Objectives

Basics of electrophysiology

1. Know the meaning of Ohm’s Law

2. Know the meaning of ionic current

3. Know the basic electrophysiology terms

6. Understand the terms ‘activation’ and ‘inactivation’

4. Know the effects of changing membrane potential in excitable cells

5. Know the effects of changing ionic conductances in excitable cells

antiarrhythmics

What do the following categories of drugs have in common?

anxiolytics

anticonvulsants

sedatives/hypnotics

anestheticsantidiabetics

antihypertensives

They all include drugs that act on ion channels

Ion channels are interesting to pharmacists

Therefore...

Channel selectivity

Na+

K+Ca2+

Cl- molecules

Voltage

Extracellular ligand

Intracellular ligand

Channel gating

Ligand-gated ion channels

(Dr. Ishmael)

Voltage-gated ion channels

Voltage-gated ion channels

• Voltage sensor• Inactivation• Voltage-dependent block

Voltage sensor

Inactivation

+

-intracellular

extracellular

Voltage-dependent block+

+

-intracellular

extracellular

+

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

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.

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.

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.

Inward current causes depolarization

Outward current causes repolarization/hyperpolarization

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

2 msec

mV

Excitable cells fire action potentials

A nerve cell (neuron)

Cell body

axon

Hodgkin and Huxley

Voltage clamp

Depolarization changes the conductance of the membrane

Inward current is carried by Na+ ions

Outward current is carried by K+ ions

Hodgkin & Huxley reconstructed the action potential

Electrochemical gradients

Which way will they go?

At what rate will they go through?

Ion channels allow ions to pass through

Why would ions want to pass through?

Concentration gradient (chemical gradient)

Net flow

Membrane potential (electrical gradient)

+ -

+

-

Anion channel

Cation channel

Membrane potential (electrical gradient)

+ -

+

- Anion channel

Cation channel

Electrochemical gradient

-

--

--

-

--- -

-

-

--

-

--

-

-- -

-

+

+

+

+

+ ++

+++++

+

+

++

+ ++ +

+

+

+

-

The Nernst potential

[Xi] = Ionic concentration inside the cell[Xo] = Ionic concentration outside the cellzX = ionic valence (number and sign (+ or -) of charges on ion)

( )EX = 60zX

. log [Xo]

[Xi]

(At physiological temperature)

EX is in millivolts (mV) [Xo] and [Xi] are in millimolar (mM)

ionExtracellular concentration (mM)

Intracellular concentration (mM)

Nernst potential (mV)

Na+

K+

Ca2+

Cl-

145

4

1.5

123

12

155

0.0001

4.2

67

-98

129

-90

If Cl- is passively distributed (not pumped), ECl = resting potential

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.

Ohm’s law: V=IR; I=GV

V or E = potential (Volts); I = current (Amps);R = resistance (Ohms); G = 1/R = conductance (Siemens)

The cell membrane is a resistor

I

V

High G

Low G

Slope = conductance (G)

Ohm’s Law

I=GV

I

V

Ohm’s Law

INa=G(Em-ENa)

IK=G(Em-EK)

ENa = 67 mV

EK = -98 mV

+++++++++

-- --------

INa

IKICl

ATPase

At rest, ionic gradients are maintained by the Na+-K+ ATPase

2 K+

3 Na+

membrane potential = -90 mV GNa is low

GK is high

+++++++++

-- --------

INa

IK

-INa = -((-90mV)-ENa) x GNa = (IK) = ((-90mV)-EK) x GK

ICl

ECl = -90 mV

outside

inside

(Ca2+ channels not shown)

If the membrane potential is not changing,

membrane is depolarizingGNa is very high

GK is high

+ + + + +

- - - - -

INa > -(IK)

Na+ channels just opened

outside

inside

INa

IKICl

(no significant effect on concentration)

GNa is very high

GK is high

+ + + + +

- - - - -

INa = (30mV-ENa) x GNa = -(IK + ICl)

= -[(30mV-EK) x GK + (30mV-ECl) x GCl ]

membrane potential = +30 mV

INa

IK

ICl

outside

inside

(outward current, inward Cl flow)

membrane potential = -90 mV

+++++++++

-- --------

INa

IKICl

ECl = -90 mV

outside

inside

What will happen to the membrane potential if we open more Cl- channels?

What will happen to excitability if we open more Cl- channels?

mT T

X

TXE

g

gE

g

gE

g

gE 1

12

2( ) ( ) ... ( )

chord conductance equation

mK

TK

C l

TC l

N a

TN a

C a

TC aE

g

gE

g

gE

g

gE

g

gE ( ) ( ) ( ) ( )

Electrical signaling changes intracellular Ca2+

[Na+]i, [K+]i, [Cl-]i don’t change significantly.

Depolarization opens Ca2+ channels. [Ca2+]i increases.

Ca2+

Action potential

axon

Postsynaptic cell

receptor

Neurotransmitter

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.

In an excitable cell, depolarization causes activation of Na+ channels, followed by inactivation of Na+ channels and activation of K+ channels.

Inward current depolarizes the membrane.Outward current repolarizes/hyperpolarizes the membrane.

These processes underlie the action potential of the nerve axon.

Ion selectivity varies among ion channels.

Net movement of ions through channels is always down the electrochemical gradient.

The membrane potential depends on the relative conductance of the membrane for K+, Na+, Cl- and Ca2+ ions.

In cells that don’t actively transport Cl-, opening Cl- channels decreases excitability by stabilizing the membrane potential.

Concentration gradients are maintained by ATPases and ion exchangers

The intracellular response to electrical signaling is a change in cytoplasmic Ca2+.