Ways in which the meninges function to maintain shape and position of CNS:
Dural layer is thick layer that binds to arachnoid and anchors meninges to skull. Arachnoid binds to dural layer and builds arachnoid trabeculae to cross subarachnoid space and anchor the pia. The pia is a thin layer that holds all the CNS in one space. This way, with CSF filling the spaces, and the multiple layers in between extracellular and CNS space, the brain has ample protection and is preserved in its shape.
Epidural hematoma: Formed when meningeal artery ruptures. Meningeal arteries reside in the skull, when they rupture, they pour blood between the cranium and dura causing an epidural hematoma.
Subdural hematoma: Formed when cerebral vein ruptures. Cerebral veins dump into venous sinuses in between dural layers in the brain. Dural sinuses don’t move, so when vein ruptures, vein will pour blood into the area “below” the dura, and cause a subdural hematoma between the dura and arachnoid layers.
3. Spinal meninges vs. crainial meninges:
a. similarities: all the same layers with differences in the connections between them. In both there are periosteum, dura, arachnoid, and pial layers
cranial: between dura and periosteum there is no clear demarcation and they act as one layer. There are arachnoid trabeculae arising from the arachnoid layer bridging the gap between arachnoid and pial layers.
spinal: dura surrounds spinal cord and pia surrounds vertebrae; this causes a real epidural space in the spinal cord. There are dentate ligaments arising from the pial layer bridging the gap between arachnoid and pial layers
4. Cellular composition of choroid plexus: Choroidal capillary, pial, and ependymal layers.
Where pial and ependymal layers meet without CNS in between. In these areas, choroidal capillaries invaginate areas (forming a “choroid fissure”) and leak contents into pial area. Ependymal cells form tight junction and thus, form a diffusion barrier (the “choroid epithelium”).
Location in ventricular system: Found in roof of inferior horn continues through body through intraventricular foramen into roof of third ventricle. Also found in floor of fourth ventricle.
Mechanism of CSF production: Produces CSF when ependymal cells actively pumping CSF components across ependymal membrane.
Composition: isotonic saline like plasma but low in protein
5. CSF pathway: (Production) Choroid plexus Ventricles Lateral or Median Aperture Cisterna Magna and Pontine Cistern Subarachnoid space Arachnoid villi (Absorption)
Mechanism of CSF return to venous system: Arachnoid villi. Arachnoid villi are formed when arachnoid layer breaks through dural layer around sinus to directly communicate with sinus. CSF flows following bulk-flow mechanics into sinus; sinuses are part of venous system.
Communicating hydrocephalus: Blockage of CSF pathway whereby ventricles can “communicate.”
Examples: Arachnoid villi (subarachnoid hemmorage), tentorial notch (blocks pathway of CSF to superior sagittal sinus)
Noncommunicating hydrocephalus: Blockage of CSF pathway whereby ventricles cannot “communicate.”
Examples: Intraventricular foramen (thalamic tumor; lateral ventricle), aqueduct (pineal tumor; lateral and 3rd ventricle), all three apertures (both lateral and single median)