Meninges and Dural Venous Sinuses

Meninges and Dural Venous Sinuses

  • Meninges are layers of connective tissue that cover the brain and spinal cord (central nervous system).
  • These layers provide anchoring and protection for the central nervous system.

Layers of Meninges (Outer to Inner)

1. Dura Mater

  • The outermost, thickest, and toughest layer (\"tough mother\").
  • In the skull, it has two layers:
    • Periosteal layer: Superficial layer closest to the bone (cranium).
    • Meningeal layer: Deeper layer continuous with the spinal cord.
  • Composed of dense fibrous connective tissue.

2. Arachnoid Mater

  • Middle layer with spider web-like projections (\"spider mother\").
  • Adhered to the meningeal layer of the dura mater.
  • Contains arachnoid granulations, which are structures that reabsorb cerebrospinal fluid (CSF) into venous blood.
  • CSF provides nutrients and electrolytes to the brain and removes waste.

3. Pia Mater

  • The innermost, tender layer (\"tender mother\").
  • Directly adhered to the surface of the brain, covering all gyri and sulci.
  • A thin layer of loose connective tissue that cannot be dissected off without damaging the brain.

Layers Approaching the Brain (Outside to Inside)

  1. Skull (bone)
  2. Dura Mater
    • Periosteal layer
    • Meningeal layer
  3. Arachnoid Mater (with web-like projections creating space for CSF, veins, and arteries)
  4. Pia Mater

Spaces Around Meninges

  • Extradural (Epidural) Space
    • Potential space between the skull and dura mater.
  • Subdural Space
    • Potential space between the dura mater and arachnoid mater.
  • Subarachnoid Space
    • Actual space between the arachnoid mater and pia mater.
    • Contains cerebrospinal fluid (CSF), arteries, and veins.

Dural Septa (Dural Partitions)

  • Double layers of the meningeal layer of the dura mater that partition the brain.
  • Insert into fissures to limit movement and support the brain.

Types of Dural Septa:

  • Falx Cerebri
    • Located in the longitudinal fissure, separating the two cerebral hemispheres.
  • Falx Cerebelli
    • Separates the two cerebellar hemispheres.
  • Tentorium Cerebelli
    • Located between the cerebrum and cerebellum and supports the weight of the cerebrum.
    • Forms a tent over the cerebellum.

Dural Venous Sinuses

  • Spaces created by the separation of the meningeal and periosteal layers of the dura mater.
  • Receive venous blood from small veins in the brain and drain CSF.
  • These are sinuses, not veins, because they are open spaces without the specific histological layers of veins.

Key Sinuses and Flow of Venous Blood

  1. Superior Sagittal Sinus
    • Runs along the longitudinal fissure within the falx cerebri.
  2. Inferior Sagittal Sinus
    • Runs along the inferior side of the falx cerebri.
  3. Straight Sinus
    • Connects the inferior sagittal sinus to the superior sagittal sinus.
  4. Occipital Sinus
    • Runs along the falx cerebelli.
  5. Confluence of Sinuses
    • The point where the superior sagittal, straight, and occipital sinuses converge.
  6. Transverse Sinuses
    • Two sinuses that extend laterally from the confluence of sinuses.
  7. Sigmoid Sinuses
    • S-shaped sinuses that continue from the transverse sinuses.
    • Lead to the jugular foramen.
  8. Internal Jugular Vein
    • The sigmoid sinus becomes the internal jugular vein after passing through the jugular foramen.

Spaces and Fluid Around the Brain

  • Dural Venous Sinus: Contains deoxygenated blood.
  • Extradural Space: Potential space; fluid is NOT desirable.
  • Subarachnoid Space: Contains cerebrospinal fluid (CSF); fluid is OK.
  • Subdural Space: Potential space; fluid is NOT desirable.

Clinical Significance: Hemorrhages

1. Epidural (Extradural) Hemorrhage

  • Occurs outside the dura mater, between the skull and periosteal layer.
  • Typically arterial in origin (e.g., middle meningeal artery damage).
  • Often caused by head trauma or skull fracture.
  • Blood collects and pushes the periosteal layer away from the skull, forming a lens-like shape on CT scans.

2. Subdural Hemorrhage

  • Occurs between the dura mater (meningeal layer) and arachnoid mater.
  • Typically venous in origin.
  • Caused by tearing of veins as they pass through the meningeal layer to enter the dural venous sinuses.
  • Blood pools, forming a crescent shape on imaging.

3. Subarachnoid Hemorrhage

  • Occurs within the subarachnoid space.
  • Typically arterial in origin.
  • Often caused by the rupture of a brain aneurysm (dilated artery).
  • Blood spreads throughout the subarachnoid space, making it harder to identify a specific bleeding point on imaging.

Ventricular System and Cerebrospinal Fluid (CSF)

  • Ventricles: Spaces within the brain that contain cerebrospinal fluid (CSF).

Ventricles of the Brain

  • Lateral Ventricles: Two ventricles, one in each cerebral hemisphere.
  • Third Ventricle: Located along the midline; receives CSF from the lateral ventricles.
  • Cerebral Aqueduct: A long, narrow tube that connects the third and fourth ventricles.
  • Fourth Ventricle: Located between the brainstem and cerebellum; continuous with the central canal of the spinal cord.

Flow of Cerebrospinal Fluid (CSF)

  1. Lateral Ventricles (2)
  2. Third Ventricle
  3. Cerebral Aqueduct
  4. Fourth Ventricle
  5. Central Canal of Spinal Cord (minor amount of CSF)

Choroid Plexus

  • A structure located in all ventricles that produces cerebrospinal fluid (CSF).
  • Specialized tissue that filters fluid from the blood, creating a specialized fluid containing electrolytes, oxygen, and other essential substances for the brain.

Functions of CSF

  • Surrounds the brain and spinal cord, providing cushioning and protection.
  • Acts as a shock absorber.
  • Transports nutrients and removes waste products.
  • Helps maintain ion balance for optimal neuronal function.

CSF Flow Pathway

  1. Choroid Plexus Produces CSF in the Ventricles
  2. CSF Flows from Lateral Ventricles to the Third Ventricle
  3. CSF Flows from the Third Ventricle through the Cerebral Aqueduct to the Fourth Ventricle
  4. CSF Exits the Fourth Ventricle via Median and Lateral Apertures into the Subarachnoid Space
  5. CSF Washes Around the Brain in the Subarachnoid Space
  6. CSF is Reabsorbed into the Dural Venous Sinuses via Arachnoid Granulations

Hydrocephalus

  • \"Water on the brain\"
  • Caused by an obstruction to the circulation of cerebrospinal fluid or overproduction of CSF.
  • Leads to accumulation of CSF and compression of the brain.