Hey Guys! as now we have enlightened ourselves with basic neuroanatomy and special features of spinal cord lets us move to the special fluid present within and around the CNS and the special barriers present there. Hope you would have guessed about the topics on which we would be discussing. So, lets go ahead. As we have known and come across that our nervous system is bathing not only with the vascular flow but also with a special type of fluid that surrounds it called as “cerebrospinal fluid” as it is present both in the brain and the spinal cord.
Cerebrospinal fluid (CSF)
Some of the key features of CSF are:-
- It is clear, colorless and odourless fluid.
- This CSF fluid is present within the cavities of the brain and in the subarachnoid space around the CNS.
- It is actively secreted by choroid plexuses situated in the lateral, third and the fourth ventricles.
There are four interconnected cavities present in the brain called as ventricles.
- Two lateral ventricles present on the right and left cerebral hemisphere are the cavities of telencephalon.
- The lateral ventricles are connected to the other cavity known third ventricle present in between the two thalami through the foramen of Monro. Third ventricle is the cavity of diencephalon.
- The third cavity is connected to another cavity called as fourth ventricle through an ependymal lined tube called as the aqueduct of Sylvius, cavity of the midbrain or mesencephalon.
- At the lower end the aqueduct of Sylvius, the CSF filled cavity expands and form the fourth ventricle which continues as a small cavity called as the central canal in the spinal cord. Fourth ventricle is the cavity of rhombencephalon.
From the fourth ventricle the CSF drains via three orifices (two foramen Luschka and a single foramen of Magendie ) into the subarachnoid space, where it equilibrates with the extracellular fluid present in the perivascular spaces. Latter the CSF drains into the venous sinuses via the arachnoid villi.
Secretion of CSF
CSF is secreted by choroid plexuses present in the roof of the lateral, third and fourth ventricles. Choroid plexus consists of cuboidal epithelium covering a core of highly vascular pia matter. The whole thing can be imagined as steps, lets us start with the entry of a choroidal artery getting into the brain matter covered with pia matter, after that the artery form tuft or folds of capillaries lined with pia matter and exits out as choroidal vein. This structure which is formed of vascular folds and pia matter covering is called as tela choroidea. In the ventricles the tela choroidea is covered by ependymal layer and becomes a special secretory structure called as Choroid plexus.
In adult humans about 500cm3/day of CSF is secreted and a steady volume of 100-150 cm3 is maintained throughout out of which around 30cm3 is present in the ventricles and the rest is present in the subarachnoid space. There is a turnover of CSF every 5-7 hours.
- Apical border of the epithelial cells of choroid plexuses have active Na+-K+ATPase pump.
- Na+ is actively pumped into the CSF generating a sodium gradient that drive two secondary active transport mechanisms (Na+-H+ exchange and Na+– Cl– symport) bringing Na across the basolateral membrane.
- Cl– influx in turn drives a Cl-HCO3 antiport. HCO3 is also produced intracellularly by hydration of CO2 in the presence of carbonic anhydrase richly present in the choroid plexus. This bicarbonate diffuses into the CSF.
- Aquaporins in the apical membrane help in the transport of water in order to balance the osmotic gradient in the CSF.
- There is also small transportation of glucose but is less than the blood. Thus, the result is that in comparison to the blood plasma CSF has higher Na+,Cl– and HCO3– concentration but lower K+, urea, glucose and amino acids concentration.
Although in the figure above we can see the path of circulation of CSF let me summarize once again.
Choroid plexus present in the lateral ventricle secrete CSF which goes into the third ventricle via foramen of Monro → CSF from third ventricles passes through the aqueduct of Sylvius to the fourth ventricle → choroid plexus of fourth ventricle add further CSF → fills central canal of the spinal cord till the terminal ventricle. This explains the circulation but the exit of the CSF from the ventricular system is via the foramen of Magendie and Luschka located in the fourth ventricle to the subarachnoid space. The dilated space of subarachnoid space near the foramen of Luschka located laterally in the fourth ventricle, having excess of CSF is known as cerebellopontine cistern. The dilated space of subarachnoid space near the foramen of Magendie, located medially in the fourth ventricle, having excess of CSF is known as cerebellomedullary cistern.
It is like as if you are standing on the floor of fourth ventricles where your head is down to the aqueduct of Sylvius and your two arms are stretching out to close the two lateral foramina of Luschka, and your one leg is bent and blocking the foramen of magendie while your other leg is on the top of the central canal of spinal cord as illustrated in the diagram below.
