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Chapter 8a

Chapter 8a. Neurons: Cellular and Network Properties. About this Chapter. Organization of Nervous System Cells of the nervous system Electrical signals in neurons Cell-to-cell communication in the nervous system Integration of neural information transfer. Physiologically :

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Chapter 8a

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  1. Chapter 8a Neurons: Cellular and Network Properties

  2. About this Chapter • Organization of Nervous System • Cells of the nervous system • Electrical signals in neurons • Cell-to-cell communication in the nervous system • Integration of neural information transfer

  3. Physiologically: Afferent (Sensory) receptors Efferent (Motor) somatic autonomic Sympathetic Parasympathetic Anatomically: Central: Brain Spinal Cord Peripheral: Nerves Receptors Ganglia

  4. Organization of the Nervous System Figure 8-1

  5. The Neuron

  6. Model Neuron Input signal Dendrites • Dendrites receive incoming signals; axons carry outgoing information Integration Cellbody Nucleus Axon hillock Axon (initialsegment) Myelinsheath Presynapticaxon terminal Outputsignal Synapticcleft Synapse Postsynapticdendrite Postsynapticneuron Figure 8-2

  7. Anatomic and Functional Categories of Neurons Sensory neurons Neurons forsmell and vision Somatic senses • Neurons can be classified according to function or structure Dendrites Neurons can be categorized by the number of processes and function Schwanncell Axon Pseudounipolar Bipolar (a) (b) Figure 8-3a-b

  8. Anatomic and Functional Categories of Neurons Interneurons of CNS Axon Dendrites Axon Anaxonic Multipolar (c) (d) Figure 8-3c-d

  9. Anatomic and Functional Categories of Neurons Efferent neuron Dendrites Axon Axonterminal Multipolar (e) Figure 8-3e

  10. Cells of NS: Glial Cells and Their Functions GLIAL CELLS • Glial cells provide physical and biochemical support for neurons. are found in Peripheral nervous system contains Satellitecells Schwann cells forms Myelin sheaths secrete Supportcell bodies Neurotrophicfactors (b) Glial cells and their functions Figure 8-5b (1 of 2)

  11. Cells of NS: Glial Cells and Their Functions GLIAL CELLS are found in Central nervous system contains Microglia (modifiedimmune cells) Ependymalcells Oligodendrocytes Astrocytes forms act as Myelin sheaths Scavengers create provide help form take up secrete Blood-brainbarrier Source ofneuralstem cells Barriersbetweencompartments Substrates forATP production Neurotrophicfactors K+, water,neurotransmitters (b) Glial cells and their functions Figure 8-5b (2 of 2)

  12. Amyotrophic Lateral sclerosis (ALS • ALS has been linked to a mutation on the gene coding for superoxide dismutase. • Microglia use reactive oxygen species (superoxides) to destroy, may lead to oxidative stress and neurodegeneration • A-myo-trophic comes from the Greek language. "A" means no or negative. "Myo" refers to muscle, and "Trophic" means nourishment–"No muscle nourishment." When a muscle has no nourishment, it "atrophies" or wastes away.

  13. Cells of NS: Glial Cells and Their Functions Ependymalcell Interneurons Microglia Capillary Astrocyte Myelin(cut) Axon Section of spinal cord Node Oligodendrocyte (a) Glial cells of the central nervous system Figure 8-5a

  14. Nucleus Schwann cell wraps aroundthe axon many times. Axon Schwann cell nucleusis pushed to outsideof myelin sheath. Myelin consistsof multiple layersof cell membrane. (a) Myelin formation in theperipheral nervous system Cells of NS: Schwann Cells • Sites and formation of myelin Figure 8-6a

  15. Cell body 1–1.5 mm Node of Ranvier is a section ofunmyelinated axon membranebetween two Schwann cells. Schwann cell nucleusis pushed to outsideof myelin sheath. Myelin consistsof multiple layersof cell membrane. Axon (b) Each Schwann cell forms myelin arounda small segment of one axon. Cells of NS: Schwann Cells Figure 8-6b

  16. Multiple Sclerosis Nystagmus - involuntary eye movement

  17. Electrical Signals: Nernst Equation • Describes the membrane potential that a single ion would produce if the membrane were permeable to only that ion • Membrane potential is influenced by • Concentration gradient of ions • Membrane permeability to those ions

  18. Electrical Signals: GHK Equation • Predicts membrane potential that results from the contribution of all ions that can cross the membrane

