In this article we will discuss about:- 1. Composition of CSF 2. Circulation of CSF 3. Functions.
Composition of CSF:
1. Clear, colorless, transparent fluid
2. Alkaline in nature
3. Around 0.3 ml of CSF is produced/min
4. Specific gravity 1.005
6. Solids—0.87 %
7. K+ = 3.00 mEq, whereas plasma has 5.0 mEq
8. CI– = 114.0 mEq whereas plasma has 100 mEq
9. Glucose: 60 mg% whereas plasma has 60-90 mg%
10. Proteins: 20 mg% whereas plasma has 6-8 gm%
In brain, there are four ventricles. They are two lateral ventricles (one each in cerebral hemispheres) and third and fourth ventricles. Choroid plexuses present more abundantly in lateral ventricles than in 3rd or 4th ventricles. These choroid plexuses (capillary network) are main source of formation of CSF.
CSF is formed by two different processes, namely active secretion and ultrafiltration:
a. Active secretion is brought about by choroid plexus.
b. Ultrafiltration or dialysis is brought about by blood vessels that are present in ventricular regions
Circulation of CSF:
i. From lateral ventricles, CSF reaches 3rd ventricle through foramina of Monroe.
ii. From 3rd ventricle to 4th ventricle, it reaches through cerebral aqueduct or aqueduct of Sylvius.
iii. From 4th ventricle, it reachesspinal canal through foramen of Magendie and into subarachnoid spaces throughout CNS through foramina of Luschka.
CSF formed gets absorbed after its function is over. Most of CSF gets absorbed into arachnoid villi and granulations which dip into subdural venous sinuses.
Subarachnoid spaces can be reached either by lumbar or cisternal puncture. Lumbar puncture is preferred because cisternal puncture requires insertion of needle into subarachnoid spaces near brainstem region. In the process, there are chances of brainstem neurons getting damaged. It is much easier to perform lumbar puncture to get a sample of CSF for diagnostic purposes.
A special needle is introduced from posterior aspect into subarachnoid spaces of spinal cord between L3 and L4 vertebrae. Between L1 and L2 vertebrae, spinal cord ends and, therefore, when needle is introduced between L3 and L4 vertebrae injury to spinal nerves is prevented (Fig. 9.50).
i. To obtain a sample of CSF for chemical, physical and histological examination.
ii. Measure intracranial tension and if intracranial tension has increased, some amount of CSF can be removed to decrease tension. However, while relieving raised intracranial tension by lumbar puncture; there are chances of herniation of brain. This should be borne in mind before lumbar puncture is desired to relieve raised intracranial tension.
iii. To inject antibiotics that cannot pass through blood brain barrier.
iv. To induce spinal anesthesia.
Functions of CSF:
1. CSF provides buoyancy effect to brain. Normal weight of brain is about 1500 gm. Because of buoyancy effect provided by CSF, the effective weight of brain is reduced to about 50 gm. This facilitates maneuverability of head over neck region.
Weight of brain acts on lumbar region because CSF extends into spinal cord along the course of vertebral column. From vertebral column, it gets radiated to ground along support legs provide to body. So the person would feel 50 gm out of total weight of 1500 gm. Total weight does not decrease only effective weight borne by head is reduced by almost 30 times.
2. Acts as an effective interstitial fluid so that exchange of substances can take place between blood and brain tissues.
3. Interstitium of brain (microenvironment in brain) is maintained so that excitability of neuronal tissue is achieved and properly maintained.
4. Acts as shock absorber: Prevent damage to neuron in CNS because any damage to neurons in CNS is almost irreparable. When there is any impact on cranium, since brain is suspended in fluid environment of CSF, CSF acts as water cushion. The impact on head is dissipated to wider area and thereby direct impact on underlying brain tissue is minimized.
5. Tries to maintain contents of intracranial cavity as constant (relative volume of blood, neuronal tissue and CSF). Cranial vault is made of bony structures and hence cranial cavity volume is fixed. In this fixed volume, neuronal tissue, blood and CSF are present. If there is an increase or decrease in volume of either neuronal tissue or blood, corresponding changes in volume occupied by CSF is brought about.
When trephan blue (an acidic dye) is injected into blood all tissues of body get stained except for certain regions in brain tissue. There is a barrier preventing diffusion of dye from blood into brain tissues and this barrier is known as blood-brain barrier.
a. Present in CNS.
b. Provides protection to maintain constant internal environment in CNS.
c. Tight junctions at adjacent endothelial cells of brain capillaries contribute for formation of blood-brain barrier.
d. In addition to this, foot process of astrocytes reinforces this barrier.
e. Some of regions of CNS which are devoid of this barrier are (hypothalamus region):
i. Posterior pituitary
ii. Area postrema
iii. Organum vasculosum of lamina terminals
iv. Subfornical organ
Area postrema, organum vasculosum of lamina terminalis and subfornical organ are together called as circumventricular organs.
In newborn infants, blood-brain barrier is not completely developed. It is developed completely only after birth. In about 1 ½ to 2 years of life, it is developed completely.
Hence during postpartum jaundice (neonatal), bilirubin can get deposited on neurons in basal ganglia resulting in kernicterus.
Knowledge of blood-brain barrier is important for doctors because while administering certain drugs they should know whether the drug has ability to cross the barrier to reach neurons in CNS in order to bring about actions.
Hydrocephalus is a condition in which there will be abnormal accumulation of CSF in skull leading to enlargement of head.
Internal/non-communicating hydrocephalus is where one of foramina for passage of CSF into ventricles is blocked. This leads to dilation of ventricle.
When subarachnoid villi are blocked, it is known as communicating or external hydrocephalus. This can lead to atrophy of brain, mental weakness and convulsions.