🧠 Interpeduncular Fossa | Boundaries, Structures & Clinical Anatomy ✨Brainstem Anatomy for USMLE

🧠 Interpeduncular Fossa Simplified | Boundaries, Structures & Clinical Anatomy Explained ✨ | Brainstem Anatomy for MBBS, NEET PG & USMLE 👩‍⚕️📚
This session simplifies its boundaries, floor, contents, and clinical importance using a conceptual, visual, and exam-oriented explanation — perfect for MBBS, NEET PG, FMGE, and USMLE Step 1/2 CK aspirants.

✨ What You’ll Learn in This Lecture
• Introduction – The Key Landmark Between the Cerebral Peduncles 🧩
 The interpeduncular fossa (also called the fossa interpeduncularis) is a deep depression located on the inferior surface of the midbrain, between the two cerebral peduncles.
 It forms part of the floor of the third ventricle anteriorly and serves as a critical region where cranial nerves, arteries, and perforating vessels converge — making it both anatomically beautiful and clinically vital.

🧱 Location and Boundaries of the Interpeduncular Fossa
Superiorly:
 • Continuous with the posterior perforated substance and roof of the midbrain.
Inferiorly:
 • Merges with the optic chiasma and mammillary bodies region.
Laterally:
 • Bounded by the cerebral peduncles (crura cerebri) on each side — thick bundles of descending motor fibers connecting the cerebral cortex to the brainstem and spinal cord. 🧠⬇️
Anteriorly:
 • Limited by the optic chiasma and optic tracts (CN II).
Posteriorly:
 • Bounded by the pons and upper part of basilar artery.

🧠 Floor of the Interpeduncular Fossa
The floor is formed by a gray matter region called the posterior perforated substance, which is pierced by numerous small arteries.
 • These are posterior perforating arteries that supply deep brain structures such as the thalamus and internal capsule.
 • The perforated appearance gives the fossa its distinctive name.

🧩 Structures Present in the Interpeduncular Fossa
The interpeduncular fossa contains several key neural and vascular structures:

1️⃣ Cranial Nerve Roots:
 • Oculomotor Nerve (Cranial Nerve III):
  – Emerges from the anterior aspect of the midbrain, precisely from the interpeduncular fossa region.
  – These fibers pass forward between the posterior cerebral and superior cerebellar arteries. 👁️
  – Clinical correlation: Compression of CN III → pupil dilation (mydriasis) and ptosis due to paralysis of levator palpebrae superioris.

2️⃣ Arteries:
 • Basilar artery (upper end) – divides into posterior cerebral arteries, which form the upper boundary of the fossa.
 • Posterior communicating arteries – form lateral limits.
 • Circle of Willis – anterior parts (optic chiasma and infundibulum) lie nearby, making this region part of the vascular hub of the brain’s base.

3️⃣ Other Structures:
 • Mammillary bodies – rounded elevations posterior to the optic chiasma, forming part of the limbic circuit (Papez circuit) for emotion and memory. ❤️🧠
 • Posterior perforated substance – contains small perforating branches from posterior cerebral and posterior communicating arteries that supply the thalamus and internal capsule.

🧭 Relations of the Interpeduncular Fossa
Structure Relation / Function
Cerebral peduncles Form lateral walls; contain corticospinal, corticobulbar, and corticopontine fibers
Oculomotor nerve (CN III) Emerges from its floor
Posterior perforated substance Forms floor; contains small perforating arteries
Mammillary bodies Form posterior boundary
Optic chiasma and tracts Lie anterior to the fossa
Basilar artery Lies just inferior and posterior to the fossa
🩺 Clinical Correlations

1️⃣ Oculomotor Nerve Palsy:
 • Lesions in the interpeduncular fossa (e.g., due to aneurysm of the posterior communicating artery) may compress CN III.
 • Signs: Ptosis, lateral deviation of eyeball, and pupil dilation. 👁️⚡

2️⃣ Weber’s Syndrome (Midbrain Stroke):
 • Caused by infarction in the cerebral peduncle region supplied by posterior cerebral artery.
 • Results in contralateral hemiplegia and ipsilateral oculomotor nerve palsy.

3️⃣ Tumors / Aneurysms:
 • Posterior communicating artery aneurysm can bulge into the interpeduncular fossa → CN III compression → pupil-sparing or pupil-involving third nerve palsy.

4️⃣ Basilar Tip Aneurysm:
 • Aneurysm at the junction of basilar and posterior cerebral arteries can distort structures in this fossa and cause visual or motor deficits.

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