101_12AI_neuro_2_extrac_fluid

101_12AI_neuro_2_extrac_fluid - topic #2: interstitial...

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topic #2: interstitial fluids in CNS & PNS choroid plexus blood-brain barrier recall that the nervous system… a) transduces environmental stimuli into electrical signals b) sends electrical signals to various tissues rationale for today’s lecture : a) these signals result from changes in the permeability of nerve cell membranes to ions inside & outside these cells. b) before studying how these signals are generated, & because incorrect ion concentrations can result in abnormal electrical activity, let’s look at how extracellular fluid is formed .
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similar to Figs 9-1 & 10-1 imagine Na + & glucose enter your blood (after a meal). how would these reach the fluid surrounding your brain , spinal cord , & nerves in your arm ? 1) first, notice the blood flow from the heart to capillaries in the “head & brain” and “arms” 2) next, ask yourself “hmm… what happens at capillaries?”
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Fig 10-18 in the arm, leg, torso: a) small ions (inorganic and organic), sugars, and amino acids are physically small enough to diffuse through small openings (“ fenestrae ”) between neighboring endothelial cells that form the walls of the capillaries. b) these materials can diffuse freely between the blood and the fluid surrounding these capillaries. c) these capillaries are therefore said to be “ leaky ”. leaving capillary entering capillary
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thus, material can leave fenestrated capillaries 2 ways: a) lipid-soluble species can diffuse through the endothelial cell membranes b) water-soluble species diffuse through fenestrae (labeled “pores” here) Fig 10-18 y: amount in brain vs standard x: oil/water partition coefficient Goldstein Betz 86 Sci Am cross-section thru capillary lipid soluble: O 2 , CO 2 , H 2 O, some drugs e.g., heroin, nicotine, valium, & ethanol cross easily, Na + doesn’t
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how does material leave the capillaries & reach peripheral nerves? 1) materials leak out of fenestrated capillaries ( labeled “blood vessels here ) 2) both lipid- and water-soluble materials diffuse through a leaky connective tissue ( “perineurium” ) to the extracellular space around nerve fibers. we will see a similar connective tissue in the brain. Fig 5-30
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circulatory system extracellular fluid summary:
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Netter’s Atlas of the Human Body (2006), p.90 in the head: 1) 2 major arteries carry blood to the brain : one passing in front of the bones of the neck (internal carotid artery) & one passing along the side of these (basilar artery). this allows them to enter the skull and extend upwards into the brain. 2) there, they branch into arterioles, then into capillaries . back side basilar artery internal carotid artery Perez’s Atlas of Human Anatomy (2006), p.163
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unlike the arm, etc: 1) most capillaries in the brain are not fenestrated : the neighboring endothelial cells are fused together at “tight junctions” 2) because the membranes block the free diffusion of water-soluble
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101_12AI_neuro_2_extrac_fluid - topic #2: interstitial...

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