Excellent explanation, Ivan. I can see why you were
such a good teacher.      Carole

--- Ivan M Suzman <[log in to unmask]> wrote:
> ^^^^^^  WARM GREETINGS  FROM  ^^^^^^^^^^^^  :-)
>  Ivan Suzman        49/39/36       [log in to unmask]
> :-)
>  Portland, Maine    land of lighthouses        52
> deg. F   :-)
>
********************************************************************
> Adapted from: Human Anatomy and Physiology
> Benjamin/Cummings
> by Elaine N. Marieb,  1992
>
> Revised by Ivan Suzman.
>
> Dear list members,
>
>     I hope the following will assist all of you in
> understanding how
> levodopa reaches the brain.
>
>
> The blood-brain barrier is a protective anatomical
> arrangement
> that helps the brain's environment to remain stable.
> No other
> organ is so absolutely dependent on a constant
> internal chemistry.
> In other parts of the body, extra-cellular
> concentrations of
> hormones, amino acids (building blocks of protein),
> and ions
> are in a state of flux, particularly after eating or
> exercising.
>
> If the brain were exposed to such chemical
> variations,.its
> nerve cells would fire uncontrollably. This is in
> part because
> some hormones and amino acids serve as
> neurotransmitters,
> and some ions (particularly potassium) modify the
> neuronal
> firing.
>
>     Blood-carried substances within the brain's
> capillaries are
> separated from the tissue of the brain by (1) the
> structure of the
> capillary walls ; (2) a relatively thick basal
> lamina on the external
> surfaces of these capillaries, and (3) the structure
> of the astrocytes,
>  which are star-shaped projections from the neurons
> of the brain
> tissues to the capillaries.
>
> The relative impermeability of brain capillaries
> results in a
> virtually complete blood-brain barrier.
>
> The blood-brain barrier is selective. A point of
> great importance in
> Parkinson's Disease, is that essential amino acids
> like
> phenylalanine (which is the important ingredient in
> Sinemet),
>  glucose, and some electrolytes diffuse into the
> brain.
>  Blood-borne wastes such as urea,  as well as large
> proteins
> are prevented from entering the brain tissue. Small
> non-essential
> amino acids and potassium ions are also prevented
> from
> entering the brain.
>
> The blood-brain barrier is ineffective against fats,
> fatty acids,
> oxygen and carbon dioxide. Fat-soluble
> molecules such as blood-borne alcohol, nicotine,
> anesthetics,
> and THC (from marijuana) easily pass by diffusion to
> the
> brain. In some areas of the brain, the blood-brain
> barrier is
> either partial or absent. In these areas, blood-born
> molecules
> gain easy access to brain tissue. One such area is
> the
> vomiting sensor of the brain stem.
>
> Another is the hypothalamus which is extremely
> important in
> PD. Hydration (water balance), body temperature and
> many
> metabolic activities are regulated here. The lack of
> a blood-
> brain barrier in these areas allows "quick-testing"
> of the
> composition of the blood.
>
> The blood-brain barrier in infants and fetuses is
> incomplete.
> Biochemical injury to the brain by toxic exposure is
> more
> likely in children than in adults. Traumatic injury
> to the brain
> may cause localized breaches in the blood-brain
> barrier.
> Medical management of the biochemistry of the brain
> can
> be achieved in some cases by infusing drugs through
> gateways in the blood-brain barrier. The drug
> manitol is one
> that is used to create these gateways.
>
> October 23, 1999  Portland, Maine  :-)
>


=====

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