Lesson 13 Cont 12 5 Action Potential Action Potentials Propagated changes in membrane potential Affect an entire excitable membrane Link graded potentials at cell body with motor end plate actions 2015 Pearson Education Inc 12 5 Action Potential Initiating Action Potential Initial stimulus A graded depolarization of axon hillock large enough 10 to 15 mV to change resting potential 70 mV to threshold level of voltage gated sodium channels 60 to 55 mV 2015 Pearson Education Inc 12 5 Action Potential Initiating Action Potential All or none principle If a stimulus exceeds threshold amount Action potential is triggered The action potential is the same no matter how large the stimulus 2015 Pearson Education Inc Figure 12 14 Generation of an Action Potential Part 3 of 9 RESTING POTENTIAL 70 mV The axolemma contains both voltage gated sodium channels and voltage gated potassium channels that are closed when the membrane is at the resting potential KEY Sodium ion Potassium ion 12 5 Action Potential Four Steps in the Generation of Action Potentials Step 1 Depolarization to threshold Step 2 Activation of Na channels Step 3 Inactivation of Na channels and activation of K channels Step 4 Return to normal permeability 2015 Pearson Education Inc 12 5 Action Potential Step 1 Depolarization to Threshold Step 2 Activation of Na Channels Rapid depolarization Voltage gated Na channels open allowing net flow of Na ions into cytoplasm Inner membrane of that area changes from negative to positive 2015 Pearson Education Inc Figure 12 14 Generation of an Action Potential 1 Depolarization to Threshold 60 mV Local current The stimulus that initiates an action potential is a graded depolarization large enough to open voltage gated sodium channels The opening of the channels occurs at the membrane potential known as the threshold KEY Sodium ion Potassium ion 2 Activation of Sodium Channels and Rapid Depolarization 10 mV When the sodium channel activation gates open the plasma membrane becomes much more permeable to Na Driven by the large electrochemical gradient sodium ions rush into the cytoplasm and rapid depolarization occurs The inner membrane surface now contains more positive ions than negative ones and the membrane potential has changed from 60 mV to a positive value KEY Sodium ion Potassium ion 12 5 Action Potential Step 3 Inactivation of Na Channels and Activation of K Channels At 30 mV Inactivation gates close Na channel inactivation K channels open net flow of K out of cell Repolarization begins 2015 Pearson Education Inc Figure 12 14 Generation of an Action Potential Part 6 of 9 3 Inactivation of Sodium Channels and Activation of Potassium Channels 30 mV As the membrane potential approaches 30 mV the inactivation gates of the voltage gated sodium channels close This step is known as sodium channel inactivation and it coincides with the opening of voltage gated potassium channels Positively charged potassium ions move out of the cytosol shifting the membrane potential back toward resting levels Repolarization now begins KEY Sodium ion Potassium ion 12 5 Action Potential Step 4 Return to Normal Permeability K channels begin to close When membrane reaches normal resting potential 70 mV K channels finish closing Membrane is hyperpolarized to 90 mV Membrane potential returns to resting level Action potential is over 2015 Pearson Education Inc Figure 12 14 Generation of an Action Potential Part 7 of 9 4 Closing of Potassium Channels 90 mV The voltage gated sodium channels remain inactivated until the membrane has repolarized to near threshold levels At this time they regain their normal status closed but capable of opening The voltage gated potassium channels begin closing as the membrane reaches the normal resting potential about 70 mV Until all these potassium channels have closed potassium ions continue to leave the cell This produces a brief hyperpolarization KEY Sodium ion Potassium ion 12 5 Action Potential The Refractory Period The time period From beginning of action potential to return to resting state during which membrane will not respond normally to additional stimuli 2015 Pearson Education Inc 12 5 Action Potential Absolute Refractory Period Sodium channels open or inactivated No action potential possible not even with a very strong stimulus Relative Refractory Period Membrane potential almost normal Very large stimulus can initiate action potential 2015 Pearson Education Inc 12 5 Action Potential Powering the Sodium Potassium Exchange Pump To maintain concentration gradients of Na and K over time Requires energy 1 ATP for each 2 K 3 Na exchange Without ATP Na and K concentration gradient would disappear very slowly after tens of thousands of action potentials Neurons stop functioning 2015 Pearson Education Inc Figure 12 14 Generation of an Action Potential Part 9 of 9 ABSOLUTE REFRACTORY PERIOD RELATIVE REFRACTORY PERIOD 3 Sodium channels close voltage gated potassium channels open and potassium ions move out of the cell Repolarization begins D E P O L A R I Z A T I O N R E P O L A R I Z A T I O N 2 Voltage gated sodium channels open and sodium ions move into the cell The membrane potential rises to 30 mV Potassium channels close and both sodium and potassium channels return to their normal states 4 30 0 40 60 70 V m l a ti n e t o p e n a r b m e M Resting potential Threshold 1 A graded depolarization brings an area of excitable membrane to threshold 60 mV During the absolute refractory period the membrane cannot respond to further stimulation During the relative refractory period the membrane can respond only to a larger than normal stimulus 0 Time msec 1 2 12 5 Action Potential Propagation of Action Potentials Propagation Moves action potentials generated in axon hillock along entire length of axon towards the axon terminal s Two methods of propagating action potentials Continuous propagation unmyelinated axons Saltatory propagation myelinated axons 1 2 2015 Pearson Education Inc 12 5 Action Potential Continuous Propagation action potentials along an unmyelinated axon Affects one segment of axon at a time Steps in propagation Step 1 Action potential in segment 1 Depolarizes membrane to 30 mV Local current is produced Step 2 Depolarizes second segment to threshold Second segment develops action potential 2015 Pearson Education Inc Figure 12 15 Propagation of an Action Potential As an action potential develops at the initial segment the membrane potential at this site depolarizes to 30
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