FOURTH VENTRICLE — Long Answer Question (MBBS First Year Anatomy)

Introduction

Location

• Anteriorly (floor): pons above and medulla oblongata below
• Posteriorly (roof): cerebellum

Boundaries / Extent

FLOOR (Rhomboid Fossa)

• Upper triangular part → posterior surface of the pons
• Lower triangular part → posterior surface of the upper half of the medulla oblongata

Features of the Floor

• Facial colliculus (pontine part) — overlies the nucleus of abducens nerve (CN VI) looped around by the facial nerve (CN VII) fibres
• Hypoglossal triangle (trigonum hypoglossi) — overlies the nucleus of CN XII
• Vagal triangle (trigonum vagi) — overlies the dorsal nucleus of vagus (CN X)

ROOF

Superior Part (Anterior Medullary Velum)

• Thin sheet of white matter
• Stretches between the two superior cerebellar peduncles
• The decussation of the trochlear nerve (CN IV) occurs within it
• Thin → pathological stretching here causes dorsal midbrain compression signs

Inferior Part (Posterior Medullary Velum)

• Formed by two thin sheets of white matter
• Completed by the tela choroidea (a double layer of pia) and the choroid plexus of the fourth ventricle
• The tela choroidea hangs down like a tent, and the choroid plexus projects into the ventricle as a T-shaped structure

LATERAL BOUNDARIES

1. Inferior cerebellar peduncle (restiform body) — upper part
2. Gracile tubercle and cuneate tubercle — lower part

COMMUNICATIONS / APERTURES (Foramina)

• Above: communicates with the third ventricle via the cerebral aqueduct (of Sylvius)
• Below: continuous with the central canal of the spinal cord via the obex

CHOROID PLEXUS

BLOOD SUPPLY

CSF CIRCULATION (Related to 4th Ventricle)

APPLIED ANATOMY (Clinical Correlations)

1. Hydrocephalus — Obstruction of the cerebral aqueduct or blockage of the foramina of Magendie/Luschka prevents CSF outflow from the fourth ventricle → non-communicating (obstructive) hydrocephalus → raised intracranial pressure.
2. Dandy-Walker Malformation — Developmental failure of the foramen of Magendie (and sometimes Luschka) to open → cystic dilatation of the fourth ventricle + cerebellar vermis hypoplasia + enlarged posterior fossa.
3. Medulloblastoma — Most common posterior fossa tumour in children; arises from undifferentiated neuroectodermal cells of the roof (superior/inferior medullary velum, vermis) of the fourth ventricle → compresses and fills the ventricle → obstructive hydrocephalus.
4. Ependymoma of the 4th Ventricle — Arises from the ependymal lining of the ventricular floor; tends to squeeze out through the foramen of Magendie ("plastic" ependymoma).
5. Area postrema lesions — The area postrema is outside the blood-brain barrier (a circumventricular organ). Damage or stimulation causes vomiting. It is a target for antiemetic drugs (e.g., ondansetron) and is affected in conditions like neuromyelitis optica spectrum disorder (NMOSD), causing intractable hiccups and vomiting.
6. PICA occlusion (Lateral medullary/Wallenberg syndrome) — The PICA supplies the lateral medulla and the inferior fourth ventricle. Occlusion causes ipsilateral cerebellar signs, crossed sensory loss, Horner syndrome, dysphagia, and nystagmus.
7. Tumours of the 4th ventricle — Can present with cerebellar dysfunction (ataxia, nystagmus, dysmetria) due to compression of the cerebellar peduncles, along with features of raised ICP (headache, vomiting, papilloedema).
8. Nystagmus — The vestibular nuclei lying in the floor → tumours/inflammation in the floor cause nystagmus, vertigo, and disequilibrium.

