NESC 2570 Lecture Notes - Lecture 3: Equilibrium Point, Tetrodotoxin, Axon Hillock
27 Jun 2018
School
Department
Course
Professor
September 23rd 2015
Action Potential Form and Nomenclature
Membrane potential at rest is -60mV
○
This gives way to the rising phase and goes above zero (depolarized - less
polarized)
This overshoot gives way to the falling phase which for a brief time
undershoots and goes below the resting potential
○
The undershoot is the hyperpolarization
○
○
They all use the similar scheme to activate an action potential
○
The Role of Na+in the generation of an action potential
Potassium, sodium, chloride and calcium
Potassium is high concentration inside and low concentration outside
○
Sodium and Chloride are abundant on the outside to begin with (salty
banana)
○
Calcium is more abundant inside than it is on the outside
○
○
During the action potential, sodium gradient increases significantly and much
bigger than the potassium
During depolarization, the permeability will increase until it reaches
threshold
○
○
Voltage Clamp
Clamp the voltage and study the current
○
Submerged the axon into a saline solution and inserted a passing electrode and a
recording electrode
○
They wanted to know if they inserted a current, would there be any change in the
membrane
○
Instead of hyperpolarizing, if they depolarizing the membrane potential,
Inward current means that you put a positive current into the cell (whether
that was taking away negative or adding positive)
○
The outward current means that you are putting in a negative charge to the
cell
○
Other than that, Hodgkin and Huxley didn't know what caused these action
potentials
○
They did a series of experiment and the results were, the larger the
depolarization, the larger the inward current and the flatter the outward
current
The larger and larger the depolarization gets, the inward current will
eventually not be curved
○
○
The inward current will change, what about the late phase?
There is not a significant change for the outward (later current) to do
with sodium
○
The delayed outward current isn't affected but the inward one is
because sodium channels are blocked
○
No sodium? No rising phase
○
Tetrodotoxin
○
Tetraethyl-ammonium will not affect the early inward current
(sodium) but it will eliminate the delayed outward current
(potassium)
○
○
○
Hodgkin and Huxley
Wanted to describe this inward and outward current mathematically
○
Ohms law -voltage is the result of current and reistance (v= IR)
○
They came up with an equation
Iion=gion (Vm - Eion)
○
○
Sodium conductance
If you use a small depolarization then the sodium conductance will also be
small
○
The larger the depolarization, the larger the conductance
○
It will inactivate and then deactivate
○
Depolarization increases sodium and potassium conductance of the axon
○
○
Potassium conductance
Potassium conductance is similar but will consistently be activated
○
The potassium conductance takes slightly longer to activate
○
Both sodium and potassium conductance's are voltage dependent
○
○
They mathematically constructed an action potential
○
Feedback Loops
Open sodium channels allows sodium to flow into the neuron and depolarize the
membrane potential
○
This depolarization will continue until all of the sodium channels have opened
and let in enough sodium for the cell to reach the threshold and the all or none
response is activated
Positive feedback loop
○
○
Once this all or none response is activated, the potassium channels will open and
potassium will rush
○
Refractory Period
Very small numbers of ions actually move across the membrane
○
During the absolute refractory potential, a second action potential cannot be
elicited because of the inactivation of the potassium channels
○
During the resting refractory potential, a second action potential can occur but
the stimulus must be larger than the first
○
September 28th 2015
Membrane Properties Continued
The Role of Na + in the generation of an action potential
The main current that is responsible for the depolarization of the action potential
is sodium
○
The relationship is described by the Nerst equation
○
During an action potential, there were dramatic changes in the membrane
permeability to these ions
When at rest, the conductance is to potassium as it is to equilibrium
○
The peak of the action potential is determined by the sodium permeability
which means that the membrane potential was more permeable to sodium,
however when the cell hyperpolarized down to the refractory position this
was the result of potassium
○
○
Voltage Clamp
The depolarization of the membrane increases the permeability to sodium,
causing more depolarization and more opening up of the conductance's which
allows for the all or none response
○
The voltage clamp allowed the ability to study the relationship between voltage
and conductance (permeability)
○
Voltage Clamp
Recording electrode One internal electrode measures membrane potential
