1. Introduction To Action Potential
Physiology
page 1
Introduction To Action Potential
About words first. We have action movies, action heroes, action auctions… Whatever the word “potential” refers to, why “action”?
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

2. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Content
Terminology
What action potential really means
Where we can find it
Neurones
Networks of neurones
Processing of information
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

3. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Action potential definition
A temporary reversal in the potential difference across a plasma membrane that occurs when a cell has been activated by a stimulus.
This explains the word “action”. Now, what kind of potential is it?
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

4. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
U = m x g x h
The potential we are talking about is a ‘potential energy’.
Potential energy = potential to deliver work
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

5. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Tsunami
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

6. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
= 1.5 V
~ 220 V
U = Q1 x Q2 / r
U … voltage
It’s an electric potential or voltage. When the battery is strong, it can be compared to a large dam. It has a potential energy ready to be delivered and generate work (that we can measure).
Q1 … charge
Q2 … charge
r … distance
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

7. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential - +
Electric potential
is a voltage that
we measure with
a voltmeter.
… that we measure with a voltmeter.
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

8. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential - +
Electric potential
is a voltage that
we measure with
a voltmeter.
It doesn’t matter whether we measure on a battery or neurone.
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

9. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential - +
Electric potential
is a voltage that
we measure with
a voltmeter.
It doesn’t matter whether we measure on a battery or neurone.
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

10. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential - +
Electric current
is a movement of
free electrons.
… that we measure with a voltmeter.
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

11. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential - +
Ions exert force
on free electrons
which move the
voltmeter pointer.
Ions: anion, cation, onion.
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

12. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential - +
Measuring electric potential is only one way of looking at the complex mechanism.
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

13. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
So, under “action potential”, remember the whole mechanism rather than the movement of a voltmeter pointer.
Better even, remember everything! 
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

14. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Where can we see action potential?
Neurones
Pacemaker
cells
In its resting state, a neuron is polarized, with its inside at about -70mV relative to its surroundings. When an excitatory neurotransmitter is released by the presynaptic neuron and binds to the postsynaptic dendritic spines, ligand-gated ion channels open, allowing sodium ions to enter the cell. This may make the postsynaptic membrane depolarized (less negative). This depolarization will travel towards the axon hillock, diminishing exponentially with time and distance. If several such events occur in a short time, the axon hillock may become sufficiently depolarized for the voltage-gated sodium channels to open. This initiates an action potential that then propagates down the axon.
Neuromuscular junction
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

15. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

16. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Action potential
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

17. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Conduction in myelinated vs. non-myel. neurones
10 – 120 m/s
m/s
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

18. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Saltatory conduction
Saltatory conduction is the mechanism by which signals move more quickly in myelinated neurons
* Metaphor: Cars (signals) move faster even with fewer lanes (smaller fibers) in the express lane (saltatory conduction)
o less merging
o still need to fill up car at a gas station
Key points to this metaphor, it is the less frequent merging that allows the cars to move faster, in those long stretches where there are no on-ramps (ion channels), there is no exchange of charges leaving the cars free to just shoot quickly to the next on-ramp. But there are some on-ramps (nodes of ranvier) that allow an influx of cars that have full tanks of gas (amplification/depolarization of membrane). Without these periodic gas stations, the cars would run out of gas (the signal would decay away too much to trigger depolarization at the next section with ion channels).
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

19. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
5 m
18 m
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

20. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

21. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Synaptic transmission
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad

22. Introduction To Action Potential
Physiology
page 2
Introduction To Action Potential
Physiology, 2nd Faculty of Medicine, Charles University Copyright © 2010 Ludek Nerad