1. Introduction to electrophysiology 1.
Department of Pharmacology and Pharmacotherapy
University of Szeged, Faculty of Medicine

2. Topics Biophysical basis Resting membrane potential
Basis of impulse propagation
Electrotonic potentials
Basic properties of ion channels
Action potentials

3. Potential = Driving force (B1-A1)<(B2-A2)<(B3-A3)
Potentials in general A3 B3 A2 B2 A1 B1
Potential = Driving force
(B1-A1)<(B2-A2)<(B3-A3)
A potential difference makes a possibility for a ball to roll down (or a charge is able to work in a given system). If there is no potential difference there is no (charge) movement
The larger the potential difference the faster the movement of the ball (or the charge)
The potential difference does not necessarily mean that a movement will happen. Other factors could be important!

4. Chemical potential Part ‘A’ Part ‘B’
The number of glucose molecules in part A is larger than in part B
The membrane is permeable for glucose
This system is not in equilibrium : a concentration gradient exists from A to B
Chemical pot.
The chemical potential is the driving force for the simple diffusion of the uncharged molecules. Essentially it is identical with the concentration difference
The diffusion is effective only on short distances
Eg.: O2-CO2 gas exchange in the lung 10
Part ‘A’
Part ‘B’
The number of glucose molecules in part A is equal with the number of part B
The system is in dynamic equilibrium
No concentration gradient, no chemical potential 5 5

5. The membrane is permeable only for K-ions
20 Cl-
0 K+
0 Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl-
Chemical (K+) K+
Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl-
The membrane is permeable only for K-ions K+
Cl- K+
Cl-
Chemical (Cl-) K+
Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl- K+
Cl-
20 Cl-
0 Cl-
14 K+
6 K+
Chemic.
- 20 mV
Electrical - +

6. - + - + Δ µ= RTlg [cA] [cB] + zF(EA-EB) 0= RTlg Chemic.
In equilibrium the chemical potential and electrical potential is equal and opposite
THERE IS EQUILIBRIUM BUT
The concentrations are not equal
There is voltage difference between the two parts
Cl-
Cl- K+ K+ - +
Cl- K+
Cl- K+
Cl- K+ - +
Electrical
Δ µ= RTlg
[cA]
[cB]
+ zF(EA-EB)
0= RTlg
Chemical
Electrical
Electrochemical potential
It is zero in equilibrium

7. - + - + [cA] EA-EB= -RT lg [cB] zF [XA] EX= -60mV lg [XB] Chemic.
THERE IS EQUILIBRIUM BUT
The concentrations are not equal
Cl-
Cl- K+ K+ - +
There is voltage difference between the two parts
Cl- K+
Cl- K+
Cl-
In equilibrium the chemical potential and electrical potential is equal and opposite K+ - +
Electrical
[cA]
EA-EB= -RT lg
Equation of chemical potential
[cB] zF
[XA]
Nernst-potential,
For monovalent cations
EX= -60mV lg
[XB]

8. [XA] EX= -60mV lg [XB] [5] EK= -60mV lg [133] EK= - 85 mV
The extracellular side of heart cells there are 5 mM, in the intracellular side there are 133 mM K+
EX= -60mV lg
[XB]
[5]
EK= -60mV lg
[133]
Since the resting membrane potential is about – 80 mV, we can suggest the resting membrane potential is mainly defined by the concentration difference of K ions
EK= - 85 mV
The Nernst potential for any given ionic species is the membrane potential at which the ionic species is in equilibrium; i.e., there is no net movement of the ion across the membrane. Therefore, the Nernst potential for an ion is referred to as the equilibrium potential for that ion.

9. The Donnan-potential in all cases: ~ - 17 mV
Donnan-equilibrium K+ K+ K+ K+
[K]A x [Cl]A = [K]B x [Cl]B K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+
There is no equilibrium for the pressure! Osmotic potential will develop A- A- A- A-
Cl-
Cl-
Cl-
Cl- A- A- A- A-
Cl-
Cl-
Cl-
Cl-
The Donnan-potential in all cases: ~ - 17 mV
K > K+
Cl < Cl-
Negative Positive

10. The Donnan-potential is the membrane potential of the death cell
The Donnan-potential does not explain the development of the resting membrane potential
17 mV versus – 80 mV
In a living cell, the continuous activity of the Na/K pump prevent the development of the Donnan-potential
The Donnan-potential is the membrane potential of the death cell

