47. Brainstem

Question 1

Medulla

• where are the junctions?
• what are the two parts of the medulla?
• what feature is in the ventral medulla?
• what syndromes are associated here (ventral medulla)?

Answer 1

Junctions: caudal (upper rootlet of C1 = SC); rostral (pons/inf edge of middle cerebral peduncles
Parts: Closed, and Open (contains caudal half of 4th ventricle boundary)

Ventral: PYRAMID: between ventral median fissure and ventrolateral sulcus, contains CS tract (lesion = medial medullary syndrome = contralateral hemiparesis)

Decussation of pyramids at spinomedullary junction: decussating fibers are Lateral CS tract (limb muscles; lesion = brown-sequard = ipsilateral hemiparesis), uncrossed fibers = ventral CS tract (trunk muscles)

Question 2

Medulla

• what features are in the Lateral medulla? what are their functions?
• what syndromes are associated in lateral medulla?

Answer 2

1. Olive - formed by inferior olivary nucleus, where olivocerebellar fibers decussate and project to opposite inferior cerebellar peduncle, fx: motor learning
2. Trigeminal Tubercle: between olive and dorsolateral sulcus, consists of spinal tract of CN V and nucleus of spinal tract (Ad and C fibers from CN V - facial pain/temp)
   lateral medullary syndrome (wallenberg’s) = ipsilateral pain/temp loss in face
   pain/temp neurons of CN V from nucleus of spinal tract decussate along trigeminothalamic fibers to VPM (ventral posterior medial) nucleus of thalamus

Question 3

Medulla

• what features are in the DORSAL medulla?
• what are these features’ functions
• what syndromes are associated in the dorsal medulla?

Answer 3

Fasciculus Gracilis + Cuneatus: from SC dorsal column (lesion of FG/FC = brown-sequard = ipsilateral loss of touch/proprio)
Gracile Tubercle: produced by nucleus gracilis (medial, legs)
Cuneate Tubercle: produced by nucleus cuneatus (lateral, arms)
Post-synapse 2o neurons decussate and ascend medial lemniscus
(lesion of medial lemniscus = medial medullary syndrome = contralateral loss of touch/proprio)

Question 4

What are the features/functions of the Reticular Formation?

Answer 4

Lateral Reticular Area: small neurons (parvocellular) - reflexes and simple CN-mediated movements (eating, gaze)

Medial Reticular Area: large neurons (magnocellular) - long ascending/descending projection systems modulating pain, movement, posture, ANS/endocrine control, arousal, consciousness

Question 5

Pons

What are the two parts of the Pons? What do they contain?

Answer 5

Basal portion: large relay station between opposite cerebral cortices, composed of longitudinal fiber bundles, transverse fiber bundles, collections of pontine nuclei between fibers

Tegmentum: ascending/descending tracts, CN nuclei, reticular formation

Question 6

What are the function/contents of the parts of the basal portion of the pons?

Answer 6

longitudinal bundles: descending fibers from crus cerebri of midbrain

i. corticobulbar fibers: synapse BILATERALLY to LMN of CN motor nuclei
ii. CS fibers: pass thru to pyramids of medulla
iii. corticopontine fibers: from all 4 lobes to ipsilateral pontine nuclei

Transverse fibers: pontocerebellar fibers: axons from pontine nuclei decussate and enter cerebellum through middle cerebellar peduncle

Question 7

What are the function/contents of the tegmentum of the pons?

Answer 7

Principle/Chief/Main Sensory Nucleus of CN V: Aa and Ab fibers for touch/vibration of face, sends 2o neurons to decussate and follow trigeminal meniscus to VPM thalamic nuclei

Also contain ascending/descending tracts of CN nuclei + reticular formation

Question 8

List the 4 parts of the midbrain

Answer 8

1. Tectum
2. Tegmentum
3. Substantia Nigra
4. Crus Cerebri

Question 9

What are the features/functions of the tectum of the midbrain?

Answer 9

Roof of midbrain, dorsal to cerebral aqueduct
Inf Colliculi: auditory path relay to thalamus
Inf Brachii: fibers from IC travel to synapse to MGN

Sup Colliculi: reflex center for eye movements (response to visual/auditory/cutaneous stim)
Sup Brachii: fibers from visual cortex of occipital lobe travel towards SC - for pupillary light reflexes, accommodation, smooth pursuit of eye movements

Question 10

What are the features/functions of the tegmentum of the midbrain?

Answer 10

Ventral to cerebral aqueduct
Contains ascending/descending tracts, CN nuclei, reticular formation, RED NUCLEUS
Mesencephalic nucleus of trigeminal nerve: cell bodies of 1o (ONLY FIRST ORDER NUCLEI) fibers transmitting proprioception
2o neurons send axons into decussated trigeminal lemniscus to VPM of thalamus

Rubrospinal tract: magnocellular part of Red nucleus - decussates to contralateral side

Question 11

What are the features/functions of the Substantia Nigra?

Answer 11

SN: large pigmented motor nucleus between tegmentum and crus cerebri
Black = melanin - byproduct of DA synthesis

Question 12

What are the features/functions of the Crus Cerebri? What would a lesion here cause?

Answer 12

Prominent elevation on either side of ventral midbrain surface - contains CS, corticobulbar, corticopontine fibers
Lesion: contralateral hemiparesis
Interpeduncular fossa: region b/w peduncles

Question 13

What structures consist of the cerebral peduncle?

Answer 13

One half of midbrain, excluding tectum (tegmentum, SN, Crus Cerebri)

Question 14

Which CN leave what parts of the brainstem?

Answer 14

CN1+2: leave forebrain
CN3: midbrain side of interpeduncular fossa
CN4: dorsal midbrain below Inf Colliculus
CNV: attached to lateral brainstem at boundary of pons/middle cerebellar peduncle
CN6: pontomedullary junction
CN7/8: lateral surface of pontomedullary junction (cerebellopontine angle)
CN9/10: rootlets attached to medulla along retroolivary groove
CN11: arises from C1-C5 and ascends thru foramen magnum
CN12: rootlets emerge from pre-olivary groove of medulla