Brainstem and Cranial Nerves

Question 1

Where does the hypoglossal nerve exit the brainstem?

Answer 1

• Hypoglossal (XII) exits from the medulla

Question 2

In general, are local signs ipsilateral or contralateral to the lesion? Long tract signs?

Answer 2

• Local signs = ipsilateral

* Long tract signs = contralateral

Question 3

What is the hallmark of a brainstem lesion?

Answer 3

This is the hallmark of a brainstem lesion: cranial nerve signs on one side, coupled with long tract signs on the opposite side.

Question 4

What is the clinical importance of the organization of the brainstem?

Answer 4

• MODULAR with the different tracts
• lesions in the medial part of the brainstem often result in completely different deficits than a laterally placed lesion at the same level.
• Therefore, you must also keep in mind whether a given tract or nerve or nucleus lies in the lateral or medial portion of the brainstem.

Question 5

what does the inferior cerebellar peduncle do?

Answer 5

• cerebellar peduncles are white matter tracts connecting cerebellum with the brainstem
• The inferior cerebellar peduncle conveys spinal cord information to the cerebellum and interconnects the cerebellum with the vestibular nuc. and inferior olive

Question 6

What does the middle cerebellar peduncle do?

Answer 6

• cerebellar peduncles are white matter tracts connecting cerebellum with the brainstem
• The middle cerebellar peduncle is the route by which information from the cerebral cortex gets to the cerebellum via the pontine nuclei.

Question 7

What does the superior cerebellar peduncle do?

Answer 7

• cerebellar peduncles are white matter tracts connecting cerebellum with the brainstem
• The superior cerebellar peduncle is the route by which the cerebellum gets information back to the cerebral cortex (via the thalamus).

Question 8

What’s up with the deep cerebellar nuclei?

Answer 8

Dentate, interposed and fastigial nuclei

*all receive excitatory input from pons and spinal cord and inhibitory input from purkinje cells

Question 9

What neurons are in the Facial nucleus and where do they come from? where are they going?

Answer 9

*facial nucleus - innervates muslces of facial expression
From - motor cortex? trigeminal ganglion?
To - take a strange route. go up toward abducens and around in front of abducens before coming back past the facial nucleus to exit the brainstem

Question 10

What neurons are in the abducens nucleus and where do they come from? where are they going?

Answer 10

• Abducens nucleus - motor neurons and interneurons that project via the MLF to nucleus iii.
• innervates the lateral rectus muscle

Question 11

What neurons are in the Superior olivary nucleus and where do they come from? where are they going?

Answer 11

• Superior olivary nucleus - auditory system
• input from both ipsilateral and contralateral cochlear nucleus
• projects to inferior colliculus

Question 12

In the brainstem slices you see the term MLF. What is that?

Answer 12

• The medial longitudinal fasciculus (MLF) is one of a pair of crossed fiber tracts (group of axons), on each side of the brainstem.
• These bundles of axons are situated near the midline of the brainstem and are composed of both ascending and descending fibers that arise from a number of sources and terminate in different areas.
• FROM - MLF is the main central connection for the oculomotor nerve, trochlear nerve, and abducens nerve.
• The vertical gaze center is at the rostral interstitial nucleus (riMLF).
• PROJECTION - The MLF ascends to the interstitial nucleus of Cajal, which lies in the lateral wall of the third ventricle, just above the cerebral aqueduct.

Question 13

The STT runs directly (blank) to the SCP at the mid-level of the pons?

Answer 13

STT = spinothalamic tract
SCP = superior cerebellar peduncle
*answer = STT runs immediately anterior to the SCP
*if you see lots of pons, find MCP and SCP and in between them at the anterior margin of SCP is STT

Question 14

What is the destination of axons in the lateral lemniscus?

Answer 14

Lateral lemniscus axons are destined for the inferior colliculus (part of the auditory system)

• when you see lots of pons and the SCP starting to dessucate…
• the lateral lemniscus is the dorsal-lateral-most white matter in the brainstem
• the ventral border of the lateral lemniscus is the STT (spino-thalamic tract)
• immediately ventral to that and spreading out towards the midline is the medial lemnisucus

Question 15

what does the pons contain (general)?

