Hodgkin and Huxley Flashcards
(93 cards)
1
Q
two of the greatest neuroscientists of the last
century
A
Alan Hodgkin and Andrew Huxley
2
Q
they were awarded the Nobel Prize in Medicine or
Physiology in 1963
A
Alan Hodgkin and Andrew Huxley
3
Q
The Hodgkin and Huxley experiments were conducted on a most unusual animal, the ___
A
squid
4
Q
The Hodgkin and Huxley experiments were conducted on a most unusual animal, the squid,
and used a newly developed electronic method called the
A
voltage clamp
5
Q
The reason they used
the squid for their experiments is because
A
the squid has giant axons that are used for escape
6
Q
a typical axon in your body has a diameter of
about
A
2 μm
7
Q
diameter of a giant axon
A
800-1000
μm, almost a full millimeter.
8
Q
Hodgkin and Huxley used a new technique called
A
voltage clamp
9
Q
evaluate the ion currents
that generate an action potential.
A
voltage clamp
10
Q
is where the fibers
from the brain make synaptic
contacts with the giant axons.
A
giant
stellate ganglion
11
Q
relates voltage, current and conductance
A
Ohm’s law
12
Q
the degree of negativity (or
positivity) inside the cell relative to the outside and is measured in millivolts (mV).
A
Membrane potential
13
Q
flow of ions through the channels in the
membrane
A
Current
14
Q
“when positive ions flow into the cell
A
“inward current
15
Q
when positive ions flow out of the cell
A
“outward current
16
Q
Current is in units of
A
Amperes
17
Q
is the symbol for current in
equations.
A
I
18
Q
is the ease with which an ion can pass through a channel.
A
Conductance
19
Q
means the same thing as permeability, but it can be measured, represented quantitatively, and is
used in equations
A
Conductance
20
Q
symbol for conductance is
A
g
21
Q
is the net force acting on an ion that drives it into or out of a cell.
A
Driving force
22
Q
is the difference between the equilibrium potential for the ion and the
membrane potential, (Eion - Vm).
A
driving force
23
Q
four terms are related and linked together by Ohms law
A
membrane potential (Vm)
current (I)
Conductance (g)
Driving force
24
Q
Ohms law
A
Current (I) =conductance (g) x driving force (Eion -Vm) or I=g x (Eion -Vm).
25
When the
driving force is 0, as occurs when the
membrane potential is at
ENa (+55 mV),
26
conductances for Na+ and K+ are not constant during the action potential.
T/F
T
27
we have three dependent
| variables
time, conductance and voltage, all of which are changing together
28
This method has the advantage of holding the membrane potential
constant over the entire length of an axon while recording the currents that flow into and out of
the axon through ion channels.
voltage clamp technique
29
is inserted down a length of a squid giant axon.
voltage-sensing electrode
30
voltage-sensing electrode
| connected to the ______that measures the membrane potential.
oscilloscope
31
The output of the
| oscilloscope (which is the membrane potential (Vm)) is connected to one of the two inputs of a
differential amplifier
32
Input B of the differential amplifier is from a
```
variable
voltage source (
```
33
dial in the diagram that allows the voltage to be set by the experimenter
Input B of the differential amplifier
34
is an amplifier that puts out a current that is proportional
| to the difference in the voltages presented to the two inputs, A and B.
differential amplifier
35
The output of the
oscilloscope (which is the membrane potential (Vm)) is connected to one of the two inputs of a
differential amplifier
input A
36
urrent put out by the differential
| amplifier, shown by the red line in Fig. 3, is fed to is an
ammeter,
37
which measures the current put
| out by the differential amplifier, and then to a wire inserted down the length of the axon
ammeter,
38
In this way the amplifier is given the
| instruction: inject whatever current is necessary at C so that the membrane voltage becomes, and
is kept equal to, the voltage at B
39
counteracts any current flowing across the
| membrane.
current through C
40
how does this happen: the membrane potential
| is held at a constant voltage that is equal to the voltage at input B
By replacing the charges flowing out of the axon
41
Ohm's law
g=I/V, where g= conductance of
membrane, I = current measured by the ammeter, and V is the voltage set by the experimenter.
Iion = gion x (Eion -Vm)
42
measures the current that has to be injected into the axon to hold the membrane potential constant
ammeter
43
first wire
measures the charges on the inside of the axon and is attached to
a wire that if fed to an oscilloscope
44
other input of voltage clamp circuit
ground wire placed in
| the seawater.
