Central Nervous System
Cerebrospinal fluid (CSF) can be found in the subarachnoid space. The total volume os 130-150mls. There is approximately 100mls around the spinal cord and 40mls in the cerebral ventricles.
We produce around 500mls of CSF per day.
The normal CSF pressure is 0.5-1.o kPa. Any obstruction to flow will increase pressure. This condition is known as hydrocephalus.
CSF has two major functions:
- hydraullic cushion to protect the brain from violent movements of the head
- provides a stable environment for cerebral function
CSF is produced by the choroid plexus in the lateral, third and fourth ventricles.
Once produced the CSF flows from lateral ventricle to third ventricle via interventricular foramina aka Foramen of Monro.
It then flows from the third to forth ventricles via the cerebral aqueduct (of Sylvius).
From the forth ventricle CSF flows into the subarachnoid space via the Foramen of Luschka (L = lateral) and foramen of Magendie (M = midline).
CSF circulates around the subarachnoid space and is reabsorbed back into the circulation via the arachnoid villi that drains into the venous sinuses.
Blood Brain Barrier
The brain is able to maintain its ionic environment within very strict limits to allow for optimal neuronal communication. This is achieved via the blood brain barrier.
It is a barrier formed by the structure of capillaries which have very tight cell-to-cell junctions in the endothelium. This is unlike any other freely permeable fenestrated capillaries found elsewhere in the body. Additionally, the end-feet of astrocytes cover the basement membrane.
The blood brain barrier functions include:
- restrict penetration of water-soluble substances
- alternatively, lipid-soluble molecules such as O2, CO2 anaethetics and alcohol can pass the barrier freely. This is why you get drunk on a pub crawl!
- the endothelium trasnports proteins for nutrients eg sugar, amino acids
- certain proteins such as insulin and albumin may be transported across via endocytosis and transcytosis.
- unwanted lipid-soluble molecules are transported back into blood by an ‘efflux pump’
More importantly, the blood-brain barrier is not a continous structure. In some areas of the midline, it consists of fenestrated capillaries such as in the third and forth ventricels, posterior lobe of pituitary and hypothalamus.