Cns 11

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Cns 11

  1. 1. CEREBROSPINAL FLUID (CSF) AND THE BLOOD-BRAIN BARRIER (BBB)
  2. 2. Cerebrospinal Fluid <ul><li>Cerebrospinal fluid (CSF) is a clear fluid present in the ventricles of the brain, the central canal of the spinal cord, and the subarachnoid space. </li></ul><ul><li>Normal: almost no blood cells, little protein. </li></ul>
  3. 3. CSF Cont. <ul><li>Its rate of formation, flow, and absorption is sufficiently high to cause its replacement several times daily. Measuring its pressure, performing cell counts, and analyzing levels of various biochemical constituents is accomplished with a spinal tap . </li></ul>
  4. 4. CSF Cont. <ul><li>An understanding of the formation, flow, and absorption of CSF is essential to an understanding of these diagnostic procedures and the pathophysiology of hydrocephalus. </li></ul>
  5. 5. Cerebrospinal Fluid <ul><li>A clear, colorless fluid that surrounds and permeates the CNS. Offers support, protection and nourishment. </li></ul><ul><li>Functions: </li></ul><ul><ul><li>Protection of cranial contents </li></ul></ul><ul><ul><li>Modulates pressure changes (same specific gravity as brain) </li></ul></ul><ul><ul><li>Serves as a chemical buffer to maintain constant ionic environment </li></ul></ul><ul><ul><li>Serves as a transport medium for nutrients and metabolites, endocrine substances and even neurotransmitters </li></ul></ul>
  6. 6. Location of CSF <ul><li>Two lateral ventricles </li></ul><ul><li>Third ventricle </li></ul><ul><li>Fourth ventricle </li></ul><ul><li>Spinal cord central canal </li></ul><ul><li>Subarachnoid space </li></ul><ul><li>Continuous with extracellular fluid of brain parenchyma </li></ul>
  7. 7.
  8. 8. Choroid Plexus <ul><li>Choroid plexus hang from the roof of each ventricle </li></ul><ul><li>These plexuses form CSF </li></ul><ul><li>The plexuses are clusters of thin walled capillaries enclosed by a layer of ependymal cells </li></ul>
  9. 9. Formation of CSF <ul><li>Cells are believed to actively secrete Na + into the ventricular system in exchange for K + . Sodium ions electrically attract Cl - and osmotically draw water from the blood vascular system to constitute the CSF. </li></ul>
  10. 10. Composition of CSF
  11. 11. The Choroid Plexus <ul><li>In adults, the total CSF volume of about 150 ml is replaced every 3-4 hours </li></ul><ul><li>The choroid plexus also helps to cleanse the CSF by removing waste products and other unnecessary solutes </li></ul><ul><li>Once produced CSF moves freely through the ventricles </li></ul>
  12. 12. Flow of Cerebrospinal Fluid
  13. 13. Absorption of CSF <ul><li>Through the arachnoid villi, a protrusion of arachnoid membrane into the central venous sinus and other sinuses </li></ul><ul><li>A valve opens when CSF pressure exceeds venous pressure </li></ul><ul><li>Absorption by veins and capillaries of CNS </li></ul>
  14. 14. CSF Pressure <ul><li>Human- 10-12 cm of H 2 O </li></ul><ul><li>Subarachnoid hemorrhage blocks absorption, raises pressure </li></ul><ul><li>Congenital excessive formation or blockage to flow raises pressure </li></ul><ul><li>Regulated by rate of absorption and rate of formation. </li></ul><ul><li>Brain tumors lower absorption & raise pressure </li></ul><ul><li>Inflamed meninges can cause excessive formation & raise pressure </li></ul>
  15. 15. Cerebrospinal fluid collection (Lumbar puncture)
  16. 16. Cerebrospinal fluid collection <ul><li>Small animals: atlanto-occipital (cerebellomedullary) cistern </li></ul>neck
  17. 17. Cerebrospinal fluid collection <ul><li>Large animals </li></ul><ul><ul><li>cerebellomedullary cistern </li></ul></ul><ul><ul><li>Lumbar cistern (L5-L6) </li></ul></ul>
  18. 18.
  19. 19. CSF collection
  20. 20. Hydrocephalus <ul><li>An abnormal increase in the volume of CSF </li></ul><ul><li>Symptoms: depression, sleep, changes in behavior, spastic paresis, papilledema, bulging of skull in young, behavioral change, pacing, head pressing, seizures, cranial nerve deficits, vestibular signs </li></ul>
