Ventricular system

The paired lateral ventricles lie in the cerebral hemispheres; the third ventricle is a midline slit between the two thalami and hypothalamus; the fourth ventricle lies between the pons and medulla in front and the cerebellum behind.

Each lateral ventricle has a frontal horn, body, atrium (trigone), occipital horn and temporal horn. The interventricular foramen of Monro joins each lateral ventricle to the third; the cerebral aqueduct (of Sylvius) in the midbrain joins the third to the fourth. The fourth ventricle opens by the median aperture (foramen of Magendie) and two lateral apertures (foramina of Luschka) into the subarachnoid space, and continues into the central canal.

The choroid plexuses, vascular tufts in the roof of the third and fourth ventricles and in the lateral ventricles (body, atrium and temporal horn), are supplied by the anterior and posterior choroidal arteries (from the internal carotid and posterior cerebral arteries) and by the posterior inferior cerebellar artery for the fourth ventricle.

The ependyma and choroid plexus receive autonomic innervation (sympathetic from the superior cervical ganglion and some parasympathetic fibres) that modulates blood flow and secretion; the details of regulation are limited. Chemoreceptors in the floor of the fourth ventricle (area postrema) lie in the vomiting centre region.

The choroid plexus secretes CSF at about 500 mL per day, so the total volume of about 150 mL (approximate, with roughly a quarter inside the ventricles) is renewed several times daily. CSF flows from the lateral ventricles through the foramina to the third ventricle, aqueduct and fourth ventricle, then into the subarachnoid space and is reabsorbed by arachnoid granulations into the superior sagittal sinus. It cushions the brain, provides buoyancy and maintains a stable chemical environment.

Hydrocephalus is excess CSF with ventricular enlargement. Obstructive (non-communicating) forms follow blockage within the system, as in aqueductal stenosis, colloid cyst at the foramen of Monro or posterior fossa tumours; communicating forms result from impaired absorption, as after subarachnoid haemorrhage or meningitis. Normal-pressure hydrocephalus presents with gait disturbance, urinary incontinence and cognitive decline. Raised intracranial pressure is managed by cause and local protocols; shunting or endoscopic third ventriculostomy may be used.

• CSF path: lateral ventricles, foramen of Monro, third ventricle, aqueduct, fourth ventricle, apertures of Magendie and Luschka, subarachnoid space, arachnoid granulations.
• Choroid plexus secretes CSF (about 500 mL per day); total volume about 150 mL (approximate).
• The cerebral aqueduct is the narrowest point; stenosis causes obstructive hydrocephalus with dilated lateral and third ventricles.
• Communicating hydrocephalus: impaired absorption (post-haemorrhage, post-meningitis).
• Normal-pressure hydrocephalus triad: gait disturbance, incontinence, dementia.

1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
3. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier, 14th edition (2020) — Tier 3 (textbook)

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