fourth ventricle

The fourth ventricle is part of the ventricular system responsible for CSF production, circulation, and cushioning of the CNS. Its floor forms the rhomboid fossa, containing important cranial nerve nuclei (V-XII) and structures like the facial colliculus (CN VII fibers looping around CN VI nucleus), hypoglossal and vagal trigones, and area postrema (chemoreceptor trigger zone, lacks blood-brain barrier, important for vomiting reflex). Its roof is formed by the superior and inferior medullary vela and involves the cerebellum. CSF flows from the lateral ventricles through the interventricular foramina of Monro to the third ventricle, then via the cerebral aqueduct to the fourth ventricle, and exits through the foramen of Magendie (midline) and foramina of Luschka (bilateral) into the subarachnoid space (cisterna magna and cerebellopontine angle cisterns respectively). Clinical significance: Obstruction at this level (e.g., by tumors like medulloblastoma or ependymoma in children, Arnold-Chiari malformation, or Dandy-Walker malformation) causes obstructive (non-communicating) hydrocephalus, presenting with signs of increased intracranial pressure. Medulloblastomas classically arise from the roof of the fourth ventricle in children. Ependymomas often arise from the ventricular lining. Dandy-Walker malformation involves cystic dilation of the fourth ventricle with cerebellar vermis agenesis. Damage to the floor can affect multiple cranial nerve nuclei, causing complex brainstem syndromes.