Neuroscience: 2:00 - 3:00Scribe: Melissa Precise

Monday, January 4, 2010Proof: Matthew Davis

Dr. GamlinCytologyPage 1 of 4

  1. Learning Objective #4 [S23]
  2. In Class Question:
  3. All are true for astrocytes EXCEPT:
  4. Buffer K+
  5. Uptake Glutamate
  6. Form Myelin
  7. Induce the Blood Brain Barrier (BBB)
  8. Answer is C. Form Myelin
  9. These are all things that glia cells do – but we have different types of glia cells
  10. Astrocytes being one type of glia cell when answering the question
  11. Astrocytes are not the glia cells responsible for making myelin, but do the other things listed.
  12. Wants us to be able to map functions to particular types of glia cells
  13. Categorize glia cells and talk about their functions.
  14. Classification [S24]
  15. Macroglia and Microglia
  16. Macroglia
  17. Should Know – lists four types
  18. Radial Glia
  19. Important during development for appropriate layering of structures like the cortex.
  20. The cortex is a layering structure for neurons to get to the right place and find their final destination.
  21. Astrocytes
  22. Might see differentiated between Type I and Type II
  23. Will not make us differentiate between these two!
  24. Schwann Cells
  25. Primary function is mylenation
  26. Found in the PNS
  27. Oligodendrocytes or known as Oligodendroglia
  28. Primary function is mylenation
  29. Found in the CNS
  30. Microglia
  31. Didn’t talk about this on this slide
  32. Key Roles of Glia [S25]
  33. These are the main roles of glia cells
  34. Remove glutamate
  35. This is the principal excitatory neurotransmitter which means releasing glutamate into the extracellular space all of the time.
  36. Glutamate is known for some of its actions on receptors to be a neurotoxin because it can excite the neuron to death.
  37. Even if not killing the neuron – excessive glutamate can trigger epilepsy.
  38. So its extracellular level needs to be regulated and that is one of the major roles of astrocytes.
  39. Going to make Myelin
  40. Homeostatic Functions
  41. Buffering of other ions for example
  42. Potassium is the key one regulated
  43. Involved in the development or induction of the Blood Brain Barrier (BBB)
  44. Developmental roles that will be discussed by another professor.
  45. Cells can proliferate postnatally
  46. Neurons do not divide postnatally (with few exceptions) – once you get that set of neurons then ythat is the set you have!
  47. Glia cells will divide.
  48. Microglia [S26]
  49. Not going to talk much about this slide.
  50. Just know that these are the macrophages of the brain.
  51. They are going to eat up dying cells, etc.
  52. Remove debris if there has been injury and can get rid of pathogens
  53. Radial Glia [S27]
  54. Not going to talk about development.
  55. Just showing the cortex developing.
  56. Radial glia are arranged to allow cells which start off near the ventricle (sub-ventricular zone) to climb along and get to the outer layer of the cortex.
  57. Learning Objective #5 [S28]
  58. Role of glia in terms of neuronal glia
  59. Glia cells work together with the neuron.
  60. Work in two important ways
  61. Action potential propagation
  62. That is the mylenation site
  63. Homeostatic Buffering
  64. Take up glutamate and other ions to maintain the extracellular media at appropriate ion and other substance concetrations
  65. Schwann Cell [S29]
  66. Myelination
  67. Two types of cells
  68. Schwann Cells
  69. Oligodendrocytes
  70. Schwann Cells
  71. Myelinating neural cells in PNS
  72. So sensory cells and motor cells are heavily myelinated because information that we need fast is somatosensory information telling where our body is in space.
  73. We want this information quick so we can make rapid adjustments and to make rapid adjustments – we need to send our motor signals fast.
  74. Can see these are myelinated.
  75. The Schwann cell makes this layer and wraps around the neuron.
  76. Can wrap around neuron up to 50 times insulating that axon.
  77. Must be important if wants to insulate that much.
  78. Myelin Sheet [S30]
  79. Another thing about Schwann cells that differs from oligodendrocytes is that they Schwann cell has a one to one relationship with the neuron.
