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Graded Potentials Time Taking care of business: 1.) Meet with me for some points :O If you set up a time to meet with me over the next two weeks, I will give you 15% of the points you missed on exam 1 back. Visit https://bio072.acuityscheduling.com/ to set up appointment 2.) Discussion on Wednesday: Submit notes on videos and textbook readings before 9am. Big Picture Look at The Nervous System Signal Processing Signal Transmission Signal Output Sensation Response Nervous System: Sensors on the periphery Central Integrating Center Effectors Na+/K+ Pump K+ Leak Channel Potential Over Time Plot Okay, so how do we “wake up” a cell at rest Depolarization and send signals via neurons to/from brain? Threshold Potential Action Potential Na+ Enters K+ Exits Voltage Sensors Summary Membrane Potential Plot: Resting Potential Resting Potential Na+/K+ Pump + + Potential Over _ Time Plot More Positive Depolarization Charge Inside + Threshold Potential 0 Action Potential Time Na+ Enters More Charge Difference Membrane Potential: Membrane Potential: Negative K+ Exits Across Cell Membrane Charge Inside Voltage Sensors Summary Step 1: Resting Potential Depolarization/graded potential Na+/K+ Pump + + Potential Over _ Time Plot More + Positive Depolarization Charge Inside + + Threshold Potential 0 Action Potential Time Na+ Enters More Potential Difference Potential Negative K+ Exits Across Cell Membrane Charge Inside Voltage Sensors Summary Depolarization: making cell less “polarized”. Making inside more + Step 1: Resting Potential Depolarization/graded potential Na+/K+ Pump + + Potential Over _ Time Plot More + Positive Depolarization Charge + Inside + Threshold Potential 0 Action Potential Time Na+ Enters More Potential Difference Potential Negative K+ Exits Across Cell Membrane Charge Inside More + charge in = more Voltage Sensors depolarization Summary Membrane Potential Plot: Resting Potential Hyperpolarization Na+/K+ Pump Potential Over _ Time Plot More - - Positive Depolarization Charge Inside Threshold Potential 0 Action Potential Time Na+ Enters More Charge Difference Negative Membrane Potential: Membrane Potential: K+ Exits Cell Membrane Across Charge Inside Voltage Sensors Summary Hyperpolarization: making cell more “polarized”. Getting more - below RMP Membrane Potential Plot: Resting Potential Hyperpolarization Na+/K+ Pump Potential Over _ Time Plot - More - Positive Depolarization Charge Inside Threshold Potential 0 Action Potential Time Na+ Enters More Charge Difference Negative Membrane Potential: Membrane Potential: K+ Exits Cell Membrane Across Charge Inside Voltage Sensors Summary These are examples of “graded potentials” What triggers the initial graded potential of the in the first place? How do we get to threshold? Dendrites Cell Function We Want: Cell Body Receive info well Axon Transmit info well and FAST Axon Terminals Capable of operating over long distance Neurotransmitters: Released by neurons to affect target cells Many other stimuli (temperature, pressure, light) can stimulate neurons. Not just neurotransmitters. Figure 8.7a ESSENTIALS – Graded Potentials Influx of positive charge creates “graded potential” Graded potential acts like a wave generated by a rock in a pond: 1.) Wave gets weaker as it emanates from the source 2.) Bigger the rock/stimulus, the bigger the initial wave Figure 8.7c ESSENTIALS – Graded Potentials Above threshold Graded Potential A stronger stimulus at the same point on the cell body creates a graded potential that is still above threshold by the time it reaches the trigger zone, so an action potential results. Stimulus −40 −55 T −70 mV Stimulus Time −40 −55 T −70 mV Time Axon hillock −40 Graded potential a.k.a. “Trigger above threshold T Zone” −55 −70 Action mV potential Time Graded Potentials: 1.) Make neuron more + or more -. Therefore, can be excitatory (make an AP more likely) or inhibitory (make an AP less likely). EPSP/IPSP= Excitatory/ Inhibitory Post-Synaptic Potential 2.) Stronger stimulus (e.g. more neurotransmitters) = bigger change in membrane potential 3.) Tend to lose strength over distance, due to current leak and cytoplasmic resistance to ion flow 4.) If graded potential reaches trigger zone (e.g. axon hillock) and is above threshold, the high density of voltage-gated Na+ channels at “trigger zone” open, triggering an action potential. Graded Potentials: 1.) Stronger stimulus = bigger change in membrane potential 2.) Tend to lose strength over distance, due to current leakT/F and: A certain cytoplasmic stimulus resistance has caused to ion a flow neuron to open its dendritic chloride channels. An action potential is 3.) Can