Date post: | 26-May-2015 |
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Health & Medicine |
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MICROENVIRONMENT OF NEURONS-REVIEW OF BLOOD BRAIN BARRIER &
IMPORTANT
RK GOIT, LECTURER
DEPARTMENT OF PHYSIOLOGY
Cerebral blood flow• the principal arterial inflow to the brain in human is via
four arteries: two internal carotids & two vertebrals
• blood flow to the brain is about 750 ml/min (15% of the cardiac output)
• cerebral blood flow is 50ml/100g/min of brain tissue
• three metabolic factors have potent effects in controlling cerebral blood flow: • CO2 concentration• H+ concentration• O2 concentration
Autoregulation of cerebral blood flow • cerebral blood flow is “autoregulated” extremely well
between arterial pressure limits of 60 & 140 mm Hg
Cerebral microcirculation• the number of blood capillaries in the brain is greatest
where the metabolic needs are greatest
• the number of capillaries & rate of blood flow are about four times as great as that of white matter in the gray matter
• brain capillaries are much less “leaky” than the blood capillaries in almost any other tissue of the body
• the capillaries are supported on all sides by “glial feet,” & provide physical support to prevent overstretching of the capillaries
Cerebrospinal fluid• a clear, colorless, almost protein free filtrate of blood
• volume is about 150 ml & rate of production is 550ml/d
• 50-70% of the CSF is formed in the choroid plexuses & the remainder is formed around blood vessel & along ventricular walls
• is present around the brain & spinal cord (in the subarachnoid space) as well as inside the brain & spinal cord (in its ventricles & the central canal respectively)
• all these chambers are connected with one another, & the pressure of the fluid is maintained at a constant level
Functions of CSF1. Mechanical protection • serves as a cushion between the CNS & surrounding bones, thus
protecting it against mechanical trauma• also buoys the brain so that it “floats” in cranial cavity
2. Homeostatic function• pH of the CSF affects pulmonary ventilation & cerebral blood flow • CSF also serves as a transport system for polypeptide hormones
secreted by hypothalamic neurons
3. Circulation• CSF is a medium for minor exchange of nutrients & waste products
between the blood & adjacent nervous tissue
FormationCSF is formed mainly in the choroid plexuses of the lateral, third, & fourth ventricles; some originates from the ependymal cells lining the ventricles & from the brain substance through the perivascular spaces
Absorption some of the CSF is absorbed directly into the veins in the subarachnoid space, & some escapes through the perivascular spaces into the subarachnoid spaces
• brain requires a very stable environment in order to function normally
• barriers, called the blood-cerebrospinal fluid barrier & the blood-brain barrier, exist between the blood & cerebrospinal fluid & brain fluid, respectively (except in some areas of the hypothalamus, pineal gland & area postrema)
• barriers are highly permeable to water, carbon dioxide, oxygen, & most lipid-soluble substances; slightly permeable to electrolytes; & almost totally impermeable to plasma proteins & most non–lipid-soluble large organic molecules
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