and is connected to the voltage amplifier
○
Command voltage- Voltage clamp amplifier compares membrane potential
to the desired (command) potential
○
Voltage clamp amplifier - When the membrane potential is different from
the command potential, the clamp amplifier injects current into the axon
through a second electrode. This feedback arrangement causes the
membrane potential to become the same as the command potential
○
The current flowing back into the axon and thus across its membrane can
be measured at the measure current
○
○
You artificially set the membrane potential and keep that transmembrane voltage
while the membrane does what it once. The currents that flow through the
membrane are measured but your command voltage is held constant so you can
measure the difference
○
Current is the actual amount of voltage going across while permeability is how
much is able to go across
V=IR
○
I = V/R
○
○
Current flow across a axon membrane during the voltage clamp experiment
Capacitor - any device that can store charge
Opposite charges attract each other, so if you wanted to store a
bunch of positively charged charges, they will repel each other
○
Two sheets of conductors (one positive and one negative)
surrounded by insulation is hooked up to a battery
If these two sheets are close enough, then the negative charges
will hold the positive
○
The amount of charge stored is related to the voltage applied -
the more voltage applied the more charge is stored
○
C=Q/V
○
If you have more area on the plates, then you will have a larger
capacitor. You can also increase the capacity by decreasing the
distance in between it
○
Dialector = insulator
○
○
The membrane serves as a capacitor
The combination of the membrane acts as an insulator
○
The charged plates are the extracellular and intracellular
solutions or cytoplasm
○
○
○
○
When the cell was depolarized, it didn't reach equilibrium
They saw an initial movement of positive charges into the cell followed by
an outward movement of charge from the membrane
○
This was dramatically different from the hyperpolarization
○
How these current flows alter when they change the magnitude of the
initial depolarization
The late part of the current steadily got larger as the depolarization
got larger
○
The inward current got larger up until about zero and then reversed
○
They found that indeed getting rid of sodium on the outside made the
current reverse
No more sodium to go in so the current reversed
○
This experiment confirmed that the early currents in response
to depolarization is carried exclusively by sodium
○
There are two specific mediators (permeability sites) one for
the early sodium and one for the later potassium
○
○
○
Looking at all their experiments, they were able to dissect the currents
induced by different amplitude depolarization into sodium and potassium
induced currents
Sodium conductance - permeability of membrane to sodium got
bigger and bigger as the depolarization got larger
But current flow reversed when the step was beyond the
equilibrium point for sodium because of the ratio of
concentrations and therefore where the ions must go
○
Flow of ions is an electrochemical equilibrium
○
If you impose a voltage across the membrane, you will force
the ions to move
○
○
Sigmoid behavior for both sodium and potassium conductance
Low conductance of permeability below rest (below -60mV)
○
Further depolarization leads to no further increase in
conductance - it is limited to sodium around 20
○
○
○
○
They tried to find the simplest equation for the time varying dependency on
voltage
In figuring out the simplest form, they determined that the conductance's of
sodium and potassium varied in some exponential power
○
Sodium was the third power that reflected the opening of the channel
Closing the channel was slower
○
○
Potassium was the fourth power
○
This suggested information about the structure
○
Keep in mind that this was in the early 1950's so this was way before the
isolation and discover of DNA
○
○
There were two elements (ionic and molecular) involved in the action potential
Rapid process that involved depolarization activation of the opening of the
sodium channels which allows the positive ions to come in which
encourages more depolarization which opens more sodium channels
causing a positive feedback
○
Two slower elements that works in the other direction - as potassium
moves in, the cell is repolarized even hyperpolarized. This is much slower
○
As a consequence of the inactivation of the sodium channels, after an
action potential the cell will not have another action potential right away
○
○
Refractory period
It takes a few milliseconds in the resting potential to regroup
○
During this inactivation time, it doesn't matter how much stimulus is given,
it is blocked
Absolute refractory period
○
○
During this refractory period more and more sodium's realign themselves
