Thread by @AaronLBerkowitz

Back with another #neurology #neuroanatomy #tweetorial.
This one goes out to all the #MedStudentTwitter studying
for the steps:

The BRAINSTEM and CRANIAL NERVES

#MedEd #NoMoreNeurophobia cc: @MadSattinJ @Tracey1milligan @StaceyLClardy @DxRxEdu @CPSolvers

This is often considered one of the most complex parts of neuroanatomy.

Let’s break it down:

First, the brainstem is divided into 3 levels, from superior to inferior:

MIDBRAIN
PONS
MEDULLA

These are MRI view so anterior on top, posterior on the bottom

Midbrain= looks like Mickey Mouse

Pons=two huge cables connecting it to the cerebellum (middle cerebellar peduncles)

Medulla=looks almost like the spinal cord

There’s lots of stuff going on in the brainstem, let’s categorize the clinically relevant structures:

1 Descending motor pathways for the body
2 Ascending sensory pathways from the body
3 Cerebellar connections
4 Cranial nerve nuclei
5 Reticular activating system etc

Let’s knock out 1 and 2

The corticospinal tracts (motor) are the most VENTRAL/ANTERIOR structure (blue)

The ascending sensory pathways (dorsal columns (green) and spinothalamic tracts (red)) are DORSOLATERAL (except for the medial lemnisci briefly being medial in the medulla)

Cerebellar peduncles run posteriorly, makes sense because the cerebellum is posterior to the brainstem. We’ll discuss these in another tweetorial.

For now, just remember those big cables connecting pons to the cerebellum are the middle cerebellar peduncles.

Reticular activating system/ascending neurotransmitter projections=fascinating but rarely relevant for localization aside from locked in syndrome (occurs when lesion of upper pons leaves RAS intact but nothing below midbrain so preserved consciousness/blinking, but nothing else)

Now for the main course, the cranial nerve nuclei!

These always seemed so confusing to me as a student. But there are two main rules to help categorize these:

First: 12/3 = 4

Second: motor medial, sensory lateral

12/3 = 4. Remember when you took calculus to get into med school? Well good news, you don’t need it! Add, subtract, divide, multiply—all you need to do.

12 cranial nerves, 3 levels of brainstem= ~ 4 nuclei per level

Midbrain: 1-2-3-4
Pons: 5-6-7-8
Medulla: 9-10-11-12

There are just a few exceptions!!

1 (olfactory) and 11 (spinal accessory) don’t connect to the brainstem.

5 has nuclei at all 3 levels

8: cochlear at pontomedullary junction; vestibular in medulla

But if you just remember 1-4/5-8/9-12 you can figure most localization issues!

Now the 2nd rule:
Motor CN nuclei=medial, sensory=lateral.

Neuroanatomy textbooks divide nuclei into somatic/branchial/ visceral motor & somatic/visceral/ special sensory

But brainstem=small, and lesions rarely respect 1/6 of it!
So let’s keep it simple: motor v sensory!!

So let's go nerve by nerve

1= olfactory: doesn't go to brainstem!

2: optic-we'll cover the visual pathways in another tweetorial, but they don't go to the brainstem...
but the afferent limb of the pupillary reflex does, and goes to midbrain so far so good

3,4,6= eye movements = MOTOR. We'll discuss in detail in another tweetorial.

3,4 = midbrain
6 = pons
as expected from 1-4/5-8/9-12

ALL RIGHT AT MIDLINE = MOTOR

5: sensory to face + motor to muscles of mastication. TRIgeminal = 3 branches (= also nuclei @ all 3 levels)

5proprioception = in midbrain (minimal clinical relevance)
5lt (light touch) = in pons
5pt (pain temperature)= in MEDULLA* (important for lateral medullary syndrome!)

7: facial motor (+ lots of other less clinically relevant stuff)= PONS (5-6-*7*-8):

7 is lateral to 6 (since technically = branchial motor), and nerve loops around CN6 nucleus on way out

All down hill from here!
8 = pons (auditory)/medulla (vestibular)
9-10=medulla
11 = doesn't go to brainstem
12 = medulla

So most medial = MOTOR
3
4
6
12

Slightly more lateral = BRANCHIAL MOTOR
5m
7
9/10

Most lateral = SENSORY
5
8

So remembering 1-4/5-8/9-2 and motor medial (3,4,6,12; branchial: 5,7,9,10); sensory lateral 5/8,

behold:

The brainstem triangle!

So MEDIAL brainstem syndrome affect the MOTOR cranial nerve nuclei at that level and the descending motor pathways for the body.

Since the pathways for the body are not yet crossed we get ipsilateral weakness above the neck (eyes, face, or tongue) contralateral below the neck

Examples of MEDIAL brainstem syndromes:

Midbrain–Ipsilateral 3rd nerve palsy and contralateral hemiparesis

Pons–ipsilateral 6 and 7 with contralateral hemiparesis

Medulla–ipsilateral tongue deviation/contralateral hemiparesis

Note, no sensory CNs, no cerebellar!

LATERAL brainstem syndromes affect the sensory/special sensory CN nuclei, sensory for the body, and cerebellar peduncles.

The most common lateral syndrome = lateral medullary (Wallenberg) from PICA stroke:

5lt (pain/temp ipsi face)
8vestib
9/10: dysarthria/dysphagia (branchial motor)
Cerebellum
Spinothalamic: pain/temp contra body

Note NO weakness (in face or body)

And now blood supply, all derived from vertebrobasilar system:

Midbrain: SUPERIOR cerebellar arteries (and PCAs)
Pons: ANTERIOR INFERIOR cerebellar arteries
Medulla: POSTERIOR INFERIOR cerebellar arteries

Medial syndromes = from midlines vessels: BASILAR (or rarely anterior spinal for medulla)

Lateral syndromes = circumferential arteries: SCA, AICA, PICA

Now why did that take a WHOLE WEEK in med school....?
#noMoreNeurophobia🧠❤️👍

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