Meninges and Cerebrospinal Fluid Overview

Meninges and Cerebrospinal Fluid

• The dura mater is a tough, fibrous membrane that lines the cranial cavity. Its layers are fused except in dural sinuses. Septa compartmentalize the cranial cavity to reduce or prevent displacement.
• The periosteal layer is highly vascular and innervated, only found within the cranial cavity.
• The meningeal layer is smooth, avascular, and continuous with the spinal dura mater.
• The falx cerebri separates the two cerebral hemispheres.
• The tentorium cerebelli separates the occipital lobe from the cerebellum.
• The falx cerebelli separates the cerebellar hemispheres.
• The middle meningeal artery supplies the dura mater.

Arachnoid Mater

• The arachnoid mater is a delicate, avascular membrane located between the dura and pia mater.
• It's loosely attached to the dura.
• Arachnoid villi protrude into venous sinuses, allowing cerebrospinal fluid (CSF) to flow into the venous system.
• Arachnoid granulations are clusters of arachnoid villi.
• Granular foveolae are impressions of arachnoid granulations in the calvaria.

Subarachnoid Space

• The subarachnoid space is the area between the arachnoid and pia mater, filled with CSF.
• The space has a pressure of 200mm H2O (adults) and 80mm H2O.
• Arachnoid trabeculae connect the arachnoid to the pia mater to maintain the brain's position within the subarachnoid space.
• Cerebellomedullary cisterns (cisterna magna) are large subarachnoid cisterns important for imaging.

Pia Mater

• The pia mater is a vascular membrane that tightly adheres to the surface of the brain and spinal cord.
• The denticulate ligament anchors the spinal cord to the dura mater.
• The filum terminale is the terminal portion of the spinal cord, anchoring the spinal cord inferiorly.
• The spinal cord has its own dura, arachnoid, and pia, the layers are fused in places

Cerebrospinal Fluid (CSF)

• Clear fluid formed by the choroid plexus in each ventricle.
• Circulates through ventricles and into the subarachnoid space, then absorbed into the venous circulation.
• Functions include buoyant support, cushioning, removal of metabolic waste, and maintaining stable ionic environment.
• CSF pressure is typically 100-150 mm H2O in adults. Pressure in children is 60-150mmHg of H2O.
• Formation of CSF by choroid plexus accounts for approximately 70% of total production.
• Production rate in adults is approximately 500 ml/day. Total CSF volume in adults is 90-150ml.

Disorders Associated with Meninges

• Meningitis: Infection of the meninges, usually bacterial, viral, or fungal. Symptoms include headache, stiff neck, and/or seizures. CSF often shows increased protein concentration, white blood cells, and/or cloudy appearance.
• Meningioma: Benign or malignant tumors arising from meninges, frequently located in the dura.
• Hematoma: Bleeding into potential spaces between meninges, potentially life-threatening. Epidural and subdural hematomas related to traumatic injury
• Subarachnoid hemorrhage: Bleeding into subarachnoid space (often caused by aneurism rupture).
• Hydrocephalus: Ventricular enlargement due to CSF blockage, causing increased intracranial pressure.

Spinal Cord vs Brain

• Spinal dura mater ends at S2.
• A space called the spinal epidural space houses venous plexuses and fatty tissue

Blood-Brain Barrier

• A specialized barrier between capillary walls and brain tissue. prevents passage of large molecules and many pathogens.
• This barrier protects the brain and its function but also prevents some drugs or antibodies from reaching the brain.
• Endothelial cells create the blood-brain barrier.
• Astrocytes and capillary basement membrane play a role in maintaining the barrier.

Circumventricular Organs (CVOs)

• Structures surrounding ventricles lack a blood-brain barrier.
• They allow for the detection and circulation of hormones and other substances into the brain.

Ventricles and CSF Circulation

• There are 4 ventricles in the brain that produce CSF, circulate it, and absorb it back into the bloodstream.
• Ventricles communicate with each other
• CSF flows unidirectionally through the ventricles and into the subarachnoid space.
• Arachnoid villi and granulations absorb CSF into the venous circulation.