and it gets to a point where an action potential can be instigated again, but
only if the stimulus is larger than the first
Relative refractory period
○
○
At rest, no lumin exits in which ions can pass through the membrane
a change in the reconfiguration of the molecule, allows molecules to
pass through the membrane
○
Inactive stage is plugged open by a piece of a molecule
○
○
These ion flows are actually very small, it only takes a fraction of the
available ions moving across the membrane
Even if you turn off of the ion pump, you can still get many action
potential
○
○
**Look up difference between absolute and relative refractory period
○
○
Passive Current Flow In Action
Action potentials start in the greatest density of sodium channels which is at the
beginning of the axon, called the axon hillock
Axons initial segment
○
○
Having started here, it will race down the axon
Cause depolarization by poking in a microelectrode
○
If you are injecting this positive current and at different spots, the current
will decay over distances
Some of the injected current leaks out because the membrane is not
impermeable to currency
○
Resulting voltage will get smaller and smaller
○
This gives us an asymptotic curve - exponential decay over distance
○
Voltage at different distances can be described by a fraction of the
peak distance - the more distance increases, the smaller the resulting
voltage
○
Length constant = lambda and time constant = tao
○
The more the current can flow across the membrane- the smaller the
length constant
○
The more the resistance to current flow in the extracellular space, the
smaller the length constant
○
The more the resistance to transmembrane flow, the longer the
length constant
○
○
○
Action Potential Conduction Requires Both Active and Passive Current Flow
Action potential causes the membrane to depolarize at some point
○
Therefore this depolarization will spread to the neighbouring membrane
○
If the part of that membrane reaches threshold, the action potential will occur and
continue down the axon
This is dependent on time and length
○
The longer the length constant, the faster the depolarization will propagate
○
○
If lambda increases, then the action potential can propagate faster down the
axon - this is why squid have such large axons
○
We want the time constant to be as small as possible and lambda to be as large as
possible so then the depolarization can go further lengths in shorter time
○
By wrapping myelin around the axons we reduce the time constant
However, it is only wrapped in segments so the current can flow
○
All the voltage gated ion channels are concentrated at the nodes of Ranvier
○
Length constant only needs to bring the next node from threshold
○
This will happen rapidly because of the small time constant - increases
velocity 100 fold from a non-myelinated axon
We need postural reflexes rapidly
○
○
Loss of myelination is devastating
○
○
(Resistance across the membrane RO)+ (Resistan
○
Saltatory Action Potential Conduction Along a Myelinated Axon
○
Membrane Properties
September 23, 2015
2:35 PM
September 23rd 2015
Action Potential Form and Nomenclature
Membrane potential at rest is -60mV
○
This gives way to the rising phase and goes above zero (depolarized - less
polarized)
This overshoot gives way to the falling phase which for a brief time
undershoots and goes below the resting potential
○
The undershoot is the hyperpolarization
○
○
They all use the similar scheme to activate an action potential
○
The Role of Na+in the generation of an action potential
Potassium, sodium, chloride and calcium
Potassium is high concentration inside and low concentration outside
○
Sodium and Chloride are abundant on the outside to begin with (salty
banana)
○
Calcium is more abundant inside than it is on the outside
○
○
During the action potential, sodium gradient increases significantly and much
bigger than the potassium
During depolarization, the permeability will increase until it reaches
threshold
○
○
Voltage Clamp
Clamp the voltage and study the current
○
Submerged the axon into a saline solution and inserted a passing electrode and a
recording electrode
○
They wanted to know if they inserted a current, would there be any change in the
membrane
○
Instead of hyperpolarizing, if they depolarizing the membrane potential,
Inward current means that you put a positive current into the cell (whether
that was taking away negative or adding positive)
○
The outward current means that you are putting in a negative charge to the
cell
○
Other than that, Hodgkin and Huxley didn't know what caused these action
potentials
○
They did a series of experiment and the results were, the larger the
depolarization, the larger the inward current and the flatter the outward
current
The larger and larger the depolarization gets, the inward current will
eventually not be curved
○
○
The inward current will change, what about the late phase?
