Objectives

1
Basics of electrophysiology
Objectives
1. Know the meaning of Ohms Law
2. Know the meaning of ionic current
3. Know the basic electrophysiology terms
4. Know the effects of changing membrane
potential in excitable cells
5. Know the effects of changing ionic
conductances in excitable cells
6. Understand the terms activation and
inactivation
2
What do the following categories of drugs have in
common?
antiarrhythmics
anxiolytics
antihypertensives
anticonvulsants
sedatives/hypnotics
antidiabetics
anesthetics
They all include drugs that act on ion channels
3
Therefore...
Ion channels are interesting to pharmacists
4
Channel selectivity
Na
K
Ca2
molecules
Cl-
5
Channel gating
Voltage
Extracellular ligand
Intracellular ligand
6
Ligand-gated ion channels
(Dr. Ishmael)
Voltage-gated ion channels
7
Voltage-gated ion channels

• Voltage sensor
• Inactivation
• Voltage-dependent block

8
Voltage sensor
9
Inactivation
extracellular

-
intracellular
10
Voltage-dependent block

extracellular

-
intracellular

11
A guide to Electrophysiologese
Membrane potential (Em) The voltage difference
across the cell membrane (inside vs outside)
(millivolts)
Resting potential The membrane potential at
which the membrane spends most of its time
Action potential The transient change in
membrane potential due to active properties of
the membrane
Electrotonic potential A change in membrane
potential due to passive properties of the
membrane
12
A guide to Electrophysiologese
Depolarization A change of membrane potential in
the positive direction.
Repolarization Return of the membrane potential
to the resting potential after a depolarization.
Hyperpolarization A change of membrane potential
to a more negative value than the normal resting
potential.
13
A guide to Electrophysiologese
Inward current Net movement of positive ions
into the cell, or net movement of negative ions
out of the cell. By convention, plotted as
negative current.
Inward current causes depolarization
Outward current Net movement of positive ions
out of the cell, or net movement of negative ions
into the cell. By convention, plotted as positive
current.
Outward current causes repolarization/hyperpolariz
ation
14
A guide to Electrophysiologese
Excitable cell A cell that can fire action
potentials
Excitability The ability to fire action
potentials
Threshold potential The membrane potential at
which an action potential fires
15
Excitable cells fire action potentials
mV
2 msec
16
A nerve cell (neuron)
axon
Cell body
17
Hodgkin and Huxley
Voltage clamp
18
Depolarization changes the conductance of the
membrane
19
Inward current is carried by Na ions
Outward current is carried by K ions
20
Hodgkin Huxley reconstructed the action
potential
21
Electrochemical gradients
Ion channels allow ions to pass through
Why would ions want to pass through?
Which way will they go?
At what rate will they go through?
22
Concentration gradient (chemical gradient)
Net flow
23
Membrane potential (electrical gradient)
Anion channel
Cation channel

-
24
Membrane potential (electrical gradient)
Anion channel
Cation channel

-
25
Electrochemical gradient
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-

-
-
-
-
26
The Nernst potential
27
Extracellular concentration (mM)
Intracellular concentration (mM)
Nernst potential (mV)
ion
Na
145
12
67
K
4
155
-98
Ca2
1.5
0.0001
129
Cl-
123
4.2
-90
If Cl- is passively distributed (not pumped), ECl
resting potential
28
The different concentrations of physiological
ions means that they have different Nernst
potentials.
Therefore, at any membrane potential, there is a
driving force on at least some of the
ions. (driving force membrane potential
Nernst potential)
At physiological membrane potentials, the driving
force is inward for Na and Ca2 ions and outward
for K ions.
Therefore, at physiological membrane potentials,
there are inward Na and Ca2 currents and
outward K currents.
29
Ohms law VIR IGV
V or E potential (Volts) I current (Amps) R
resistance (Ohms) G 1/R conductance
(Siemens)
The cell membrane is a resistor
30
Ohms Law
High G
I
IGV
Low G
V
Slope conductance (G)
31
Ohms Law
I
IKG(Em-EK)
EK -98 mV
V
ENa 67 mV
INaG(Em-ENa)
32
At rest, ionic gradients are maintained by the
Na-K ATPase
INa
2 K









ATPase
-
-
-
-
-
-
-
-
-
-
ICl
IK
3 Na
33
membrane potential -90 mV
GNa is low
GK is high
INa
outside









-
-
-
-
-
-
-
-
-
-
inside
ICl
IK
If the membrane potential is not changing,
-INa -((-90mV)-ENa) x GNa (IK) ((-90mV)-EK)
x GK
ECl -90 mV
(Ca2 channels not shown)
34
Na channels just opened
GNa is very high
membrane is depolarizing
GK is high
INa
outside





-
-
-
-
-
inside
ICl
IK
(no significant effect on concentration)
INa gt -(IK)
35
GNa is very high
membrane potential 30 mV
GK is high
ICl
INa
-
-
-
-
-
outside





inside
IK
(outward current, inward Cl flow)
INa (30mV-ENa) x GNa -(IK ICl)
-(30mV-EK) x GK (30mV-ECl) x GCl
36
membrane potential -90 mV
ECl -90 mV
INa
outside









-
-
-
-
-
-
-
-
-
-
inside
ICl
IK
What will happen to the membrane potential if we
open more Cl- channels?
What will happen to excitability if we open more
Cl- channels?
37
chord conductance equation
38
Electrical signaling changes intracellular Ca2
39
Here are the main points again
Nerves, muscles and other excitable cells use
electrical signaling
Physiologically, Na channels always pass inward
current K channels always pass outward current.
Inward current depolarizes the membrane. Outward
current repolarizes/hyperpolarizes the membrane.
In an excitable cell, depolarization causes
activation of Na channels, followed by
inactivation of Na channels and activation of K
channels.
These processes underlie the action potential of
the nerve axon.
40
Net movement of ions through channels is always
down the electrochemical gradient.
Concentration gradients are maintained by ATPases
and ion exchangers
The membrane potential depends on the relative
conductance of the membrane for K, Na, Cl- and
Ca2 ions.
Ion selectivity varies among ion channels.
In cells that dont actively transport Cl-,
opening Cl- channels decreases excitability by
stabilizing the membrane potential.
The intracellular response to electrical
signaling is a change in cytoplasmic Ca2.