Answer 15

The pons contains nuclei that receive axons from various cortical areas.

• Projections from the axons of these pontine neurons form large transverse fiber bundles which traverse the pons and ascend to the contralateral cerebellum via the middle cerebellar peduncles.
• Also, within the pons base and tegmentum are longitudinally ascending and descending fibers. The nuclei of the 5th (trigeminal), 6th (abducens), 7th (facial) and the 8th (vestibulocochlear) nerves are located in the pons tegmentum.

Question 16

What does the medulla contain (general)?

Answer 16

• The medulla tegmentum contains ascending and descending fibers and nuclei from the 9th (glossopharyngeal), 10th (vagus), 11th (accessory) and the 12th (hypoglossal) nerves.
• The corticospinal fibers (pyramid) are alongside the anterior median fissure, and decussate (cross the midline) to the contralateral side on their way to the spinal cord.
• Other prominent structures in the medulla are the inferior olive, and the inferior cerebellar peduncle.
• The medulla contains nuclei which regulate respiration, swallowing, sweating, gastric secretion, cardiac, and vasomotor activity.

Question 17

What are the boundaries of the medulla?

Answer 17

*The medulla lies between the pons rostrally and the spinal cord caudally. It is continuous with the spinal cord just above to foramen magnum and the first spinal nerve. The posterior surface of the medulla forms the caudal half of the fourth ventricle floor and the cerebellum, its roof (fig. 12). The base of the medulla is formed by the pyramidal-descending fibers from the cerebral cortex.

Question 18

what actions do the nuclei contained with the medulla perform?

Answer 18

*The medulla contains nuclei which regulate respiration, swallowing, sweating, gastric secretion, cardiac, and vasomotor activity.

Question 19

which cranial nerves are associated with the medulla?

Answer 19

The glossopharyngeal (IX), vagus (X), spinal accessory (XI), and hypoglossal (XII) are associated with the medulla,

Question 20

which cranial nerves are associated with the pons?

Answer 20

vestibulocochlear (VIII), facial (VII),abducens (VI), and trigeminal (V) nerves are associated with the pons.

Question 21

What are the 9 discrete axial segments of the brainstem you should know on sight?

Answer 21

1. Upper midbrain
2. Lower midbrain
3. Upper pons
4. mid pons
5. lower pons
6. upper medulla- at rostral pole of the olive
7. upper-mid medulla- at the level of the olive
8. lower-mid medulla -below the level of the olive
9. lower medulla-level of the pyramidal decussation

Question 22

What can mess with the function of CN I?

Answer 22

Cranial nerve 1, the olfactory nerve, originates from numerous cells in the olfactory mucosa, which traverse the cribiform plate to enter the olfactory bulb.

• Impairments of olfaction can result from failure of odorants to reach the olfactory mucosa (a “conduction deficit”) as with nasal polyps, or sensorineural deficits resulting from injury to the olfactory pathways through the CNS.
• In general unilateral loss of smell is more often sensorineural in origin, while bilateral loss is more often a conduction problem.
• A careful history is important, and history of head injury, smoking, upper respiratory infection, toxin or drug exposure may be relevant. To examine olfactory function, first make sure passages are open with visual inspection (speculum and illumination helpful), and test each nostril separately with non-irritating stimuli. (Chemical irritants can instead activate nerve endings in the trigeminal system and will give misleading results about olfaction). Useful compounds to test with include coffee, cinnamon, cloves, or wintergreen.

Question 23

Pupillary examination tests the function of which cranial nerves?

Answer 23

• Pupillary testing examines function in both CN2 and CN3.
• Examination of pupillary size, shape, equality, and reactivity to light and accommodation should be documented.
• If the inequality is greater in the dark, this indicates sympathetic dysfunction. If greater in the light, parasympathetic dysfunction is implicated.