CSF present in the subarachnoid space around the spinal cord near L4 and L5 can be taken for sampling by lumbar puncture.
The CSF coming out from the fourth layer ascends up to the cerebral hemisphere and reaches to the top where the dura mater layer separate and then diffuse latter creating a triangular space lined by endothelial cells like veins called as dural venous sinuses. e.g sagittal sinus concerned with venous drainage via internal jugular vein.
In these dural sinuses arachnoid mater form finger like projections called as arachnoid villi. Arachnoid villi have special endothelial cells which allows the unidirectional bulk flow fluid from subarachnoid space to dural sinuses. These channels present in the arachnoid villi allows the entry of CSF into the venous system.
- It acts as cushion around the central nervous system and protects it from injury.
- As the specific gravity of CSF and brain is same, brain floats in the CSF. This reduces the weight of the brain from 1400 gm to 50 gm due to buoyancy of the CSF. As the weight is less it can be kept suspended with thin delicate structures such as nerves and arteries and veins.
- CSF acts as a reservoir and regulates intracranial content. e.g. during old age as the brain tissue slightly shrinks , the CSF volume increases.
- It acts as a nourishing fluid for the CNS.
- It also acts as a vehicle for removing metabolic waste products.
- It transports hormones and hormonal products.
Some important features to remember regarding CSF are:-
- Normal volume is 150 ml
- Rate of production is 550 ml/day, and CSF turns over 3.7 times a day.
- Pressure is approx 60-180 mm H2O, with average pressure of 112 mm H2O.
Had there been no CSF, we would not have been withstand even the minor traumas of everyday life. Brain damage is mostly common when the skull is injured by blow on the head and it is driven against the skull or the tentorium at the opposite of the site of injury. Such type of injury is called as contrecoup injury. When the skull is fractured at the site of injury and the bone is driven into the neural tissue it is called as depressed skull fracture causing coup injury.
It is a procedure to collect the CSF for diagnostic sampling. It is obtained by making the patient lie at left lateral position with bent knees or may be on sitting posture and then insert a needle into the central canal at the interspinous area at the L4 level. Patients with raised intracranial pressure are excluded from the procedure as it may lead to brain hernia.
Alteration of the CSF in different conditions
It is a condition when there is excessive CSF within the cranial cavity or skull. It is also referred as ‘water on the brain’.
Causes in simple words are:-
- Abnormally increased production of CSF. e.g. Hyperfunctioning tumor of choroid plexus which is a rare condition
- Abnormal circulation.
- Blockage of the foramen Monro due to development of cyst in the lateral ventricle leading to enlargement of lateral ventricle which damages the brain substances.
- Congenital aqueductal stenosis leads to hydrocephalus due to the expansion of the third and lateral ventricle leading to head enlargement.
- Tumor of cerebellum blocking the foramina of fourth ventricles may lead to the expansion of fourth, third and lateral ventricles causing hydrocephalus
- Severe infection of meninges e.g. tuberculous meningitis may lead to scar formation in the subarachnoid spaces which may lead to hydrocephalus.
- Reduced drainage due to blockage of arachnoid villi as a result of accumulation of cells near the arachnoid villi due to subarachnoid hemorrhage or infections.
What we have talked about in the above section we can list them under these headings of communicating hydrocephalus or non-communicating hydrocephalus.
- Communicating hydrocephalus or non- obstructive hydrocephalus. Examples- Reduced drainage due to scarring or fibrosis of subarachnoid space, or functional impairment of the arachnoidal granulations or due to thrombosis of the sagittal sinus. Here the CSF is able to communicate with subarachnoid space and the problem is beyond.
- Non- communicating hydrocephalus or obstructive hydrocephalus due to blockage of the aqueduct of Sylvius or the different foramina of lateral, third and fourth ventricles. Here the CSF is unable to communicate with the subarachnoid space.
Sometimes we may come across a peculiar type of hydrocephalus where pressure is normal with impaired drainage system. Here the cerebral hemisphere shrinks and thus the pressure remains normal. Clinical symptoms may include dementia, urinary incontinence with gait abnormality.
Hydrocephalus can be treated by placing a drainage tube (shunt) between the brain ventricles and abdominal cavity or the arterial system where there is shunt placed between the brain and the heart. Although this procedure does involves certain risk factors. There lot other conditions where the CSF pressure is altered and ot is beyond the scope of the section to discuss about them. Any clarification or query is always welcome. See you then in the next section.