Nerve signaling


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Nerve signaling

  1. 1. Nerve Signaling Membrane Potential Action potential Synapse NeurotransmitterChapter 48 page 1026-1038
  2. 2. Membrane Potential Voltage (V)  difference in electrical potential (charge separation)  difference in the amount of energy in charged ions (E/Q) between two points Membrane potential (Vm): voltage across a membrane  Outside of nerve cell: excess of cations (+)  Inside of nerve cell: excess anions (-)
  3. 3. Measuring membrane potential Voltmeter Vm = Vin – Vout  Vm: membrane potential  Vin: potential on inside of cell  Vout: potential on outside of cell By convention, Vout is defined as zero Resting potential (VR): membrane potential of a neuron at rest (–70mV)
  4. 4. Maintenance of RestingPotential1. Ion distribution: large pool of negatively charged molecules (e.g. proteins) inside the neuron2. Membrane permeability: more permeable to K+ (efflux) than Na+ (influx)3. Na+/K+ pump: moves 3 Na+ out for every 2 K+ in
  5. 5. 1. Ion Distribution Large internal pool of negatively charged molecules: proteins, amino acids, sulfate, phosp hate etc. large molecules that cannot cross membrane
  6. 6. 2. Membrane Permeability Charged ions cannot directly diffuse across cell membrane (lipid, nonpolar) Transmembrane proteins regulate movement of ions  Ion Channels: facilitated diffusion (passive transport)  Pumps: active transport
  7. 7. Passive transport of ions Dependent on electrochemical gradient Chemical gradient: concentration of the ion  chemical force: movement from high to low ion concentration Electrical gradient: relative electrical charge  electrical force: movement of positive ion to area of negative ion concentration and vice versa (membrane potential)
  8. 8. Equilibrium potential (Ex) Potential at which there is no net movement of an ion (at equilibrium)
  9. 9. Establishing Equilibrium withK+ If assume cell only permeable to K+ Chemical force drive K+ diffusion out of cell Inside cell becomes more negative Electrical force “pull” K+ back into cell Equilibrium when chemical and electrical forces are:  in opposite directions  equal in magnitude
  10. 10. Establishing Equilibrium withK+ If only permeable to K+:  EK = –85 mV  more negative than resting potential of –70mV Thus, must involve movement of some cations into the cell
  11. 11. Establishing Equilibrium withNa+ Na+ permeability  Na+ chemical force: influx  Na+ electrical force: influx
  12. 12. Establishing Equilibrium withK+ and Na+ Resting potential maintained with movement of both K+ and Na+ But to maintain –70mV, neuron has more K+ efflux than Na+ influx
  13. 13. Establishing Equilibrium withK+ and Na+ Neuron is more permeable to K+ than Na+ More opened K+ channels than Na+ channels
  14. 14. Animation: Resting MembranePotential content/electricalsignaling.html
  15. 15. Establishing Steady State Overtime if cell left unchecked:  influx of Na+ makes cell less negative  Drives steady efflux of K+  Concentration gradient dissipates This doesn’t happen. If it does, you’re dead.  Want to keep the gradient and avoid equilibrium
  16. 16. 3. Na+/K+ pump use ATP to drive active transport 3 Na+ out of cell, 2 K+ into cell maintain ionic gradients, restoring steady state
  17. 17. Animation: Na+/K+ pump http://highered.mcgraw-
  18. 18. Types of ion channels Ungated: open all the time Voltage-gated: open or close in response to changes in membrane potential  Examples: K+ and Na+ channels in axons Chemically-gated: open or close in response to chemicals (i.e. neurotransmitters)  Examples: receptors on dendrites are channels that open when neurotransmitters bind
  19. 19. Polarization Hyperpolarization: an increase in voltage across the membrane  More negative  E.g. K+ outflow, Cl- inflow Depolarization: reduction in voltage across the membrane  Less negative, more positive  E.g. increased Na+ inflow
  20. 20. Graded Potentials
  21. 21. Graded Potential Magnitude of polarization depends on the strength of the stimulus E.g. larger stimulus opens more channels, increasing the cells permeability for that ion, producing a larger change in membrane potential
  22. 22. Characteristic of a gradedpotential Decremental: magnitude decays/degenerates as it spreads
  23. 23. Threshold Potential The potential at which an action potential occurs Around –50 to –55 mV After threshold is reached:  Stimulus intensity plays no role in the magnitude  No longer graded potential but an action potential
  24. 24. Action Potential Only generated in the axon Trigger zone: region on axon in which the first action potential occurs An example of positive feedback: depolarization to threshold potential triggers a larger depolarization to action potential
  25. 25. Action Potential: All-or-nothing Non-graded, all-or- none event Magnitude of action potential is independent of stimulus strength once threshold is reached Amplitude (magnitude) of all action potentials is constant
  26. 26. Action Potential: All-or-nothing Question: So how is stimulus strength translated into an action potential if the stimulus strength can’t change the magnitude?