Question 24

The basis for the light reflex in the pupillary exam is in what part of the brain?

Answer 24

this reflex illustrates the most rostral brainstem segment: the upper midbrain
*The afferent limb of the reflex is the optic pathway, and illumination in each eye will innervate both of the output nuclei (the Edinger-Westphal nuclei in the oculomotor complexes. Identify the oculomotor nuclei and fibers of the third nerves as they exit the midbrain- they will traverse the red nuclei and exit in the interpeduncular cistern.

Question 25

What’s the bad eye in the swinging flashlight test?

Answer 25

• testing for APD - afferent pupillary defect
• When swinging the flashlight to the “bad” eye, the pupil will dilate slightly (as will its counterpart), as the direct response in this “pathological” eye is not as great as the consensual response to the immediately preceding stimulation of the “normal” eye

Question 26

What are the different classifications for eye position?

Answer 26

One can observe the eyes in primary position (looking straight ahead),

• secondary positions (bilateral up, down, left, and right gaze), and
• tertiary positions (up and right, up and left, down and right, down and left).
• The cardinal positions of gaze refer to the four tertiary positions plus left and right gaze. These provide the best assessment of each EOM’s independent function.

Question 27

How does a CN III palsy present?

Answer 27

Cranial nerve 3 palsy will manifest as a spectrum of deficits depending on whether it is a complete or incomplete palsy.

• A complete palsy will consist of mydriasis (a fixed, dilated pupil) from loss of parasympathetic innervation to the pupilloconstrictors,
• ptosis (from loss of innervation to the levator palpebrae superioris), and
• paralysis of the extraocular muscles innervated by CN3.
• In primary position, the eye will lie down and out, owing to unopposed action of the lateral rectus and superior oblique (a partial depressor of the eye).

Question 28

What is Horner Syndrome and what does it indicate?

Answer 28

Another condition that can cause ptosis, as well as pupillary inequality is Horner’s syndrome.
*Damage to the sympathetic outflow from the hypothalamus to the spinal cord preganglionic sympathetic neurons (e.g. with cervical or upper thoracic spinal cord injury), from the spinal cord to the sympathetic ganglia (e.g. pancoast tumor), or from the ganglia to the target structures in the orbit/face/eye (e.g. internal carotid artery dissection/trauma) can cause loss of sympathetic innervation leading to ipsilateral ptosis (loss of sympathetic tone to smooth muscle of levator), miosis (unopposed parasympathetic input to pupil) and anhidrosis (loss of innervation of sweat glands).

Question 29

How does a CN 4 palsy present?

Answer 29

A cranial nerve 4 palsy will weaken the superior oblique muscle in the orbit.

• A fourth nerve palsy can be tricky to diagnose, and the action of the SO muscle depends on the direction of the eye.
• When the eye is pointed medially, the SO is a depressor (so a 4th nerve palsy will have the eye elevating as it turns medially).
• When the eye is turned laterally, the SO intorts the eye (rotates it around the axis of the globe).
• To compensate for an extorted eye, the patient will often tilt his or her head away from the damaged side, and intort the “good” eye to a greater degree, in order to align the retina and compensate for the diplopia that would otherwise result.

Question 30

What’s special about the anatomy of the 4th CN?

Answer 30

Cranial nerve 4 is unusual. The motor nucleus in the brainstem (difficult to visualize- seen on lower midbrain slice, similar location as CN3 nuclei on upper brainstem slice) sends its axon dorsally, over to the contralateral side of the midbrain, where it exits beneath the inferior colliculus, and travels around the midbrain in the subarachnoid space to the cavernous sinus and then the orbit. This long intracranial course makes it susceptible to injury in trauma.

Question 31

Lateral gaze is initiated by the frontal eye field (FEF) contralateral to the direction of gaze. Trace the pathway.

