Membrane Potentials Flashcards
What is the composition of the ICF?
High in K, low in Na and Cl
What is the composition of ECF?
High in Na and Cl but low in K Composed of blood plasma and interstitial fluid E.g. blood/serum
What is the barrier that divides the ICF and ECF?
Plasma membrane
The difference in ion concentrations between ICF and ECH is maintained and regulated by what?
Control mechanisms within the plasma membrane (e.g. ion channels)
What are the three types of sensors that control the gates on ion channels?
Ligand, voltage and second messenger
What are voltage gated channels?
Have gates controlled by changes in membrane potentials Ex. Voltage gated Na channels or potassium channels
What are ligand gated channels?
Have gates controlled by the binding of a ligand such as neurotransmitter Ex. ACh receptors
What is the diffusion potential?
The potential difference generated across a membrane when an ion diffuses down its concentration gradient
What does the magnitude of the diffusion potential depending on?
The size of the concentration gradient (it is a driving force measured in mV)
How is an equilibrium potential reached?
- Na ions move down their concentration gradient (side 1 to 2) but Cl ions remain on side 1 2. A positive charge now builds up on side 2 3. This positive charge will now prevent further movement of Na and the electrical force drives Na out of side 2 to side 1 4. At equilibrium the Na ions move from side 1 to side 2 down its concentration gradient for every Na ion that moves from side 2 to side 1 down its electrical gradient (no further change in Na concentration occurs and no further change in electrical potential)
What is the equilibrium potential for an ion determined by?
Chemical forces (concentration gradient) and electrical forces therefore movement of an ion across the membrane is determined by both of these factors across the membrane
What is the composition of Na in ECF and ICF?
ECF: 140 ICF: 14
What is the composition of K in the ECF and ICF?
ECF: 4 ICF: 120
What is the equilibrium potential for Na in skeletal muscle?
+65 mV
What is the equilibrium potential for K in skeletal muscle?
-95 mV
What is the equilibrium potential for Ca2+ in skeletal muscle?
+120 mV
What is the equilibrium potential for Cl- in skeletal muscle?
-90 mV
For uncharged substances such as sucrose what is the only factor we consider to determine their diffusion equilibrium?
Only consider the influence of the concentration gradient
The driving force for net diffusion of ions must account for what?
Both the concentration gradient and the electrical potential across the membrane
If the driving force is negative (Em is more negative than the Ex) then..
Cation will enter the cell and anion will leave the cell
If the driving force is positive (Em is more positive than the Ex) then..
Cation will leave the cell and anion will enter the cell
If the driving force is zero (Em = Ex) then..
No net movement of the ion in either direction
In a solution of ions a flow of current is carried by the movement of what?
Ions
What does the movement of ions between the ICF and ECF across the cell memrbane constitute?
An electrical (ionic) current
What are the factors that determine the ionic current across the cell membrane?
Difference between the equilibrium potential for a given ion and the actual membrane potential Permeability of the membrane to a given ion
The larger the difference between Em and Ex the larger the what?
Imbalance between the electrical and concentration gradients and the larger the net movement of the given ion
If the permeability of the membrane to a given ion is high then the ionic current at a particular value of driving force will be what?
Higher than if the permeability was low
What is permeability closely related to?
Conductance - for a given driving force the greater the conductance the greater the current flow
What is conductance?
An index of the ability of an ion to carry current across a membrane
What are the two factors that determine the membrane potential?
Ion concentrations (across the cell membrane) Relative ion permeabilities
What is the Na/K ATPase?
A primary active pump that largely develops and maintains the concentration gradients for Na and K
Ions with the highest permeabilities or conductances at rest (steady state) will make the greatest contribution to what?
The resting membrane potential
What do the relative ion permeabilities determine?
The relative importance of a particular ion in governing where Em lies
What does the Goldman-Hodgkin-Katz equation give?
A quantitative relation between Em on one hand and the ion concentrations and permeabilities on the other hand
What is the interpretation of the Goldman equation?
The combination of an outwardly directed K gradient (product of Na/K ATPase activity) and the high permeability of the membrane to K makes the ICF electrically negative with respect to the ECF However the finite/limited permeability of the membrane to Na (and Cl) prevents the membrane potential from reaching the Nernst potential for K
What is the role of the Na/K ATPase?