  27. 27. FrequencyCoding Stimulus strength & duration correlates to frequency of action potential
  28. 28. Graded & Action Potentials
  29. 29. Phases of action potential Rapid depolarization Rapid repolarization Undershoot Return to resting
  30. 30.  Step 1: Resting State.Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
  31. 31.  Step 2: Threshold.Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
  32. 32.  Step 3: Depolarization phase of the action potential.Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
  33. 33.  Step 4: Repolarizing phase of the action potential.Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
  34. 34.  Step 5: Undershoot.Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
  35. 35. Permeability of ions during anaction potential
  36. 36. Gates in Action PotentialGates Speed Resting Upon depolarizationK Slow ClosedNa activation Rapid ClosedNa inactivation Slow Open
  37. 37. Gates in Action PotentialGates Speed Resting Upon depolarizationK Slow Closed OpensNa activation Rapid Closed OpensNa inactivation Slow Open Closes
  38. 38. Mechanism of action potential Na+ Na+ K+ gate activation inactivation Membrane Phases (slow) potential Gate (fast) Gate (slow) Resting Closed Closed Open -70 mV Some Threshold Closed Open -50 to –55 mV open Opening ClosingDepolarization All open +35 mV slowly slowlyRepolarization All open Open All Closed -70 mV Closing Undershoot Closed Closed -75 mV slowly
  39. 39. Animation: Action Potential npotential.html n_potential.html (unmyelinated axon)
  40. 40. Propagation Action potential “travels” by repeated regeneration along axon Depolarization by influx of Na depolarizes neighboring region above threshold Unidirectional
  41. 41. Propagation Why can’t the action potential be propagated backwards (bidirectional)? Recall:  Undershoot  Na inactivation gate
  42. 42. Refractory Period Period when neuron is insensitive to depolarization Occurs during undershoot when both Na gates are closed Can only respond to another stimulus when Na inactivation is reopened Prevents action potential from moving backwards Limits maximum frequency with which action potential can be generated
  43. 43. Unmyelinated Conduction
  44. 44. Factors affecting speed ofconduction Axon diameter  Larger  faster  Due to less resistance Myelination  Voltage-gated ion channels only in nodes of Ranvier (unmyelinated region)  Axon only exposed to ions in ECF at nodes  No action potential in regions between nodes
  45. 45. Saltatory Conduction Current generated by action potential at a node “leaps” to next node to stimulate new action potential
  46. 46. Saltatory Conduction Na+ and K+ exchange can only occur where the axons are exposed to the extracellular fluid. allows for faster signal conduction along the axon
  47. 47. Myelinated Conduction
  48. 48. Animation: Conduction /actionp.html
  49. 49. Synapse A cell junction that controls communication between a neuron and another cell 3 main types of connections where synapse would be observed:  Sensory input (afferent pathway): sensory receptor & sensory neuron  Integration: neuron & neuron (interneuron)  Motor output (efferent pathway): motor neuron & muscle cells, neurons & glands
  50. 50. Electrical Synapse Current from presynaptic cell flows directly to the postsynaptic cell through gap junctions Gap junctions: pores, channels between adjacent cells, through which ions and other small molecules can pass Direct communication through physical connection
  51. 51. Electrical Synapse Found in giant axons in crustaceans Not common in vertebrates Advantage: rapid transmission of action potential from cell to cell Disadvantage: more difficult to regulate
  52. 52. Anatomy of a chemicalsynapse
  53. 53. Synaptic cleft narrow gap separating pre- and postsynaptic cell Thus, cells are not electrically coupled Signal conversion: electrical  chemical  electrical
  54. 54. Synaptic Vesicles sacs at the synaptic terminal that contains neurotransmitters Neurotransmitter: substance released by presynaptic cell as an intercellular messenger into synaptic cleft Each neuron usually secretes only one type of neurotransmitter
