Electophysilology II - the basis of bioelectricity Flashcards
What is the difference between ion transporters and ion channels?
Ion Transporters/Pumps
- maintain concentration gradients over the long term
- not directly involved in neuronal signalling
Ion Channels
- allow selective movement of ions across the membrane
- direction depends on both concentration and electrical gradient
- downhill= no energy consumption
- ion flow via ion channels creates ionic currents which cause electrical changes that drive neuronal signalling
What are the Electrical Terms?
Q is charge (units: coulombs)
I is current (units: amps)
-current is movement of charge, with respect to time (i.e. I= dQ/dt).
V is voltage (units: volts)
-voltage (potential) is separation of charge
R is resistance (units: ohms) and G is conductance
- resistance is the resistance to current flow
- number of open channels constitutes the resistance/conductance
- R= 1/G
C is capacitance (units: farads)
- capacity to store and separate charge, C= Q/V
- lipid bilayer constitutes the capacitor
What are the ion distributions across the membrane?
The ion distributions across the membrane are uneven. The total amount of charge on either side of the membrane is equal (principle of electro-neutrality), barring some tiny fraction of ions just lining the surface on either side.
· Na+ → high outside
· K+ → high inside
· Cl- → high outside
What do the concentration gradients of ions across the membrane set up?
Ionic equilibrium potentials
Ionic equilibrium for Na+ (ENa)= +56
Ionic equilibrium for K+ (EK)= -102
Ionic equilibrium for Cl- (ECl)= -76
What maintains the concentration difference of ions across the membrane?
Ionic pumps maintains concentration gradients across the membrane, and therefore maintain the ionic equilibrium potentials
What causes membrane potential?
separation of positive and negative charges across the cell membrane
What is the equilibrium potential?
The membrane potential for an ion where there is no net flow of that ion from one side of the membrane to the other
What counterbalances the concentration gradient across the membrane?
Depending on what the concentration difference is, there will be some voltage difference (electrical force) that will be enough to exactly counterbalance that.
Therefore, as ions diffuse out down their concentration gradient, this sets up an electrical gradient.
The equilibrium potential will be the voltage at which the electrical gradient exactly counterbalances (equal and opposite to) the concentration gradient
What is the equilibrium potential proportional to?
The equilibrium potential is proportional to the log of the concentration ratio.
What is the Nernst equation?
The Nernst equation calculates the equilibrium potential for an ion based on the charge on the ion and its concentration gradient across the membrane.
What is the K+ equilibrium potential with normal physiological concentration (Ek)?
Concentration gradient= high K+ in the cell, low outside of the cell.
-concentration gradient is driving K+ outwards
In order to counteract this, we need a force that is equal and opposite to the concentration gradient. This force is generated by the electrical (voltage) gradient that is generated. Therefore, an inward electrical gradient is needed to balance the outward concentration gradient
This means we need a negative equilibrium potential to generate an opposite driving force. The way you do quantify this is by the Nernst equation.
What is the Na+ equilibrium potential with normal physiological concentration (ENa)?
Concentration gradient= high Na+ outside the cell, low inside the cell.
-concentration gradient is driving Na+ inwards
In order to counteract this, we need a force that is equal and opposite to the concentration gradient. This force is generated by the electrical (voltage) gradient that is generated. Therefore, an outward electrical gradient is needed to balance the inward concentration gradient
This means we need a positive equilibrium potential to generate an opposite driving force. The way you do quantify this is by the Nernst equation.
How would Vm be equal to Eion?
if the membrane potential was only permeable to that specific ion
-however this is not the case, as the membrane is permeable to other ions
Why is the resting membrane potential (-70mV) closer to the equilibrium potential of K+ (-96mV) than to the equilibrium potential of Na+ (+60mV)?
Each ionic equilibrium potential is going to influence the membrane potential, but in proportion to that ion’s permeability.
-the membrane is more permeable to K+ than Na+, the K+ equilibrium potential of -96mV has more influence on the membrane potential than the Na+ equilibrium potential of +60mV
What happens if Vm is different from Eion?
