Brainstem & Cranial Nerves

Question 1

Three regions of the Brainstem

Answer 1

1 midbrain
2 pons
3 medulla

Question 2

Basic functions of the Brainstem

Answer 2

1 cranial nerve function
- similar to lower motor & sensory neurons in spinal cord
2 conduit
- ascending and descending tracts
3 integration
- mostly by reticular formation

Question 3

Midbrain

Answer 3

Dorsal surface
▪ superior & inferior colliculi
▪ superior cerebellar peduncle
Ventral surface
▪ cerebral peduncles
CN rootlets
▪ CN III (oculomotor)
➢ ventral surface, between cerebral peduncles
▪ CN IV (trochlear)
➢ dorsal surface at junction of pons & midbrain
▪ (only CN on dorsal surface)

Question 4

Pons

Answer 4

Dorsal surface
▪ middle cerebellar peduncle
➢ superior cerebellar peduncle forms roof of 4th ventricle
Ventral surface
▪ swelling of basal pons
CN rootlets
▪ at midpons
➢ CN V (trigeminal)
▪ at junction of medulla and pons
▪ from medial to lateral
➢ CN VI (abducens)
➢ CN VII (facial)
➢ CN VIII (vestibulocochlear)

Question 5

Medulla

Answer 5

Dorsal surface
▪ obex
➢ where 4th ventricle narrows into central canal
▪ inferior cerebellar peduncle
Ventral surface
▪ olive
▪ pyramids
▪ pyramidal decussation
➢ interrupts anterior fissure of spinal cord
➢ site of cerebrospinal tract crossing
CN rootlets
▪ CNs IX, X, & XII (glossopharyngeal, vagus, & hypoglossal

Question 6

Three anterior/posterior regions

Answer 6

1 Tectum: most posterior layer
▪ primarily in midbrain
- contains superior & inferior colliculi
2 Tegmentum: middle layer
- contains most brainstem nuclei
3 Basis: anterior layer
- contains tracts
▪ e.g., cerebral peduncle, pyramids

Question 7

In general, sensory nuclei are more _______ & motor are more _______

Answer 7

Lateral; medial

Question 8

Oculomotor nucleus (CN III)

Answer 8

Located in rostral midbrain
▪ ventral edge of periaqueductal gray
Function (all ipsilateral)
▪ supplies eyelid (eyelid elevator)
▪ ocular muscles
➢ superior rectus
➢ medial rectus
➢ inferior oblique
➢ inferior rectus

Question 9

Damage to CN III

Answer 9

▪ lateral strabismus
▪ cannot move eye medially
▪ vertical eye movements impaired
▪ diplopia
➢ double vision (due to misaligned eyes)
▪ ptosis
➢ drooping of eyelid

Question 10

Edinger-Westphal nucleus (subdivision of oculomotor nucleus)(CN III)

Answer 10

Located in rostral midbrain
Function
▪ parasympathetic control of pupillary sphincter & ciliary muscle
➢ pupil constriction & focus (near vision)
Damage
▪ mydriasis
➢ dilated pupil, does not react to light (failed pupillary light reflex)
➢ cannot focus near vision

Question 11

Substantia nigra

Answer 11

Located in rostral midbrain
Function
▪ site of dopamine production (compact part)
▪ functional part of basal ganglia
➢ initiating/coordinating/modifying movements and muscle tone
Damage (see basal ganglia notes)
▪ Parkinsonism type of hypokinetic disorder

Question 12

Reed nucleus

Answer 12

Located in rostral midbrain
Function
▪ coordinating motor movement via cerebral cortex/cerebellar connections
▪ origin of rubrospinal tract….facilitates flexor musculature (predominate UE)
➢ fibers project (contralateral) to all spinal levels but predominate in cervical region
(prox UE)…thus flexor spasticity of UE

Question 13

Damage to red nucleus

Answer 13

▪ isolated lesion to red nucleus or rubrospinal tract is rare
▪ lesion rostral to red nucleus
➢ contribute to decorticate rigidity
▪ flexion of elbows, wrists & finders
▪ extension & internal rotation of legs
▪ lesion caudal to red nucleus
➢ contribute to decerebrate rigidity
▪ extension of arms & legs (esp. elbow)
▪ internal rotation of both arms & legs
▪ cerebellar signs may also be associated with red nucleus damage

Question 14

Trochlear nucleus (CN IV)

Answer 14

Located in caudal midbrain near midline
▪ only cranial nerve to emerge from dorsal surface
Function
▪ motor control of contralateral superior oblique
Damage
▪ less noticeable than other regions controlling eye movements
➢ diplopia while attempting to move eye down & laterally

Question 15

Mescencephalic nucleus of trigeminal (CN V)

Answer 15

Located from rostral pons to midbrain
▪ not a true nucleus
▪ is a sensory ganglion embedded in brainstem
Function
▪ proprioceptive information from jaw (and teeth)
▪ primary sensory information for jaw jerk reflex
➢ tapping chin causes mouth to close (vis masseter, mostly)
Damage
▪ loss of jaw jerk reflex

Question 16

Trigeminal motor nucleus (CN V)

