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Electrical Signaling

Electrical Signaling. Resting Membrane Potential Action Potential Post-Synaptic Potential. Signal Processing. Neurons send and receive receive electrical & chemical signals. Resting Membrane Potential. Difference in electrical charge across membrane when neuron is at rest.

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Electrical Signaling

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  1. Electrical Signaling Resting Membrane Potential Action Potential Post-Synaptic Potential

  2. Signal Processing Neurons send and receive receive electrical & chemical signals

  3. Resting Membrane Potential Difference in electrical charge across membrane when neuron is at rest

  4. Hodgekin & Huxley ( 1952) • Developed a mathematical model for how electrical signals ( action potentials) are produced and transmitted down the axon. • Describes flow of ions across membrane

  5. Hodgekin & Huxley Expt

  6. Results • Inside of neuron negatively charged • Differential distribution of ions ( electrically charged particles)

  7. Background & Vocabulary of Electrical Signals • Polarized: difference in electrical charge between the inside and the outside of the cell • Ions—electrically charged molecules • Anions are negatively charged • Cations are positively charged • Ions are dissolved in intracellular fluid, or cytoplasm, and are separated from the extracellular fluid by the cell membrane, a lipid bilayer

  8. Ion: Electrically charged molecule

  9. Electrons: negatively charged • Proton: positively charged • Total number of electrons is not equal to number of protons

  10. Ions differentially distributed across cell membrane

  11. Figure 3.2The Distribution of Ions Inside and Outside of aNeuron

  12. Reasons for Differential Distribution of Ions • Selective permeability: • Diffusion • Electrostatic Force • Sodium-potassium pump

  13. 1. Selective Permeability • Potassium ions (K+) move freely • Sodium Ions kept outside • WHY??? • Ion channels: Open for K+, Closed for NA+

  14. Figure 3.2The Distribution of Ions Inside and Outside of aNeuron

  15. Ion Channels Regulate Flow of Ions • Gated Channels: Sodium Ions ( Na+) kept outside • Selectively Permeable Channels: Potassium ions ( K+) concentrated inside (we’ll see why soon) • Nodes Of Ranvier: location of Channels on unmyelinated parts of axon

  16. 2. Diffusion • Ions move from a high concentration to a low concentration in order to create equilibrium

  17. Diffusion & Ions • Na+: • Concentrated outside • Can’t diffuse inside because channels are closed • K+: • Concentrated inside • Moves outside, but other force moves K+ back in

  18. Figure 3.3Ionic Forces Underlying Electrical Signaling in Neurons

  19. 3. Electrostatic Pressure • Ions of similar charges repel • Ions of opposite charge attract

  20. Electrostatic Force & Ions • Na+: • attracted to the inside of the neuron • but can’t enter due to closed channels • K +: • are attracted to inside of neuron • can enter freely

  21. 4. Sodium-Potassium Pump • “Leaky” channels: permit a few Na+ ions to enter • Na+/K+ Pump: exchanges 3 Na+ ions from inside for them for 2 K+ ions from outside

  22. Resting Membrane Potential Summary • Polarization: inside of axon is negative relative to outside • Na+ ions located outside ; K+ ions inside • Na+ ions under pressure to move inside due to: • Diffusion- concentration gradient • Electrostatic force- attraction to oppositely charged area • But, Na+ ions kept outside by: • Selectively permeable membrane ( Na+ gates closed) • Sodium-Potassium-Pump ( actively pumps out Na+ ions) • Sets Up Potential Energy, like battery

  23. Action Potential Rapid reversal of RMP, that is transmitted down axon towards next neuron

  24. Changes in Electrical Potential Lead to Action Potential • Neurons receive chemical signals from nearby neurons • Excitatorysignals depolarize the cell membrane (i.e., reduce polarization) • Make inside less negative • Inhibitorysignals hyperpolarize the cell (i.e., increase polarization) • Make inside more negative

  25. All inputs integrated at Axon Hillock

  26. If Net Inputs Inhibitory: No Action Potential

  27. If Net Inputs Excitatory, Action Potential Triggered

  28. Action Potential • rapid reversal of inside potential conveyed down the axon to the next neuron • Triggered when membrane reaches the threshold • Critical level of depolarization—about –40 mV • The membrane potential reverses and the inside of the cell becomes positive

  29. Figure 3.5The Effects of Hyperpolarizing and Depolarizing Stimuli on a Neuron

  30. Local vs. Action Potentials

  31. Local potential • Graded Potential: Size of change in electrical potential coded by strength of input stimulus • The greater the stimulus, the greater the change in membrane potential • Size diminishes as it moves away from the point of stimulation • Occurs at dendrites

  32. Action Potential • All-or-none Potential: • The neuron fires at full amplitude or not at all—the size (amplitude) is independent of stimulus size • Rate Law: Information is encoded in changes in the number of action potentials—with increased stimulus strength more are produced, but the size is the same

  33. Steps in Action Potential • Depolarization: gradual entry of positive ions due to input from other neurons • Threshold: critical level of depolarization; causes all voltage-gated Na+ gates to open • Sodium-Influx: Na+ ions rush in • pushed by diffusion & Electrostatic forces • Membrane potential inside reverses; becomes positively charged • Potassium-Efflux: K+ ion channels open, allowing K+ to leave

  34. Peak: point at which Na+ channels close • Repolarization: K+ still leaving axon, leads to membrane potential returning to negative state • Hyperpolarization: Potassium channels close: membrane slightly more negative than RMP • Sodium-Potassium Pump Kicks in: return to RMP

  35. Action Potentials Spread Along Axon • Propagation: process in which Depolarization travels along an axon like a wave • Uni-directional: Action potentials always move away from the cell body to the terminal buttons. • Saltatory Conduction: Action potential regenerated at node of ranvier- rapid conduction

  36. Axons(Part 2)

  37. Multiple Sclerosis

  38. What Happens at End of Axon?

  39. Chemical Signaling Synapse Neurotransmitters Neuropharmacology

  40. Electrical Signals into Chemical Signals

  41. Parts of a Synapse • Presynaptic membrane—on the axon terminal of the presynaptic neuron • Synaptic cleft—a gap that separates the membranes • Postsynaptic membrane—on the dendrite or cell body of the postsynaptic neuron

  42. Steps in Synaptic Transmission • Action potential arrives at the presynaptic axon terminal • Ca++ Influx: Voltage-gated calcium channels in the terminal membrane open and calcium ions (Ca2+) enter • Exocytosis: Release of Neurotransmitter • Calcium ions cause synaptic vesicles fuse with the presynaptic membrane • Vesicles rupture, releasing transmitter into the synaptic cleft

  43. Receptor Binding: Transmitters bind to specific postsynaptic receptor molecules, causing the opening of ion channels and leading to an EPSP or IPSP • Post-synaptic Potential: EPSPs or IPSPs spread toward the postsynaptic axon hillock—if threshold is reached, an action potential will occur

  44. Termination of signal: Synaptic transmission is rapidly stopped • Re-uptake • Enzymatic Breakdown • Pre-synaptic modulation: Transmitter may activate presynaptic receptors, decreasing transmitter release

  45. Post-synaptic Potentials

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