The electrical gradient and concentration gradient are therefore different, and a net driving force (electrochemical gradient) is set up
In the case of Na+ when the membrane potential is -70mV, the concentration gradient is inward, and the electrical gradient is outward. Because the inward concentration force is greater than the outward electrical force, the net driving force (electrochemical driving force) will be pointed inwards, and will be large. The permeability of Na+ is relatively low, so the actual movement of Na+ according to the electrochemical driving force will be relatively slow, even though there is a large driving force.
In the case of K+, the concentration gradient is outward, and the electrical gradient is inward. Because the outward concentration force is stronger than the inward electrical force, the net driving force (electrochemical driving force) will be pointed outwards, and will be small. The permeability of K+ is high, so the movement of K+ according to the electrochemical driving force will be fast, even though there is a small driving force
What is the electrochemical driving force?
The electrochemical driving force is a potential (voltage) that drives ion flow (current) across the membrane
- it is the difference between Vm and Eion
- the greater the absolute of this difference, the greater the driving force
What does an ionic current try to change the membrane potential closer to?
An ionic current ‘tries’ to change the membrane potential (Vm) in a way that brings it closer to the equilibrium (reversal) potential Eion for that ion.
This means that if there is an increase in conductance/permeability for a given ion, then the change in membrane potential Vm will always be towards the equilibrium potential for that ion.
If there was a decrease in the conductance of the membrane to K+ (we’ve closed up potassium channels), Vm will move away from the potassium equilibrium potential (EK+)
What is Ohm’s law?
Ohm’s law is a law stating that current is proportional to voltage and inversely proportional to resistance.
I= V/R= gV
Since conductance (g) is the reciprocal of resistance (R): g= 1/R
We can use Ohm’s law to work out what the value of an ionic current would be, assuming we know the value of the conductance and the driving force.
Ionic current is membrane conductance for that ion times the driving force on that ion:
Iion= gion (Vm-Eion)
With this, we can tell whether a current is going to be positive or negative, and therefore whether it is going to be inward or outward.
What is current defined as?
movement of positive charge
· Outward current is +ve charge LEAVING the cell → hyperpolarisation
· Inward current is +ve charge ENTERING the cell → depolarisation
INa+ is an inwardly flowing current → depolarising
IK+ is an outwardly flowing current→hyperpolarising
What is the Equivalent Circuit Model?
This model solves for Vm at steady state
In steady state (resting Vm), there is no net inward or outward current, but since membrane potential is neither at K+ or Na+ equilibrium potential, there is small outflow of K+ and small inflow of Na+. These have to cancel each other out so that the net inward/outward current is 0
-IK + INa = 0
Vm is going to be influenced by ENa and EK, but to the extent of their conductances. Suppose gNa is very low, then ENa is going to be quite small. EK on the other hand will have a bigger effect on Vm because it has higher conductance.
In a membrane with various ionic equilibrium potentials (e.g. Na, Cl, K), increasing the permeability of Cl- will…
cause the membrane potential to move towards ECl
Does an IPSP hyperpolarise or depolarise the cell?
Reversal potential of an IPSP at ECl
An IPSP typically hyperpolarises the post-synaptic cell and therefore makes it less likely to fire an action potential. But an IPSP can still depolarise a cell. It doesn’t become an EPSP. just a depolarising IPSP.
-e.g. if the membrane potential is more negative than the conventional chloride equilibrium potential
In this case, when increasing Cl permeability, the membrane potential will be depolarising and will move towards the ECl. But, even though the IPSP is depolarising, it has no chance of getting to AP threshold, because it can only drive the membrane potential to the reversal potential (chloride equilibrium potential)
Therefore, if the reversal is negative to threshold, then it is going to be inhibitory.
If the reversal is positive to threshold, it will be excitatory.
If the reversal potential of an IPSP is at ECl, what is the reversal potential of an EPSP at?
Reversal potential of an EPSP is not at ENa. An EPSP typically has a reversal potential of around 0 (+/- 10).
This is because the channels mediating the EPSP are both permeable to Na and K
Therefore, reversal potential will be some way between the Na and K equilibrium potentials
Are equilibrium potentials constant?
YES!!
