies lie in the mesencephalic nucleus; the mesencephalic nucleus projects to the nearby motor nucleus of V which sends an impulse to the masseter muscle to contract.
How can you test motor CN V?
by palpating the masseter and temporalis muscles when the patient bites down on a tongue blade. The muscles closing the jaw, however, are powerful and mild or moderate weakness will be difficult to detect.
The muscles opening the jaw are much weaker. Thus, jaw opening against resistance provided by the examiner is a more sensitive test. The strong lateral pterygoid will cause the jaw to deviate toward the side of the weakened pterygoid muscle (analogous to a tongue deviating toward the weaker side on protrusion).
The masseter muscle has muscle spindles and can be tested via a muscle stretch reflex known as the jaw jerk
T or F. The upper motor neuron input to the trigeminal motor nuclei is largely bilateral,
thus unilateral lesions to motor cortex or corticobulbar fibers do not produce unilateral weakness of jaw opening or closing but the jaw jerk may be increased.
Lesions of motor CN V or its nuclei will cause what?
unilateral weakness of jaw closure, reduced jaw jerk, and atrophy of the temporalis and masseter muscles.
The rules of UMN and LMN change apply here as well as in the spinal cord. The jaw jerk is especially valuable at the bedside when there is concern of cord compression in the neck, typically by cervical spondylosis. Why?
Because the leg fibers descending in the corticospinal tract lie in the periphery, they are squeezed first and hence the patient develops weakness in the legs and problems with gait. The muscle stretch reflexes in the legs increase. The upper extremities may have combined UMN and LMN picture as explained in another session.
If the jaw jerk is normal or low under these conditions, then the lesion is likely to be in the neck and you would order a cervical MRI to rule out cord compression there. If the jaw jerk is increased as well, then the lesion is not in the neck but above that level, for example a parasagittal meningioma You would order MRI of the head rather than of the neck.
An MRI study costs about $1000, and as a physician, you will be monitored by insurance companies for cost effectiveness in ordering expensive studies.
Cranial nerve VII, the facial nerve, is a mixed motor, parasympathetic, and sensory nerve. The motor component is served by the _____ nucleus (shown in the red box) that lies at the mid pontine level and innervates the muscles of facial expression and several smaller muscles such as the stapedius muscle.
facial
The _______ _______ nucleus is seen just below the facial motor nucleus.
superior salivatory
What does the superior salivary nucleus do?
This efferent nucleus sends preganglionic, parasympathetic fibers through the intermediate nerve of CN VII. These fibers synapse in parasympathetic ganglia serving the lacrimal and salivary glands.
CN VII and its adjoining intermediate nerve exit the brainstem where?
at the pontomedullary junction in a region called the cerebellopontine angle.
NOTE: the intermediate nerve lies lateral to the facial nerve as it emerges from this important area.
What does this MRI show?
The MRI shows a right cerebellopontine angle mass found to be an acoustic neuroma, as is the case about 80% of the time. The pathology shows this tumor to be a Schwannoma and it usually arises from the vestibular and not the acoustic nerve, so the purists call it a vestibular Schwannoma while everyone else is used to acoustic neuroma.
This slide depicts a cross section of the brainstem at the level of the facial nucleus shown in orange.
Note how the axons initially pass dorsal medially and loop over the abducens nucleus shown in red before turning ventrally to exit the brainstem at the pontomedullary junction.
Note that the facial nucleus axons create a bulge on the floor of the fourth ventricle called the ____ ______
facial colliculus.
The facial nerve and the nervus intermedius (intermediate nerve) that accompanies it exit the calvarium via what?
Motor VII exits brainstem at pontomedullary junction. It traverses the cerebellopontine angle to enter the internal auditory meatus to travel in the auditory canal of the petrous bone. It bends ventrally to enter the facial canal and exits the cranium via the stylomastoid foramen.
How do the fibers (nervus intermedius) of the superior salivatory nucleus run?