Answer 31

• The signals from the FEF travel down the internal capsule through the crus cerebri to the pons where they innervate a group of cells in the contralateral pons called the paramedian pontine reticular formation (PPRF)
• the PPRF is composed of neurons that each innervate two cells located within the abducens nucleus
• In the abducens nucleus, the PPRF neuron innervates a motor neuron that will send its axon out the abducens nerve to the ipsilateral lateral rectus muscle. Another branch of the same PPRF neuron will innervate an interneuron in the abducens nucleus, that will send its axon across the midline, and up a thin white matter band called the medial longitudinal fasciculus (MLF). The MLF transmits this axon rostrally to the upper midbrain, where it innervates certain neurons of the oculomotor nuclei
• coordinates lateral and medial rectus muscle movement

Question 32

What can happen to the MLF in MS?

Answer 32

• The MLF, as a pathway composed of myelinated axons, is susceptible to demyelinating diseases, and can be unilaterally damaged in multiple sclerosis
• A unilateral injury to one MLF produces a condition called an “internuclear ophthalmoplegia”, in which lateral gaze to the side opposite the lesion results in failure of the ipsilateral eye to follow medially (the resulting diplopia induces nystagmus in the contralateral eye that is gazing laterally).
• demonstrate this is MLF and not oculomotor nucleus problem by having them converge the eyes medially (should do this fine)

Question 33

A lesion in the RIGHT frontal eye field will look like what lateral gaze exam finding?

Answer 33

nystagmus on lateral gaze to the RIGHT (ipsilateral to the MLF lesion)
*inability of LEFT eye to go medial and follow the RIGHT eye (stays central)

Question 34

Describe the path of proprioceptive fibers in the trigeminal nerve

Answer 34

Unlike the spinal cord, proprioception (from muscles of mastication) follows its own unique course.

• These axons travel through V3 into the brainstem and up to the midbrain, where the cell bodies lie.
• This is the only example of a sensory neuron with its cell body inside the neural tube (all others are in a ganglion outside the CNS). You will not need to identify the mesencephalic nucleus of CN V, but you should know that the efferent part of the axon descends to the motor nucleus of V, where it forms a synapse on the motorneurons that innervate the muscles of mastication. This reflex arc forms the jaw-jerk reflex, which can be tested clinicall

Question 35

trace the pathway of touch and vibration sensation through the trigeminal nerve

Answer 35

Touch and vibration sense is transmitted via V1, V2, and V3, via cells with their cell bodies in the large Gasserian ganglion.

• These axons enter the brainstem dorsolaterally, with the large sensory root of the trigeminal nerve, where the fibers briefly traverse the fibers of the middle cerebellar peduncle, to innervate neurons in the principal sensory nucleus of V.
• This is found at the level of the mid pons just lateral to the motor nucleus of V.

Question 36

Describe the pathway of pain and temperature fibers in the trigeminal nerve

Answer 36

Pain and temperature fibers also travel via V1, V2, and V3 into the brainstem and their cell bodies also lie in the Gasserian ganglion.

• These axons enter the brainstem at mid pons but then descend to more caudal segments where they innervate neurons in the spinal trigeminal nucleus.
• This nucleus spans from the pons all the way to the caudal medulla, and is actually a column of grey matter continuous with the substantia gelatinosa of the spinal cord.
• Like the substantia gelatinosa, the spinal trigeminal nucleus is the location of second order neurons conveying pain and temperature signals to higher centers

Question 37

What is the spinal trigeminal nucleus?

Answer 37

This nucleus spans from the pons all the way to the caudal medulla, and is actually a column of grey matter continuous with the substantia gelatinosa of the spinal cord.
*Like the substantia gelatinosa, the spinal trigeminal nucleus is the location of second order neurons conveying pain and temperature signals to higher centers

Question 38

How do you clinically evaluate the function of the trigeminal nerve?

Answer 38

Testing trigeminal function includes the corneal blink reflex, which involves the V1 branch of the trigeminal nerve for the input, and requires function of the facial nerve for the output.

