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Bi/CNS 150 Lecture 4
Monday, October 6, 2013
Action potentials
Henry Lester
If you’ve not requested a section, email Teagan
Electricity is also a Language of the Brain.Intracellular recording with sharp glass electrodes.
1. A current applied by the experimenter increases firing rates
V, I
http://info.med.yale.edu/neurobio/mccormick/movies/rly_exp.avi
Prof. David McCormick’s data
Intracellular recording with sharp glass electrodes.2. Artificially applied acetylcholine acts on muscarinic receptors to
change the membrane potential, increasing action potential frequency.
(The spikes in these examples are about 100 mV in amplitude)
http://info.med.yale.edu/neurobio/mccormick/movies/ach_fin.avi
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Prof. David McCormick’s data
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http://www.krl.caltech.edu/Projects/physicscourses/index.htm
Monday’s lecture employs electrical circuits
See also Appendix A in Kandel
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Atomic-scale structure of (bacterial) Na+ channels (2011, 2012)
Views from the
extracellular solution
electrically, open channel = conductor
Views from the
membrane plane
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The miniature single-channel conductors add in parallel
ENa
(+60 mV)
GNa = Na
=GNa
Na
Na
outside
cytosol = inside
mostly K+mostly Na+
GK = K
EK (- 60 mV)
GKK
K
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C
E
G
Na+
VE G E G E G
G G GK K Na Na Cl Cl
K Na Cl
K+
At DC, IC = CdV/dt = 0,
so
Cl-
peak of action
potential: Na+ channels
open too
resting potential:
K+ channels open
outside
cytosol = inside
The membrane potential at steady state(not at equilibrium)
“after-hyperpolarization”: more K+ channels open
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http://nerve.bsd.uchicago.edu/
Simulation of the nerve impulse (“unclamped”)
Francisco Bezanilla's simulation program at the Univ of Chicago:
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Spatially homogeneous membrane (“membrane AP”). Either spherical, or patch, or wire in axon.
First, show passive properties of membraneTurn off conductances. Ampl ± 2, delay 10, duration 15, total time 40
Now back to default (“reset parameters”)Note threshold. Vary pulse amplitude (2 to 20 A).Note constant amplitude
Note hyperpolarization. Plot G(K), G(Na) and note that hyperpolarization is caused by G(K).
“Refractory” period 30 ms total time, vary pulse 2 duration, pulse 3 = 30 A. Plot G(K)
simultaneously.
Simulation of the nerve impulse (“unclamped”)
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Repetitive firing: the frequency code
total time to 40 ms; lengthen pulse 1 to 30 ms,
Vary pulse amp from 2, 5, 10.
Note the smaller AP’s—the squid axon is not specialized for repetitive
firing.
(For robust frequency encoding, we require at least one additional type of K+ channel.)
Simulation of the nerve impulse (“unclamped”)
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http://nerve.bsd.uchicago.edu/nerve1.html
Click on Voltage Plot, V vs T.StartParameter edits are not useful.
Cable properties of the Axon
Francisco Bezanilla's simulation program at the Univ of Chicago:
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http://nerve.bsd.uchicago.edu/nerve1.html
Propagation (V vs. t)
Measure propagation velocity: set blue electrode at 2 cm6.18 ms – 3.88 ms = 2.3 ms30 mm/2.3 ms = 11 mm/ms = 13 m/s. Pretty fast!
At 30o C, 2.89 ms - 1.47 ms = 1.42 ms30 mm / 1.42 ms = 21.1 m/s. Faster
Simulation of the nerve impulse (“unclamped”)
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There are dozens of V-gated channels,Causing the variety of action potential waveforms
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An approximate explanation for the electrocardiogram, slide 1
The left ventricle pumps against the greatest resistancetherefore it has thickest walls; therefore its currents are the largest; therefore it contributes most of the ECG.
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An approximate explanation for the electrocardiogram, slide 2
CE
G
Na+ K+ Cl-extracellular
cytosol
CE
G
Na+ K+ Cl-
ClKNai
gEVdt
dVCI
iii
,,
;)(
extRextext IRV
The capacitive currents are largest
An extracellular
electrode pair
records IR drops
proportional to the
(absolute value) of
the 1st derivative of
membrane potential.
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chest
leg
extRextext IRV
Only a small fraction of the current flows across the resistance between chest and a limb.
This produces a V ~ 103 times smaller than the transmembrane potential.
The ECG records this signal
An approximate explanation for the electrocardiogram, slide 3
CE
G
Na+K+
Cl-
extracellular
intracellular
CE
G
Na+K+
Cl-
extRextext IRV
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Action potentials and the electrocardiogram
Electrocardiogrammeasured on the skin
Action Potentialmeasured with
intracellularelectrode
P
S
R
T
Q
K+ channels conductNa+ channels conduct
~ 100 V
~ 100 mV
~ 1 sec
ST depression is a common anomaly,implying that additional current flows between sections of the heart during the “plateau”
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-from sense organs to the brain
-within the brain
-from the brain to muscles
-even in a muscle or in the heart
-even in the pancreas
The frequency of impulses represents signaling among cells
in the
nervous system.
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End of Lecture 4
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Intracellular recording with sharp glass electrodesA cell is receiving stimuli from other cells, not from the experimenter
(The spikes in these examples are about 100 mV in amplitude)
Same data;choice of formats.
Media player required
http://info.med.yale.edu/neurobio/mccormick/movies/rly_exp.mpg
http://info.med.yale.edu/neurobio/mccormick/movies/rly_exp.avi
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