An electrogenic contribution (3 Na ions pumped out for every 2 K ions pumped into the cell) Maintains the concentration gradient of Na and K
What is the Chord conductance equation?
Alternative equation to the Goldman equation Considers the equilibrium potential for each ion by its relative conductance
According to the Chord conductance equation, ions with the highest conductance drive the membrane potential towards what?
Their own equilibrium potentials whereas those with low conductance have little influence on the membrane potential
What underlies the generation of the AP?
A transient dramatic increase in Na permeability (and also conductance)
What is the basis for the transient depolarization of the membrane potential associated with the neuronal AP?*
A large increase in pNa (increase in conductance) due to the activation of voltage gated Na channels
What are some characteristics of an AP?
Triggered by depolarization All or none events Propagation of APs from one site to the next is non decremental (doesnt decrease) At the peak of the AP the membrane potential reverses signs becoming positive inside
What must be reached in order to trigger an AP?
A threshold level of depolarization
After a neuron cell fires an AP there is a brief period of time in which what occurs?
It is impossible to trigger another AP called the absolute refractory period
What is the resting membrane potential?
Steady state Inside of the cell is negative (-70mV)
What is the threshold potential of an AP?
Membrane potential at which the depolarization level is enough to trigger an AP
What is the depolarizing phase of an AP?
When the membrane potential becomes less negative than the resting membrane potential Characterized by an inward current of Na associated with an increased conductance of Na
After threshold of an AP has been reached depolarization becomes what?
Self sustained giving rise to the upstroke
What is the peak of an AP?
Maximum amplitude (depolarization) of the AP
What is the overshoot of an AP?
The membrane potential transiently overshoots zero and the inside of the cell becomes positive with respect to the outside for a brief period of time
What is the repolarization phase of the AP?
The upstroke is terminated and the membrane repolarizes to the resting level
What is the undershoot (hyperpolarizing after potential) of the AP?
When the AP repolarizes toward the normal resting membrane potential and transiently becomes more negative than normal
What is the hyperpolarizing phase of the AP?
The membrane potential becomes more negative than the resting membrane potential
Explain the ionic basis for the explosive nature of the AP
When the membrane is depolarized pNa increases allowing Na ions to carry + charges into the cell This depolarizes the cell further causing greater increase in pNa and more depolarization The process is explosive and tends to continue until all Na channels are open and the membrane potential has been driven up to near ENa
What causes the Em (membrane potential) to return to rest after the AP?
The depolarization induced increase in pNa is transient There is a delayed voltage dependent increase in pK
When the Em is equal to or more negative than the usual resting membrane potential, the activation gate of the Na channel is what?
Closed to prevent Na from entering the cell at resting potential
When is the inactivation gate of the Na channel open?
At resting membrane potential (when the activation gate is closed)
Both the inactivation and activation gates of Na channels respond to what?
Depolarization but with different speeds and in opposite directions Activation gate opens rapidly in response to depolarization but the inactivation gate slowly closes in response
When are both the activation and inactivation gates for Na channels open?
Immediately after a depolarization allowing Na to enter the cell A few seconds after the depolarization the activation gate is still open but the inactivation gate is closing
Due to the inactivation and activation gates the pNa first increases in response to what?
Depolarization then declines again even if the depolarization were maintained in some way
What is Na channel inactivation?
The delayed decline in Na permeability upon depolarization
Explain the voltage dependent K channels
Closed at resting membrane potential Activation gate opens upon depolarization however it responds slowly to depolarization (slow activation) pK increases with a delay and K current is outward
What is the absolute refractory period?
When the inactivation gates of voltage gated Na channels are closed No amount of depolarization can cause the cell to fire an AP preventing the influx of Na necessary to trigger the AP
What is the relative refractory period?
Mainly overlaps with the hyerpolarization phase (undershoot) during which conductance to K is higher than normal levels During this period an AP can be evoked but only if a greater than usual depolarizing current is applied
What is the net driving force equation?
mV = Em - Ex
What is Em?
Membrane potential (mV)
What is Ex?
Equilibrium potential for a given ion (mV)
What is the Nernst equation?
What is the Goldman-Hodgkin-Katz equation?
What is the equation for ionic current?
What is the Chord conductance equation?