thereforebe excitatory imminent! (make an Dun AP moredun DUNNN..... likely) or inhibitory (make an AP less likely). EPSP/IPSP= Excitatory/Inhibitory Post-Synaptic Potential T/F: A certain stimulus has caused a neuron to open its 4.)dendritic If graded Na+potential channels. reaches The trigger membrane zone (e.g. potential axon near hillockthe dendrites) and is above is -55mV. threshold, An actionthe high potential density ofis thereforeNa+ channels at “triggerimminent! zone” Dun open dun triggering DUNNN..... action potential. Step 2: Resting Potential Action potential Na+/K+ Pump + + Potential Over _ Time Plot More + Positive Depolarization Charge + Inside + Threshold Potential 0 Action Potential Time Na+ Enters More Potential Difference Potential Negative K+ Exits Across Cell Membrane Charge Inside Voltage Sensors Summary Membrane Potential Plot: Resting Potential Threshold Potential Na+/K+ Pump Potential Over Time Plot More Positive Depolarization Charge Inside Threshold Potential 0 Action Potential Time Threshold Potential Na+ Enters More Potential Difference Potential Negative K+ Exits Across Cell Membrane Charge Inside Voltage Sensors Summary Threshold potential: voltage at which an action potential will be triggered Action potential: rapid, strong depolarization (and repolarization) signal of cell Used to communicate signals rapidly within and between neurons Membrane Potential Plot: Resting Potential Threshold Potential Na+/K+ Pump Potential Over Time Plot More Positive Depolarization Charge Inside Threshold Potential 0 Action Potential Time Threshold Potential Na+ Enters More Potential Difference Potential Negative K+ Exits Across Cell Membrane Charge Inside Voltage Sensors Summary Threshold potential: -55 mV If threshold is reached, we get an: Resting Potential Action Potential Na+/K+ Pump Potential Over Time Plot More Positive Depolarization Charge Inside Threshold Potential I II 0 Action Potential Time Na+ Enters More Potential Difference Potential Negative K+ Exits Across Cell Membrane Charge Inside III Voltage Sensors Summary What triggers each of these phases? So, if graded potential takes axon hillock to threshold, why does an action potential start -55mV Na+ and K+ voltage gated channels react Resting Potential to changes in potential Na+/K+ Pump Potential Over Time Plot Depolarization --- Threshold Potential Action Potential Na+ Enters +++ K+ Exits Voltage Sensors Summary Na+ and K+ voltage gated channels react Resting Potential to changes in potential Na+/K+ Pump Potential Over Time Plot Depolarization --- Threshold Potential Action Potential Na+ Enters +++ K+ Exits Voltage Sensors Depolarization > Threshold Triggers Opening Summary of Na+ and K+ volt. gated channels Step 1: Na+ influx depol. neuron Resting Potential Na+/K+ Pump Potential Over Na+ Time Plot Inactivates Depolarization Exits Threshold Potential K+ Action Potential Na+ Enters Na+ Enters K+ Exits Voltage Sensors Summary Figure 8.10a (1 of 5) Na+ channel has two gates Na+ ECF +30 0 mV −55 −70 ICF Activation gate Inactivation gate (Voltage-Sensitive!) (Time-Sensitive!) At the resting membrane potential, the activation gate closes the channel. Figure 8.10b (2 of 5) Na+ channel has two gates Na+ +30 0 mV −55 −70 Depolarizing stimulus arrives at the channel. Activation gate opens. Figure 8.10c (3 of 5) Na+ channel has two gates Na+ +30 0 mV −55 −70 With activation gate open, Na+ enters the cell and creates a positive feedback loop, opening more Na+ channels! How does the cell stop this? Figure 8.10d (4 of 5) Na+ inactivation gate won’t reset/open until membraneNa+ has channel repolarized has two (gotten gates sufficiently negative) Na+ +30 0 mV −55 −70 Inactivation gate closes and Na+ entry stops. This is part of the absolute refractory period. The activation gate is still open, but another action potential cannot be triggered until the inactivation gate resets to the open position. Figure 8.11 POSITIVE FEEDBACK Na+ entry during an action ACTION POTENTIAL potential creates a positive Rising phase Na+ enters Peak Falling phase feedback loop. The positive cell feedback loop stops when the Na + channel inactivation gates close. Na+ channel To stop cycle, slower Na+ channel activation Feedback cycle inactivation gate gates closes (see Fig. 8.10). open rapidly More depolarization Depolarization triggers + + Slow K K leaves Repolarization channels open cell Step 2: Na+ inact. gate closes, volt. gated K+ channels open Resting Potential Na+/K+ Pump Potential Over Na+ Time Plot Inactivates Depolarization Exits Threshold Potential K+ Action Potential Na+ Enters Na+ Enters K+ Exits Voltage Sensors Summary.
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