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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