Brainstem 1: Long Tracts Flashcards
Well... Brainstem 2 makes Brainstem 1 seem easy, at least...
Which cranial nerves exit medially?
III, IV, VI, and XII
What do the cranial nerves that exit medially* all have in common?
*corrected from “laterally.” Sorry, that was a really bad typo.
All are motor only.
Which CNs exit laterally?
V, VII, IX, and X.
What is similar about CNs that exit laterally?
V, VII, IX and X are mixed sensory and motor.
What’s the only CN that exits dorsally?
IV trochlear
What’s in the pyramids?
Corticospinal tract.
Why is the medial/lateral CN distinction clinically relevant?
They have different blood supplies.
What’s the only completely crossed CN?
IV
Where does IV run when it emerges from the midbrain?
Posterior to the inferior to the inferior coliculus.
What separates the dorsal pons from the cerebellum?
The 4th ventricle.
Where do axons that have ascended to the lower medulla synapse?
In the dorsal column nuclei.
What’s the medial lemniscus? (where are things coming from / going)
2nd neuron axons for proprioception/mechanoreception. Came from the dorsal column nuclei, headed to the VPL in the thalamus.
Which is more medial, the medial lemniscus or the spinothalamic tract?
Medial lemniscus is more medial.
In what structure is each corticospinal tract running right before it reaches the pons? Within that structure, where does it run?
In the cerebral peduncle. In the middle of the ventral-ish side.
Where do corticospinal fibers run in the brain (i.e. before the pyramidal decussation)? Dorsal/ventral, medial/lateral?
Ventral and medial
Where must a vascular lesion be to mess up corticospinal fibers?
ventral and medial
What would a lesion in the medial-most, ventral-most part of the basis pontis cause? (remember to specify ipsi/contralateral)
Contralateral spastic weakness. These are corticospinal fibers above the pyramidal decussation.
Review: Which vertebral levels send afferent axons to the fasiculus / nucleus gracilis? Fasciculus / nucleus cuneatus?
T6 and below-> Gracilis
T5 and above -> Cuneatus
What happens after afferent axons synapse in the nucleus cuneatus or gracilis?
Cross mid-line immediately, then travel up the medial lemniscus -> thalamus VPL nucleus
What would a lesion to the medial lemniscus cause?
Contralateral loss of mechano/proprioception (in neck and below).
How are axons within the medial lemiscus arranged with regard to their origin? Does it change?
Begins arranged, dorsal to ventrally, Arm-Trunk-Leg (ATL). As fibers ascend, leg and trunk fibers run laterally, so that the arrangement, from lateral to medial is Leg-Trunk-Arm (LTA).
Imagine a pair of legs (where… arm = thigh and trunk = knee, leg = foot..) that does a split as it ascends.
Why does the spinothalamic tract give off axons as it ascends toward the VPL nucleus?
Gives off pain fibers to areas of the brain that suppress pain.
What does a lesion of the spinothalamic tract in the brainstem do?
Contralateral loss of pain / temp sensation. (same as when it’s in the spinal cord)
How, geographically, does the spinothalamic tract travel up the brainstem?
Starts lateral, stays lateral (but not all the way to the edge) until it hits the VPL nucleus.
What fibers course right next to the spinothalamic tract (just lateral to them)?
Descending hypothalamic fibers controlling sympathetics.
Where do descending sympathetics run in the brain stem? Review: In the spinal cord?
Way laterally in the brain stem. In the spinal cord, intermediolateral columns.
Is Central Horner’s Syndrome ipsilateral or contralateral? How might you determine that it’s Central, not peripheral Horner’s?
Ipsilateral (for both central and peripheral). In central Horner’s, nearby spinothalamic tract may be lesioned, and thus may have contralateral pain / temp loss in trunk and limbs.
What would a lateral vascular lesion in the brainstem damage (that we discused in this lecture)?
Spinothalamic tract and descending sympathetics.
What does the medial longitudinal fasciculus (MLF) do?
Links vestibular nuclei and centers for conjugate gaze with abducens, trochlear, and occulomotor nuclei (most important for conjugate horizontal gaze).
What two diseases is the MLF particularly susceptible to?
MS and neurosyphilis.
Where does the MLF run?
Floor of 4th ventricle -> adjacent to central canal / cerebral aqueduct
What 4 things will be found in every brain stem cross-section?
A long tract
A CN nucleus or nerve
A trigeminal nucleus
A cerebellar peduncle or fibers forming one
Do muscles innervated by CNs typically get unilateral or bilateral innervation?
Bilateral
What are the UMN axons of cranial nerves called?
Corticobulbar or corticonuclear axons.
Where do LMNs of cranial nerves have their cell bodies?
In the corresponding CN nucleus (as covered in the following lecture).
If you damage a whole side of CN UMNs, do you get weakness? (note that there’s an exception)
No, one side appears to suffice, except for some of CNVII.
How do UMN and LMN lesions to CNVII appear differently? Why?
Only muscles that shut eyes and wrinkle forehead are innervated bilaterally. Thus… UMN lesion: paralysis/weakness in everything on that side except forehead wrinkling/eye-closing.
LMN lesion: weakness / paralysis of whole side of face (recall: Bell’s Palsy)
What does a unilateral lesion to corticobulbar fibers do?
No CN deficits except contralateral weakness of lower face muscles. (repetition is good…)
What does a lesion to the spinothalamic tract do? (2 thing, one less obvious)
Contralateral pain/temp loss.
Ipsilateral Central Horner’s Syndrome
