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4 Today’s lecture Today’s Lecture Restingpotential Potential Resting Action potential Potential Action Ionic basis basis of of the theaction actionpotential potential A review of the Resting Membrane Potential Resting Membrane Potential Est. Cytoplasm and CSF- Seperated by Plasma membrane PRIMARILY PASSIVE POTASSIUM CHANNELS Two Natural Factors influencing the DISTRIBUTION of Ions: a. Diffusion b. Electrostatic gradient Potassium Permeable membrane Cytoplasm CSF A review of the Resting Membrane Potential Resting Membrane Potential Two Natural Factors influencing the DISTRIBUTION of Ions: a. Diffusion b. Electrostatic gradient Blue = +ive Red = - ive A review of the Resting Membrane Potential Resting Membrane Potential A review of the Resting Membrane Potential Resting Membrane Potential Four Key Ions are Unequally distributed Na+, Cl-, K+, Poly-anions such as amino acids and proteins. Small concentrations A review of the Resting Membrane Potential Resting Membrane Potential Two Channels allow for passage of ions between Cytoplasm and CSF a. Ion Channels (Passive) Namely K+ since the others are quite scarce b. Na-K ATPase pump (Active) Components of an action potential Review! Small amount of ions actually involved (10-12 mol /cm2 of Plasma membrane) -> not enough to change the concentrations significantly At the immediate boundary of the Plasma membrane RMP NOT Unique to neurons Thought experiment: If we reversed the situation? The voltage would be inverted! Components of an action potential Components of an Action Potential 1. Threshold: Minimum strength of current required 2. All or none phenomena: - Either a complete action potential that propagates along the axon or no response at all - once generated, moves along the axon without a drop or gain in amplitude 3. Always followed by a refactory period Action Potential Membrane Potential (mV) end of rising phase Na+ channels inactivate; Na+ stops entering 50 0 -50 -65 rising phase Na+ channels open; Na+ enters falling phase K+ channels open; K+ leaves threshold resting potential undershoot 0 1 2 Time (ms) Voltage Changes in neurons Hyperpolarization vs Depolarization Voltage Changes in neurons: Hyperpolarization vs Depolarization Voltage-Gated Sodium Channel The Voltage-Gated Sodium Channel gating charge + 3) channel opens Na+ 1) sensory receptor or synaptic activity Na+ Axon 2) depolarization Action Potential Rising Phase The action potential rising phase occurs because the neuronal membrane contains voltage-gated Na+ channels that open in a regenerative manner if depolarization passes the threshold level: Na+ rushes in POSITIVE FEEDBACK LOOP voltage-gated Na+ channels open BUT – It is a temporally isolated event Voltage-Gated Sodium Channel The Voltage-Gated Sodium Channel Has Three States closed open inactivated + + + + + + Axon End of Rising Phase The action potential rising phase ends because the voltage-gated Na+ channels inactivate Na+ no longer enters voltage-gated Na+ POSITIVE channels inactivate LOOP ENDS Action Potential Voltage-Gated Sodium Channel The Voltage-Gated Potassium Channel gating charge + 3) K+ channel opens, K+ leaves “Creaky” door (slow) 1) Na+ entry (rising phase) Na+ K+ Axon 2) depolarization Action Potential Action Potential voltage-gated Na+ channels refractory closed open inactivated closed + + + + + + time velocity voltage + closed + closed voltage-gated K+ channels + open closed Action Potential Action Potential Summary 1. The membrane is depolarized past threshold, by a receptor potential or synaptic potential. 2. Voltage-gated sodium channels open in a positive-feedback loop, causing the rising phase. 3. The sodium channels inactivate, ending the rising phase and initiating the refractory period. 4. Voltage-gated potassium channels open, causing the falling phase and the undershoot (after-hyperpolarization). 5. The sodium channels close, ending the refractory period. 6. The potassium channels close, ending the undershoot and restoring the membrane to its resting potential.