They exit the brainstem as a small root adjacent to motor VII. These fibers travel with motor VII to synapse in the sphenopalatine and submandibular ganglia with secondary neurons innervating the salivary glands.
This slide presents an artist’s idea of an upper motor neuron lesion (Lesion A or B) and a lower motor neuron lesion (Lesion C) involving CN VII. Note sparing of the forehead muscles with an upper motor neuron lesion because of bilateral innervation of CN VII nuclei.
Two examples of LMN lesions due to Bell’s palsy are shown on the right and one example of an UMN lesion, a patient recovering from a right cortical stroke, is shown on the left.
Cranial nerve IX, the glossopharyngeal nerve, is a mixed motor, parasympathetic, and sensory nerve. Its motor component is served by what nucleus?
the nucleus ambiguus (shown by the red box) that lies near the junction of the pons and medulla and innervates the stylopharyngeus muscle.
What does the inferior salivatory nucleus (shown in the purple box) do?
lies near the midline of the medulla and sends preganglionic, parasympathetic fibers through CN IX to the otic ganglion with secondary innervation of the parotid gland.
Note that a small yellow box was placed in the figure to block out the line going to the superior salivatory nucleus because it is a component of CN VII. The red arrow points to CN IX.
CN IX exits the brainstem at the junction between the pons and the medulla.
The glossopharyngeal nerve exits the calvarium via what?
the jugular foramen.
What does the nucleus ambiguus do again?
Nucleus ambiguus fibers exit the calvarium via the jugular foramen and innervate the stylo- pharyngeal muscle to raise the pharynx during talking and swallowing.
Cranial nerve X, the vagus nerve, is a mixed motor, parasympathetic, and sensory nerve. Its motor component is served by what nucleus?
nucleus ambiguus (red box) that lies near the lateral medulla and innervates muscles of the soft palate, pharynx, and larynx.
What does the dorsal motor nucleus of the vagus (purple box) do?
lies near the midline of the medulla and sends preganglionic, parasympathetic fibers to the intramural ganglia associated with the heart, lung, and digestive tract. The red arrow points to the nerve.
CN X exits the brainstem as a series of rootlets between what structures?
between the inferior olive and the inferior cerebellar peduncle.
Where does CN X exit the skull?
CN X exits the calvarium via the jugular foramen along with CN IX and XI.
Anatomy of motor CN X
The nucleus ambiguus fibers travel in CN X to innervate the pharyngeal laryngeal, and palate muscles.
Preganglionic fibers of the dorsal motor nucleus travel with CN X to synapse in intramural ganglia in the walls of the heart, lungs, and gut
Cranial nerve XI (shown by the red arrow) is called the spinal accessory nerve. It is a purely motor nerve that controls head turning through innervation of the sternocleidomastoid muscles and shoulder elevation through innervation of the upper trapezius muscles. It receives motor axons from where?
both the caudal portion of the nucleus ambiguus and the spinal accessory nucleus.
This slide shows CN XI axons exiting the lower medulla and upper spinal cord and joining together to form the spinal accessory nerve.
Motor anatomy of CN XI
The caudal end of nucleus ambiguus sends fibers through the proximal portion of CN XI before joining CN X.
Fibers from the spinal accessory neurons in the C1-C5 cervical cord ascend through the foramen magnum and exit through the jugular foramen to innervate the sternocleidomastoid and the trapezius muscles.
Cranial nerve XII is called the hypoglossal nerve and it is a pure motor nerve that controls tongue movement. It is served by what nucleus?
the hypoglossal nucleus that lies near the midline in the mid to posterior portion of the medulla.
This slide shows CN XII axons as a group of nerve fibers exiting the brainstem between the inferior olives and pyramids and then joining together to form the hypoglossal nerve.Note the location of the nerve lying at the junction of the medulla and spinal cord.
CN XII exits the calvarium through its own foramen called ______
the hypoglossal foramen that lies along the posterior aspect of the petrous temporal bone.
