CSF: Production, Circulation and Pathology Flashcards
What is cerebrospinal fluid (CSF)?
Clear, colourless liquid composed mostly of water
Production of CSF?
By the secretory epithelium of the choroid plexus
500-600 ml are produced per day and, in the CNS, there is 150ml that is replaced 3-4 times daily
What does the CSF supply and remove from the CNS?
Supplies water, amino acids and ions
Removes metabolites
3 major functions of the CSF?
- Mechanical protection - it is a shock-absorbing medium in which the brain floats
- Homeostatic functions:
• pH of CSF affects pulmonary ventilation and cerebral blood flow
• Transports hormones - Circulation - it is a medium for minor exchange of nutrients and waste products between blood and brain tissue
How can clinical CSF analysis be carried out and why is this useful?
Obtained via a lumbar puncture; aids in the diagnosis of brain, meninges, and spinal cord issues
Characteristics of normal CSF?
Clear and colourless
Contains little protein, few Igs and only 1-5 cells
Consequences of conditions that cause CSF accumulation?
Serious neurological deficits, e.g: hydrocephalus (fetus or newborn)
Embryonic development of the brain and ventricular system?
At 3 weeks, the developing NS consists of a tube (neural canal); its cavity gives rise to the adult brain ventricules and spinal cord’s central canal
Choroid plexus develops from cells in the walls of the ventricules
Function of the choroidal cells?
Specialised secreting cells that produce CSF
What is the choroid plexus?
Networks of capillaries in the walls of the ventricles
Development of the choroid plexus?
Developing arteries invaginate the roof of the centricle to form the choroid fissure
Involuted ependymal cells along with the vessel enlarge into villi and form the choroid plexus
Location of the choroid plexus in the adult brain?
Found in the 3rd, 4th and lateral ventricles
Label the choroid plexus?
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Ions involved with CSF production?
CSF secretion inv. transport of ions across the epithelium, from blood to CSF:
Na+
Cl-
HCO3-
Ion movement drives water into the CSF
How is CNS secretion possible?
Due to the polarised distribution of specific ion transporters in the apical or basolateral membrane of the epithelial cells
Action of the active Na+ transporter in CSF production?
Electrical gradient pulls along CL- and both ions drag water by osmosis
Compare ion concentrations in CSF to plasma?
Lower K+, glucose and much lower protein
Higher Na+ and Cl-
Relationship of CSF production to arterial blood pressure?
CSF production in the choroid plexuses is an active secretory process
It is not directly dependent on the arterial BP
Label the ventricular system
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How are the ventricles connected to one another?
Intraventricular foramina (of Monroe) - connects lateral ventricles to 3rd ventricle
Cerebral aqueduct (of Sylvius) - connects 3rd ventricle to the 4th
How are the ventricles connected to the subarachnoid space?
Foramen of Magendie (median aperture) - connects the 4th ventricle to subarachnoid space
Foramina of Luschka (lateral aperture) - connects the 4th ventricle to the subarachnoid space
Pathway of CSF circulation following its production?
- Produced in the choroid plexuses of each lateral ventricle
- Flows to third ventricle through 2 interventricular foramina
- More CSF added by choroid plexus in roof of third ventricle
- Then flows through cerebral aqueduct (midbrain) and into the 4th ventricle
- Another choroid plexus in the 4th ventricle adds more CSF
- Enters the subarachnoid space through the roof of the 4th ventricle, via:
• Single median aperture
• Paired lateral apertures - Circulates in the central canal of the spinal cord
Where is the subarachnoid space?
Between the pia and dura mater
Outflow of the CSF?
- Returns to venous blood through arachnoid granulations into the superior sagittal sinus (SSS)
- Brain intersitial fluid makes up the final portion of the CSF and drains into the CSF via perivascular spaces
How is the blood separated from the CSF?
By the blood-CSF barrier
Why is the blood supply to the majority of the brain restricted?
By the blood-brain barrier; it consits of the capillary endothelium, basal membrane and perivascular astrocytes
Function of tight junctions in the BBB?
Prevent paracellular movement of molecules
Which areas of the brain do not have a BBB?
Circumventricular organs or pineal gland
Function of the BBB?
Protects the brain from common bacterial infections and toxins
Main obstacle for drug delivery to the CNS
Tumours that can affect the ventricles, choroid plexus and CSF?
Colloid cyst (often found at the interventricular foramen)
Ependymomas (arising from the ependymal cells lining the ventricles)
Choroid plexus tumours
What is a ventricular haemorrhage? Different types?
Accumulation of blood in the ventricles:
• Epidural haematoma - arterial bleed between skull and dura
• Subdural haematoma - venous bleed between dura and arachnoid
• Subarachnoid haemorrhage
What is hydrocephalus?
Accumulation of CSF in the ventricular system or around the brain, either de to obstruction OR over-production
There is subsequent enlargement of 1/more ventricles and an increase in CSF pressure
What is idiopathic intracranial hypertension?
AKA pseudotumour cerebri - unknown aetiology but symptoms include headache and visual disturbance due to papilloedema
There are no imaging features of hydrocephalus, despite the increased CSF features
What is papilloedema?
Optic disc swelling due to increased IC pressure being transmitted to the subarachnoid space surrounding the optic nerve
Visual symptoms of papilloedema?
- Enlarged blind spot
- Blurring of vision
- Visual obscurations
- Loss of vision
What is aqeuous humor?
Specialised fluid that bathes structures within the eyes
Functions of aqueous humor?
Provides oxygen, metabolites
Contains HCO3, which buffers H+ produced in the cornea and lens by anaerobic glycolysis
Production of aqueous humor?
Produced by an energy-dependent process in the epithelial layer of the ciliary body
IT IS NOT SIMPLY AN ULTRAFILTRATE OF PLASMA
Pathway of aqueous humor flow?
- From ciliary body, it enters the posterior chamber of the eye and then the anterior chamber
- Drains to the scleral venous sinus, via a trabecular meshwork and the canal of Schlemm
- Small amount diffuses through the vitreous and is absorbed across the retinal pigment epithelium
Where is the angle of Schlemm located?
In the angle between the iris and cornea (iridocorneal angle)
Organisation of the ciliary epithelium?
Ciliary body and posterior surface of the iris are covered by 2 juxtaposed layers of epithelial cells:
- A forward continuation of the pigment epithelium of the retina
- Overlying this is an inner non-pigmented layer
How are ions involved in the production of aqueous humor?
HCO3- and H+ (formed in the epithelial cells by hydration of CO2, catalysed by carbonic anhydrase) transported across the basolateral membrane of pigment epithelium
exchanged for
Cl- and Na+ which diffuse into NPE cells (via gap junctions); then enter the aqueous humor
Accompanied by water, via aquaporins
Replacement of the aqueous humor in the anterior and posterior chambers of the eye?
Posterior chamber - completely replaced in ~30 mins
Anterior chamber - longer turnover time so ~120 mins
Importance of the balance between secretion and drainage of aqueous humor?
Maintains the intraocular pressure
Cause of glaucoma?
Raised intra-ocular pressure is caused by an imbalance between rates of secretion and removal aqueous humor
Drugs that reduce production of aqueous humor? Side effects?
Carbonic anhydrase inhibitors (used to reduce ocular pressure in glaucoma):
• Dorzolamide - administered as eye drops (avoids systemic side effects)
• Acetazolomide - administered orally (can affect kidneys and cause an acidosis)
