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
Channels allow ions to diffuse across membranes
Voltage-gated: Na+ channels, K+ channels, Ca2+ channelsLigand-gated: neurotransmitters (acetylcholine, glutamate)
Figure 5-34a
Potassium Equilibrium Potential
Figure 5-34b
Figure 5-34c
Resting membrane potential is due mostly
to high potassium permeability
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).
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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Typical ion concentrations
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]
Sodium Equilibrium Potential
ENa = 61 x 1 = +61 mV
ENa 61
1 log
[150]
[15]
The Na+-K+-ATPase (“sodium pump”) works to keep intracellular K+ high and Na+ low
• 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
Real neurons and “Dynamic Polarization”
Pyramidal cellLayer V neocortex
Purkinje cellCerebellum
Axon
Axon
DendritesDendrites
Santiago Ramon y Cajal, 1900
Axon collateralsCollateralbranch
Input
Output
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
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).
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
Graded potentials decay as they move over distance.
Cable theory
“Overshoot”
mV
+40
-80
0
1 ms
Action Potential•All-or-none•Not due to “membrane breakdown”
Shock
Na+-dependence of AP
Voltage-clamp
Voltage-clamp of squid giant axon
Isolation of Na and K currents
I/V relationship of Na and K channels
HH model
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (1 of 9)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (2 of 9)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (3 of 9)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (4 of 9)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (5 of 9)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-9 (6 of 9)
Electrical Signals: Action Potentials
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (7 of 9)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Action Potentials
Figure 8-9 (8 of 9)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Figure 8-9 (9 of 9)
Electrical Signals: Action Potentials
Why is AP peak < ENa?
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
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10c
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10d
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Refractory Period
Figure 8-14
How does an AP travel down an axon?
AP propagation
Figure 8-15, step 5
Speed of AP conduction is governed by:
•Diameter of the axon
•Resistance of the axon membrane to ion leakage
Myelin sheath “insulates” axons
Saltatory conduction
1 mm
Axon size matters
Myelination increases conduction velocity
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Graded Potentials
Subthreshold and suprathreshold graded potentials
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Graded Potentials
Figure 8-8b
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Signals: Coding for Stimulus Intensity
DendriteAP
trigger zoneAxon
terminal
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
Single channel recordings“stochastic behavior”
Characterize channels by their:conductance (pS)selectivitykinetics
Whole-cell recording of different types of K channels
Channels are comprised of multiple subunits
Ligand-gated ion channels