Action Potential Flashcards
Action potential components
)
* Overshoot (> 0 mV)
* Rising phase
* Threshold (all-or-none)
* Falling phase
Undershoot (below RMP)
An action potential is the result of
selective (voltage- and time-dependent) increases in the membrane’s permeability to Na+ and K+
Alan Hodgkin realized that to understand ion flux across the membrane…
it was necessary to eliminate differences in membrane potential.
In the 1940s Kenneth Cole and George Marmount had developed the
concept of
the voltage clamp.
Cole discovered that it was possible to use two electrodes and a feedback circuit to kp the cell’s membrane potential at a level set by the experimenter
The voltage clamp allows the
membrane voltage to be manipulated
independently of the ionic currents, allowing the current-voltage
relationships of membrane channels to be studied.
Hodgkin and Huxley used the voltage clamp to
outline the ionic
mechanisms that underlie the action potential (published 1952).
They received the Nobel Prize in Physiology or Medicine in 1963.
current flow aqccross the membrane was observed in
response to depolarization, but not hyperpolarization.
Effect is voltage dependent
direction of current flow is
defined as
net movement of
positive charge.
inward current
Flow of positive charge into
the cell (or negative charge
out of the cell)
In voltage-clamp experiments the flow of positive charge into the cell (the inward current) is typically shown as
a
downward deflection.
The reversal- (equilibrium) potential gives clue about the ionic nature of the early inward current:
The equilibrium potential is the
potential at which the electrical force offsets the movement of an ion due to its concentration gradient → no
net current flows
Action potential depolarization/amplitude
depends
greatly on Na+ gradient (but the resting potential does not)
depolarization of the membrane has 2 effects:
- an early influx of Na+
into the neuron, (produces transient/inactivating inward current) - followed by a delayed efflux of K+ (produces non-inactivating outward current)
The current through each class of voltage-gated channel can be calculated from
Ohm’s law
Calculating conductances of an active channel population
total ionic current as the sum of separate Na+
, K+
, and leak currents:
Hodgkin and Huxley
predictions for the probability of channels being open.
Calculation of Membrane Conductances From Voltage-Clamp Data
m
activation of gNa
Calculation of Membrane Conductances From Voltage-Clamp Data
n
inactivation of gNa
Calculation of Membrane Conductances From Voltage-Clamp Data
h
activation of gK
Membrane conductance
(g)
Vm
(resting potential)
controlled through the voltage clamp
I(ion)
measured by isolating the current in
ion-substitution experiments (or through
pharmacological blockade)
(Vm – Eion)
the electrochemical driving force
M, n (and h) describe
the probability of “particles” to be in the correct position to open (or to inactivate, respectively)
a channel.
Ƭ (tau)
time constant of change
“delayed rectifier”.
require time to turn on, but K+ much slower, requiring several ms to reach its peak
Green Arrow is
the resting potential of
the cell.
As the K+ channel is virtually the only one
open at these negative voltages, the cell will rest very
near to the equilibrium potential for potassium Ek
.
Yellow Arrow is…
the equilibrium potential
for Na+
(ENa).
In this two-ion system, ENa is the
natural limit of membrane potential beyond which a cell cannot pass.
Current values illustrated in this graph that exceed ENa are measured by artificially pushing the cell’s voltage past its natural limit.
However, ENa could only be reached if the opposing
potassium current was absent.
Blue Arrow is…
the maximum voltage that the peak of the action potential can approach.
his is the
actual natural maximum membrane potential that this cell can reach.
It cannot reach ENa because of the counteracting influence of the potassium current.
Red arrow is…
the action potential threshold.
This is where I
sum becomes net-inward.
Note that this is a zero-current crossing, but with a negative slope.
Any such “negative slope crossing” of the
zero current level in an I/V plot is an unstable point.
At any voltage negative to this crossing, the current is outward and so a cell will tend to return to its resting potential.
At any voltage positive of this crossing, the current is inward and will tend to depolarize the cell.
This depolarization leads to more inward current, thus the
sodium current becomes regenerative.
The point at which the green line reaches its most negative value is the point where all sodium channels are open. Depolarizations beyond that point thus decrease the sodium current as the driving force decreases as the membrane potential approaches ENa.
