141. Cerebrospinal Fluid – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Cerebrospinal fluid (CSF) is a clear, colorless fluid present in the cerebral ventricles and subarachnoid space. It protects the brain and spinal cord, maintains a stable neuronal environment, and helps remove metabolic waste.
• The ventricular system consists of two lateral ventricles, third ventricle, and fourth ventricle. The lateral ventricles communicate with the third ventricle through the foramina of Monro, and the third ventricle communicates with the fourth ventricle through the cerebral aqueduct.
• CSF reaches the subarachnoid space from the fourth ventricle through one median and two lateral apertures. It then circulates around the brain and spinal cord and is absorbed mainly through arachnoid granulations into the dural venous sinuses, especially the superior sagittal sinus.
• The choroid plexus is the principal site of CSF formation. Formation involves ultrafiltration followed by active secretion. Sodium is actively transported into the ventricles by sodium–potassium adenosine triphosphatase; chloride follows sodium and water follows osmotically.
• Approximately 500 mL of CSF is formed daily, while the total CSF volume is about 150 mL. Thus, the entire CSF volume is renewed about three to four times daily.
• CSF has an osmolarity and sodium concentration similar to plasma, but it contains much less protein, potassium, glucose, and amino acids, while chloride concentration is higher.
• Major functions of CSF include mechanical protection, buoyancy, maintenance of the extracellular environment, transport of substances, waste removal, and regulation of intracranial pressure. Buoyancy reduces the effective weight of the brain from about 1,400 g to approximately 50 g.
• Normal CSF pressure is approximately 8–15 mmHg in the supine adult. Cerebral venous pressure is an important determinant of CSF pressure. Raised intracranial pressure may cause headache, vomiting, altered consciousness, neurological deficits, and papilledema.
• Lumbar puncture is usually performed at L3–L4 or L4–L5 because the spinal cord has already terminated. It is used for CSF examination but should be avoided when markedly raised intracranial pressure and possible brain herniation are suspected.
• CSF examination is useful in diagnosing neurological disorders. Bacterial meningitis typically shows neutrophils, high protein, and low glucose; viral meningitis shows lymphocytes with normal glucose; subarachnoid hemorrhage may produce blood-stained or xanthochromic CSF.
• Hydrocephalus may be communicating, due mainly to impaired absorption, or non-communicating, due to obstruction within the ventricular system. The cerebral aqueduct is an important site of obstruction.
• The blood–brain barrier (BBB) is formed mainly by tight junctions between cerebral capillary endothelial cells, supported by astrocyte end-feet. It restricts harmful substances while permitting selected nutrients such as glucose to enter the brain.
• Circumventricular organs such as the area postrema, subfornical organ, organum vasculosum of the lamina terminalis, median eminence, posterior pituitary, and pineal gland have an absent or highly permeable BBB.
• The blood–CSF barrier is located mainly at the choroid plexus and depends on tight junctions between choroid plexus epithelial cells.