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Neuroscience > Hodgkin and Huxley > Flashcards

Hodgkin and Huxley Flashcards

1
Q

two of the greatest neuroscientists of the last

century

A

Alan Hodgkin and Andrew Huxley

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2
Q

they were awarded the Nobel Prize in Medicine or

Physiology in 1963

A

Alan Hodgkin and Andrew Huxley

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3
Q

The Hodgkin and Huxley experiments were conducted on a most unusual animal, the ___

A

squid

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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

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5
Q

The reason they used

the squid for their experiments is because

A

the squid has giant axons that are used for escape

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6
Q

a typical axon in your body has a diameter of

about

A

2 μm

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7
Q

diameter of a giant axon

A

800-1000

μm, almost a full millimeter.

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8
Q

Hodgkin and Huxley used a new technique called

A

voltage clamp

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9
Q

evaluate the ion currents

that generate an action potential.

A

voltage clamp

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10
Q

is where the fibers
from the brain make synaptic
contacts with the giant axons.

A

giant

stellate ganglion

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11
Q

relates voltage, current and conductance

A

Ohm’s law

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12
Q

the degree of negativity (or

positivity) inside the cell relative to the outside and is measured in millivolts (mV).

A

Membrane potential

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13
Q

flow of ions through the channels in the

membrane

A

Current

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14
Q

“when positive ions flow into the cell

A

“inward current

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15
Q

when positive ions flow out of the cell

A

“outward current

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16
Q

Current is in units of

A

Amperes

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17
Q

is the symbol for current in

equations.

A

I

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18
Q

is the ease with which an ion can pass through a channel.

A

Conductance

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19
Q

means the same thing as permeability, but it can be measured, represented quantitatively, and is
used in equations

A

Conductance

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20
Q

symbol for conductance is

A

g

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21
Q

is the net force acting on an ion that drives it into or out of a cell.

A

Driving force

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22
Q

is the difference between the equilibrium potential for the ion and the
membrane potential, (Eion - Vm).

A

driving force

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23
Q

four terms are related and linked together by Ohms law

A

membrane potential (Vm)
current (I)
Conductance (g)
Driving force

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24
Q

Ohms law

A

Current (I) =conductance (g) x driving force (Eion -Vm) or I=g x (Eion -Vm).

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25
Q

When the
driving force is 0, as occurs when the
membrane potential is at

A

ENa (+55 mV),

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26
Q

conductances for Na+ and K+ are not constant during the action potential.
T/F

A

T

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27
Q

we have three dependent

variables

A

time, conductance and voltage, all of which are changing together

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28
Q

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.

A

voltage clamp technique

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29
Q

is inserted down a length of a squid giant axon.

A

voltage-sensing electrode

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30
Q

voltage-sensing electrode

connected to the ______that measures the membrane potential.

A

oscilloscope

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31
Q

The output of the

oscilloscope (which is the membrane potential (Vm)) is connected to one of the two inputs of a

A

differential amplifier

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32
Q

Input B of the differential amplifier is from a

A 
variable
voltage source (
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33
Q

dial in the diagram that allows the voltage to be set by the experimenter

A

Input B of the differential amplifier

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34
Q

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.

A

differential amplifier

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35
Q

The output of the
oscilloscope (which is the membrane potential (Vm)) is connected to one of the two inputs of a
differential amplifier

A

input A

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36
Q

urrent put out by the differential

amplifier, shown by the red line in Fig. 3, is fed to is an

A

ammeter,

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37
Q

which measures the current put

out by the differential amplifier, and then to a wire inserted down the length of the axon

A

ammeter,

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38
Q

In this way the amplifier is given the

instruction: inject whatever current is necessary at C so that the membrane voltage becomes, and

A

is kept equal to, the voltage at B

39
Q

counteracts any current flowing across the

membrane.

A

current through C

40
Q

how does this happen: the membrane potential

is held at a constant voltage that is equal to the voltage at input B

A

By replacing the charges flowing out of the axon

41
Q

Ohm’s law

A

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
Q

measures the current that has to be injected into the axon to hold the membrane potential constant

A

ammeter

43
Q

first wire

A

measures the charges on the inside of the axon and is attached to
a wire that if fed to an oscilloscope

44
Q

other input of voltage clamp circuit

A

ground wire placed in

the seawater.

45
Q

measures the membrane potential (Vm)

A

oscilloscope

46
Q

which is the difference in

charge between the inside and outside of the axon.

