8/13/2019 The Chemical Environment of the Brain
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The chemical environment of the brain
The brain has the consistency of firm jelly, and therefore is
protectively encased in a thick, bony
skull. The brain literally floats in about 150 millilitres (mL)
of CerebroSpinal Fluid(!")
secreted by the choroid ple#us. $ppro#imately 500 mL of !" is
secreted daily, %hich slo%ly
circulates do%n throu&h the four ventricles, up throu&h
the subarachnoid spaceand e#its into the
cerebral veinsthrou&h the arachnoid villi. The brain has no
lymphatic system, so the !" serves asa partial substitute.
The dura materis a tou&h, protective connective tissue %hich
is ti&htly bound to the skull, but
%hich encases the cerebral veins. 'nder the dura mater is the
subarachnoid spacecontainin& !",
arteriesand %eblike strands of connectivesupportive tissue
called the arachnoid(*spiderlike*)
mater. The pia materis a permeable membrane of colla&en,
elastin fibers+ fibroblasts on the
floor of the subarachnoid space %hich allo%s diffusion bet%een
the !" and the interstitial fluidof
the brain tissue. The pia mater lies on a membrane that is
infiltrated %ith astrocyte processes. The
dura mater, the arachnoid mater and the pia mater are
collectively referred to as the meninges.
hile the brain + !" are separated by the some%hat permeable pia
mater, the blood-
cerebrospinal fluid barrierand the blood-brainbarrier(---)
represent substantial protection for
the brain a&ainst undesirable blood substances. These
barriers are very permeable to %ater, o#y&en,
carbon dio#ide and small lipidsoluble substances. They are also
some%hat permeable to small
electrolytes and special transport systems e#ist for some other
specific molecules such as
essential amino acids. The barriers are the result of
endothelial cells%hich line capillary %alls
and &lial cellscalled astrocytes%hich %rap the capillaries
%ith fibers.
The brain is not only a functionally distinct or&an, it is a
chemically distinct one. 50/ of dry brain
%ei&ht is lipid (in contrast to 0/ for other or&ans).
2ost of the brain lipid is structural (in
myelinor membranes) in contrast to the tri&lycerides and
free fatty acids constitutin& the fat of
other or&ans. The bloodbrain barrier creates a protected
chemical environment for the brain
%herein certain molecules can perform functions independent of
the functions those molecules
perform in the rest of the body. This is particularly important
for the neurotransmitters serotonin
(%hich is hi&hly concentrated inplatelets+ the intestine)
and norepinephrine(%hich affects blood
pressure + metabolism). $ll of the kno%n aminoacid
neurotransmitters are non-essentialamino
acids.This means that they can be manufactured in the brain,
%ithout needin& to be supplied from
outside the brain. -ut in the major area of the brain %hich does
not have a bloodbrain barrier the
hypothalamus the primary neurotransmittersarepeptides.The three
major cate&ories of substances that act as neurotransmitters
are (1) amino acids(primarily &lutamic acid, 3$-$, aspartic
acid + &lycine), () peptides(vasopressin, somatostatin,
neurotensin,etc.) and (4) monoamines(norepinephrine, dopamine+
serotonin) plus acetylcholine.
The major *%orkhorse* neurotransmitters of the brain are
&lutamic acid (&lutamate) and 3$-$.
The monoamines + acetylcholine perform speciali6ed
modulatin& functions, often confined to
specific structures. The peptides perform speciali6ed functions
in the hypothalamus or act as co
factors else%here in the brain. 7"or a %ellor&ani6ed
cate&ori6ation of neurotransmitters, see
8eurotransmitter (ikipedia).9
(-en -est, Chapter 10: Brain Neurotransmitters, at
http:%%%.benbest.comscienceanatmindanatmd10.html)