DIAGRAM

SUMMARY TABLE

FOURTH VENTRICLE — Parts and Boundaries

(MBBS First Year Anatomy — LAQ)

1. INTRODUCTION (2–3 lines — always write this)

2. PARTS OF THE FOURTH VENTRICLE

3. FLOOR (Rhomboid Fossa) — Most Important Part ⭐

• Upper triangular part → posterior surface of pons
• Lower triangular part → posterior surface of upper medulla oblongata

Features of the Floor (Learn these — high yield):

• Facial colliculus (pontine part) → nucleus of CN VI + fibres of CN VII loop around it
• Hypoglossal triangle → nucleus of CN XII below
• Vagal triangle → dorsal nucleus of CN X

• Superior fovea — depression at pontine level
• Inferior fovea — depression at medullary level

4. ROOF (Tent-shaped) ⭐

Superior part — Anterior Medullary Velum

• Thin sheet of white matter
• Bridges the two superior cerebellar peduncles
• Trochlear nerve (CN IV) decussates within it

Inferior part — Posterior Medullary Velum + Tela Choroidea

• Two thin crescentic sheets of white matter
• Completed by tela choroidea (double fold of pia mater)
• Choroid plexus hangs from here → produces CSF
• Has a central opening = Foramen of Magendie (median aperture)

5. LATERAL BOUNDARIES

1. Superior cerebellar peduncle (upper part)
2. Inferior cerebellar peduncle (restiform body) (middle part)
3. Gracile tubercle + Cuneate tubercle (lower part)

6. LATERAL RECESSES

• The ventricle extends laterally as two lateral recesses
• They curve around the brainstem toward the cerebellar flocculus
• Each lateral recess opens at its tip as the Foramen of Luschka (lateral aperture)

7. COMMUNICATIONS / APERTURES (Floor of CSF Flow) ⭐

8. CSF FLOW THROUGH THE 4TH VENTRICLE

(For the question "describe the floor/flow of the 4th ventricle")

9. APPLIED ANATOMY (Write 2–3 — gives extra marks)

10. DIAGRAM (Draw in exam — easy, earns marks)

         Cerebral aqueduct (above)
                  ↓
    ┌─────────────────────────────┐
    │  Superior cerebellar peduncle│ ← Lateral boundary (upper)
    │                             │
    │   FLOOR (Rhomboid Fossa)    │
    │  ┌─────────────────────┐    │
    │  │ Facial colliculus   │    │
    │  │ Striae medullares   │    │
    │  │ Vagal/XII triangle  │    │
    │  │ Area postrema/Obex  │    │
    │  └─────────────────────┘    │
    │  Inferior cerebellar ped.   │ ← Lateral boundary (lower)
    └─────────────────────────────┘
                  ↓
          Central canal (below)

ROOF: Ant. medullary velum (sup.) + Post. medullary velum + Tela choroidea (inf.)
OPENINGS: Foramen of Magendie (midline) + Foramina of Luschka (×2 lateral)

HOW TO SCORE FULL MARKS — Quick Checklist

• Introduction (shape, location, lining) — 2 marks
• Floor + named features (facial colliculus, striae, obex, area postrema) — 4 marks
• Roof (ant. + post. medullary velum, tela choroidea) — 3 marks
• Lateral boundaries (3 structures each side) — 2 marks
• Apertures/communications (Magendie, Luschka, aqueduct, central canal) — 3 marks
• CSF flow — 2 marks
• Applied anatomy (2–3 points) — 2 marks
• Diagram — 2 marks

Tip: Examiners love when you mention the facial colliculus and explain why it matters (CN VI nucleus + CN VII fibres loop around it) — it shows conceptual clarity, not just rote learning. Similarly, explaining area postrema = outside BBB = vomiting centre gets you extra marks every time.