  21. 22. Hydrocephalus <ul><li>Communicating – due to excessive formation of CSF or lack of absorption </li></ul><ul><li>Non-communicating – due to obstruction of flow through ventricle system </li></ul>
  22. 23. Blood-Brain Barrier (BBB) <ul><li>Direct injection of large molecules (e.g., a dye) into the venous system will often penetrate all tissues except the brain. This observation led to the notion of a physiological barrier to flow of substances from blood to brain tissue – the blood brain barrier. </li></ul><ul><li>The BBB functions to preserve a stable environment for neurons and glia of the CNS </li></ul>
  23. 24. Blood-Brain Barrier <ul><li>The barrier is a protective mechanism that helps maintain a stable environment for the brain </li></ul><ul><li>The brain is very dependent on a contant internal environment </li></ul><ul><li>Fluctuations in the concentration of ions, hormones, or amino acids, would alter the brain’s function </li></ul><ul><ul><li>Hormones and amion acids / neurotransmitters </li></ul></ul><ul><ul><li>Ions / neuron thresholds (K+) </li></ul></ul>
  24. 25. Blood-Brain Barrier <ul><li>Bloodborne substances within the brain’s capillaries are separated from the extra- cellular space and neurons by </li></ul><ul><ul><li>Continuous endothelium of the capillary walls </li></ul></ul><ul><ul><li>Relatively thick basal lamina surrounding the external face of the capillary </li></ul></ul><ul><ul><li>To a limited extend the “feet” of the astrocytes that cling to the capillaries </li></ul></ul>
  25. 26. Differences between brain and general capillaries <ul><li>General capillary </li></ul><ul><ul><li>small solutes can diffuse through intercellular clefts </li></ul></ul><ul><ul><li>pinocytosis independent of molecular size (pass large molecules) </li></ul></ul><ul><ul><li>fenestrated </li></ul></ul><ul><li>Brain capillary </li></ul><ul><ul><li>tight junctions </li></ul></ul><ul><ul><li>reduced pinocytosis </li></ul></ul><ul><ul><li>no fenestra </li></ul></ul><ul><ul><li>astrocyte foot processes </li></ul></ul>
  26. 27. General vs. Brain Capillaries
  27. 28. Brain capillary <ul><li>Tight junctions </li></ul><ul><li>Surrounded by astrocytes </li></ul><ul><li>Few vesicles </li></ul><ul><li>Carrier mediated transport of glucose and amino acids </li></ul>
  28. 29. Blood-Brain Barrier <ul><li>The blood-brain barrier is a selective, rather than absolute barrier </li></ul><ul><li>Nutrients, such as glucose, essential amino acids, and some electrolytes, move passively by facilitated diffusion through the endothelial cell membranes </li></ul><ul><li>Bloodborne metabolic wastes, such as urea and creatinine as well as proteins, certain toxins, and most drugs, are prevented from entering brain tissue </li></ul>
  29. 30. Blood-Brain Barrier <ul><li>Lipid soluble molecules will cross </li></ul><ul><li>Negatively charged molecules cross more freely than positively charged ones </li></ul><ul><li>High CO 2 /low O 2 produce vasodilation and decrease resistance of BBB </li></ul><ul><li>Injury or inflammation decreases resistance of BBB (allows some antibiotics to be used for treatment) </li></ul>
  30. 31. Blood-Brain Barrier (BBB) <ul><li>Selected brain areas are not protected by the BBB – the circumventricular organs (CVO) bordering on the 3 rd and 4 th ventricles </li></ul><ul><ul><li>parts of the hypothalamus - median eminence </li></ul></ul><ul><ul><li>neurohypophysis </li></ul></ul><ul><ul><li>pineal gland </li></ul></ul><ul><ul><li>area postrema </li></ul></ul><ul><ul><li>subfornical organ </li></ul></ul><ul><ul><li>subcommissural organ </li></ul></ul>
  31. 32. Blood – CSF Barrier <ul><li>Barrier from blood to the CSF </li></ul><ul><li>Similar structural mechanism to the BBB </li></ul>

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