Action potential in a neuron part 3 voltage gated sodium channel

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  • 1. Action Potential in a Neuron. Rricketson 11.16.2012 Copyright © 2012 Seaspine. All rights reserved. 1
  • 2. THE MEMBRANE POTENTIAL-THE NA-K ATPASE PUMPThe Na+/K+-ATPase pump helps maintain membrane potential. Moves 3 sodium ions out and 2 potassium ions in, (removes 1 + charge) against concentration gradient Responsible for up to 2/3 of the neuron’s energy expenditure. 2
  • 3. THE MEMBRANE POTENTIAL-THE NA-K ATPASE PUMPMechanism (PDB ID 2B8E, 2ZKE biological assembly) K+ K+ Unphosphorylated form binds 3 intracellular Na+ ions while binding ATP Hydrolysis of ATP  ADP + Phosphorylation of the pump at a highly conserved Aspartate residue; ADP + Pi. Phosphorylated form has a low affinity for Na+  Release Pump binds 2 extracellular K+ ions  Dephosphorylation. Dephosphorylation high affinity for Na+ ions. K+ (2)  Released Na+ K+ ions are transported into the cell. Na+ Na+ 3
  • 4. THE MEMBRANE POTENTIAL-THE POTASSIUM CHANNELMechanism (PDB ID 2EKW, biological assembly)A neuron is permeable to K+Allows K+ to move from high [K+]o =5 mmolconcentration in the cytoplasm to low K+concentration in the extracellularenvironment K+ Potassium diffuses out of the cell from its higher to lower concentration (chemical force). The Electrical potential or Vm= -90 mV electrical force pulls K+ in to the cell. At equilibrium Vm= -90 mV The forces are equal and opposite K+ [K+]i =150 mmo K+ K+ 4
  • 5. THE RESTING POTENTIALResting potential (EK )refers tothe difference between thevoltage inside and outside theneuron when not propagating an Cl-impulse. The resting potential of K+the average neuron is around -70 Na+mVT The inside of the cell is 70 Na+mV less than the outside of the Na+cell. Na+ K+ Na+ K+ K+ Cl- K+
  • 6. VOLTAGE GATED SODIUM CHANNEL (NAV)Voltage Gated Na Channel(PDB ID 2EKW, biological assembly) 6 helical segments. S1-S6 S4-Voltage Sensor 4 Arg + charged residues. Depolarization phase of AP Conformational change via linker S4-S5 “P” loops between S5 and S6 form pore - charged Glu and Asp line pore and attract Na+ ions. Inactivaton Gate (S5 and S6) closes pore (repolarization) Three states:  Deactivated (closed)  Activated (open)  Inactivated (closed)- 6
  • 7. GENERATING THE ACTION POTENTIALGenerated in axon hillock and Nodes ofRanvier of neurons by special types ofvoltage gated ion channels (Na+, Ca++)embedded in a cells plasma membrane Threshold is –55 mV for Na+ channel recruitment Amplitude is independent of the amount of current (all or nothing) Rising Phase: • Rapid influx of Na+ causes a: • Depolarization of membrane potential • Inactivation of Na+ channels (ENa≈ +55 mV) Falling Phase: • K+ channel activation (repolarization) Afterhyperpolarization • K+ channels slow to close 7
  • 8. VOLTAGE GATED SODIUM CHANNEL (NAV)Interactive walk-through of the Action Potential. 8
  • 9. THANK YOU 9