FOURTH VENTRICLE

(As per your Textbook — Anatomy LAQ)

INTRODUCTION

BOUNDARIES

1. ROOF (Posterior Wall) ⭐

• Formed by the superior medullary velum (anterior medullary velum)
• It is a thin sheet of white matter stretching between the two superior cerebellar peduncles

• Formed by the inferior medullary velum — a thin sheet of nervous tissue
• Completed by the tela choroidea — a double layer of pia mater
• The choroid plexus of the 4th ventricle is carried by the tela choroidea
• The cavity of the 4th ventricle communicates with the subarachnoid space through the foramen of Magendie (median aperture) in the midline, and through the foramina of Luschka (lateral apertures) on each side
• The choroid plexus receives blood supply from the posterior inferior cerebellar artery (PICA)

2. LATERAL BOUNDARIES ⭐

• Superior cerebellar peduncle — upper part
• Inferior cerebellar peduncle — lower part (restiform body)

3. FLOOR (Rhomboid Fossa) ⭐⭐ Most Important

• Upper part → posterior surface of pons
• Lower part → posterior surface of upper medulla oblongata

FLOOR OF THE 4TH VENTRICLE — FEATURES

Sulcus Limitans

• Runs along each side of the floor
• Divides the floor into medial (motor) and lateral (sensory) parts

Medial Eminence

• Lies medial to the sulcus limitans
• In the pontine part: contains the facial colliculus — a rounded elevation produced by fibres of CN VII looping over the nucleus of CN VI (abducens nucleus)
• In the medullary part: contains the hypoglossal triangle (overlies CN XII nucleus) and vagal triangle (overlies dorsal nucleus of CN X)

Striae Medullares

• Transverse fibres that cross the floor from the median sulcus to the lateral recesses
• Divide the floor into pontine (upper) and medullary (lower) parts

Area Vestibularis

• Lateral to the sulcus limitans in the pontine part
• Overlies the vestibular nuclei

Area Postrema

• Triangular area at the lower end of the floor, near the obex
• Chemoreceptor trigger zone — outside the blood-brain barrier

Obex

• Small plate of nervous tissue at the inferior angle of the rhomboid fossa
• Marks the junction of the 4th ventricle with the central canal of the spinal cord

Calamus Scriptorius

• The inferior triangular part of the floor
• Resembles the tip of a writing pen ("calamus" = pen)
• Bounded below by the obex

FIGURE FROM YOUR TEXTBOOK

• Median sulcus
• Facial colliculus — Superior fovea
• Striae medullares
• Sulcus limitans
• Hypoglossal triangle — Inferior fovea
• Vagal triangle
• Area postrema
• Obex
• Taenia
• Funiculus separans

CHOROID PLEXUS OF THE 4TH VENTRICLE

• The choroid plexus is a tuft of capillaries that project into the cavity of the 4th ventricle
• It enters the 4th ventricle through the roof (tela choroidea)
• Through these three apertures (Magendie + 2× Luschka), the plexus opens the subarachnoid space of the lateral aperture (foramen of Luschka) into the subarachnoid space and the foramen of Magendie around the arachnoid space
• The entire plexus of the 4th ventricle is T-shaped, where the horizontal limb of T is double — one on each side entering the lateral recess and protruding through the foramen of Luschka into the subarachnoid space

APPLIED ANATOMY / CLINICAL CORRELATIONS

• Tumours in this region are not gliomas
• The glioma which occurs rarely in this region is ependymoma
• More common tumours are poorly differentiated primitive neuroectodermal tumours (medulloblastoma)
• The glioma in this region is a hazard, strongly associated with hydrocephalus because tumours can obstruct CSF flow
• The glioma which forms in this region may cause obstructions, leading to disorders of respiration and vasomotor disturbances
• Upon the ventricular floor (rhomboid fossa), tumours become tightly bound there secondarily
• This may lead to formation of internal hydrocephalus

COMMUNICATIONS OF THE 4TH VENTRICLE

HOW TO WRITE IN EXAM — SEQUENCE TO FOLLOW

Follow this exact order — same as your textbook:

1. Introduction (diamond-shaped, posterior cranial fossa)
2. Boundaries — heading first:
   • Roof (superior part + inferior part)
   • Lateral boundaries
   • Floor (rhomboid fossa)
3. Features of the floor (sulcus limitans → medial eminence → striae medullares → area vestibularis → area postrema → obex → calamus scriptorius)
4. Choroid plexus (T-shaped, PICA supply, apertures)
5. Communications (table or list)
6. Applied anatomy (2–3 points from clinical correlation box)
7. Diagram of rhomboid fossa