11.23 A Flashcards
Define a current as it relates to action potentials.
the movement of ions across the cell membrane
What is V as it relates to action potentials?
the membrane potential
The membrane potential is a measure of the potential of _____ relative to _____.
the cytoplasm relative to the bath
What are the two forces that drive ion movement?
- electrical due to the membrane potential
- chemical due to an ion concentration gradient
When membrane potential is negative, there is a force pulling what sorts of ions into the cell?
cations
When membrane potential is positive, there is a force pushing what sorts of ions out of the cell?
cations
The Nernst potential of an ion is a measure of what force driving it’s movement?
the chemical force
What is the equation for Nernst potential?
E = (60/z)log(extracellular/intracellular concentration)
There will be no net movement of a particular ion across a membrane when what is true?
the electrical and chemical forces driving it’s movement are balanced out
Under normal conditions, in which direction does potassium move relative to the cytoplasm?
potassium moves out of the cell
Are currents defined in terms of movement of positive or negative ions?
positive ions
Positive currents have what affect on V?
they make it more negative because cations are leaving the cytoplasm
Sodium ions will generate a (positive/negative) current?
negative
Describe the flow of ions across a membrane that will generate a negative current?
cations moving into the cell
A repolarizing current is a (positive/negative) one.
positive
Define flux.
the direction of movement of an ion
How is current different from flux?
flux is the direction of movement of an ion where as current is strictly the direction of movement of a cation
The magnitude of an ion’s current will depend on what two things?
the forces driving the movement (chemical and electrical) and the conductance
What is conductance?
how easily the ion can move across the membrane, it is the inverse of resistance
Membrane potential describes the ___ force driving ion movement whereas Nernst potential describes the ____ force driving ion movement.
membrane = electrical, Nernst = chemical
Conductance is dependent on what three things?
- number of open ion channels
- the presence of leak channels
- ion concentration
How does conductance of potassium change if the extracellular potassium concentration is increased and all other factors remain unchanged?
the potassium conductance will also increase, and ions will be able to leave the cell more easily
The conductance of potassium is proportional to what other factor?
the extracellular concentration of potassium
Give an equation for the current of an ion moving across a cell membrane.
I = g (V - E) = conductance (membrane potential - Nernst potential)
The sign of what defines the sign of an ion’s current?
the sign of (V - E)
What is the typical resting membrane potential in a cell?
-70 mV to -80 mV, very close to the Nernst potential of potassium
If the conductance of a particular ion is increased, what will happen to the membrane potential?
it will move closer to that ion’s Nernst potential
The current across a cell membrane is zero when what is true?
there is no net movement of ions across that membrane
Describe the structure of the voltage-gated sodium channel?
it is a channel with two gates, an activation gate and an inactivation gate; if either gate is closed, sodium cannot pass through the channel
Describe a resting voltage-gated sodium channel under normal conditions.
the activation gate is closed and teh inactivation gate is open
At resting potential, how can the magnitude of the sodium current be equal to the potassium current?
although the conductance of potassium is significantly greater, the (V - E) of sodium is larger than that of potassium
If at resting membrane potential, the membrane potential is stable, what must be true about the currents of potassium and sodium?
they must be equal but opposite
If a cell is only permeable to sodium and potassium, what is true about their combined fractional conductances?
they equal 1
Give an equation relating the value of the membrane potential to the fractional conductance and Nernst potential of potassium and sodium.
V = (fg1 x E1) + (fg2 x E2)
