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 :-) > ===== __________________________________________________ Do You Yahoo!? Bid and sell for free at http://auctions.yahoo.com