The document discusses the ionic basis and recording of action potentials. It begins with an introduction to excitable tissues and how they generate electrical signals. It then covers the history of discoveries in this area including Galvani's work in the 18th century and more recent work by Hodgkin, Huxley, Neher and Sakmann. The document discusses resting membrane potential, graded potentials, action potentials and the ionic basis involving sodium, potassium and chloride ions. It describes how action potentials are recorded using microelectrodes, amplifiers and oscilloscopes. Finally, it covers some applied aspects like tetrodotoxin poisoning and channelopathies.

1. 1
IONIC BASIS AND RECORDING OF
ACTION POTENTIAL
Dr.Anu Priya.J.
12/11/2013

2. 2









Introduction
History
Resting membrane potential
Graded potential
Action potential
Ionic basis
Types
Recording
Applied aspects
12/11/2013

3. Introduction
3
• Nerve and muscle are excitable tissues
• Can undergo rapid changes in their membrane potentials
• Change their resting potentials into electrical signals that aid
in cellular communication
• These signaling events are mediated by ion channels
12/11/2013

4. History
4

Since the 18th century, when Galvani introduced the
concept of "animal electricity", electric potentials have
been observed and recorded in different nerves and
muscles.
12/11/2013

5. 5

Illustration of Italian physician Luigi Galvani's experiments, in
which he applied electricity to frogs legs; from his book De
Viribus Electricitatis in Motu Musculari (1792).
12/11/2013

6. History
6


1963- A. L. Hodgkin and A. F. Huxley - Nobel prize in Physiology or
Medicine- study of sodium and potassium channels – voltage
clamp method
Sir John Carew Eccles-shared-work on synapse
12/11/2013

7. History
7


The patch clamp technique - Erwin Neher and Bert
Sakmann - Nobel Prize in Physiology or Medicine in 1991
Record the currents of single ion channels for the first
time, proving their involvement in fundamental cell
processes such as action potential conduction.
12/11/2013

8. History
8

Sir A. F. Huxley passed away on 30 May 2012 – age 94 years
12/11/2013

9. Resting Membrane Potential
9



It is the potential difference existing across the cell
membrane at rest
Interior of the cell is negatively charged in relation to the
exterior
State of polarisation
12/11/2013

10. Resting Membrane Potential
10
RMP is maintained by:
1.
Natural concentration gradient
2.
Selective permeability of cell membrane
3.
Impermeable anions
4.
Sodium-potassium ATPase pump
12/11/2013

11. Resting Membrane Potential
11







Neurons have a selectively permeable membrane
During resting conditions membrane is:
 permeable to potassium (K+) (channels are open)
 impermeable to sodium (Na+) (channels are closed)
Diffusion force pushes K+ out (concentration gradient)
This creates a positively charged extra-cellular space.
Electrostatic force pushes K+ in
Thus, there is a ‘dynamic equilibrium’ with zero net
movement of ions.
The resting membrane potential is negative
12/11/2013

12. Graded potential
12
Subthreshold stimuli cause sensory receptors to depolarize and
produce a voltage called a generator potential(Receptor
Potential)

Does not obey all or none law

Graded response

it is not propagated

Summation

No refractory period

Duration(5-10 ms)
12/11/2013

13. Graded potential & Action potential
13
12/11/2013

14. Action potential
14





An action potential is a rapid change in the membrane
potential in response to a threshold stimulus followed by
a return to the resting membrane potential.
The size and shape of action potentials differ considerably
from one excitable tissue to another.
An action potential is propagated with the same shape and
size along the whole length of a cell.
The action potential is the basis of the signal-carrying
ability of nerve cells.
Voltage-dependent ion channel proteins in the plasma
membrane are responsible for action potentials.
12/11/2013

15. 15
12/11/2013

16. 16
12/11/2013

17. Hodgkin cycle
17
12/11/2013

18. Graded potential & Action potential
18
12/11/2013

19. 19
12/11/2013

20. 20
12/11/2013

21. 21
12/11/2013

22. 22
Role of other ions
 Impermeable Anions
 Calcium ions
12/11/2013

23. Properties
23

Voltage inactivation

Refractory period

All or none law

Propagative

depolarization and repolarization

No summation
12/11/2013

24. Recording of action potential
24


Mammalian axons less than 20 μm diameter
Squid-giant cells-largest axon in neck regionabout 1 mm diameter
12/11/2013

25. Recording of action potential
25

a)
b)
Requirements of instrument used :
It should be capable of responding extremely rapidly
The potential changes which are in millivolts has to be
amplified before being recorded
12/11/2013

26. Recording of action potential
26

1.
2.
3.
The instruments used are:
Microelectrodes
Electronic amplifiers
Cathode ray oscilloscope (CRO)
12/11/2013

27. Microelectrodes
27

Micropipette – tip size less than 1 mm diameter

Filled with strong electrolyte solution- KCl

Resistance – 1 billion Ω

The tip of the micropipette is passed through the cell membrane of
the nerve fibre

Indifferent electrode – in extracellular fluid

Connected to cathode ray oscilloscope through amplifier
12/11/2013

28. Electronic amplifier
28

magnify the potential changes of the tissue to be
recorded on the oscilloscope screen
12/11/2013