  80. One Schwann cell interacts with one axon
  81. One axon, however, can have many Schwann cells along its length
  82. A single Schwann cell only contacts a single axon
  83. Myelin sheet wraps around the neuron – one to one.
  1. Gap Junctions and Disease [S31]
  2. Just to highlight why it is important and what can go wrong.
  3. Can see Schwann cell with its layer wrapped around the axon
  4. If you are a axon – then you will have myelin sheath which is a flat sheet that is going to wrap around the axon over and over again.
  5. Would think that this would cause a problem of getting ions and other nutrients from the soma to the distal part if it doesn’t have parts like axonal transport.
  6. This is the way glia cells does this.
  7. Glia cells sort of short circuit the root so that as the layer is wrapped around – the different layers talk to each other via gap junctions.
  8. These channels make direct links between the layers.
  9. So instead of something or a substance having to go all the way around to the end – it just jumps across the layers.
  10. This offers a fast way of getting something from one part of the myelin sheath to the next part.
  11. Know this is important because can have mutations in these channels which make them not transmit ions and nutrients.
  12. When have these mutations, the myelin cell will die and then ultimately end up with loss of the axons and weakness and atrophy.
  13. Probably know of a lot of diseases that affect myelination and are all pretty bad because they affect signaling.
  14. These mutations specifically affects these channels which then effects the myelin cell being able to maintain itself.
  15. Nodes of Ranvier [S32]
  16. Can see in the figure, that there are junctions between the myelination.
  17. These are really important and are known as the Nodes of Ranvier.
  18. These are needed for fast AP propagation.
  19. Oligodendrocytes [S33]
  20. Can more clearly see the layers going around the axon.
  21. Showing oligodendrocytes
  22. So now in the CNS – in the brain or spinal cord.
  23. Don’t have Schwann cells in the CNS – only have oligodendrocytes.
  24. Oligodendrocytes and Schwann cells do basically the same job
  25. More than one oligodendrocyte can contact the axon
  26. This can cause more serious problems than if affect oligodendrocytes than if affected Schwann cells
  27. Since more than one oligodendrocyte can contact more than one axon, affecting an oligodendrocyte can affect numerous axons.
  28. For example, with MS which affects myelination
  29. If affecting myelination in the CNS, there will be much more affect.
  30. One oligodendrocyte can effect 10-50 axons.
  31. 1:10 to 1:50 [S34]
  32. Can see in this figure that one oligodendrocyte is touching two axons
  33. Two things about myelinating cells
  34. Two types depends on whether in the PNS or in the CNS
  35. Have slightly different numerical relationship with the axons
  36. Unmyelinated CNS Fibers [S35]
  37. There are a lot of unmyelinated fibers in the CNS (brain and spinal cord)
  38. Even though might think that myelination is needed to help with action potential signaling, it is not practical to myelinate everything.
  39. If brain myelinated every axon in the brain – the head would be as big as the room.
  40. So brain has to choose which axons it myelinates.
  41. Will see some of these when doing neural anatomy gross lab.
  42. Can see massive fiber tracts of heavily myelinated neurons.
  43. Motor information coming from the motor cortex down the spinal cord is heavily myelinated as well as the important sensory information.
  44. Information going from one side of the cortex to the other cortex in order for both sides of the body to be coordinated need to be extremely fast – so these are heavily myelinated.
