Neuro Objectives 5

Neuro Objectives 5

1. Meningeal coverings:

1. 
   Dura mater
   
   1. 
      Thick, tough, and collagenous
      
   2. 
      Forms two layers with outside layer as the periosteum
      
2. 
   Arachnoid
   
   1. 
      Thin collagenous layer
      
3. 
   Pia mater
   
   1. 
      Thin collagenous layer
      

Spaces:

1. 
   Epidural space (space between periosteum and dura)
   
   1. 
      Brain – Potential epidural space (periosteum and dura are bound closely and there is no real clear demarcation between the two)
      
   2. 
      Spinal Cord – Real epidural space (periosteum surrounds vertebrae, dura surrounds spinal cord; THINK! Epidural for pregnancy given in REAL epidural space!)
      
2. 
   Subdural space (space between dura and arachnoid)
   
   1. 
      Brain – Potential subdural space (dura and arachnoid layer are always bound)
      
   2. 
      Spinal Cord – Potential subdural space (dura and arachnoid layer are always bound)
      
3. 
   Subarachnoid space (space between arachnoid and pia)
   
   1. 
      Brain – Real subarachnoid space (arachnoid and pia always have space between them held apart by arachnoid trabeculae)
      
   2. 
      Spinal Cord – Real subarachnoid space (arachnoid and pia always have space between them held apart by arachnoid trabeculae)
      
      1. 
         Note: Cisterns are areas where arachnoid bridges depressions in brain. Major cisterns are cisterna magna and pontine cistern.
         
4. 
   Subpial space (space between pia and CNS)
   
   1. 
      Brain – Never there
      
   2. 
      Spinal Cord – Never there
      

General Configuration of Major Dural Septa:

Two major dural septa:

1. 
   falx cerebri (sickle shape filling intracerebral fissure)
   
2. 
   tentorium cerebelli (“tent” over cerebellum)
   
3. 
   configuration looks like this:
   

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.

Three most common sites of brain herniation:

1. 
   Across falx cerebri
   
   • 
     Clinical consequence: no serious consequences
     
   • 
     Causes: subdural hematoma, compresses contralateral gyri
     
2. 
   Through tentorium notch
   
   • 
     Clinical consequence: coma or death
     
   • 
     Causes: temporal lobe tumor pushes on midbrain (consciousness center)
     
3. 
   Through foramen magnum
   
   • 
     Clinical consequence: immediate fatality
     
   • 
     Causes: cerebellar tumor compresses medulla (respiration/cardiovascular system)
     

2. Mechanisms of hematoma:

1. 
   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.
   
2. 
   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

3. 
   differences:
   

• 
  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 3^rd ventricle), all three apertures (both lateral and single median)