There is not a significant change for the outward (later current) to do
with sodium
○
The delayed outward current isn't affected but the inward one is
because sodium channels are blocked
○
No sodium? No rising phase
○
Tetrodotoxin
○
Tetraethyl-ammonium will not affect the early inward current
(sodium) but it will eliminate the delayed outward current
(potassium)
○
○
○
Hodgkin and Huxley
Wanted to describe this inward and outward current mathematically
○
Ohms law -voltage is the result of current and reistance (v= IR)
○
They came up with an equation
Iion=gion (Vm - Eion)
○
○
Sodium conductance
If you use a small depolarization then the sodium conductance will also be
small
○
The larger the depolarization, the larger the conductance
○
It will inactivate and then deactivate
○
Depolarization increases sodium and potassium conductance of the axon
○
○
Potassium conductance
Potassium conductance is similar but will consistently be activated
○
The potassium conductance takes slightly longer to activate
○
Both sodium and potassium conductance's are voltage dependent
○
○
They mathematically constructed an action potential
○
Feedback Loops
Open sodium channels allows sodium to flow into the neuron and depolarize the
membrane potential
○
This depolarization will continue until all of the sodium channels have opened
and let in enough sodium for the cell to reach the threshold and the all or none
response is activated
Positive feedback loop
○
○
Once this all or none response is activated, the potassium channels will open and
potassium will rush
○
Refractory Period
Very small numbers of ions actually move across the membrane
○
During the absolute refractory potential, a second action potential cannot be
elicited because of the inactivation of the potassium channels
○
During the resting refractory potential, a second action potential can occur but
the stimulus must be larger than the first
○
September 28th 2015
Membrane Properties Continued
The Role of Na + in the generation of an action potential
The main current that is responsible for the depolarization of the action potential
is sodium
○
The relationship is described by the Nerst equation
○
During an action potential, there were dramatic changes in the membrane
permeability to these ions
When at rest, the conductance is to potassium as it is to equilibrium
○
The peak of the action potential is determined by the sodium permeability
which means that the membrane potential was more permeable to sodium,
however when the cell hyperpolarized down to the refractory position this
was the result of potassium
○
○
Voltage Clamp
The depolarization of the membrane increases the permeability to sodium,
causing more depolarization and more opening up of the conductance's which
allows for the all or none response
○
The voltage clamp allowed the ability to study the relationship between voltage
and conductance (permeability)
○
Voltage Clamp
Recording electrode One internal electrode measures membrane potential
and is connected to the voltage amplifier
○
Command voltage- Voltage clamp amplifier compares membrane potential
to the desired (command) potential
○
Voltage clamp amplifier - When the membrane potential is different from
the command potential, the clamp amplifier injects current into the axon
through a second electrode. This feedback arrangement causes the
membrane potential to become the same as the command potential
○
The current flowing back into the axon and thus across its membrane can
be measured at the measure current
○
○
You artificially set the membrane potential and keep that transmembrane voltage
while the membrane does what it once. The currents that flow through the
membrane are measured but your command voltage is held constant so you can
measure the difference
○
Current is the actual amount of voltage going across while permeability is how
much is able to go across
V=IR
○
I = V/R
○
○
Current flow across a axon membrane during the voltage clamp experiment
Capacitor - any device that can store charge
Opposite charges attract each other, so if you wanted to store a
bunch of positively charged charges, they will repel each other
○
Two sheets of conductors (one positive and one negative)
surrounded by insulation is hooked up to a battery
If these two sheets are close enough, then the negative charges
will hold the positive
○
The amount of charge stored is related to the voltage applied -
the more voltage applied the more charge is stored
○
C=Q/V
○
If you have more area on the plates, then you will have a larger
capacitor. You can also increase the capacity by decreasing the
distance in between it
○
Dialector = insulator
○
○
The membrane serves as a capacitor
The combination of the membrane acts as an insulator
○
The charged plates are the extracellular and intracellular
solutions or cytoplasm
○
○
○
○
When the cell was depolarized, it didn't reach equilibrium
They saw an initial movement of positive charges into the cell followed by
an outward movement of charge from the membrane
○
This was dramatically different from the hyperpolarization
○
How these current flows alter when they change the magnitude of the
initial depolarization
The late part of the current steadily got larger as the depolarization
got larger
○
The inward current got larger up until about zero and then reversed
○
They found that indeed getting rid of sodium on the outside made the
current reverse
No more sodium to go in so the current reversed
○
This experiment confirmed that the early currents in response
to depolarization is carried exclusively by sodium
○
There are two specific mediators (permeability sites) one for
the early sodium and one for the later potassium
○
○
○
Looking at all their experiments, they were able to dissect the currents
induced by different amplitude depolarization into sodium and potassium
induced currents
Sodium conductance - permeability of membrane to sodium got
bigger and bigger as the depolarization got larger