  19. Electrical Signals: Ion Movement • Resting membrane potential determined primarily by • K+ concentration gradient leak channels open • Cell’s resting permeability to K+, Na+, and Cl– • Gated channels control ion permeability • Mechanically gated • Pressure or stretch • Chemical gated • Ligands, NTs • Voltage gated • Membrane potential change • Threshold voltage varies from one channel type to another (minimum to open or close)

  20. Electrical Signals: Channel Permeability Table 8-3

  21. Electrical Signals: Graded Potentials • Graded potentials decrease in strength as they spread out from the point of origin Figure 8-7

  22. Electrical Signals: Graded Potentials • Subthreshold and (supra)threshold graded potentials in a neuron Figure 8-8a

  23. Electrical Signals: Graded Potentials Figure 8-8b

  24. 5 4 5 6 1 2 3 7 1 8 2 3 4 6 9 7 8 9 Electrical Signals: Action Potentials Resting membrane potential Depolarizing stimulus Membrane depolarizes to threshold.Voltage-gated Na+ channels open quickly and Na+ enters cell. Voltage-gatedK+ channels begin to open slowly. Rapid Na+ entry depolarizes cell. Na+ channels close and slowerK+ channels open. K+ moves from cell to extracellularfluid. K+ channels remain open andadditional K+ leaves cell, hyperpolarizing it. Voltage-gated K+ channels close,less K+ leaks out of the cell. Cell returns to resting ion permeabilityand resting membrane potential. Threshold Figure 8-9 (1 of 2)

  25. Electrical Signals: Action Potentials Figure 8-9 (2 of 2)

  26. Electrical Signals: Voltage-Gated Na+ Channels • Na+ channels have two gates: activation and inactivation gates Na+ ECF ICF Activationgate Inactivationgate (a) At the resting membrane potential, the activation gatecloses the channel. Figure 8-10a

  27. Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10b

  28. Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10c

  29. Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10d

  30. Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10e

  31. Electrical Signals: Ion Movement During an Action Potential Figure 8-11

  32. Electrical Signals: Refractory Periods Bothchannelsclosed Na+channelsopen Bothchannelsclosed Na+ channels close andK+ channels open Na+ channels reset to original positionwhile K+ channels remain open Na+ Na+ and K+channels K+ K+ K+ Absolute refractory period Relative refractory period Action potential Na+ Ion permeability Membrane potential (mV) K+ High High Increasing Excitability Zero Time (msec) Figure 8-12

  33. Electrical Signals: Coding for Stimulus Intensity • Na+ and K+ [ ]’s change very little • 1 in 100000 K+ leave to shift from +30 to -70mVolts • Na/K pump will re-establish, but neuron without pump can still 1000x Figure 8-13a

  34. Electrical Signals: Coding for Stimulus Intensity Figure 8-13b

  35. Electrical Signals: Trigger Zone • Graded potential enters trigger zone • Voltage-gated Na+ channels open and Na+ enters axon • Positive charge spreads along adjacent sections of axon by local current flow • Local current flow causes new section of the membrane to depolarize • The refractory period prevents backward conduction; loss of K+ repolarizes the membrane

  36. Electrical Signals: Trigger Zone Figure 8-14

  37. Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. 4 Local current flow from the active region causes new sections of the membrane to depolarize. 5 The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane. Inactive region Refractory region Active region Figure 8-15

  38. Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon Figure 8-15, step 1

  39. Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. Figure 8-15, steps 1–2

  40. Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. Figure 8-15, steps 1–3

  41. Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. 4 Local current flow from the active region causes new sections of the membrane to depolarize. Inactive region Refractory region Active region Figure 8-15, steps 1–4

  42. Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. 4 Local current flow from the active region causes new sections of the membrane to depolarize. 5 The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane. Inactive region Refractory region Active region Figure 8-15, steps 1–5

  43. Electrical Signals: Action Potentials Along an Axon Figure 8-16b

  44. Electrical Signals: Speed of Action Potential • Speed of action potential in neuron influenced by • Diameter of axon • Larger axons are faster • Resistance of axon membrane to ion leakage out of the cell • Myelinated axons are faster

  45. Electrical Signals: Myelinated Axons • Saltatory conduction Figure 8-18a

  46. Electrical Signals: Myelinated Axons Figure 8-18b

  47. Electrical Signals: Chemical Factors • Effect of extracellular potassium concentration of the excitability of neurons Figure 8-19a

  48. Electrical Signals: Chemical Factors Figure 8-19b

  49. Electrical Signals: Chemical Factors Figure 8-19c

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