45
measures the membrane potential (Vm)
oscilloscope
46
which is the difference in
| charge between the inside and outside of the axon.
membrane potential (Vm)
47
The output of the oscilloscope, the Vm, is fed to the
A
| input of the differential amplifier.
48
is set
| by the experimenter
voltage fed to the other input (B) of the differential amplifier
49
The voltage he feeds to the B input is determined by
turning the dial on the variable
| voltage source
50
puts out a current that is proportional to the difference in
| voltage at its two inputs, A and B.
differential amplifier
51
That current put out by the differential amplifier is fed to an
ammeter,
52
That current put out by the differential amplifier is fed to an
ammeter,
| and second wire in the axon (red wire).
53
changes the membrane potential along the entire length of the axon at exactly the same time
until the membrane potential, Vm, has the same value as the voltage set by the experimenter.
The current though the
38
red wire
54
At that
membrane potential (set by voltage clamp circuit), no further current flows into axon because both inputs to the differential amplifier
have exactly the same voltage. T/F
true
55
By virtue of its design, the amplifier
| will inject current until
VA = VB.
56
the current injected into the axon under voltage
| clamp exactly reflects the
reflects the ionic currents flowing across the membrane as a result of the
depolarization voltage set by the experimenter.
57
the inward
| current could well be due to
Na+
58
delayed outward current might be due to
K+
59
One way to sort out the contributions of the ions is by
substitution experiments
60
substitute for sodium
choline
61
This
preserves the osmolarity and the total charge of the extracellular solution, but will block current
through the Na+ channel
choline
62
We would expect that when the membrane is clamped to -10 mV and when ENa is -10 mV, the
_____ should disappear
inward current
63
a powerful toxin that selectively blocks Na+
| channels.
tetrodotoxin (TTX),
64
inward current disappears uif
seawater with TTX
tetrodotoxin
or Na replaced by choline in seawater
65
a neurotoxin isolated
| primarily from the eggs and ovaries of the Japanese puffer fish
TTX
66
action is to block the voltage
| sensitive Na+ channels
Tetrodotoxin
67
Thus, when TTX is used to treat a squid giant axon,
the inward current
| disappears
68
delayed outward current is carried by
K+
69
K+ permeability is selectively blocked by
| the drug,
tetraethylammonium (TEA).
70
Addition of TEA to the fluid bathing an axon under
| voltage clamp results in the
loss of the delayed outward current
71
total current measured during voltage clamp
| is simply the linear addition of the
inward current, carried by Na+, and the outward current,
| carried by K+.
72
action potential is composed entirely of these currents
Na+ and K+ currents
73
action potential can be explained completely on the basis of
Na+ and K+ currents.
T/F
T
74
provides a direct measure of how many channels of
| a particular type are opened by a particular membrane potential
conductance
75
is the voltage acting to drive the ion through the membrane, i.e.
the driving force.
V
76
is a measure of how far the voltage acting on an ion is from equilibrium
driving force
77
what is measured in the voltage
| clamp,
Iion,
78
voltage set by the experimenter
command voltage or holding
| voltage
79
equilibrium potential for a particular ion (Na+ or K+).
"Eion
80
Na+ channel is conductive for
brief instant, less than 1.0 ms
81
Na+ channel rendered non-conductive by
closing of the inactivation gate.
82
K+ also has an activation gate opened by
depolarization
83
K+ channels only have an activation gate. T/F
T
84
do not have an inactivation gate.
T
85
The activation gates of K+ channels
| remain open as long as the membrane remains
depolarized.
86
The activation gates of Na+ channels react more quickly than
Na+ inactivation gate or the K+ activation gate
87
accounts for the upstroke of the action potential.
delay in the opening of K+
channels allows the initial opening of Na+ to completely dominate the membrane for less than a
millisecond, thereby evoking a large influx of Na+ with a minimum of K+ efflux
88
In other words, the more depolarized the membrane is from rest, the
larger are both the Na+ and K+ conductances. T/F
T
89
In other words, the more depolarized the membrane is from rest, the
larger are both the Na+ and K+ conductances. Relationship is
sigmoidal
90
once the membrane potential is around ___, even small changes in membrane
potential cause conductances of both Na+ and K+ to increase markedly. In
-30 mV
91
there is
| a steep increase in conductance for each small change in membrane potential, from
about -30 to
| +10 mV
92
This steep increase in conductance explains the
positive feedback that creates the
| explosive, all or none nature of the action potential
93
What the graph also shows is that the maximum conductance is reached when the
membrane potential is
+20 mV.
