Exam Notes: Meninges, Cerebrospinal Fluid, Skull, and Blood-Brain Barrier

Protection Part 1: Meninges

  • The meninges are protective membranes surrounding the brain and spinal cord.

Dura Mater

  • The Dura Mater is the most superior layer.
  • It's tough and inflexible due to collagen-rich fibrous tissue, making it the most durable layer.
  • Consists of two layers:
    • Periosteal (Fibrous):
    • Meningeal (Smooth):

Spaces

  • Potential Spaces: Areas where spaces can occur between structures usually pressed together.
Epidural Space
  • Located above the Dura Mater.
  • It's a potential space between the spinal cord and vertebral canal wall.
  • Contains arteries and lymphatics.
Subdural Space
  • Located between the Dura Mater and the Arachnoid Mater.
  • It is a potential space.

Arachnoid Mater

  • The Arachnoid Mater is the middle meningeal layer.
  • It sometimes extends into sinuses formed by the dura mater.
  • These extensions are called Arachnoid Villi/Arachnoid Granulations.
  • They transfer Cerebrospinal Fluid (CSF) from ventricles to the bloodstream.
Subarachnoid Space
  • Located between the arachnoid mater and pia mater.
  • Filled with Cerebrospinal fluid.
  • In the brain, it has the blood vessels and cranial nerves.

Pia Mater

  • The Pia Mater is the innermost meninges layer.
  • It's soft.
  • Runs down the sulci and fissures of the cortex.
  • Creates a membranous layer for ventricles.
  • Fuses with Ependymal cells, which line fluid-filled spaces in the brain and spinal cord.
  • Forms Choroid Plexi, a bundle of blood vessels that produce cerebrospinal fluid.

Part 2: Cerebrospinal Fluid

Cerebrospinal Fluid (CSF)

  • A clear liquid, filtrate of blood.
  • Produced in ventricles in the brain.
  • Found in the subarachnoid space and central canal in the spinal cord.
  • Acts as a cushion for the brain and spinal cord.
  • It brings nutrients to the brain and removes waste.
  • 500500 ml of CSF is produced daily.
  • CSF Circulation: Venous Sinuses

Venous Sinuses

  • Venous channels located within the dura mater (between the Periosteal and Meningeal layers).
  • Receive blood from cerebral veins and CSF from the subarachnoid space.
  • Dural Venous sinuses are surrounded by endothelium.
  • Pump blood towards the Jugular veins.
  • Have no valves.

Astrocytes

  • Most common Glial Cell in the Central Nervous System (CNS).
  • Surrounds neurons, synapses, and capillaries.
  • Functions:
    • Guide migration of neurons.
    • Controls the permeability of capillaries for ions.
    • Maintain the chemical micro-environment in the brain.
    • Maintain neuronal spacing.
    • Provide mechanical support.

Glymphatic System

  • A macroscopic system that transports wastes and nutrients in the brain using perivascular channels.
  • Delivers CSF to the parenchyma (functional tissue) of the brain.
  • Astrocytes are a part of the glymphatic system and use their aquaporin-4 water channels (AQP4) to regulate the perivascular (outside of vessels) environment.
  • CSF and Interstitial Fluid (ISF) enter the perivascular space.
  • Wastes removed by the glymphatic system include:
    • Beta-amyloid peptide (associated with Alzheimer’s disease).
    • Metabolic byproducts of neural activities.

Ependymal Cells

  • Are cells that outline the brain or spinal cord.
  • Similar to epithelial cells.
  • Most have cilia (hair-like projections).
  • Found in Choroid Plexi and Ventricles (CSF production).
  • Filter blood from ions to form CSF.
  • Separates ISF and CSF in cavities (Ventricles).

Choroid Plexi

  • The part of the ventricle where blood is filtrated through the ependymal cells to form CSF.

Part 3: The Skull (Cranium)

Cranial Cavity

  • A rigid cavity containing the brain, important blood vessels, and CSF.
  • The Foramen Magnum connects the Cranial Cavity with the Vertebral Canal.
  • The Cranial Cavity communicates with tissues outside the skull via Foramina.
  • Due to the rigid structure of the Cranial Cavity, growing tumors or extensive bleeding will have no room to expand, causing Intracranial Pressure.

Part 4: Blood-Brain Barrier

Blood-Brain Barrier (BBB)

  • A highly selective membrane between the extracellular fluid in the CNS and the blood capillaries.
  • Formed by endothelial cells of blood capillaries.
  • Made up of 3 layers:
    • Endothelial cells
    • Basal membrane
    • Astrocyte feet
  • Prohibits the filtration of materials into the CSF in Choroid Plexi.

Circumventricular Organs

  • Structures that are not protected by the BBB.
  • Interact directly with the peripheral blood.
  • Sense blood chemistry and monitor it.
  • Release Hormones
  • Examples: Area Postrema / Pituitary gland / Pineal gland

Multiple Sclerosis (MS)

  • Autoimmune neurological disease.
  • Autoreactive T-cells, B-cells, and Macrophages pass the BBB.
  • T-cells and Macrophages produce cytokines, damaging oligodendrocytes.
  • B-cells produce myelin-specific antibodies, damaging the myelin sheath.
  • Overactivation of microglia (producing cytokines) and astrocytes (enlarge and produce cytokines).