Surface Anatomy Of The Brainstem

Name the three components of the brainstem in order from superior to inferior.

The three components of the brainstem from superior → inferior are:

• Midbrain — the top part, connecting the brainstem to the diencephalon

• Pons — the middle bulging segment

• Medulla oblongata — the lowest part, continuous with the spinal cord

What is the tectum of the midbrain, and what does the term mean?

The tectum of the midbrain is the dorsal (“roof”) surface of the midbrain

• Location: Forms the roof over the cerebral aqueduct at the midbrain level

• Key feature: Contains the four colliculi (2 superior, 2 inferior)

What are the cerebral peduncles, and what structure lies between them?

• Cerebral peduncles = the two big motor “stalks” on the front (ventral) midbrain carrying major descending motor tracts

• Between them: the interpeduncular fossa

What is the interpeduncular fossa, and what emerges from it?

Interpeduncular fossa = the midline dip on the front (ventral) midbrain between the two cerebral peduncles

What emerges from it:

• Oculomotor nerves (CN III) come straight out of this fossa

Summarise the key surface landmarks of the midbrain on both its dorsal and ventral surfaces.

Dorsal (back) surface of the midbrain:

• Tectum = “roof”

• Contains the four colliculi (2 superior + 2 inferior)

Ventral (front) surface of the midbrain:

• Paired cerebral peduncles (big motor stalks)

• Interpeduncular fossa between them

• CN III (oculomotor nerve) exits from this fossa

What are the two main parts of the pons?

The pons has two main parts:

• Basal pons — the front (ventral) bulging part

• Tegmentum — the back (dorsal) “covering” part

What is the tegmentum of the pons, and what structure does it contribute to?

The tegmentum of the pons is the back (dorsal) “covering” part of the pons

• It forms part of the floor of the 4th ventricle together with the medullary tegmentum

• It contains cranial nerve nuclei and ascending sensory tracts

What is the basal pons, and what is its distinguishing surface feature?

The basal pons is the front (ventral) bulging part of the pons

• It creates the rounded, protruding surface you see on the ventral brainstem

• It contains transverse pontocerebellar fibres and descending corticospinal fibres

What is the 4th ventricle, and which brainstem structures contribute to its floor?

The 4th ventricle is the CSF‑filled cavity behind the pons and medulla, connecting the cerebral aqueduct to the central canal

Its floor is formed by:

• Pontine tegmentum → upper floor

• Open/rostral medulla → lower floor

Describe the anatomical boundaries of the medulla oblongata.

The medulla oblongata sits between the pons and the spinal cord

• Superior boundary: the inferior border of the pons

• Inferior boundary: the foramen magnum, where it becomes the spinal cord

• It is the lowest part of the brainstem

Distinguish between the "open" (rostral) and "closed" (caudal) portions of the medulla.

Open medulla = 4th ventricle present
Closed medulla = only the central canal

• Open (rostral) medulla: the 4th ventricle is visible on the back → forms the lower floor of the ventricle.

• Closed (caudal) medulla: the ventricle narrows into the central canal, just like the spinal cord.

What is the inferior olive, where is it located, and why is it clinically important as a landmark?

The inferior olive is the oval bulge on the side of the medulla.

Why it matters: it’s the key landmark for locating four cranial nerves.

• Above the olive: CN IX, X, XI

• Below the olive: CN XII

What are the medullary pyramids, and what do they contain?

The medullary pyramids are the two long ridges on the front (ventral) surface of the medulla.

What they contain:

• The corticospinal tract — the main motor pathway coming from the primary motor cortex.

Summarise the key surface landmarks visible on the medulla, specifying whether each is on the anterior, lateral, or posterior surface.

Anterior (front) surface

• Medullary pyramids — long motor ridges

• Anterior median fissure — groove between the pyramids

Lateral surface

• Inferior olive — the big oval bulge

• Above the olive: CN IX, X, XI

• Below the olive: CN XII

Posterior (back) surface

• Upper part: forms the floor of the 4th ventricle (open medulla)

• Lower part: becomes closed, like the spinal cord

Describe the CN I (olfactory nerve) — what structures are visible on the inferior brain, and does CN I actually attach to the brainstem?

CN I (olfactory nerve) does not attach to the brainstem.

What you actually see on the inferior brain:

• Olfactory bulb — small oval structure sitting on the cribriform plate

• Olfactory tract — the band running back from the bulb

Key point:
The true olfactory nerves are tiny filaments from the nose that synapse in the bulb. The bulb and tract belong to the inferior frontal lobe, not the brainstem.

Describe the CN II (optic nerve) landmarks visible in the inferior view, and where is the optic chiasm?

CN II (optic nerve) is seen from below as the two optic nerves coming from each orbit.

• They meet at the optic chiasm — the X‑shaped crossing where nasal retinal fibres cross and temporal fibres stay on the same side.

• The optic chiasm sits just above the pituitary gland.

Where does CN III (oculomotor nerve) attach to the brainstem?

CN III (oculomotor nerve) attaches to the midbrain.

• It emerges from the interpeduncular fossa — the midline dip between the two cerebral peduncles on the ventral midbrain.

• Then it travels forward toward the orbit through the superior orbital fissure.

Where does CN V (trigeminal nerve) attach to the brainstem?

CN V (trigeminal nerve) attaches to the lateral side of the pons.

• It’s the largest cranial nerve

• Has a big sensory root + small motor root

• It’s the dominant nerve on the mid‑lateral pons

Where does CN VI (abducens nerve) attach to the brainstem?

CN VI (abducens) attaches at the pontomedullary junction.

• It exits at the midline, just above the medullary pyramids

• Sits right at the border between pons (top) and medulla (bottom)

Where does CN VII (facial nerve) attach relative to CN VI?

CN VII (facial nerve) attaches at the pontomedullary junction, just like CN VI — but more lateral.

• CN VI: exits at the midline

• CN VII: exits lateral to CN VI at the same level

Where does CN VIII (vestibulocochlear nerve) attach relative to CN VII?

CN VIII (vestibulocochlear) attaches right behind CN VII at the lateral pontomedullary junction.

• CN VII: more anterior

• CN VIII: just posterior/dorsolateral to CN VII

Which three cranial nerves attach to the lateral surface of the medulla, and what is their relationship to the inferior olive?

CN IX, X, XI all attach to the lateral medulla, and they all sit above the inferior olive.

• They emerge as rootlets in the posterolateral sulcus

• They lie just behind the olive’s bulge

Where does CN XII (hypoglossal nerve) attach in relation to the inferior olive?

CN XII (hypoglossal) attaches just below the inferior olive.

• Sits in the anterolateral sulcus

• Emerges as rootlets between the pyramid and the olive

• Lies below and slightly medial to CN IX, X, XI

What do the medullary pyramids contain, and from where do their constituent fibres originate?

The medullary pyramids contain the corticospinal tract — the main voluntary motor pathway.

• Their fibres come from primary motor cortex (precentral gyrus)

• These fibres are the axons of upper motor neurons heading down to control contralateral body movement

A stroke affecting the anterior medullary surface on one side might damage the medullary pyramid. What type of motor deficit would this produce on the opposite side of the body?

A stroke hitting one medullary pyramid damages the corticospinal tract before it crosses.

Result: Contralateral UMN weakness with:

• Spasticity

• Hyperreflexia

• Babinski sign

• Clonus

Why: The fibres haven’t decussated yet, so injury on one side affects the opposite body.