Page 170 - The Miracle in the Cell Membrane

The Miracle in the Cell Membrane

3. Brain Cell Selectivity: "The Blood-
Brain Barrier"
In the brain, spe cial sen tries admit the nec es sa ry nutri -
ents in the blood but keep out sub stan ces that would keep
nerve cells from func tion ing. These sen tries form a bar ri er
between the brain's nerve tis sues and pre vent sub stan ces in
the blood from enter ing. This bar ri er is con sti tut ed by
endo the li um cells that line the brain's blood ves sels. The
bar ri er's impor tance stems from the fact that nerve cells
need a spe cif ic chem i cal envi ron ment. Lacking such an
obsta cle, when you ate foods that increase the con cen tra -
tions of glu cose, amino acids, hor mones or other com -
pounds, or when you engaged in exer cise, nerve func tions
would move out of con trol and would even suf fer sei zures.
In the brain, count less cap il la ries bring nutri ents and
carry away waste pro ducts. The brain's endo the li um cells
have spe cial con nec tions that pre vent sub stan ces in the
blood from pass ing through the cell mem brane and reach -
ing the nerve tis sue. For that rea son, the endo the li um cells
are almost total ly imper me a ble. But did the bar ri er not
allow any thing pass, then the brain would be deprived of
the need ed oxy gen, glu cose and amino acids and would
die. Yet the blood-brain bar ri er's spe cial mech a nisms keep
unwant ed sub stan ces out, but carry vital mol e cu les to the
brain.
Molecules sol u ble in fat can gen er al ly pass through the
blood-brain bar ri er imme di ate ly. These include nic o tine,
eth a nol and her o in. However, if charged mol e cu les are not
sol u ble in fat and must depend on spe cial trans port sys -

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