Study sheet: Fourth Ventricle and CSF Circulation

📋 Course Outline

1. Fourth Ventricle Location and Structure
2. Roof and Cerebrospinal Fluid Openings
3. Rhomboid Fossa and Floor Landmarks
4. Choroid Plexus of the Ventricle
5. Cerebrospinal Fluid Circulation
6. Cerebrospinal Fluid Absorption

📖 1. Fourth Ventricle Location and Structure

🔑 Key Concepts & Definitions

• Fourth ventricle : A tent-shaped cavity filled with cerebrospinal fluid, located anterior to the cerebellum and posterior to the pons and the superior half of the medulla oblongata.

★ Must-know

• The fourth ventricle is lined with ependyma and is continuous superiorly with the cerebral aqueduct and inferiorly with the central canal of the medulla oblongata and spinal cord.

Further detail

• The fourth ventricle has lateral boundaries, a roof, and a rhomboid-shaped floor; the caudal part of each lateral boundary is formed by the inferior cerebellar peduncle and the cranial part by the superior cerebellar peduncle.

💡 Memory Hook

The fourth ventricle lies anterior to the cerebellum but posterior to the pons and medulla.

📖 2. Roof and Cerebrospinal Fluid Openings

🔑 Key Concepts & Definitions

• Inferior medullary velum : A thin sheet devoid of nervous tissue, formed by ventricular ependyma and a posterior covering of pia mater, that forms the inferior part of the fourth ventricle roof.

📝 Essential Points

• The superior part of the fourth ventricle roof is formed by the medial borders of the superior cerebellar peduncles and the superior medullary velum.

• The fourth ventricle communicates with the subarachnoid space through one median aperture, the foramen of Magendie, and two lateral apertures, the foramina of Luschka.

📌 The median and lateral apertures of the fourth ventricle permit cerebrospinal fluid to flow from the ventricular system into the subarachnoid space.

💡 Memory Hook

One median aperture plus two lateral apertures connects the ventricle to the subarachnoid space.

📖 3. Rhomboid Fossa and Floor Landmarks

🔑 Key Concepts & Definitions

• Rhomboid fossa : The diamond-shaped floor of the fourth ventricle, formed by the posterior surface of the pons and the cranial half of the medulla oblongata.

★ Must-know

• The median sulcus divides the floor of the fourth ventricle into symmetrical halves, and each medial eminence is bounded laterally by the sulcus limitans.

• The vestibular area lies lateral to the sulcus limitans, and the vestibular nuclei lie beneath it.

• Inferior to the stria medullaris, the hypoglossal triangle overlies the hypoglossal nucleus, the vagal triangle overlies the dorsal motor nucleus of the vagus, and the area postrema lies between the vagal triangle and the lateral ventricular margin.

Further detail

• The facial colliculus is an elevation at the inferior end of the medial eminence produced by facial nerve motor fibers looping over the abducens nucleus.

💡 Memory Hook

Medial triangles mark hypoglossal and vagal nuclei, whereas the vestibular area lies laterally.

📖 4. Choroid Plexus of the Ventricle

🔑 Key Concepts & Definitions

• Tela choroidea : A two-layered fold of pia mater that projects through the roof of the fourth ventricle and is covered by ependyma.

★ Must-know

• The choroid plexus of the fourth ventricle has a T shape, with a double vertical part, and is suspended from the inferior half of the roof.

📌 The choroid plexus produces cerebrospinal fluid.

Further detail

• The blood supply of the fourth ventricular choroid plexus comes from the posterior inferior cerebellar arteries.

💡 Memory Hook

A T-shaped vascular plexus hangs from the inferior roof.

📖 5. Cerebrospinal Fluid Circulation

★ Must-know

• 🔄 Cerebrospinal fluid follows this ventricular route:

  1. It passes from the lateral ventricles through the interventricular foramina to the third ventricle.
  2. It passes through the cerebral aqueduct to the fourth ventricle.
  3. It passes through the median and lateral apertures into the subarachnoid space.

• After entering the subarachnoid space, cerebrospinal fluid moves through the cerebellomedullary and pontine cisterns, flows over the cerebral hemispheres, and some descends around the spinal cord and cauda equina.

Further detail

• Cerebrospinal fluid circulation is aided by arterial pulsations of the choroid plexuses and by cilia on the ependymal cells lining the ventricles.

• Further circulation around the spinal cord and cauda equina relies on:

  • pulsations of the spinal arteries
  • movements of the vertebral column
  • respiration
  • coughing
  • changes in body position

💡 Memory Hook

Lateral ventricles → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space.

📖 6. Cerebrospinal Fluid Absorption

🔑 Key Concepts & Definitions

• Arachnoid villi : Diverticula of the subarachnoid space that pierce the dura mater and project into the dural venous sinuses, especially the superior sagittal sinus.

★ Must-know

• Cerebrospinal fluid is absorbed into venous sinuses when cerebrospinal fluid pressure exceeds venous pressure in the sinus.

• Arachnoid villi act as valves because increased venous pressure compresses the villous tips, closes endothelial tubules, and prevents blood from refluxing into the subarachnoid space.

• Because cerebrospinal fluid production by the choroid plexuses is constant, the rate of absorption through the arachnoid villi controls cerebrospinal fluid pressure.

Further detail

• Arachnoid villi tend to cluster into arachnoid granulations, which increase in number and size with age and may become calcified in advanced age.

💡 Memory Hook

When cerebrospinal fluid pressure exceeds venous pressure, arachnoid villi permit absorption into the venous sinuses.

📊 Synthesis Tables

Fourth Ventricle Floor Landmarks

Landmark	Location	Underlying structure
Medial eminence	Medial to the sulcus limitans	Not specified
Vestibular area	Lateral to the sulcus limitans	Vestibular nuclei
Hypoglossal triangle	Medial to the vagal triangle	Hypoglossal nucleus
Vagal triangle	Lateral to the hypoglossal triangle	Dorsal motor nucleus of the vagus
Area postrema	Between the vagal triangle and lateral ventricular margin	Not specified