• There should be a direct and consensual response.
• Testing pain, temperature, touch, vibration is identical to techniques used for sensory testing in the rest of the body, understanding the anatomic boundaries for V1,V2, and V3.
• Muscles of mastication can be assessed for atrophy and strength, with unilateral nerve injury causing the jaw to open and deviate toward the side of the injury (see figure below). The jaw-jerk reflex tests proprioception as well.

Question 39

Describe the anatomy of the facial nerve

Answer 39

Motorneurons of cranial nerve 7, the facial nerve, originate in the facial nucleus, found on the “lower pons” slice, just ventrolateral to the abducens nucleus.
*The axons of these cells curve around the abducens nucleus before exiting the pons at the cerebello-pontine angle, just below the middle cerebellar peduncle

Question 40

Describe the innervation pathway of the the facial nucleus

Answer 40

The facial nucleus is itself innervated by upper motor neurons from the primary motor cortex.

• These innervate the nucleus in a peculiar way that has important clinical implications.
• The facial nucleus can be divided into a rostral and caudal half. The rostral half innervates the upper half of the face, and the caudal half the lower face.
• Upper motor neurons from the motor cortex innervate the rostral half of the nucleus bilaterally, but they only innervate the contralateral caudal facial nucleus.
• What this means is that damage to the motor cortex (or UMNs as they traverse the internal capsule) will cause weakness of the contralateral lower face only (as the upper face will still have innervation from the ipsilateral motor cortex). A facial palsy in which only the lower face is weak is therefore likely from UMN injury, and called a central facial palsy. One in which the entire half of the face is weak- upper and lower- usually involves injury to the facial nucleus or nerve (the lower motor neurons), and is called a peripheral facial palsy.

Question 41

What is indicated in asymmetric motor function in the face?

Answer 41

*What this means is that damage to the motor cortex (or UMNs as they traverse the internal capsule) will cause weakness of the contralateral lower face only (as the upper face will still have innervation from the ipsilateral motor cortex). A facial palsy in which only the lower face is weak is therefore likely from UMN injury, and called a central facial palsy. One in which the entire half of the face is weak- upper and lower- usually involves injury to the facial nucleus or nerve (the lower motor neurons), and is called a peripheral facial palsy.

Question 42

what is the Rinne test?

Answer 42

• for conduction vs. sensorineural hearing loss
• The Rinne test is performed by placing a low frequency (512Hz) vibrating tuning fork against the patient’s mastoid bone and asking the patient to tell you when the sound is no longer heard. Once they signal they can’t hear it, quickly position the still vibrating tuning fork 1–2cm from the auditory canal, and again ask the patient to tell you if they are able to hear the tuning fork
• normally hear the fork in the air for a couple seconds after they lost it in the bone

Question 43

What’s the weber test?

Answer 43

• auditory exam, testing sensorineural hearing loss
• In the Weber test, the vibrating tuning fork is placed on the midline of the head, and the patient asked if it is louder on one side or the other. Normally it will be equal, but in case of sensorineural hearing loss it will be diminished on the affected side. In the case of conductive hearing loss, the sound will seem greater on the affected side, possibly because of increased sensitivity of the cochlea or possibly because of acoustic properties related to the obstruction.

Question 44

A right sided vestibular lesion would produce what kind of nystagmus?

Answer 44

The lesion disturbs the equilibrium and the result is the intact side drives the eyes to the right as the slow phase of nystagmus. The brain senses the movement and rapidly corrects with a saccade in the opposite direction. This is the fast phase. *Thus a right sided vestibular lesion would produce left-beating nystagmus.

Question 45

Cranial Nerve 9 sends what fibers where?

Answer 45

The 9th nerve transmits taste from the posterior 1/3 of the tongue to the nucleus solitaries,

• some pain and temperature fibers from the posterior 1/3 of the tongue to the spinal trigeminal nucleus, and
• chemosensation from the carotid body to the nucleus solitaries.

Question 46

Where do the fibers of the 10th cranial nerve come from?

