Neuroanatomy: Fourth Ventricle Structure

Fourth Ventricle Structure

• Surrounded by pons and upper medulla, the fourth ventricle is an expanded cavity.
• Its roof is primarily ependyma and pia mater, shaped like a tent, covered by cerebellum.
• Upper roof over pons, covered by superior medullary velum (white matter) bounded by superior cerebellar peduncles.
• Lower roof over medulla, covered by inferior medullary velum (white matter from flocculus base) or just ependyma and pia in the lower part.
• Lower margin connected to gracile and cuneate tubercles; a median aperture (foramen of Magendie) pierces it, allowing CSF to enter the cerebellomedullary cistern.
• Lateral recesses extend as narrow tubes that open into lateral apertures (foramen of Luschka), situated anteriorly behind the 8th nerve, into the pontine cistern.
• Three apertures enable CSF exit to subarachnoid space for absorption by arachnoid villi; blockages cause hydrocephalus.

Choroid Plexus of Fourth Ventricle

• Small, bilateral, L-shaped structure indenting the medullary part of the roof.
• Originates at the lateral aperture, a branch of the posterior inferior cerebellar artery, near the flocculus.
• Extends medially, joining its counterpart, forming a T-shaped structure.
• Venous drainage back into the occipital sinus.

Fourth Ventricle Floor (Rhomboid Fossa)

• Diamond-shaped area, with boundaries like superior cerebellar peduncles, gracile and cuneate tubercles, and inferior cerebellar peduncles.
• A median sulcus runs from the midbrain aqueduct opening to the central canal.
• Floor symmetrical on either side of the sulcus; Medullary striae (white fibers) cross transversely at the widest part, connecting pontine and medullary areas. Aberrant fibers from pontine nuclei to cerebellum via inferior peduncle.
• Pontine part features medial eminence (adjacent to median sulcus), slightly more prominent at its lower end (facial colliculus) overlying facial nerve and abducent nucleus.
• Lateral angle houses the vestibular area, above vestibular nuclei.
• A superior fovea connects to aqueduct opening; contains locus caeruleus (pigmented noradrenergic cells) in its upper part.
• Medullary part smaller than pontine part, with lower vestibular area.
• Inferior fovea separates into hypoglossal trigone (medial),containing hypoglossal nucleus, and vagal trigone (lateral) overlying dorsal vagal nucleus.

Cerebrospinal Fluid (CSF)

• Largely produced by choroid plexuses of third, lateral, and fourth ventricles, with ~30% from other brain capillaries.
• Total volume ~130 mL (approx. 130 mm H2O pressure), with ~30 mL in ventricles and ~100 mL in subarachnoid space (~75 mL spinal/ ~25 mL cranial).
• Production exceeds 500 mL daily, with continuous circulation and resorption primarily via arachnoid granulations.
• Partial drainage through cribriform plate into nose and cervical lymphatics.
• CSF pressure is largely unaffected by arterial pressure but reflects venous pressure changes.
• Lumbar puncture can reduce pressure; continuous leakage maintains low pressure.
• Protective buffer for neural tissue, allowing brain to float (reducing 1500g weight to 50g), and crucial in metabolite removal.
• Ependymal cells in ventricles create selective transport mechanisms and tight junctions for CSF/blood barrier.