CSF

Question 1

What is CSF?

Answer 1

Cerebrospinal fluid (CSF) is a clear, watery fluid that surrounds the brain and the spinal cord. It is an ultrafiltrate of blood plasma and is contained within the subarachnoid space and the central canal of the spinal cord.

Question 2

What are the properties of CSF?

Answer 2

• it is a clear watery fluid
• It is produced at a rate about 400mils to 600mils a day;
• Adult CSF volume is estimated to be about 150 ml with a distribution of 125 ml within the subarachnoid spaces and 25 ml within the ventricles
• It is produced continuously and is constantly renewed four to five times per 24-hour period in the average young adult.

Question 3

How is CSF formed?

Answer 3

The CS is produced by the choroid plexus which covers two lateral ventricles, and the roof of the third and fourth ventricles.
The choroid plexus is composed of fenestrated capillary loops, covered by a layer of specialized ependymal cells. The blood plasma freely permeates through the capillary loops. However, a barrier exists at the ependymal cells as they are connected by tight junctions. Hence, the ependymal cells form the blood–cerebral spinal fluid barrier, regulating the composition of CSF.
Several different mechanisms play a role in the transport of ions and micronutrients via ependymal cells, including active and passive transport. Additionally, as water follows the osmotic gradient created by the active transport of ions, blood plasma and CSF have approximately the same osmolarity.

Question 4

Outline the formation of CSF at a cellular level

Answer 4

1. Movement of Na+ down its chemical gradient into the epithelial cell through the Na+/H+ antiporter.
   -a (H+)proton is pumped out into the choroid isf
2. The proton reacts with HCO3 to form CO2
3. CO2 freely diffuses into the ependymal cell through the basolateral membrane.
4. CO2 reacts with water to form carbonic acid.
   -the proton produced acts as a substrate for the Na+/H+ antiporter and recycles to promote the movement of Na+ from the choroid if and into the ependymal cell.
5. HCO3 moves back into the choroid isf
   - The amount of HCO3 that moves into the ventricles is dependent upon the PH of csf.
6. Na+ moves out into the ventricles via the Na+/K+ ATPase pump
   - This creates an osmotic gradient that draws water from the choroid isf into the ventricles.
   - the positive charge of Na+ also pulls negative charged ions such as chloride into the csf.

Question 5

Briefly describe the circulation of CSF

Answer 5

CSF is produced continuously which keeps the fluid in circulation around the central nervous system. CSF flows from the lateral ventricle to the third ventricle through the interventricular foramen (also called the foramen of Monro).
The third ventricle and fourth ventricle are connected to each other by the cerebral aqueduct (also called the Aqueduct of Sylvius).
CSF then flows into the subarachnoid space through the foramina of Luschka (there are two of these) and the foramen of Magendie (only one of these).
Absorption of the CSF into the blood stream takes place in the superior sagittal sinus through structures called arachnoid villi.

Question 6

Explain the absorption of CSF

Answer 6

CS gets drained into the superior sagittal venous sinus through the arachnoid villi, small protrusions of arachnoid matter into the venous sinus.
Physiologically, the pressure of CS within the subarachnoid space is greater than that within the venous sinus. Hence, the CSF will drain into the venous sinuses.

Question 7

What are the functions of CSF?

Answer 7

• Buoyancy/Support - the brain weighs ~1400g, but due to the presence of CSF creating a bath, it only has a net weight of 50g.
• Protection - CSF acts as a shock absorber preventing damage caused by the brain hitting the cranium during trauma.
• Homeostasis - regulates the distribution of metabolites surrounding the brain, keeping the external environment stable. Maintains stable intrinsic CNS temperature.
• Clearing waste - waste products produced by the brain cells are excreted into the CSF, which then drains into the bloodstream.
• Chemical stability - the CS creates an environment to allow for proper functioning of the brain, e.g. maintaining low extracellular K+ for synaptic transmission
• Diagnosis of diseases such as meningitis, encephalitis etc
• Nutrition - The CSF contains glucose, proteins, lipids, and electrolytes, providing essential CNS nutrition.
• Immune function - The CSF contains immunoglobulins and mononuclear cells.

Question 8

What diseases may arise from dysfunctional CSF?

Answer 8

1. Hydrocephalus: is a condition of abnormal accumulation of CSF caused by increased CSF production, blockage of flow, or decreased absorption. The ventricles distend in order to accommodate elevated CSF volumes, potentially causing damage to the brain by pressing its tissue against the boney skull.
2. CSF Leak is a condition in which CSF is able escape from the subarachnoid space through a hole in the surrounding dura. The volume of CSF lost in a leak is very variable, ranging from insignificant to very substantial amounts.
3. Syringomyelia due to blockage of CSF circulation.
4. Meningitis is a condition in which the coverings of the brain become inflamed.
5. Subarachnoid Hemorrhage (SAH) is the leakage of blood into the subarachnoid space where it mixes with the CSF. SAH is most commonly caused by trauma with 80% of nontraumatic SAHs being caused by aneurysm rupture.
6. Papilledema: edema of the optic disc due to increased cf pressure.

Question 9

Lumbar puncture

Answer 9

L4 L5