A

membrane potential (Vm)

47
Q

The output of the oscilloscope, the Vm, is fed to the

A

A

input of the differential amplifier.

48
Q

is set

by the experimenter

A

voltage fed to the other input (B) of the differential amplifier

49
Q

The voltage he feeds to the B input is determined by

A

turning the dial on the variable

voltage source

50
Q

puts out a current that is proportional to the difference in

voltage at its two inputs, A and B.

A

differential amplifier

51
Q

That current put out by the differential amplifier is fed to an

A

ammeter,

52
Q

That current put out by the differential amplifier is fed to an

A

ammeter,

and second wire in the axon (red wire).

53
Q

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.

A

The current though the
38
red wire

54
Q

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

A

true

55
Q

By virtue of its design, the amplifier

will inject current until

A

VA = VB.

56
Q

the current injected into the axon under voltage

clamp exactly reflects the

A

reflects the ionic currents flowing across the membrane as a result of the
depolarization voltage set by the experimenter.

57
Q

the inward

current could well be due to

A

Na+

58
Q

delayed outward current might be due to

A

K+

59
Q

One way to sort out the contributions of the ions is by

A

substitution experiments

60
Q

substitute for sodium

A

choline

61
Q

This
preserves the osmolarity and the total charge of the extracellular solution, but will block current
through the Na+ channel

A

choline

62
Q

We would expect that when the membrane is clamped to -10 mV and when ENa is -10 mV, the
_____ should disappear

A

inward current

63
Q

a powerful toxin that selectively blocks Na+

channels.

A

tetrodotoxin (TTX),

64
Q

inward current disappears uif

A

seawater with TTX
tetrodotoxin

or Na replaced by choline in seawater

65
Q

a neurotoxin isolated

primarily from the eggs and ovaries of the Japanese puffer fish

A

TTX

66
Q

action is to block the voltage

sensitive Na+ channels

A

Tetrodotoxin

67
Q

Thus, when TTX is used to treat a squid giant axon,

A

the inward current

disappears

68
Q

delayed outward current is carried by

A

K+

69
Q

K+ permeability is selectively blocked by

the drug,

A

tetraethylammonium (TEA).

70
Q

Addition of TEA to the fluid bathing an axon under

voltage clamp results in the

A

loss of the delayed outward current

71
Q

total current measured during voltage clamp

is simply the linear addition of the

A

inward current, carried by Na+, and the outward current,

carried by K+.

72
Q

action potential is composed entirely of these currents

A

Na+ and K+ currents

73
Q

action potential can be explained completely on the basis of
Na+ and K+ currents.
T/F

A

T

74
Q

provides a direct measure of how many channels of

a particular type are opened by a particular membrane potential

A

conductance

75
Q

is the voltage acting to drive the ion through the membrane, i.e.
the driving force.

A

V

76
Q

is a measure of how far the voltage acting on an ion is from equilibrium

A

driving force

77
Q

what is measured in the voltage

clamp,

A

Iion,

78
Q

voltage set by the experimenter

A

command voltage or holding

voltage

79
Q

equilibrium potential for a particular ion (Na+ or K+).

A

“Eion

80
Q

Na+ channel is conductive for

A

brief instant, less than 1.0 ms

81
Q

Na+ channel rendered non-conductive by

A

closing of the inactivation gate.

82
Q

K+ also has an activation gate opened by

A

depolarization

83
Q

K+ channels only have an activation gate. T/F

A

T

84
Q

do not have an inactivation gate.

A

T

85
Q

The activation gates of K+ channels

remain open as long as the membrane remains

A

depolarized.

86
Q

The activation gates of Na+ channels react more quickly than

A

Na+ inactivation gate or the K+ activation gate

87
Q

accounts for the upstroke of the action potential.

A

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
Q

In other words, the more depolarized the membrane is from rest, the
larger are both the Na+ and K+ conductances. T/F

A

T

89
Q

In other words, the more depolarized the membrane is from rest, the
larger are both the Na+ and K+ conductances. Relationship is

A

sigmoidal

90
Q

once the membrane potential is around ___, even small changes in membrane
potential cause conductances of both Na+ and K+ to increase markedly. In

A

-30 mV

91
Q

there is

a steep increase in conductance for each small change in membrane potential, from

A

about -30 to

+10 mV

92
Q

This steep increase in conductance explains the

A

positive feedback that creates the

explosive, all or none nature of the action potential

93
Q

What the graph also shows is that the maximum conductance is reached when the
membrane potential is

A

+20 mV.

Neuroscience (6 decks)

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