Ventricular System and CSF Circulation

The Ventricular System

• The ventricular system is a series of interconnected cavities derived from the neural tube cavity.
• It produces and circulates cerebrospinal fluid (CSF) to the subarachnoid space.
• The ventricular system consists of:
  • Lateral ventricles in the cerebral hemispheres
  • 3rd ventricle in the diencephalon
  • 4th ventricle in the hindbrain

CSF Production

• 80% of CSF is formed by the choroid plexus of the lateral ventricle.
• 20% of CSF is formed by the choroid plexuses of the 3rd and 4th ventricles and possibly by the capillaries on the surface of the brain and spinal cord.
• The choroid plexus is a mass of blood capillaries that enters the choroid fissure, invaginating the pia matter and ependyma.

CSF Circulation

• CSF circulation involves the flow of CSF from the ventricles to the subarachnoid space.
• The subarachnoid space contains several cisterns, including:
  • Cisterna magna: the largest cistern, located between the dorsal surface of the medulla oblongata and the anteroinferior aspect of the cerebellum.
  • Interpeduncular cistern (cisterna basalis): located over the interpeduncular fossa.
  • Cisterna pontis: located in front of the basilar part of the pons.
  • Cisterna ambiens: located between the splenium of the corpus callosum and the superior surface of the cerebellum.

CSF Absorption

• CSF is absorbed into the superior sagittal sinus via arachnoid villi and granulations.
• Some CSF drains directly into the dural lymphatics.
• Some CSF follows the CN I nerve fibers to the nasal lymphatics.
• The rest of the CSF follows the spinal nerves to the cervical lymphatics.

Arachnoid Villi and Dural Lymphatics

• The arachnoid villi act as one-way valves, allowing the drainage of CSF from the subarachnoid space to the venous sinuses whenever the CSF pressure exceeds the blood pressure inside the sinuses.

Hydrocephalus

• Congenital hydrocephalus is most commonly caused by aqueductal stenosis.
• Acquired hydrocephalus can be classified into:
  • Communicating (non-obstructive) hydrocephalus: caused by impaired CSF reabsorption by arachnoid villi (e.g., meningitis).
  • Non-communicating (obstructive) hydrocephalus: caused by obstruction to the flow of CSF by a tumor, cyst, blood clot, etc.
  • Relative obstruction: caused by overproduction of CSF (e.g., choroid plexus papilloma).
  • Hydrocephalus ex vacuo: caused by enlargement of cerebral ventricles and subarachnoid spaces due to brain atrophy (e.g., brain injuries).

Ventricles

Lateral Ventricle

• The lateral ventricle has three parts: the anterior horn, body, and posterior horn.
• The anterior horn projects anterolaterally downwards into the frontal lobe.
• The body of the lateral ventricle lies behind the level of the interventricular foramen.
• The posterior horn projects posteromedially into the occipital lobe.
• The inferior horn is the largest horn, located in the temporal lobe.

Third Ventricle

• The third ventricle has a lateral wall, anterior wall, posterior wall, roof, and floor.
• The lateral wall is formed by the thalamus and hypothalamus.
• The anterior wall is formed by the anterior column of the fornix, anterior commissure, and lamina terminalis.
• The posterior wall is formed by the pineal body, posterior commissure, and cerebral aqueduct.
• The roof is formed by the ependyma lining the under-surface of the choroid plexus.
• The floor is formed by the optic chiasma, tuber cinereum, infundibulum, and mammillary bodies.
• The third ventricle has several recesses, including the suprapineal recess, pineal recess, infundibular recess, and optic recess.

Fourth Ventricle

• The fourth ventricle is located in the pontine and medullary parts of the brainstem.
• The roof of the fourth ventricle appears lifted upwards and backwards like a tent, giving it a triangular shape.
• The floor of the fourth ventricle is formed by the rhomboid fossa, which has a median sulcus, striae medullares, and several trigones.
• The fourth ventricle has several recesses, including the dorsal recess, dorsolateral recesses, and lateral recesses.
• The fourth ventricle communicates with the subarachnoid space through the lateral foramina of Luschka and the median foramen of Magendie.
• The cerebral aqueduct of Sylvius connects the fourth ventricle to the third ventricle.