Ventricular system

The ventricular system is a series of interconnected, fluid-filled cavities within the brain that produce, circulate, and contain cerebrospinal fluid (CSF). This system includes the two lateral ventricles, third ventricle, cerebral aqueduct, and fourth ventricle. It is lined by ependymal cells and plays a vital role in protecting the brain, regulating intracranial pressure, and maintaining homeostasis within the central nervous system.

Components of the Ventricular System

Lateral Ventricles

The lateral ventricles are the largest and most anterior parts of the ventricular system. They are paired and located in the cerebral hemispheres, one in each hemisphere. Each lateral ventricle has four parts:

• Anterior (frontal) horn: Extends into the frontal lobe.
• Body: Located in the parietal lobe.
• Posterior (occipital) horn: Extends into the occipital lobe.
• Inferior (temporal) horn: Extends into the temporal lobe.

The lateral ventricles communicate with the third ventricle via the interventricular foramina (foramina of Monro).

Third Ventricle

The third ventricle is a narrow, midline cavity located between the two halves of the diencephalon. It is bordered laterally by the thalamus and hypothalamus and is connected to the lateral ventricles by the interventricular foramina. The third ventricle continues posteriorly into the cerebral aqueduct.

Cerebral Aqueduct (Aqueduct of Sylvius)

The cerebral aqueduct is a narrow channel that connects the third ventricle to the fourth ventricle. It runs through the midbrain and is prone to obstruction due to its narrow diameter. Aqueductal stenosis can lead to obstructive hydrocephalus.

Fourth Ventricle

The fourth ventricle is located between the brainstem (pons and medulla) anteriorly and the cerebellum posteriorly. It is diamond-shaped in cross-section and has several key openings:

• Median aperture (foramen of Magendie): Opens into the cisterna magna of the subarachnoid space.
• Two lateral apertures (foramina of Luschka): Open into the pontine cistern on each side.

The fourth ventricle tapers caudally to become the central canal of the spinal cord.

Circulation of Cerebrospinal Fluid (CSF)

CSF is primarily produced by the choroid plexus, a network of specialized ependymal cells and capillaries found within the lateral ventricles, third ventricle, and fourth ventricle. The flow of CSF follows this path:

1. Lateral ventricles
2. Interventricular foramina (of Monro)
3. Third ventricle
4. Cerebral aqueduct
5. Fourth ventricle
6. Median and lateral apertures
7. Subarachnoid space surrounding brain and spinal cord
8. CSF is absorbed into venous circulation via arachnoid granulations into the superior sagittal sinus

Functions of the Ventricular System

• Protection: CSF cushions the brain and spinal cord from trauma by acting as a fluid buffer.
• Buoyancy: The brain is suspended in CSF, which reduces its effective weight and prevents it from compressing its own vasculature.
• Homeostasis: CSF helps regulate the chemical environment, remove waste products, and distribute nutrients and signaling molecules.
• Intracranial pressure modulation: Changes in CSF volume can accommodate small fluctuations in blood volume to maintain stable intracranial pressure.

Histology and Lining

The ventricular system is lined by a single layer of ciliated ependymal cells, which assist in the circulation of CSF. The choroid plexus is composed of ependymal cells surrounding capillaries and loose connective tissue and is responsible for actively secreting CSF. Tight junctions between choroid plexus epithelial cells contribute to the blood-CSF barrier.

Clinical Significance

• Hydrocephalus: Abnormal accumulation of CSF due to obstruction (non-communicating) or impaired absorption (communicating), leading to increased intracranial pressure, ventriculomegaly, and potentially brain damage. Common causes include aqueductal stenosis, tumors, hemorrhage, or infection.
• Ventriculitis: Inflammation of the ventricular system, often due to bacterial meningitis or intraventricular shunt infections.
• Colloid cyst: A benign lesion commonly found at the roof of the third ventricle near the foramen of Monro, which can obstruct CSF flow and cause acute hydrocephalus.
• Intraventricular hemorrhage: Common in premature infants due to fragile germinal matrix vessels; may also occur in adults with trauma or aneurysmal rupture.
• Ventricular shunting: A surgical intervention (e.g., ventriculoperitoneal shunt) used to drain excess CSF in cases of hydrocephalus.

Imaging

CT and MRI are the primary imaging modalities used to assess the ventricular system. Enlargement of ventricles, displacement, or distortion can indicate pathologies such as hydrocephalus, mass effect from tumors, or congenital malformations. MRI provides high-resolution visualization of the ventricles, choroid plexus, and flow dynamics using sequences like phase-contrast imaging.

Development

The ventricular system originates from the neural tube's central canal. The cavities of the prosencephalon (forebrain) form the lateral and third ventricles, the cavity of the mesencephalon becomes the cerebral aqueduct, and the cavity of the rhombencephalon forms the fourth ventricle and central canal of the spinal cord. Developmental abnormalities such as Dandy-Walker malformation or Arnold-Chiari malformation can disrupt the normal anatomy and function of the ventricular system.