Answer 16

Located mid-pons
Function
▪ motor neurons for muscles of mastication
▪ masseter, temporalis, medial/lateral pterygoids
➢ other minor inputs to tensor tympani, mylohyoid, & anterior belly of digastric
Damage
▪ unilateral lesions to nucleus have NO observable effect on chewing
➢ due to bilateral innervation
▪ exception is tensor tympani

Question 17

Trigeminal main sensory nucleus (CN V)

Answer 17

➢ aka – principal sensory nucleus of V
Located in mid-pons
▪ rostral of facial motor nucleus
Function
▪ somatosensory & conscious proprioceptive inputs from the face & head
➢ information decussates here & joins medial side of medial lemniscus
▪ minor: some oral cavity information remains uncrossed (dorsal trigeminothalamic tract)
Damage
▪ ipsilateral loss of touch sensation from face

Question 18

Abducens nucleus (CN VI)

Answer 18

Located in caudal pons
▪ closer to the midline (because it is a motor nucleus)
▪ surrounded by genu of facial nerve
➢ visible as facial colliculus on the floor of the 4th ventricle
Function
▪ supplies the ipsilateral lateral rectus m.
▪ causes abduction of the eye
Damage
▪ medial strabismus
➢ ipsilateral eye deviates slightly medially while gazing straight ahead
▪ as a result of the now-unopposed medial rectus m.
➢ may be able to get affected eye to midposition – but not past

Question 19

Facial motor nucleus (CN VII)

Answer 19

Located in caudal pons
Function
▪ somatic motor output
➢ muscles of facial expression
▪ bilateral to upper face
▪ unilateral to lower face
➢ stapedius (in middle ear)
Damage
▪ hemiplegia of ipsilateral face
➢ note: compare this to UMN damage (e.g., corticobulbar fibers) which paralyzes
the mid- and lower- contralateral face

Question 20

Cochlear nucleus (CN VIII)

Answer 20

Located in the caudal pons near the medulla/pons junction
▪ dorsolateral extreme
Function
▪ receives auditory information from the cochlear division of VIII (auditory n.)
➢ hearing
Damage
▪ may cause loss of specific frequencies from the ipsilateral ear

Question 21

Vestibular nuclei (vestibular portion of VIII)

Answer 21

Located mid-medulla to mid-pons
▪ 4 separate nuclei
Function
▪ receive input from vestibule & semicircular ducts
➢ head position, movement, & rotation
➢ primary sensors for VOR
➢ primary contributor to medial & lateral vestibulospinal tracts
Damage
▪ vertigo, nystagmus, nausea/vomitting

Question 22

Nucleus of the solitary tract (CNs VII, IX, & X)(facial, glossopharyngeal, & vagus)

Answer 22

Located from medulla to caudal pons
Function
▪ major visceral sensory system of the brainstem
➢ taste
➢ blood pressure & chemical sensation (from carotid sinus & aortic bodies)
➢ status of GI & cardiopulmonary systems
▪ e.g., lung stretch, dryness of mucous membranes
Damage
▪ loss of ipsilateral taste (esp. rostral end of NTS)
▪ increased heart rate (caudal end of NTS

Question 23

Dorsal motor nucleus of the vagus (CN X)

Answer 23

Located in medulla
Function
▪ visceral motor output (90% of parasympathetic fibers)
➢ parasympathetic output to thoracic & abdominal regions
▪ are preganglionic neurons
Damage
▪ ?loss of GI tract motility; cardiopulmonary problems

Question 24

Nucleus ambiguus

Answer 24

Located through medulla
Function – split innervation: both branchial motor & visceral motor (parasympathetic)
▪ branchial motor component
➢ CNs IX & X (glossopharyngeal & vagus)
➢ damage = difficulty swallowing (dysphagia) & speaking (dysphonia)
▪ visceral motor component
➢ CN X (vagus)
➢ cardioinhibitory neurons
▪ decreases cardiac output (e.g., during sudden blood pressure increases)
▪ damage = increased resting heart rate

Question 25

Hypoglossal nucleus (CN XII)

Answer 25

Located throughout most of medulla
Function (somatic motor)
▪ innervates extrinsic muscles of the tongue (genioglossus, styloglossus, & hyloglossus)
Damage
▪ ipsilateral weakness of the tongue (protrudes to weak side)

Question 26

Spinal trigeminal nucleus (CN V) and tract

Answer 26

Nucleus is located from upper cervical spinal cord to pons
▪ pain/temperature region is more caudal region
➢ (functions of oral & interposed are not covered here; less clear)
Tract runs from pons to caudal medulla
▪ all trigeminal neurons enter at mid-pons
Function
▪ sensations of pain and temperature from ipsilateral face & head
Damage (to nucleus or tract)
▪ loss of pain and temperature to ipsilateral face & head
➢ however, due to proximity – damage often affects both nucleus & STT
▪ = bilateral loss of pain and temperature to head; contralateral body

Question 27

Accessory nucleus (CN XI)

