Resting membrane potential 1 mV= 0.001 V membrane separates intra- and extracellular compartments...
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Resting membrane potential
•1 mV= 0.001 V
•membrane separates intra- and extracellular compartments
•inside negative (-80 to -60 mV)
•due to the asymmetrical distribution of ionsacross the cell membraneAND the differential permeability of the membrane to these ions
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Channels allow ions to diffuse across membranes
Voltage-gated: Na+ channels, K+ channels, Ca2+ channelsLigand-gated: neurotransmitters (acetylcholine, glutamate)
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Figure 5-34a
Potassium Equilibrium Potential
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Figure 5-34b
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Figure 5-34c
Resting membrane potential is due mostly
to high potassium permeability
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The Nernst equation describes an ion’s equilibrium potential
Eion RT
zF ln
[ion]out
[ion]in
where:R is the gas constant (8.314 X 107 dyne-cm/mole degree), T is the absolute temperature in o Kelvin, z is the charge on the ionF is the Faraday (the amount of electricity required to chemically alter one gram equivalent weight of reacting material = 96,500 coulombs).
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A simpler version of the Nernst equation
At 37ºC:
When ions can move across a membrane, they will bring the membrane potential to their equilibrium potential.
Eion 61
z log
[ion]out
[ion]in
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QuickTime™ and aTIFF (Uncompressed) decompressor
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Typical ion concentrations
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Calculating the membrane potential for a cell that is only permeable to K+
[K+]out = 5 mM[K+]in = 150 mM
Ek = 61 x (-1.5) = -92 mV
Eion 61
z log
[ion]out
[ion]in
EK 61
1 log
[5]
[150]
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Sodium Equilibrium Potential
ENa = 61 x 1 = +61 mV
ENa 61
1 log
[150]
[15]
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The Na+-K+-ATPase (“sodium pump”) works to keep intracellular K+ high and Na+ low
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• The membrane potential can be described by the relationship between ion permeabilities and their concentrations
• The Goldman equation:
• Vm =
PNa[Na+]out+ PK[K+]out+ PCl[Cl-]in
Predicting the membrane potential (Vm)
PNa[Na+]in+ PK[K+]in+ PCl[Cl-]out
61 log
At the resting potentiala. K+ is very close to equilibrium.b. Na+ is very far from its equilibrium.c. PK >> PNa
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Real neurons and “Dynamic Polarization”
Pyramidal cellLayer V neocortex
Purkinje cellCerebellum
Axon
Axon
DendritesDendrites
Santiago Ramon y Cajal, 1900
Axon collateralsCollateralbranch
Input
Output
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Electrical Signals: Ion Movement• Resting membrane potential determined by
– K+ concentration gradient– Cell’s resting permeability to K+, Na+, and Cl–
• Gated channels control ion permeability– Mechanically gated– Ligand gated– Voltage gated
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Current flow through ion channels leads to changes in membrane potential
Ohm’s Law: V = I * RV = voltage, I = current (Amps), R = resistance (Ohms)
I = V/R or I = V * GG = conductance (Siemens)
For current to flow, there must be a driving force (Vm - Eion) > or < 0, thus I = (Vm - Eion) * G
If current flows across a resistance--the cell membrane acts like one--there is a change in voltage (membrane potential).
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Graded potentials can be: EXCITATORY or INHIBITORY (action potential (action potential is more likely) is less likely)
The size of a graded potential is proportional to the size of the stimulus.
Graded potentials decay as they move over distance.
Graded Potentials
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Graded potentials decay as they move over distance.
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Cable theory
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“Overshoot”
mV
+40
-80
0
1 ms
Action Potential•All-or-none•Not due to “membrane breakdown”
Shock
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Na+-dependence of AP
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Voltage-clamp
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Voltage-clamp of squid giant axon
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Isolation of Na and K currents
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I/V relationship of Na and K channels
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HH model
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (1 of 9)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (2 of 9)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (3 of 9)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (4 of 9)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (5 of 9)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-9 (6 of 9)
Electrical Signals: Action Potentials
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (7 of 9)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (8 of 9)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-9 (9 of 9)
Electrical Signals: Action Potentials
Why is AP peak < ENa?
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Voltage-Gated Na+ Channels
Na+ channels have two gates: activation and inactivation gates
Figure 8-10a
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10c
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10d
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Refractory Period
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Figure 8-14
How does an AP travel down an axon?
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AP propagation
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Figure 8-15, step 5
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Speed of AP conduction is governed by:
•Diameter of the axon
•Resistance of the axon membrane to ion leakage
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Myelin sheath “insulates” axons
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Saltatory conduction
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1 mm
Axon size matters
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Myelination increases conduction velocity
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Kawasaki Z750S
Top speed=170 mphTop speed=225 mph
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Graded Potentials
Subthreshold and suprathreshold graded potentials
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Graded Potentials
Figure 8-8b
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Coding for Stimulus Intensity
DendriteAP
trigger zoneAxon
terminal
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Patch-clamp recording
Giga=109
Mega= 106
vs. sharp microelectrodePros: high resistance seal & low resistance electrode better for recording small currents and injecting large currentsCons: disrupt (“dialyze”) cellular contents
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Single channel recordings“stochastic behavior”
Characterize channels by their:conductance (pS)selectivitykinetics
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Whole-cell recording of different types of K channels
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Channels are comprised of multiple subunits
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Ligand-gated ion channels