Meninges and Ventricles

• The concept revolves around the meninges and ventricles

Objectives

• Meninges of spinal cord, including clinical concepts related to spinal cord meninges
• Cranial meninges
• Ventricular system of the brain

Meninges of Spinal Cord

• The meninges of the spinal cord consist of three layers: pia mater, arachnoid mater, and dura mater

Pia Mater

• The inner layer, it is located on the surface of the spinal cord and roots of spinal nerves
• It forms "tethers" to support the spinal cord
• Denticulate ligaments help suspend spinal cord in dural sac

Arachnoid Mater

• It is the delicate, avascular, middle layer
• The subarachnoid space, located under the arachnoid layer, contains cerebrospinal fluid (CSF)
• The dura mater and arachnoid mater directly touch

Dura Mater

• It is the tough, fibrous, outer layer
• Dural sac is a tubular sheath containing the spinal cord and formed by dura mater
• Extradural (epidural) space is located between the dura and vertebrae and contains fat and veins

Clinical Significance - Spina Bifida

• Spina bifida is a failure of fusion of more than one vertebral arch
• There are two types: cystica and occulta
• Cystica is a severe abnormality where one or more vertebral arches fail to develop and a cyst, of variable content, forms
  • Meningocele involves herniation of meninges
  • Myelomeningocele involves herniation of both meninges and spinal cord/nerve roots
• Occulta has no cyst, occurs when L5 or S1 fails to form and is indicated by a tuft of hair

Clinical Significance - Lumbar Puncture

• Lumbar puncture, also known as spinal tap, is performed to obtain CSF from the subarachnoid space
• Procedure is typically midline between L3 & L4 and/or L4 & L5, inferior to the spinal cord
• Procedure uses the expanded subarachnoid space inferior to the spinal cord

Clinical Significance - Epidural Block

• It involves introducing an anesthetic agent into the extradural space
• It affects nerve roots after exiting the dural sac
• Affected area depends on amount injected and position of the body

Cranial Meninges

• Consist of the periosteal layer, meningeal layer, dura mater, arachnoid mater, subarachnoid space, pia mater, cerebral cortex and white matter

Dura Mater - Cranium

• It comprises two fibrous connective tissue layers: periosteal and meningeal
• The external periosteal layer forms the periosteum covering internal surface of the calvaria
• The internal meningeal layer is continuous at the foramen magnum with the dura covering the spinal cord

Dura Mater - Infoldings

• Dural infoldings involve the meningeal layer of the dura occasionally separating itself from the periosteal layer
• These infoldings divide the cranial cavity into compartments, supporting brain parts
  • Falx cerebri is a fold dipping into the longitudinal fissure, dividing cerebral hemispheres, and anchoring at crista galli
  • Falx cerebelli divides the cerebellar hemispheres
  • Tentorium cerebelli extends into the transverse fissure between the cerebrum (occipital lobes) and cerebellum, connecting to the Falx Cerebri

Dura Mater - Dural Venous Sinuses

• Spaces exist between the periosteal and meningeal layers of the dura, where layers separate
• They are lined by endothelium
• All sinuses flow toward the jugular foramen as blood from brain drains through sinuses into internal jugular veins

Arachnoid Mater

• Arachnoid trabeculae is a web-like arrangement of elastic and collagen fibers that anchor arachnoid mater to pia mater
• The subarachnoid space, located between the arachnoid and pia, contains CSF
• Arachnoid villus/villi are vascular structures projecting from the subarachnoid space into the dural venous sinuses

Pia Mater - Cranium

• It contains numerous tiny arteries that supply blood to the surface of the brain

Ventricular System of Brain

• 4 hollow, membrane-lined, interconnected spaces in the central core of the brain circulate CSF throughout the CNS
• The 4th ventricle is continuous with central canal of spinal cord
• Apertures are openings from the 4th ventricle into the subarachnoid space around the brain
• The cerebral aqueduct connects the 3rd and 4th ventricles

CSF Flow

• CSF is produced by the choroid plexus in the ventricles
• CSF flows from the third ventricle through the cerebral aqueduct into the fourth ventricle
• From the fourth ventricle, CSF flows into the subarachnoid space via paired lateral apertures or the single median aperture, and into the central canal of spinal cord
• As CSF flows through the subarachnoid space, it removes waste products and provides buoyancy to support the brain
• Excess CSF flows into the arachnoid villi, then drains into the dural venous sinuses

Choroid Plexus

• It involves vascular epithelium within ventricles that produces CSF
• It forms CSF from blood plasma