But current flow reversed when the step was beyond the
equilibrium point for sodium because of the ratio of
concentrations and therefore where the ions must go
○
Flow of ions is an electrochemical equilibrium
○
If you impose a voltage across the membrane, you will force
the ions to move
○
○
Sigmoid behavior for both sodium and potassium conductance
Low conductance of permeability below rest (below -60mV)
○
Further depolarization leads to no further increase in
conductance - it is limited to sodium around 20
○
○
○
○
They tried to find the simplest equation for the time varying dependency on
voltage
In figuring out the simplest form, they determined that the conductance's of
sodium and potassium varied in some exponential power
○
Sodium was the third power that reflected the opening of the channel
Closing the channel was slower
○
○
Potassium was the fourth power
○
This suggested information about the structure
○
Keep in mind that this was in the early 1950's so this was way before the
isolation and discover of DNA
○
○
There were two elements (ionic and molecular) involved in the action potential
Rapid process that involved depolarization activation of the opening of the
sodium channels which allows the positive ions to come in which
encourages more depolarization which opens more sodium channels
causing a positive feedback
○
Two slower elements that works in the other direction - as potassium
moves in, the cell is repolarized even hyperpolarized. This is much slower
○
As a consequence of the inactivation of the sodium channels, after an
action potential the cell will not have another action potential right away
○
○
Refractory period
It takes a few milliseconds in the resting potential to regroup
○
During this inactivation time, it doesn't matter how much stimulus is given,
it is blocked
Absolute refractory period
○
○
During this refractory period more and more sodium's realign themselves
and it gets to a point where an action potential can be instigated again, but
only if the stimulus is larger than the first
Relative refractory period
○
○
At rest, no lumin exits in which ions can pass through the membrane
a change in the reconfiguration of the molecule, allows molecules to
pass through the membrane
○
Inactive stage is plugged open by a piece of a molecule
○
○
These ion flows are actually very small, it only takes a fraction of the
available ions moving across the membrane
Even if you turn off of the ion pump, you can still get many action
potential
○
○
**Look up difference between absolute and relative refractory period
○
○
Passive Current Flow In Action
Action potentials start in the greatest density of sodium channels which is at the
beginning of the axon, called the axon hillock
Axons initial segment
○
○
Having started here, it will race down the axon
Cause depolarization by poking in a microelectrode
○
If you are injecting this positive current and at different spots, the current
will decay over distances
Some of the injected current leaks out because the membrane is not
impermeable to currency
○
Resulting voltage will get smaller and smaller
○
This gives us an asymptotic curve - exponential decay over distance
○
Voltage at different distances can be described by a fraction of the
peak distance - the more distance increases, the smaller the resulting
voltage
○
Length constant = lambda and time constant = tao
○
The more the current can flow across the membrane- the smaller the
length constant
○
The more the resistance to current flow in the extracellular space, the
smaller the length constant
○
The more the resistance to transmembrane flow, the longer the
length constant
○
○
○
Action Potential Conduction Requires Both Active and Passive Current Flow
Action potential causes the membrane to depolarize at some point
○
Therefore this depolarization will spread to the neighbouring membrane
○
If the part of that membrane reaches threshold, the action potential will occur and
continue down the axon
This is dependent on time and length
○
The longer the length constant, the faster the depolarization will propagate
○
○
If lambda increases, then the action potential can propagate faster down the
axon - this is why squid have such large axons
○
We want the time constant to be as small as possible and lambda to be as large as
possible so then the depolarization can go further lengths in shorter time
○
By wrapping myelin around the axons we reduce the time constant
However, it is only wrapped in segments so the current can flow
○
All the voltage gated ion channels are concentrated at the nodes of Ranvier
○
Length constant only needs to bring the next node from threshold
○
This will happen rapidly because of the small time constant - increases
velocity 100 fold from a non-myelinated axon
We need postural reflexes rapidly
○
○
Loss of myelination is devastating
○
○
(Resistance across the membrane RO)+ (Resistan
○
Saltatory Action Potential Conduction Along a Myelinated Axon
○
Membrane Properties
September 23, 2015 2:35 PM
Document Summary
This gives way to the rising phase and goes above zero (depolarized - less polarized) This overshoot gives way to the falling phase which for a brief time undershoots and goes below the resting potential. They all use the similar scheme to activate an action potential. The role of na+ in the generation of an action potential. Potassium is high concentration inside and low concentration outside. Sodium and chloride are abundant on the outside to begin with (salty banana) Calcium is more abundant inside than it is on the outside. During the action potential, sodium gradient increases significantly and much bigger than the potassium. During depolarization, the permeability will increase until it reaches threshold. Submerged the axon into a saline solution and inserted a passing electrode and a recording electrode. They wanted to know if they inserted a current, would there be any change in the membrane. Instead of hyperpolarizing, if they depolarizing the membrane potential,
