Hodgkin and Huxley Flashcards
two of the greatest neuroscientists of the last
century
Alan Hodgkin and Andrew Huxley
they were awarded the Nobel Prize in Medicine or
Physiology in 1963
Alan Hodgkin and Andrew Huxley
The Hodgkin and Huxley experiments were conducted on a most unusual animal, the ___
squid
The Hodgkin and Huxley experiments were conducted on a most unusual animal, the squid,
and used a newly developed electronic method called the
voltage clamp
The reason they used
the squid for their experiments is because
the squid has giant axons that are used for escape
a typical axon in your body has a diameter of
about
2 μm
diameter of a giant axon
800-1000
μm, almost a full millimeter.
Hodgkin and Huxley used a new technique called
voltage clamp
evaluate the ion currents
that generate an action potential.
voltage clamp
is where the fibers
from the brain make synaptic
contacts with the giant axons.
giant
stellate ganglion
relates voltage, current and conductance
Ohm’s law
the degree of negativity (or
positivity) inside the cell relative to the outside and is measured in millivolts (mV).
Membrane potential
flow of ions through the channels in the
membrane
Current
“when positive ions flow into the cell
“inward current
when positive ions flow out of the cell
“outward current
Current is in units of
Amperes
is the symbol for current in
equations.
I
is the ease with which an ion can pass through a channel.
Conductance
means the same thing as permeability, but it can be measured, represented quantitatively, and is
used in equations
Conductance
symbol for conductance is
g
is the net force acting on an ion that drives it into or out of a cell.
Driving force
is the difference between the equilibrium potential for the ion and the
membrane potential, (Eion - Vm).
driving force
four terms are related and linked together by Ohms law
membrane potential (Vm)
current (I)
Conductance (g)
Driving force
Ohms law
Current (I) =conductance (g) x driving force (Eion -Vm) or I=g x (Eion -Vm).
When the
driving force is 0, as occurs when the
membrane potential is at
ENa (+55 mV),
conductances for Na+ and K+ are not constant during the action potential.
T/F
T
we have three dependent
variables
time, conductance and voltage, all of which are changing together
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
is inserted down a length of a squid giant axon.
voltage-sensing electrode
voltage-sensing electrode
connected to the ______that measures the membrane potential.
oscilloscope
The output of the
oscilloscope (which is the membrane potential (Vm)) is connected to one of the two inputs of a
differential amplifier
Input B of the differential amplifier is from a
variable voltage source (
dial in the diagram that allows the voltage to be set by the experimenter
Input B of the differential amplifier
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
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
urrent put out by the differential
amplifier, shown by the red line in Fig. 3, is fed to is an
ammeter,
which measures the current put
out by the differential amplifier, and then to a wire inserted down the length of the axon
ammeter,
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
counteracts any current flowing across the
membrane.
current through C
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
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)
measures the current that has to be injected into the axon to hold the membrane potential constant
ammeter
first wire
measures the charges on the inside of the axon and is attached to
a wire that if fed to an oscilloscope
other input of voltage clamp circuit
ground wire placed in
the seawater.
measures the membrane potential (Vm)
oscilloscope
which is the difference in
charge between the inside and outside of the axon.
membrane potential (Vm)
The output of the oscilloscope, the Vm, is fed to the
A
input of the differential amplifier.
is set
by the experimenter
voltage fed to the other input (B) of the differential amplifier
The voltage he feeds to the B input is determined by
turning the dial on the variable
voltage source
puts out a current that is proportional to the difference in
voltage at its two inputs, A and B.
differential amplifier
That current put out by the differential amplifier is fed to an
ammeter,
That current put out by the differential amplifier is fed to an
ammeter,
and second wire in the axon (red wire).
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
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
By virtue of its design, the amplifier
will inject current until
VA = VB.
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.
the inward
current could well be due to
Na+
delayed outward current might be due to
K+
One way to sort out the contributions of the ions is by
substitution experiments
substitute for sodium
choline
This
preserves the osmolarity and the total charge of the extracellular solution, but will block current
through the Na+ channel
choline
We would expect that when the membrane is clamped to -10 mV and when ENa is -10 mV, the
_____ should disappear
inward current
a powerful toxin that selectively blocks Na+
channels.
tetrodotoxin (TTX),
inward current disappears uif
seawater with TTX
tetrodotoxin
or Na replaced by choline in seawater
a neurotoxin isolated
primarily from the eggs and ovaries of the Japanese puffer fish
TTX
action is to block the voltage
sensitive Na+ channels
Tetrodotoxin
Thus, when TTX is used to treat a squid giant axon,
the inward current
disappears
delayed outward current is carried by
K+
K+ permeability is selectively blocked by
the drug,
tetraethylammonium (TEA).
Addition of TEA to the fluid bathing an axon under
voltage clamp results in the
loss of the delayed outward current
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+.
action potential is composed entirely of these currents
Na+ and K+ currents
action potential can be explained completely on the basis of
Na+ and K+ currents.
T/F
T
provides a direct measure of how many channels of
a particular type are opened by a particular membrane potential
conductance
is the voltage acting to drive the ion through the membrane, i.e.
the driving force.
V
is a measure of how far the voltage acting on an ion is from equilibrium
driving force
what is measured in the voltage
clamp,
Iion,
voltage set by the experimenter
command voltage or holding
voltage
equilibrium potential for a particular ion (Na+ or K+).
“Eion
Na+ channel is conductive for
brief instant, less than 1.0 ms
Na+ channel rendered non-conductive by
closing of the inactivation gate.
K+ also has an activation gate opened by
depolarization
K+ channels only have an activation gate. T/F
T
do not have an inactivation gate.
T
The activation gates of K+ channels
remain open as long as the membrane remains
depolarized.
The activation gates of Na+ channels react more quickly than
Na+ inactivation gate or the K+ activation gate
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
In other words, the more depolarized the membrane is from rest, the
larger are both the Na+ and K+ conductances. T/F
T
In other words, the more depolarized the membrane is from rest, the
larger are both the Na+ and K+ conductances. Relationship is
sigmoidal
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
there is
a steep increase in conductance for each small change in membrane potential, from
about -30 to
+10 mV
This steep increase in conductance explains the
positive feedback that creates the
explosive, all or none nature of the action potential
What the graph also shows is that the maximum conductance is reached when the
membrane potential is
+20 mV.