Answer 27

Located in ventral horn of upper spinal cord
▪ from ~C5 to junction with medulla
➢ ventrolateral portion of ventral horn
▪ rootlets ascend to exit through jugular foramen
Function (somatic motor)
▪ ipsilateral SCM & trapezius
Damage
▪ weakness to ipsilateral SCM & trapezius

Question 28

Reticular system

Answer 28

Diffuse network of nuclei located throughout brainstem
➢ reticular means “net-like”
b receive and give widespread connections
▪ respond to wide range of sensory inputs
2 Two functional components of reticular system:
-Ascending & Descending reticular formations

Question 29

Ascending reticular formation

Answer 29

Commonly referred to as ARAS (ascending reticular activating system)
Function
▪ regulates cortex
➢ sleep-wake cycle wakefulness
➢ level of consciousness and alertness
➢ modify attention…ability to tune in/out environmental stimuli
Pathways
▪ ARAS projects fibers to intralaminar thalamus which then reach cortex
Lesion
▪ coma

Question 30

Descending reticular formation

Answer 30

Function
▪ motor
▪ autonomic nervous system role
▪ pain modulation
Motor
▪ maintains/modifies tone, posture, balance, stability and antigravity reflexes during
movement
➢ reticulospinal tract
▪ pontine (medial) reticulospinal tract
* facilitates extension, inhibits flexion
 e.g., inhibits flexor reflexes = only noxious stimuli can cause reflex
▪ medullary (lateral) reticulospinal tract
* facilitates flexion, inhibits extension
▪ plays role in vestibular (eye/ear/cerebellar) function

Question 31

Autonomic NS

Answer 31

▪ pathways
➢ receives input from hypothalamus
▪ autonomic regulation of
➢ ventilation/cardiovascular systems
▪ ventilation: apneustic, pneumotaxic (pons), DRG and VRG (medulla)
➢ cardiovascular
▪ cardioinhibitory & cardiacceleratory areas in medulla
➢ other autonomic visceral functions
▪ swallowing, vomit, coughing, sneezing, sexual activity, urination/defecation

Question 32

Pain Modulation

Answer 32

▪ plays major role in the complex pathways of pain modulation
➢ anesthetics/psychotropic drugs are thought to act on reticular formation, PAG
▪ recreational drugs, painkillers, anesthetics, hallucinogens, etc

Question 33

Lesion of reticular structures

Answer 33

Damage to the ascending reticular activating system (ARAS)
- altered level of consciousness
- range from mild – coma – death
Damage may have obvious detrimental outcome on visceral/motor systems of the body
- ventilation and cardiovascular function

Question 34

Medial lemniscus

Answer 34

carries touch & conscious proprioceptive information from
- body (dorsal column nuclei)
- face (trigeminal main sensory nucleus)
Lesion
- contralateral loss of touch & conscious proprioception

Question 35

Anterolateral pathway (spinothalamic tract)

Answer 35

carries pain & temperature from
- body (combination of lateral & anterior STT)
- face [spinal trigeminal nucleus (also CNs VII, IX, & X)]
Lesion
- contralateral loss of pain & temperature

Question 36

Medial longitudinal fasciculus (MLF)

Answer 36

▪ carries both ascending & descending fibers
Ascending information carries:
▪ eye movement information from abducens nuc → oculomotor nuc.
➢ for horizontal (conjugate) gaze
▪ vestibular information to CNs III, IV, VI
Descending information
- medial vestibulospinal tract
▪ coordinates eye & head/neck movements
Lesion
- strabismus (type depends on level and severity of damage)
- nystagmus

Question 37

Lateral lemniscus

Answer 37

carries auditory information from bilateral cochlear nuclei to inferior colliculus
- located in dorsolateral extreme of the pons

Question 38

Corticospinal tracts

Answer 38

Carries
- contralateral motor information to trunk/extremities
Located in anterior regions of midbrain (cerebral peduncles), pons and medulla (pyramids)
Pyramidal decussation
- cross midline near junction of medulla/spinal cord
Damage
- above decussation
▪ contralateral hemiparesis of trunk/extremities
- below decussation
▪ ipsilateral hemiparesis of trunk/extremities

Question 39

Corticobulbar tracts

Answer 39

Carries
- motor information to cranial nerves (head/face)
▪ bilateral innervation except
➢ lower face (CN 7)
➢ tongue (CN 12)
Damage
- contralateral hemiparesis of lower face & tongue
- other head/face motor structures maintain function at some level due to bilateral
connections

Question 40

Corticopontine tracts

Answer 40

Carries
- motor information to cerebellum via pontine nuclei
▪ cerebral peduncles
Damage
- potential cerebellar signs
▪ cerebellar dyssynergias

Question 41

Spinal trigeminal tract

Answer 41

Located in pons and medulla
- descends to medulla → decussates →joins medial edge of contralateral STT
- carries
- ipsilateral pain and temp sensory input from face
Damage
- ipsilateral loss of pain/temp from face

Question 42

Descending autonomic sympathetic tracts

Answer 42

Located in dorsal lateral regions of brainstem
- hypothalamus connections to sympathetics descend through brainstem
▪ sympathetics “originate” in upper thoracic cord
Damage
- Horner’s syndrome
▪ miosis – small pupil
▪ ptosis – drooping eyelid