11. Development of the resting membrane potential
The membrane is an electric barrier and the ion transport through the ion channels is regulated
The unequal ion distribution causes potential difference. The inner side is negative
In macroscopic level both part is electroneutral
Factors maintaining the resting membrane potential
Unequal ion distribution
Selective permeability of the membrane
Operation of the Na/K pump
Donnan-potential
All living cells has any resting membrane potential

12. - + [X+]A [X+]B [X+]A [X+]B
A&B: Unequal ion distribution + selective permeability
Unequal ion distribution + semipermeable membrane = leads to development of equilibrium potential
- In the resting membrane potential the K ion distribution is important
EX=-RT lg
[X+]A
[X+]B zF
~ -90 mV
135 mM KCl
5 mM KCl
[X+]A
EX=-60mV lg - +
[X+]B
R: standard gas constant: 8,31 J/mol/K
T: absolute temperature (K)
Z: valency of the ion (K+ = 1)
F: Faraday constant C/mol
E(K)= -90 mV
E(Na) = +70 mV
E(Ca) = +120 mV

13. C) Na/K pump
Electrogenic operation: 3 Na+ out, 2 K+ in (net 1 + charge out, moves the membrane potential to the negative direction
Contributes directly to the resting membrane potential only with mV
However it supports unequal distribution for Na and K ions
Its full inhibition causes death however certain drugs inhibit this pump (in heart failure)

14. D) Donnan-potential
It is developed by the large, negatively charged, impermeable proteins in the cell
The Donnan-potential is only about mV, therefore it has marginal role in the setting of the resting membrane potential
The Donnan-potential is the potential of the death cells

15. In glial cells and in Purkinje cells of the heart the resting membrane potential is nearly equal with K-equilibrium potential = the cell in rest is permeable only to K-ions
In neurons the resting membrane potential is a bit positive than the K-equilibrium potential = The cell has resting conductance for Na ions
In the sinus-node and in the AV-node there is no stable resting potential
Cell type
Rest.memb.pot.
Hair cell, cochlea
-15 to -40mV
Skeletal muscle
−95 mV
Smooth muscle
–60 mV
Astroglia
–80 to –90 mV
Neuron
–60 to –70 mV
Erythrocita
–8.4 mV
Chondorcita
-8 mV
Aorta smooth muscle cell
-45 mV
Cardiac ventricular cell
-80 mV
Photoreceptor cell
–40 mV

16. What is the purpose of the resting membrane potential?
In neurons the basis of the impulse propagation
In smooth muscle and skeletal muscle cells the action potential generation and contraction
In heart muscle cells the action potential generation, contraction and impulse propagation
In non-excitable cells it is important in the secondary active transport systems (e.g.: in Na-glucose symporter)
40% of the energy consumption of the cells is expended to the maintenance of the unequal ion distribution
Without resting membrane potential there is no heart beating, or neuron function, or muscle contraction…

17. Change of the resting membrane potential
External stimuli (light, sound, smell, mechanic, termic…etc)
The actual value of resting mebrane potential is changing…
Ligands (neurotransmitters, signal molecules, ions, stb), voltage change
Local potential change happens (electrotonic potential)
Becomes more negative (hyperpolarization)
It is abolished
Becomes more positive
(depolarization)
Propagates, evokes new action potentials
Action potential developed

18. X Electrotonic potential changes + + + + + + + + + + + + + + + + + + +
Ligand-activated Na channels + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - -
Axon
Ligand-activated Na channels
Negative charges - X - + +
Axon + + + + + + + +
Positive charges + + + + + + + + + + + + + + + + + +

19. The potential change declines in the function of the distance Reasons:
Na-channel
Axon + -
The potential change declines in the function of the distance
Reasons:
1.: The potential change cannot activate new ion channels
2.: The Na/K pump is restoring the initial Na and K values
Time constant: The time required during the potential decreases to the 37% percent of the initial potential
Length constant: The distance required during the potential decreases to the 37% of the initial potential

20. The larger the stimulus the larger the evoked potential
The larger the stimulus the larger the evoked potential. It has no treshold potential
Depolarization or hyperpolarization
The potentials can be strengthen or weaken each other (spatial and temporal summation)
Cannot be inhibited
There is no refractory period
When the amplitude reaches the threshold it evokes action potential
Voltage-dependent fast Na channels or Ca channels are not involved

21. It has important physiological relevance
Excitatory postsynaptic potential (EPSP)
Inhibitory postsynaptic potential (IPSP)
In the retina caused by light stimulus
Its function is the signal propagation, cell-cell communication but it is effective only in small distance
In larger distance action potentials required

22. Action potentials
After meeting with appropriate stimuli, the membrane of the excitable cells respond with a special potential change which is called action potential
The morphology of the action potentials is changing in the different tissues. Furthermore a given tissue has different types of action potentials (e.g.: heart tissue)
Function: Neurons: Impulse propagation
Skeletal and smooth muscle: development of contraction
Heart muscle: Impulse propagation and contraction

23. The action potentials have different morphology even within the given tissue
It means that the underlying ion channels are largely different in different cell types
It has important physiological consequences!