  1. CNS vs. PNS Summary [S36]
  2. Summary scheme found in book
  3. Basically showing Schwann cells in PNS
  4. When get into CNS – start to use oligodendrocytes to myelinate
  5. Astrocytic End Feet….. [S37]
  6. Astrocytes
  7. Contact blood vessels which is one of the things that they do
  8. Do not form the BBB, but they help to induce the BBB.
  9. Induce the (part of) the BBB [S38]
  10. Endothelial cells form the BBB which line the capillaries
  11. These form tight junctions.
  12. Without astrocytes, the tight junctions would not form
  13. Without the tight junctions, the capillaries would become leaky
  14. So remember astrocytes have a role in forming the BBB or at least a part of forming it.
  15. Buffering of Extracellular Ions [S39]
  16. Astrocytes have a homeostatic role.
  17. The neuron signaling is shown in the figure
  18. Every time it signals, some Potassium leaks out of the cell and some sodium comes into the cell.
  19. Have a very small extracellular space
  20. So doesn’t take much potassium ions to start to change the concentration of extracellular potassium.
  21. If change concentration of extracellular potassium then will start to initially over-excite the neurons and will ultimately quiet them.
  22. Will affect signaling if don’t buffer, so that is why it is believed that glia cells are thought to buffer potassium.
  23. From Here to There….. [S40]
  24. One idea that contradicts the non-polarization theory of astrocytes.
  25. This arrangement of astrocytes being close to neurons and to capillaries is thought to be maybe that they can buffer potassium. By diffusion, can dump excess ions into the capillaries as a way of removal.
  26. So can think of astrocytes as having some sort of homeostatic/metabolic polarity in this respect.
  27. Astrocytes Are Not Really Star-Shaped [S41]
  28. Myth that astrocytes are star shaped came from the staining of filaments that they have.
  29. So when filaments are stained – they look like they are star shaped
  30. Astrocytes are really more space filling
  31. In all of the spaces between the neurons there are astrocytes
  32. There is some extracellular space between these
  33. But astrocytes tend to fill everything else
  34. Have neat relationship where don’t overlap
  35. Astrocytes cover the territory up to the dividing lines shown on the slide
  36. Past the dividing line, the other astrocyte takes over
  37. So cover the entire CNS like that
  38. Figure to right shows coloring in of the astrocyte processe around the spine (synaptic spine/vesicles of transmitter) which is completely engulfing the synapse.
  39. Can take up potassium, but can also take up glutamate
  40. This is a very important function for astrocytes
  41. Transmitter “Shuttle” [S42]
  42. When glutamate is released as a transmitter and diffuses out of the synaptic cleft, glutamate relies exclusively on astrocytes to be re-uptaken.
  43. Glutamate is shuttled to the astrocyte and converted to glutamine which is then passed back into the pre-synaptic terminal to be re-synthesized as glutamate.
  44. So have a little glutamate cycle
  45. Astrocytes playing a very important role
  46. Can see the structure and function go together
  47. Why is the astrocyte engulfing the synapse?
  48. Well probably to make its job really efficient.
  49. Nervous System Regeneration [S43]
  50. Tend to crop up with various diseases like MS
  51. For some unknown reason (have some little hints about what is going on)
  52. When have damage of CNS neurons – they are not able to regenerate.
  53. In the CNS, get reactive glia cells that actually come in and form scars where the damage has happened.
  54. So have glia scar and don’t have regeneration of CNS neurons
  55. Can get regeneration in the PNS
  56. May know people that have had Bell’s Palsy where have peripheal neurons that have died and can regenerate and re-innervate their targets
  57. Seems like the difference in glia cells reaction to damage not due to the differences in CNS and PNS neurons
  58. CNS does not repair as far as we know but PNS glia cells can repair
  59. Glia Versus Neurons – Difference? [S44]
  60. One of the most important things of glia cells compared to neurons
  61. Neurons are excitatory
  62. Glia cells do have ion channels, but the general principle is that the neurons are excitable and produce regenerative signals called action potentials.
  63. Glia cells do not do this
  64. In that way, all the functions of glia cells are supportive to the neuron being able to make excitatory signals.
  65. Don’t worry about this figure – basically showin that glia cells are not excitable.
  66. Will be talking a little about ionic currents down the road – but not right now!

[End 42 min]