The document discusses the microenvironment of neurons, focusing on the blood brain barrier and cerebrospinal fluid. It notes that the brain requires a stable environment to function normally, which is provided by barriers between blood and cerebrospinal fluid and blood and brain fluid. These barriers allow the passage of water, carbon dioxide, oxygen, and lipid-soluble substances while being impermeable to plasma proteins and large organic molecules. Cerebrospinal fluid acts as a cushion and transport system for the brain, and is formed in the choroid plexuses and absorbed into veins in the subarachnoid space. The cerebral circulation and autoregulation of blood flow is also summarized.

1. MICROENVIRONMENT OF NEURONS-
REVIEW OF BLOOD BRAIN BARRIER &
IMPORTANT
RK GOIT, LECTURER
DEPARTMENT OF PHYSIOLOGY

2. 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

3. Autoregulation of cerebral blood flow
• cerebral blood flow is “autoregulated” extremely well
between arterial pressure limits of 60 & 140 mm Hg

4. 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

5. 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

6. Functions of CSF
1. 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

7. Formation
CSF 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

9. 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

10. • 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

11. •