Answer 46

The vagus nerve similarly possesses efferents from the nucleus ambiguus to the majority of the skeletal muscle of the larynx and pharynx.
*Parasympathetic outflow from the dorsal vagal nucleus to the thoracic and abdominal viscera originates on this slice, and visceral afferents as well as a small region of pain and temperature sensation from the tympanic membrane ascend to the nucleus solitaries and spinal trigeminal nucleus, respectively.

Question 47

Describe what damage to the hypoglossal nuclei might look like?

Answer 47

Cranial nerve 12, the hypoglossal nerve, consists of motorneurons whose cell bodies lie in the hypoglossal nucleus, and send axons that emerge from the brainstem between the olive and the medulla.
*Upper motor neurons from the contralateral motor cortex innervate the hypoglossal nuclei. Damage to the hypoglossal nucleus or nerve will produce weakness, and the tongue will deviate toward the side of the lesion (the intact side “pushes” the tongue forward and away—normally the two sides are in equilibrium and the tongue protrudes in midline). Damage to upper motor neurons will produce deviation away from the side of the injury (i.e. left motor cortex injury—tongue points right. Left hypoglossal nerve injury- tongue points left). Injury to the nucleus or nerve, like other LMN injuries, produces more atrophy and fasciculations.

Question 48

UMN damage of CN 12 motoneurons would result in the tongue deviating ipsilateral or contralateral to lesion?

Answer 48

Contralateral.

• right motor cortex lesion
• innervation of left (contralateral) CN 12 nucleus messed up
• weakness of left side of tongue
• strong side pushes to the left, resulting in contralateral deviation to UMN injury
• nucleus injury means deviation to the ispsilateral side

Question 49

what are you supposed to see in the upper midbrain?

Answer 49

Upper midbrain—main features: oculomotor nuclei, CN 3, MLF, crus cerebri, red nuclei, substantia nigra, Medial lemniscus, spinothalamic tract, aqueduct, superior colliculus

Question 50

What are you supposed to see in the lower midbrain?

Answer 50

Lower midbrain—main features: trochlear nucleus (hard to see), MLF, inferior colliculus, decussation of superior cerebellar peduncle, medial lemniscus, spinothalamic tract, lateral lemniscus

Question 51

What are you supposed to see in the upper-pons area?

Answer 51

Upper pons—main features- superior cerebellar peduncle, fourth ventricle, MLF, pontocerebellar fibers and top of middle cerebellar peduncle, medial lemniscus, spinothalamic tract, corticospinal fibers (disaggregated)

Question 52

What are you supposed to see in the mid-pons area?

Answer 52

Mid pons—main features- root entry zone of CN V, principal sensory and motor nuclei of V. MLF, middle cerebellar peduncle, spinothalamic tract, medial lemniscus, pontine grey matter, corticospinal fibers (disaggregated)

Question 53

What are you supposed to see in the lower pons?

Answer 53

Lower pons—main features- abducens nucleus, facial nucleus, spinal trigeminal nucleus, superior end of vestibular complex (don’t need to resolve individual vestibular nuclei). Trapezoid body, superior olivary nucleus, corticospinal fibers, pontocerebellar fibers, middle cerebellar peduncle

Question 54

What are you supposed to see in the upper medulla?

Answer 54

Upper medulla—main features- Inferior cerebellar peduncle, vestibular nuclei, spinal trigeminal nucleus, spinothalamic tract, medial lemniscus, superior pole of inferior olivary nucleus, pyramid, vestibulocochlear nerves

Question 55

What are you supposed to see in the upper-mid medulla?

Answer 55

Upper-mid medulla—main features- hypoglossal nucleus, dorsal motor nucleus of the vagus, nucleus tractus solitaries, spinal trigeminal nucleus, inferior cerebellar peduncle, vestibular nuclei, spinothalamic tract, nucleus ambiguus, inferior olivary nucleus, pyramid, medial lemniscus

Question 56

What are you supposed to see in the lower-mid medulla?

Answer 56

Lower-mid medulla—main features- nucleus gracilis, nucleus cuneatus, spinal trigeminal nucleus, pyramids, internal arcuate fibers forming decussation of medial lemniscus.