Lecture 21: Meninges and Ventricular System Notes

Meninges

  • Protective covering for the brain.
  • Derived from the Greek word "meninx" meaning membrane.
  • Three layers:
    • Dura mater
    • Arachnoid mater
    • Pia mater

Layers of Meninges

  • Dura mater:
    • Outermost layer.
    • Dense and fibrous, providing toughness.
    • Composed of two layers; outer and inner.
    • Venous sinuses form between these layers in certain locations.
    • The inner layer forms dural folds.
  • Arachnoid mater:
    • Located beneath the dura mater.
    • Has a spider-like appearance.
    • Positioned above the pia mater.
    • Does not extend into the sulci (valleys) of the brain.
    • Special features:
      • Subarachnoid space
      • Arachnoid granulations
      • Arachnoid trabeculae
    • Contains blood vessels within the subarachnoid space, lying on top of the pia mater.
  • Pia mater:
    • Innermost layer of the meninges.
    • Transparent and delicate.
    • Blood vessels in the arachnoid sit on top of it.
    • Adheres to the brain surface, following the gyri and extending into the sulci.

Special Features of Dura Mater

  • Dural folds:
    • Formed by the inner layer of the dura mater.
    • Separate major divisions of the brain.
    • Provide stability to the brain within the cranium.
    • Three dural folds:
      • Falx cerebri: Separates the cerebral hemispheres in the median plane.
      • Falx cerebelli: Separates the cerebellar hemispheres in the median plane.
      • Tentorium cerebelli: Separates the cerebrum from the cerebellum in the horizontal plane.
  • Venous sinuses:
    • Located where the two layers of dura mater separate.
    • Act as collecting veins.
    • Collect:
      • Venous blood from the brain.
      • 'Old' CSF after cycling through the ventricular system.

Special Features of the Arachnoid Mater

  • Subarachnoid space:
    • Located between the arachnoid and pia mater.
    • Filled with cerebrospinal fluid (CSF).
  • Arachnoid granulations:
    • Perforate the inner layer of the dura mater.
    • Transport "old" CSF from the subarachnoid space into the venous sinuses.

Meninges of the Spinal Cord

  • Consist of dura mater, arachnoid mater, and pia mater.
  • The dorsal side of the spinal cord can be identified by the dorsal root ganglion and the spinous process of the vertebra.

Ventricular System

  • A network of interconnected spaces (ventricles) within the brain.
  • Filled with cerebrospinal fluid (CSF).
  • Lined with ependymal cells, which circulate the CSF via waving cilia.
  • CSF is produced by the choroid plexus.
  • Components:
    • Lateral ventricles (2): One in each cerebral hemisphere.
    • Third ventricle (1): Located in the diencephalon.
    • Cerebral aqueduct (1): Connects the 3rd ventricle to the 4th ventricle; located in the midbrain.
    • Fourth ventricle (1): Located at the level of the cerebellum.
    • Central canal: continuous of the fourth ventricle into the spinal cord.

Cerebrospinal Fluid (CSF)

  • Produced by the choroid plexus within the ventricles.
  • Surrounds the CNS within the subarachnoid space.
  • Functions:
    • Provides support and cushion.
    • Transports nutrients and waste.
  • Circulation path:
    • Lateral ventricles → 3rd ventricle → cerebral aqueduct → 4th ventricle → subarachnoid space → flows around brain and spinal cord.
    • Exits through arachnoid granulations into venous sinuses.

Composition and Function of Cerebrospinal Fluid

  • Ultrafiltrate of plasma.
  • Functions:
    • Provides protection and cushioning for the brain.
    • Nourishment (vitamins, necessary ions).
    • Hormones from blood (Leptin, Prolactin).
    • Removal of metabolic waste (e.g., from neurotransmitter metabolism).
    • Removal of foreign particles (drugs, etc.).
  • Usually considered sterile, but can contain microbes in infection.
  • Brain washing occurs during sleep.
  • Lumbar puncture:
    • Procedure to collect CSF where a needle is inserted usually between L3 and L4 to collect CSF.