  55. 55. Chemical Synapsea. Presynaptic membrane depolarized by action potentialb. Voltage-gated Ca2+ channels openc. Ca2+ enters celld. Stimulates exocytosis of synaptic vesicles
  56. 56. Effect of changing Ca Levels Voltage-gated Ca2+ channels close soon after opening Ca2+ actively transported out of axon terminal bringing it back to resting level But if another action potential arrives soon after previous, then Ca2+ levels continue to increase Frequency of AP  [Ca]  [NT]
  57. 57. Chemical Synapsereleases NT receptors for NT Presynaptic neuron Postsynaptic neuron (axon) (dendrite)
  58. 58. NeurotransmittersNT Type Description ExamplesAcetylcholine Most common Acts on motor neurons(Ach) and skeletal musclesAmino acids Affects mainly the CNS Glutamate (+ in CNS), GABA (- in brain), glycine (- in spinal cord)Neuropeptides Short chain of amino acids Endorphins (- pain), Substance P (+ pain)Biogenic amines Derived from amino acids Catecholamines (tyr), serotonin (trp), dopamine (phe/tyr)Gaseous signals Not stored in synaptic Nitric oxide vesicles, made as needed
  59. 59. NT binds to receptor Each type of receptor on the post-synaptic membrane specifically recognize one neurotransmitter Receptors are part of a gated ion channel When NT binds, gates open allowing in a specific ion (i.e. Na, K, Cl) Thus these channels are chemically gated
  60. 60. Postsynaptic Potentials (PSP) Start at the dendrites and progress to the axon hillock Are graded potentials  vary in magnitude with NT  Decremental (degenerates with distance) 2 types:  Excitatory (EPSP)  Inhibitory (IPSP)
  61. 61. Excitatory postsynapticpotential (EPSP) Excitatory synapse:  Increase chance of generating an AP  Depolarization  Net flow of positive charge into cell NT binding to receptor opens gated Na+ and K+ channels  Diffusion down electrochemical gradients: Na+ enters, K+ leaves  But force of Na+ stronger than K+ For an AP to occur, the EPSP must be strong enough to reach and depolarize to threshold potential at the axon hillock
  62. 62. Inhibitory PostsynapticPotential (IPSP) Inhibitory synapse:  Decrease chance of generating an AP  Hyperpolarization  Net flow of negative charge into cell NT binding to receptor opens gated K+ and Cl- channels  Diffusion down electrochemical gradients: K+ leaves, Cl- enters
  63. 63. Summation Additive effect of post-synaptic potentials (PSP)  Can result in both stimulatory effect (depolarization) or inhibitory (hyperpolarization)  Occurs at the axon hillock (neuron’s integrating center) Example: Single EPSP usually not strong enough to trigger AP (same application to IPSP)  Several EPSP working simultaneously on the same postsynaptic cell can have cumulative impact IPSP and EPSP can counter each other’s effects
  64. 64. Spatial Summation
  65. 65. Spatial Summation Several different synaptic terminals (usually from different presynaptic cells) stimulating the same postsynaptic cell at the same time having an additive effect on membrane potential
  66. 66. Temporal Summation
  67. 67. Temporal Summation Chemical transmission from one or more synaptic terminal occurring close together in time affecting membrane on postsynaptic membrane before the voltage can return to resting potential
  68. 68. Neurotransmitter Removal Process:  enzymatic degradation  reuptake into presynaptic cell  diffusion out of the cleft Consequence:  ensures effect of NT is brief and precise  allows transmission of the next action potential
  69. 69. Communication across asynapse 1. Action potential 2. Calcium channels open 3. Calcium enters cell and triggers release of NT by exocytosis 4. NT diffuse across synaptic cleft 5. Binds to receptor on postsynaptic cell causing a response by postsynaptic cell 6. Removal by enzymatic degradation 7. Removal by reuptake by presynaptic cell 8. Removal by diffusion away from synaptic cleft
  70. 70. Animations: Synapse http://highered.mcgraw- mations/content/synapse.html mations/content/synaptictransmission.html http://highered.mcgraw- (Ach) f