Module 4 - Intro to Resting Membrane Potential Flashcards
What are the three types of potentials to consider?
Resting membrane potential
Action potential
Graded potential
What is RMP?
The membrane voltage when the neuron/cell is at rest.
What are action and graded potentials?
Are fluctuations from RMP caused by the opening of ion channels.
What does excitation equate to?
Depolarization from RMP
What does inhibition equate to?
Hyperpolarization from RMP
What types of cells have more polarized RMPs? What is their RMP range?
Neurons, muscle and glial
-30 to -90 mV
What types of cells have less polarized RMPs? Give examples and state their range.
Non-excitable cells
e.g., epithelial, RBCs
-8 to -30 mV
What three things do ion pumps and exchangers ensure in all living cells (including bacteria)?
K+ is more abundant inside than outside
Na+ is more abundant outside than inside
Ca2+ is very low inside
What ion concentration gradient can vary during development?
Cl-
Why must Ca2+ be very low inside the cell
Toxic: precipitates proteins and organic/inorganic ions
How do sea water animals differ in terms of ion concentrations compared to mammals?
Have more than double the ion concentrations compared to mammals
How are mammals and sea water animals similar in terms of ions?
Roughly the same ratios across the membrane, K+ more abundant inside and Na+ more abundant outside.
What type of energy acts on ions in a cell?
Electrical potential energy (voltage)
Random kinetic energy
What does the total energy for moving a given ion through an aqueous cellular environment come from?
Voltage (Vm)
Concentration gradient of that ion.
What determines the direction of an ion’s flow?
The balance between its gradient and the membrane voltage.
What does the Nernst equation calculate?
The membrane voltage for a given ion concentration gradient where movement of ions stops (i.e., equilibrium potential).
What is the equilibrium potential?
A voltage that exactly opposes the diffusion energy of an ion gradient.
Give the Nernst equation and define the variables.
Eion = RT/zF ln([ion]out/[ion]in)
R - gas constant
T - absolute temperature in K
z - valence of the ion
F - faraday constant
What is the thermodynamic gas constant?
8.31 J/mol K
How is absolute temperature calculated?
celsius + 273.15
What is the Faraday constant?
9.65 E4 C/mol
What is the approximate value of RT/zF for a monovalent cation at 20 deg C?
~ 25 mV
Which ion gradients are better able to control Vm?
Those that flow more quickly across the cell membrane.
What is the RMP of most neurons and muscle cells?
Close to EK
Why are most neurons and muscle cells’ RMP close to EK?
Cells are generally more permeable to K+ at rest compared to other ions, due to an abundance of K+ leak channels.
What was the hypothesis of Julius Bernstein?
RMP depends only on [K+]out vs. [K+]in, and thus EK
Who developed the squid giant axon preparation?
John Zachary Young
Why was the squid giant axon a popular choice for electrophysiological experiments?
It was so large, one could easily insert a recording electrode along its length
Who tested Bernstein’s hypothesis with the squid giant axon?
Hodgkin and Horowicz
If Bernstein was right, what should have been the results of Hodgkin and Horowicz’s experiment?
Changing [K+]out while keeping [K+]in constant would change EK, and RMP/Vm should follow EK
What did Hodgkin and Horowicz actually see?
Hypothesis was supported at high [K+]out concentrations
But showed an exponential relationship at near physiological [K+]out
Why were Hodgkin and Horowitz’s findings different than expected?
As EK pulls away from ENa, Na+ ions are further from equilibrium, flowing faster into the cell and countering the K+ currents.
What does the driving force of an ion consist of?
Membrane voltage and that ion’s concentration gradient
What happens when Vm is not equal to Eion?
There is energy/driving force for ion flow.
What happens when Ohm’s Law and the Nernst equation are combined?
Defines the net energy acting on a given ion type in solution to produce a current.
What is the equation wed get when combining Ohm’s Law and the Nernst equation? Define the variables.
Iion = (Vm - Eion) * Gion
Iion - ion current/flow
Vm = membrane voltage
Eion = ion’s equilibrium potential
Gion = conductance for that ion
What is the equation for driving force?
Vm - Eion
Using the equations/principles of IK and INa, describe how K and Na can both influence Vm.
K+ has a higher membrane conductance (GK>GNa), permitting larger current (IK), and hence a stronger influence on Vm.
But as EK becomes more negative and pulls Vm further from ENa, the driving force for Na+ current (Vm-ENa) increases.
Thus, Na+ ions can exert more control over Vm at negative voltages to counter the K+ effect.
EXERCISE
EK is -70 mV
Vm = -70 mV
How much driving force is there for moving K+ across the cell membrane?
In which direction would K+ flow?
0 mV
No net flux of ion across membrane.
EXERCISE
EK = -70 mV
Vm = 0 mV
How much driving force is there for moving K+ across the membrane?
In which direction would K+ flow?
+70 mV
Outward: +ve values for DF equate to outward cation flow.
EXERCISE
EK = -70 mV
Vm = -90 mV
How much driving force is there for moving K+ across the cell membrane?
In which direction would K+ flow?
-20 mV
Inward: -ve values for DF equate to inward cation flow.
EXERCISE
Half the K+ leak channels are removed from the membrane.
What happens to GK?
What happens to the K+ current?
What if all K+ channels were removed?
GK and IK drop by half
No more K+ current
EXERCISE
ENa = +70 mV
Vm = 0
How much driving force is there for moving Na+ across the cell membrane?
In which direction would Na+ flow?
-70 mV
Inward: -ve values for DF equate to inward cation flow
EXERCISE
ENa = +70 mV
Vm = +90 mV
How much driving force is there for moving Na+ across the cell membrane?
In which direction would Na+ flow?
+20 mV
Outward: +ve values for DF equate to outward cation flow.
EXERCISE
EK is -50 mV
ENa is +50 mV
Vm = 0 mV
Which ion has the greatest driving force?
Which ion has the larger current?
In which direction would the net flux of ions be greatest?
What would happen to Vm?
Conductance for K+ higher than Na+, thus more DF for K+.
K+ has larger current
Flux outwards would be greatest
Vm would become more negative.
EXERCISE
EK = -50 mV
ENa = +50 mV
Vm = 0mV
GNa = 0
What would happen to Vm?
Since nothing is countering EK, Vm would drop to -50 mV.
EXERCISE
EK is -50 mV
ENa = +50 mV
Vm = 0 mV
GK - 0
What would happen to Vm?
Since nothing is countering ENa, Vm would rise to +50 mV.
