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Chapter 2

Chapter 2. Structure and functions of cells of the nervous system. Cells of the Nervous System. Supporting Cells Glia (glial cells) - Supporting cells that “glue” the nervous system together; 3 most important types are: Astrocytes Oligodendrocytes Microglia.

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Chapter 2

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  1. Chapter 2 Structure and functions of cells of the nervous system

  2. Cells of the Nervous System • Supporting Cells • Glia (glial cells) - Supporting cells that “glue” the nervous system together; 3 most important types are: • Astrocytes • Oligodendrocytes • Microglia

  3. Summary: Things to think about • Membrane potentials • Lipid bilayer • Ion types (cations and anions contributing) • Distribution of ions across the membrane • Membrane proteins • Channels • Pumps/transporters: • Passive vs active movement of ions • Action potentials • Threshold • Temporal explanation of ion movement across the membrane.

  4. An Action Potential • Temporal and sequential importance of ion transfer across the membrane. • Dependent on voltage-gated (dependent) channels Figure 2.21

  5. Factors Influencing Conduction Velocity • Saltatory conduction • High density of Na+ V-D at Nodes of Ranvier 2 advantages of Saltatory Conduction • Economical • Much less Na+ enters cell (only at nodes of Ranvier) mush less has to be pumped out. • Speed • Conduction of APs is faster in myelinated axons because the transmission between the nodes is very fast.

  6. Communication Between Neurons

  7. Some Simple Vocab

  8. Details of a Synapse Figure 2.28

  9. The Synapse • Synaptic transmission- transmission of signal from one cell to another • Neurotransmitter • Postsynaptic potentials • Excitatory • Inhibitory Scanning electron micrograph (real) shows the synapses between nerve fibres (purple) and a nerve cell (yellow). Magnified 10,000 times. NOVA

  10. Release of Neurotransmitters

  11. Small, clear Vesicles • After synthesis, NTs are stored in vesicles (lipids). • Varying numbers of vesicles at the button • Terminal button could contain both large and small sized vesicles Large, dense core Scanning electron micrograph- nerve ending (broken) with vesicles False colour electron micrograph

  12. Small vesicles (neurotransmistters) • Synthesized in the terminal button and packaged in synaptic vesicles • Large dense core (typically neuropeptides) • Assembled in the cell body, packaged in vesicles, and then transported to the axon terminal.

  13. Vesicle and Release Proteins • Vesicle Transporters: Get substances into vesicles • Each vesicle: 1000s NT molecules • Trafficking Proteins: • Docking • Release • Recycle

  14. Vesicle Pools • Very few vesicles are docked (<1%) • Most in the reserve pool (85-90%) • Recycling pool (10-15%)

  15. Neurotransmitter Release • The arrival of an AP at the terminal opens dependent Ca2+ channels • The entry of Ca2+ causes vesicles to fuse with the terminal membrane and release their contents Exocytosis

  16. Release of Neurotransmitters Figure 2.31

  17. Release of Neurotransmitters Figure 2.31 Docked • Synaptic vesicle migrates to presynaptic membrane.

  18. Release of Neurotransmitters Figure 2.31 • 2. Vesicle fuses with presynaptice membrane.

  19. Release of Neurotransmitters Figure 2.31 • 3. Neurotransmitter is released into the synaptic cleft.

  20. Vesicles After Release • Recycling of vesicle material (<1sec) • Kiss and Run (leave) • Release most NT, reseals and moves into cytoplasm to be refilled • Merge and Recycle • Vesicle fuses completely with the membrane • Bulk Endocytosis • Large pieces of the membrane fold in to reform vesicles Figure 2.33

  21. Pos Activation of Receptors I. Postsynaptic Receptors

  22. I. Postsynaptic receptors • Ligand– a molecule that binds to another • A NT is a ligand of its receptor Released NT molecules produce signals in postsynaptic neurons by binding to receptors Receptors are specific for a given NT

  23. 1) Ionotropic Receptors • Receptor that contains a binding site for a neurotransmitter and an ion channel that opens when a molecule of the neurotransmitter attaches to the binding site. Figure 2.34

  24. Ionotropic Receptors Excitatory e.g. Nicotinic (N1) receptors (depolarizes) NT binds and an associated ion channel opens or closes, causing a PSP If Na+ channels are opened, for example, an EPSP occurs If K+ or Cl- channels are opened, for example, an IPSP occurs Inhibitory e.g. BZP receptors (hyperpolarizes)

  25. 2) Metabotropic Receptors Slower variety (short cut faster than second messenger system) Actions are reliant on activation of G-proteins located in the internal membrane of the postsynaptic cell 2 basic varieties: 1) short cut 2) second messenger 2) Second messenger 1) Short cut Figure 2.35

  26. Figure 2.36 Ionic Movement During Postsynaptic Potentials

  27. Figure 2.37 1) REUPTAKE • Mediated by transporter molecules on neurons and glia • After it is taken up it may be degraded or recycled in vesicles

  28. 2) ENZYMATIC DEGRADATION • Removal at the cleft • E.g. Cholinergic synapses (ACh) • Neuromuscular junction • ED can occur in the synapse or in the cytoplasm • Used to recycle: • ACh -> choline by ACh-esterase (AChE)

  29. 3) DIFFUSION • Away from the synapse • Glia cells • Transporters for uptake

  30. II. Autoreceptors • Sensitive to neurotransmitter released by presynaptic terminal • Act as safety valve to reduce release when levels are high in synaptic cleft (autoregulation)

  31. Excitatory Post-Synaptic Potential Transmitter causes the receptor sites to open gated ion channels that permit Na+ into the cell (depolarizing event) Known as an EPSP

  32. Inhibitory Post-Synaptic Potential Transmitter causes the receptor sites to open gated ion channels that permit K+ out of the cell or Cl- into the cell (hyperpolarizing event) Known as an IPSP

  33. INTEGRATION of Input Signals Spatial Summation Temporal Summation

  34. + SPATIAL SUMMATION1. Summation of EPSPs + Two distinct synaptic inputs onto postsynaptic cell Same time EPSP + EPSP = larger EPSP Cell is depolarized

  35. - SPATIAL SUMMATION2. Summation of IPSPS Two independent inhibitory inputs Postsynaptic cell hyperpolarized -

  36. - SPATIAL SUMMATION3. Summation of EPSP and IPSP + EPSP (depolarizing) and IPSP (hyperpolarizing) input Not net change in membrane potential

  37. TEMPORAL Summation • Single synapse initiating a sequence of membrane events

  38. Presynaptic Inhibition • Axoaxonic- decreases NT released • Presynaptic facilitation can occur also (increasing NT released)

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