Bales: brainstem topography Flashcards
what are the 3 regions of the brainstem (from dorsal to ventral)?
tectum, tegmentum and basis
Where does CN III exit the brainstem? CN V? CN VI? CN XII?
III=intereduncular fossa
V=upper side of the pons (at pons-middle cerebellar peduncle junction)
VI=pontomedullary junction
XII=between pyramid and olive
on the ventral side!
What is the midbrain tectum composed of?
4 mound -like gray matter structures called colliculi (superior 2=vision, inferior 2=auditory)
What is the only cranial nerve to exit dorsally and where does it exit?
trochlear N.
exits at the pontomesencephalic junction and courses around the cerebral peduncle to the ventral side
What is the obex?
the junction between the closed medulla (inferiorly) and open medulla (superiorly)
What 2 cranial nerves exit the brainstem at the cerebellopontine angle?
CN VII and VIII
How are the nuclei positioned in the brainstem?
lateral–> medial
(like taking the neural tube and cutting the dorsal side and spreading so the middle stays ventral and the dorsal falls to the side):
GSA (general somatic)
SSA (special somatic) (vision, hearing, equilibrium)
GVA (pain and reflexes from viscera)
SVA (smell and taste)
SVE (striated skeletal
mm of V, VII, IX, and X–> chew your face and swallow)
GVE (pre and post ganglionic parasympathetics (III, VII, IX, X)
GSE striated skeletal mm of III, IV, VI, XII –> lick your eyes
How can you distinguish a caudal (inferior closed medulla) section?
*main: fibers from the pyramids are decussating (spots)–> criss-cross appearance ventrally
3 large sensory nuclei are located just internal to their respective tracts (look like 3 acorns) (gracile nuc –> cuneate nut–> spinal trigeminal nuc (medial–> lateral))
central canal in center and enclosed
How can you distinguish a mid-medulla (superior closed medulla) section?
internal arcuate fibers representing the sensory decussation
- fibers from the gracile and cuneate nuclei cross to form the contralateral medial lemniscus (rectangular midline darkening)
the gracile & cuneate nuclei are much larger and the tracts much reduced
adjacent to the midline in the dorsal tegmentum, just ventral to the central canal, is the hypoglossal nucleus
incomplete inferior olivary nuclei may be present on top of pyramids
How can you distinguish rostral medulla (open medulla) section?
inferior olivary nucleus (the large open-ended squiggly thing facing inward); it is resting on the corticospinal tracts (pyramids)
central canal is gone and the ventricular sytstem is
large and flared out (4th ventricle)
hypoglossal nucleus is prominent
inferior cerebellar peduncle present with vestibular and cochlear nuclei draped over them. (CN VIII can be seen going here)
How can you distinguish a pontomedullary junction section?
“bat wings” shaped large 4th ventricle
pyramids are assembling from the pons
middle cerebellar peduncle (large mass of white matter)
pyramidal tracts are broken up with scattered small pontine nuclei
medial lemniscus is globular (less rectangle shaped)
facial colliculus and abducens nucleus form at the 4th ventricle floor
How can you distinguish a caudal pons section?
Medial lemniscus is now horizontal
How can you distinguish a mid-pons section?
The 4th ventricle narrows rostrally to become a small cylinder in the midbrain (Bales thinks it looks like a flying squirrel?)
Two of the cerebellar peduncles are prominent (middle and superior). Superior forms the walls of the ventricle
How can you distinguish a rostral pons section?
4th ventricle looks like a “ginko leaf” or “fan”
Superior cerbellar peduncle is just beginning to decussate
central gray associated with the ventricle
How can you distinguish a pontomesencephalic junction -caudal midbrain section?
central gray surrounds the aqueduct
Medial lemniscus is more vertical again
SCP decussation in the center behind the central gray
The dense organized crus cerebri of midbrain is transitioning to fragmented pontine basis