24. Sections of the action potentials
Depolarization: Initial phase of the AP. All APs has depolarization. The underlying ion current mostly the Na+ current (In sinus and AV node the Ca2+ current)
Repolarization: The AP declines to the value of the resting potential. All AP has repolarization. The underlying ion current is K-current
Plateau phase: Exist in heart muscle and smooth muscle. The underlying current is the Ca2+ current, it has important role in the contraction
Dep.
Rep.
Plat.
Resting membrane potential:
Nearly isoelectric section between two action potentials. The majority of APs have resting potential except of sinus node and AV node, heart Purkinje cells

25. Ion conductances during the action potential
The membrane permeability is not constant
The ion channels are able to open and close during an action potential

26. Goldmann-Hodgkin-Katz equation
The Nernst-equation provides the equilibrium potential of an ion in the presence of given external and internal concentrations, when the membrane permeability is constant
In the presence of multiple ions and changing permeability we use the GHK-equation
Em=-RT lg F
PK[K+]out+PNa[Na+]out+PCl[Cl-]in
PK[K+]in+PNa[Na+]in+PCl[Cl-]out

27. Nernst-equation vs. GHK-equation
There is only 1 diffusible ion in the system
More diffusible ion can be in the system
Does not take into account the changing permeability of the channels
Counts with the changing permeability
It is cannot applicable to calculate the actual value of the action potential
The actual value of the membrane potential during an AP can be calculated in any time

28. Types of action potentials I. Neuronal action potential
Resting potential: ~ 70 mV
Duration: ~ 1-2 ms
Amplitude: mV
Overshoot: mV
Afterpotential: hyperpolarization
Ion channels: fast, voltage dependent Na channel, slow voltage dependent K channel
The refractory period is very short

29. Types of action potentials II. Skeletal muscle action potential
Resting potential: ~ 90 mV
Duration: ~ 2-3 ms
Amplitude: mV
Overshoot: mV
Afterpotential: Depolarization
Ion channels: fast, voltage dependent Na channel, slow voltage dependent K channel

30. Types of action potentials III. Smooth muscle action potential
The morphology is very variable
Resting potential: ~ -50 és -60 mV
Duration : ~ ms
Amplitude: mV
Overshot: Marginal
Afterpotential: (Depolarization)
Ion channels: Mainly slow Ca2+, slow voltage dependent K-channels, oscillating background K-conductance (slow wave)

31. Types of action potentials IV. Heart muscle slow action potentials
SINUS-NODE ACTION POTENTIAL
Amplitude
Slow diastolic depolarization
Maximal diastolic potential
Sinus node, AV-node
Resting membrane potential: No (most negative point kb -50 mV)
Duration: ms
Amplitude: mV
Overshoot: Marginal
Afterpotential: No
Ion channels: Depolarization by Ca2+ current, there is no voltage dependent Na current

32. Types of action potential IV. Heart muscle action potential
Atrial, ventricular, Purkinje cells
Resting potential: mV
Duration: ms
Amplitude: mV
Phases:
0. Depolarization (INa)
Early repolarization (Ito)
Plateau (ICaL)
Delayed repolarization (IKr, IKs, IK1)
Resting potential (IK1)
Overshoot: mV
Afterpotential: No
Ion channels: Fast voltage dependent Na-current, slow Ca2+ current, slow K+-current

33. Electrotonic potentials Action potentials
Stimuli larger than threshold evoke action potential
Depolarization or hyperpolarization depending on the stimulus. No threshold
The amplitude is a all-or-nothing phenomenon. If it is evoked, always has the same magnitude. It depends on the properties of the ion channels (Na, Ca)
The amplitude is proportional with the stimulus
The amplitude is few milivolts. < 20 mV
The magnitude of the amplitude depends on the tissue type, but mainly larger than 100 mV
It is evoked by the opening of ligand-activated, mechanosensitive, thermosensitive ion channels. No voltage dependent Na channels are involved
It is evoked by voltage-gated Na and Ca channels
No refractory period
It has absolute and relative refractory period
Spatial and temporal summation
Summation is not possible because of the refractory period and the all-or-nothing phenomenon
Propagates with decrement
Propagates without decrement