29. Cathode ray oscilloscope
29
Rapid and instantaneous recording of electrical
events of living tissues
 Parts
i.
Glass tube
ii.
Cathode
iii.
Fluorescent screen
iv.
Two sets ( horizontal and vertical ) electrically
charged plates

12/11/2013

30. Cathode ray oscilloscope
30
12/11/2013

31. Recording of action potential
31


Patch clamp method
Voltage clamp method
12/11/2013

32. Patch clamp method
32
12/11/2013

33. Voltage clamp method
33
12/11/2013

34. Types
34



Monophasic
Biphasic
Compound
12/11/2013

35. Biphasic action potential
35
12/11/2013

36. Biphasic action potential
36
12/11/2013

37. Compound action potential
37




Peripheral nerves in mammals are made up of many
axons bound together in a fibrous envelope called
the epineurium.
Potential changes recorded extracellularly from
such nerves therefore represent an algebraic
summation of the all-or-none action potentials of
many axons.
The thresholds of the individual axons in the nerve
and their distance from the stimulating electrodes
vary.
With subthreshold stimuli, none of the axons are
stimulated and no response occurs.
12/11/2013

38. 38




When the stimuli are of threshold intensity, axons with
low thresholds fire and a small potential change is
observed.
As the intensity of the stimulating current is increased,
the axons with higher thresholds are also discharged.
The electrical response increases proportionately until
the stimulus is strong enough to excite all of the axons in
the nerve.
The stimulus that produces excitation of all the axons is
the maximal stimulus, and application of greater,
supramaximal stimuli produces no further increase in
the size of the observed potential.
12/11/2013

39. Compound action potential
39
12/11/2013

40. Applied aspects
40
Hereditary spherocytosis (HS)
 Plasma membrane of red cells three times more
permeable to Na+
 The level of Na+,K+-ATPase elevated.
 When HS red blood cells have sufficient glucose to
maintain normal ATP levels, they extrude Na+ as
rapidly as it diffuses into the cell cytosol. Hence
the red blood cell volume is maintained.
12/11/2013

41. Applied aspects
41




When HS erythrocytes are delayed in the venous
sinuses of the spleen, where glucose and ATP are
present at low levels, the intracellular ATP
concentration falls.
Therefore, Na+ cannot be pumped out by the
Na+,K+-ATPase as rapidly as it enters.
The red blood cells swell - osmotic effect of
elevated intracellular Na+ concentration.
Spleen targets these swollen erythrocytes for
destruction - anemia.
12/11/2013

42. Applied aspects
42


Tetrodotoxin (TTX)- a potent poison - specifically blocks
the Na+ channel- binds to the extracellular side of the
sodium channel.
Tetraethylammonium (TEA+), another poison, blocks the
K+ channel when it is applied to the interior of the nerve
fiber.

12/11/2013

43. Applied aspects
43

The ovaries of certain species of puffer fish, also known as
blowfish, contain TTX. Raw puffer fish - Japan.
12/11/2013

44. Applied aspects
44

Saxitoxin is another blocker of Na+ channels that is
produced by reddish-colored dinoflagellates that are
responsible for so-called red tides.
12/11/2013

45. Applied aspects
45

Shellfish eat the dinoflagellates and concentrate saxitoxin in
their tissues.

A person who eats these shellfish may experience lifethreatening paralysis within 30 minutes after the meal
12/11/2013

46. Applied aspects
46




In an inherited disorder, called primary hyperkalemic
paralysis, patients have episodes of painful spontaneous
muscle contractions, followed by periods of paralysis of the
affected muscles.
Elevated levels of K+ in the plasma and extracellular fluid.
Some patients with this disorder have mutations of voltagegated Na+ channels that result in a decreased rate of voltage
inactivation.
This results in longer-lasting action potentials in skeletal
muscle cells and increased K+ efflux during each action
potential. This can raise the extracellular levels of K+.
12/11/2013

47. Applied aspects
47




The elevation of extracellular K+ causes depolarization of
skeletal muscle cells.
Initially, the depolarization brings muscle cells closer to
threshold, so that spontaneous action potentials and
contractions are more likely.
As depolarization of the cells becomes more marked, the cells
accommodate because of the voltage-inactivated Na+
channels.
Consequently, the cells become unable to fire action
potentials and are unable to contract in response to action
potentials in their motor axons.
12/11/2013

48. Applied aspects
48


Low potentials recorded in neuropathy and spinal cord
compression
INJURY POTENTIAL
The difference in electrical potential between the
injured and uninjured parts of a nerve or muscle – also
called demarcation potential
12/11/2013

49. Applied aspects
49
TETANY
 Hypocalcemia – sodium channels activated by
very little increase of membrane potential from
resting state
12/11/2013

50. 50
12/11/2013

51. References
51

Guyton and Hall Textbook of Medical Physiology 12th edition

Ganong's Review of Medical Physiology 23rd edition

Berne & Levy Physiology 6th edition

Boron and Boulpaep Medical physiology 2nd edition

Basics of Medical physiology by Dr.Venkatesh.D 3rd edition

Textbook Of Medical Physiology by Indu Khurana 1st edition

Internet references
12/11/2013