Medulla - Advanced Anatomy

The Medulla

Overview

Here, we'll learn about the medulla.

Start a table.
Denote that, from a clinician's broad perspective, the medulla is the center for the most basic generators of life.
Start a mid-sagittal section.

First, draw the different brainstem levels, from superior to inferior:

Midbrain
Pons
Medulla

Label the anterior/posterior orientational plane.

Key Structures

Include the key structures that border the brainstem:

The hyopthalamus, superiorly.
The cerebellum, posteriorly.
The cervical spinal cord, inferiorly.
And the temporal lobe, laterally.

Now, point out the:

Medullary pyramids, which contain descending motor fibers.
The inferior olive, an important motor relay center.
The gracile and cuneate nuclei, which receive large, proprioceptive sensory fibers from the spinal cord.
Shade in the CSF and indicate that the 4th ventricle lies at the level of the pons.

Radiographic Axial Section

Before we draw a detailed anatomical section, let's review an axial section in radiographic (the common clinical) perspective.

Show its anterior/posterior orientational plane.

Draw the medulla.
Cap off its most anterior region with the medullary pyramids: the medullary correlate of the cerebral peduncles, which carry the descending corticospinal tract fibers – they decussate at the cervico-medullary junction.
Now, draw the inferior olive just behind the medullary pyramids, which contains climbing fibers.
The inferior olives project to the contralateral cerebellum as part of the clinically important triangle of Guillain-Mollaret.

Clinical Correlation - Oculopalatal Myoclonus

Then, include the gracile nucleus, medially, which receives lower body sensory fibers.
And the cuneate nucleus, laterally, which receives upper body sensory fibers.

Anatomic Axial Section

Now, let's draw an anatomic axial outline of the medulla.
Indicate the anterior–posterior axis of our diagram.
Label the left side of the page as nuclei and the right side as tracts.
Draw an anatomic, axial cross-section of the medulla.
Next, draw the fourth ventricle — the cerebrospinal fluid space of the medulla.
Delineate the relatively small basis (compared to the pons, which has a large basis from the pontine nuclei and pontocerebellar tracts).
Then, the tegmentum.
Show that the basis comprises corticospinal tract fibers, which decussate at the cervico-medullary junction; they form the medullary pyramids.
Importantly, the corticonuclear and corticopontine fibers synapse either above or at the level of the medulla. The only major cortical motor pathway to descend through the medulla is the corticospinal tract.
In the anterior tegmentum, label the inferior olive.
Now, begin a table for the medulla.
In this table, we'll try to maintain the same anterior/posterior orientation as in our midsagittal section.
Indicate that the inferior olive contains climbing fibers; they project to the contralateral dentate nucleus of the cerebellum as part of the triangle of Guillain-Mollaret.
The dentate nucleus of the cerebellum projects back to the contralateral red nucleus, and then down to the inferior olive of origin via the central tegmental tract. The inferior olive (aka inferior olivary complex) comprises the main inferior olivary nucleus and the accessory olivary nuclei.
The inferior olive receives many different fiber pathways, including tracts from the spinal cord (from below) and from the red nucleus (from above).
Next, let's draw the large fiber sensory nuclei: in the dorsal medulla, medially, label the gracile nucleus.
It receives lower body sensory fibers from the gracile posterior column tract.
Then, laterally, label the cuneate nucleus.
It receives upper body sensory fibers from the cuneate posterior column tract.
Next, indicate that in the lower medulla, internal arcuate fibers decussate from the gracile and cuneate nuclei to the opposite side of the medulla (called the great sensory decussation).
Show that the internal arcuate fibers form the medial lemniscus pathway in midline.
Lateral to the medial lemniscus, label the anterior trigeminothalamic tract.

We find anterior trigeminothalamic projections throughout the brainstem because the anterior trigeminothalamic tract is formed from fibers of the spinal trigeminal nucleus, which extends inferiorly into the upper cervical spinal cord.

Posterior trigeminothalamic tracts originate and ascend from the principal sensory nucleus in the pons, so no posterior trigeminothalamic tract fibers are evident within the medulla.
Next, along the lateral wall of the medulla, draw the spinothalamic tract (of the anterolateral system).
At the medullary level, the anterolateral system contains many ascending sensory pathways in addition to the spinothalamic fibers, including the spinoreticular, spinomesencephalic, and spinotectal pathways.

It also carries spino-olivary and spinovestibular fibers, which disperse within the medulla, itself.

Then, posterior to the spinothalamic tract, draw the anterior and posterior spinocerebellar tracts.
Next, draw the rubrospinal tract, which provides upper extremity flexion.
Now, in posterior midline, draw the medial longitudinal fasciculus.
Anterior to it, draw the tectospinal tract.
The antero-posterior and medial-lateral positions of these pathways remain roughly unchanged throughout their course through the brainstem).
Then, label the inferior cerebellar peduncle (we do not find the superior and middle cerebellar peduncles in the medulla, as we did in the pons).
Along the anterior border of fourth ventricle, label the area postrema. Indicate that it is an important chemoreceptor trigger zone for vomiting, especially in obstructive hydrocephalus (or, less commonly, in a 4th ventricular tumor)
Now, in the ventral tegmentum label the reticular formation.
It serves numerous functions; the most notable one is wakefulness.
The reticular formation divides into lateral, medial, and median zones. Indicate that the raphe nuclei populate the median zone.
They are primarily serotinergic and are modulated by psychotropic medications.

Cranial Nerve Nuclei

Next, let's include the cranial nerve nuclei in an axial view of the medulla; they're in opposite orientation of our prior radiographic diagram.

Medially, draw the motor nuclei:

Start with the hypoglossal nucleus, CN 12
Then, the dorsal motor nucleus of the vagus, CN 10
Then, the inferior salivatory nucleus, CN 9
Lastly, the nucleus ambiguus, CNs 9 and 10

Then, draw the sensory nuclei:

Start with the most medial-lying of the sensory nuclei, the solitary tract nucleus of CNs 7, 9, and 10.
Then, the medullary extension of the vestibulocochlear nucleus, CN 8
Finally the spinal trigeminal nucleus, CN 5

The motor nuclei

CN 12
CN 10 (the dorsal motor nucleus)
CN 9 (the inferior salivatory nucleus)
CNs 9 and 10 (nucleus ambiguous).

The sensory nuclei

CNs 7, 9, and 10 (the solitary tract nucleus)
CN 8
CN 5 (the spinal trigeminal nucleus).

Medullary Stroke Syndromes

Key Clinical Stroke Correlations

Wallenberg's Syndrome

Dorsolateral Medullary Infarct

Dejerine's Syndrome

Medial Medullary Infarct

Medulla - Advanced Anatomy

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