c. establishes an equilibrium potential for a particular ion
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C. Establishes an equilibrium potential for a particular ion. based on Donnan equilibrium. Nernst equation. 1. What membrane potential would exist at the true equilibrium for a particular ion?. - PowerPoint PPT PresentationTRANSCRIPT
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C. Establishes an equilibrium potential for a particular ionbased on Donnan equilibrium
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Nernst equation1. What membrane potential would exist at the true equilibrium for a particular ion?
- What is the voltage that would balance diffusion gradients with the force that would prevent net ion movement?
2. This theoretical equilibrium potential can be calculated (for a particular ion).
Eion = RT ln [X]outside
zF [X]inside
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ENa,K,Cl = RT PK [K+]out + PNa [Na+]out + PCl[Cl-]in
PK [K+]in + PNa [Na+]in + PCl[Cl-]outF_____________________________ln___
Goldman Equation1. quantitative representation of Vm when membrane is permeable to more than one ion species2. involves permeability constants (P)
pp 72-73
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Resting Potential
A. Vrest
1. represents potential difference at non-excited state-30 to -100mV depending on cell type
2. not all ion species may have an ion channel3. there is an unequal distribution of ions due to active pumping mechanisms
- contributes to Donnan equilibrium- creates chemical diffusion gradient that contributes to the equilibrium potential
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Resting Potential
B. Ion channels necessary for carrying charge across the membrane1. the the concentration gradient, the greater its contribution to the membrane potential
2. K+ is the key to Vrest (due to increased permeability)
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Resting PotentialC. Role of active transport
ENa is + 63 mV in frog muscleVm is + -90 to -100mV in frog muscle
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Action Potentials
large, transient change in Vm
depolarization followed by repolarizationpropagated without decrementconsistent in individual axons“all or none”
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Action Potentials
A. Depends on1. ion chemical gradients established by active transport through channels2. these electrochemical gradients represent potential energy3. flow of ion currents through “gated” channels
- down electrochemical gradient4. different types of Na+ and K+ channels than seen in most cells
- voltage-gated
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Action PotentialsB. Properties
1. only in excitable cells- muscle cells, neurons, some receptors, some secretory cells
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Action PotentialsB. Properties
2. a cell will normally produce identical action potentials (amplitude)
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Action PotentialsB. Properties
3. depolarization to threshold
- rapid depolarization- results in reverse of polarity
- or just local response (potential) if it does not reach threshold
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Action PotentialsB. Properties
a. threshold current (-30 to -55 mV)b. AP regenerative after threshold (self-perpetuating)
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Action PotentialsB. Properties
4. overshoot: period of positivity in ICF5. repolarization
a. return to Vrest
b. after-hyperpolarization
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Action PotentialsB. Properties
6. accommodationa. time-dependent decrease in excitability b. result of a series of subthreshold depolarizationsc. threshold increasesd. the slower the rate of depolarization (current intensity), the greater the in thresholde. change in sensitivity of ion channels
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Action PotentialsC. Refractory period
1. absolute2. relative
a. strong enough stimulus can elicit another APb. threshold is increased
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Action PotentialsD. ∆ Ion conductance
- responsible for current flowing across the membrane
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Action PotentialsD. ∆ Ion conductance
1. rising phase: in gNa
overshoot approaches ENa
(ENa is about +60 mV)2. falling phase: in gNa and in gK
3. after-hyperpolarizationcontinued in gK
approaches EK
(EK is about -90 mV)
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Gated Ion ChannelsA. Voltage-gated Na+ channels
1. localization
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Gated Ion ChannelsA. Voltage-gated Na+ channels
1. localizationa. voltage-gated
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Gated Ion ChannelsA. Voltage-gated Na+ channels
1. localizationb. ligand-gated at synapses
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Gated Ion ChannelsA. Voltage-gated Na+ channels
1. localizationNa+ channels occupy only a small fraction of surface area100-500 channels/m
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Gated Ion ChannelsA. Voltage-gated Na+ channels
2. current flowa. Na+ ions flow through channel at 6000/sec at emf of -100mVb. number of open channels depends on time and Vm
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Gated Ion ChannelsA. Voltage-gated Na+ channels
3. opening of channela. gating molecule with a net charge
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Gated Ion ChannelsA. Voltage-gated Na+ channels
3. opening of channelb. change in voltage causes gating molecule to undergo conformational change
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Gated Ion ChannelsA. Voltage-gated Na+ channels
4. factors contributing to specificitya. anions at mouth of channelb. sizec. ability to dehydrate (shed water of hydration)
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Gated Ion ChannelsA. Voltage-gated Na+ channels
5. generation of AP dependent only on Na+
repolarization is required before another AP can occurK+ efflux
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Gated Ion ChannelsA. Voltage-gated Na+ channels
6. positive feedback in upslopea. countered by reduced emf for Na+ as Vm approaches ENa
b. Na+ channels close very quickly after opening (independent of Vm)
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Gated Ion ChannelsB. Voltage-gated K+ channels
1. slower response to voltage changes than Na+ channels2. gK increases at peak of AP
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Gated Ion ChannelsB. Voltage-gated K+ channels
3. high gK during falling phasedecreases as Vm returns to normalchannels close as repolarization progresses
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Gated Ion ChannelsB. Voltage-gated K+ channels
4. hastens repolarization for generation of more action potentials
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Does [Ion] Change During AP?A. Relatively few ions needed to alter Vm
B. Large axons show negligible change in Na+ and K+ concentrations after an AP.