Question 11

College Answer

Better answers had a structure with headings such as function, formation, circulation, absorption and composition with dot point facts under each heading. The second part of the question lent itself to a diagram with labelling which scored well. Precise surface anatomy and mentioning all layers from the skin to the sub-arachnoid space scored well.

Discussion

• Function
  • Barrier function (the blood-CSF barrier)
  • Chemical stability and waste removal
  • Buoyancy and mechanical cushioning of the CNS
  • Hydraulic pressure buffering of ICP with arterial pulse and respiration
• Formation:  
  • 400-600ml/day, by highly regulated ultrafiltration and active secretion
  • Ultrafiltrate of plasma is formed by the fenestrated choroidal capillaries
  • Ions are actively transported out of this ultrafiltrate, and into the CSF
  • This creates an osmotic gradient which pulls water across the membrane through aquaporin channels
• Circulation:
  • ​​​​​​​Out of the choroid plexus on the floor of the lateral ventricles
  • Through the foramen of Munro into the third ventricle
  • Through the Aqueduct of Sylvius, into the fourth ventricle
  • From the fourth ventricle, into the cisterna magna via the lateral foramina of Luschka and then up to the basilar cisterns around the pons, and then to the rest of the cortex
  • Or, from the fourth ventricle, via the medial foramen of Magendie, down to the spinal subarachnoid space
  • It is then reabsorbed from the subarachnoid space
• Reabsorption:
  • ​​​​​​​Reabsorbed at 25ml/hr (CSF secretion rate = reabsorption rate) into the dural venous blood, mainly from arachnoid granulations
  • Driving force is hydrostatic gradient between CSF and venous blood
  • Thus, CSF reabsorption stops when CSF pressure is less than ~ 7 cm H₂O
• Composition:​​​​​​​​​​​​​​
  • CSF contains minimal protein (~0.2g/L)
  • CSF sodium potassium and calcium are slightly lower than plasma
  • CSF chloride CO2 and bicarbonate are higher than plasma
  • CSF glucose is about 2/3rds of the plasma value

Sakka, Laurent, Guillaume Coll, and Jean Chazal. "Anatomy and physiology of cerebrospinal fluid." European annals of otorhinolaryngology, head and neck diseases 128.6 (2011): 309-316.

Segal, Malcolm B. "The choroid plexuses and the barriers between the blood and the cerebrospinal fluid." Cellular and molecular neurobiology 20.2 (2000): 183-196.

Brown, P. D., et al. "Molecular mechanisms of cerebrospinal fluid production." Neuroscience 129.4 (2004): 955-968.