To study the Concept of Action Potential and describe the stages of action potential. Ionic basis of Action Potential & its Propogation. Properties of Action Potential. Types action Potential

1. Action Potential
By
Dr Padmaja R Desai
Prof & Head
Department of Physiology
D Y Patil Medical College Kolhapur

2. OBJECTIVES
 To study the Concept of Action Potential
and describe the stages of action
potential.
 Ionic basis of Action Potential & its
Propogation.
 Properties of Action Potential.
 Types action Potential

3.  Def: - It is a rapid stereotyped self propagated
electrochemical change across the cell membrane when
adequately stimulated.
 It is electrical property of a tissue.
 It is expressed by graph taking time in milliseconds (ms)
because it is very rapid on X axis and Voltage in milli
volts (mv) on Y axis.

4. Studied in big diameter axon

5. Action Potential Recorded By
Cathode ray oscilloscope

8. Phases of Action Potential (AP)
 Stimulus Artifact
 Latent Period
 Depolarization
 Repolarization
 Hyperpolarization

9.  Stimulus Artifact.
It is due to current
leakage from the
stimulating electrodes to
the recording
electrodes.
It marks the point of
stimulus.

10. Latent Period
It is the time interval
taken by the impulse to
travel along the axon
from the site of
stimulation to the
recording electrodes.
It ends with the start of
AP.

11. Depolarization
- Depolarisation is the
ascending limb of the AP.
 Depolarization means
reduction in the membrane
potential from negative
value towards zero.

13. Threshold or firing level
 After initial 15 mv of
depolarization the state
of depolarization
increases.
 If the depolarization of
local potential produced
by a stimulus is of
sufficient value it is
called as Threshold or
firing level.
 The threshold required
is 30 to 40 mv above the
RMP.

14. Depolarization
-
 Depolarisation is the
ascending limb of the
AP.
 Depolarization means
reduction in the
membrane potential
from negative value
towards zero.

15. Repolarization
 The potential now drops
towards the resting level
and thus descending limb
of AP develops called as
Repolarization.
 The rapid rise and rapid fall
are the Spike Potential of
the axon.
 Initial 70 % of the
repolarisation is rapid but
latter on there is slow drop
in the potential called as
after depolarization

16. Hyperpolarization
 After reaching the
previous RMP level the
potential drops still more
and becomes still more
negative than its resting
value. This is called as
hyper polarization
 After some time the
potential comes back to
its previous resting value
i.e. RMP.

18. Duration of Action Potential
The size and duration of AP varies from tissue to tissue.
AP in a mammalian axon is not more than a few msec
but in cardiac muscle it is approximately 200 -300ms.

19. Ionic Basis

21. Depolarisation
 When the stimulus is applied at a spot the
Voltage gated sodium Na+ channels
begins to open and Na+ enters inside the
cell due to
A .Concentration or Chemical gradient
B .Electrical gradient.
This is called Na+ influx

23.  During depolarization phase the polarity
is reversed that is inside the cell
membrane which was negative ,now due
to entry of Na+ ions becomes positive .

24. Repolarisation
It is due to two factors:-
A .The inactivation gates of Na+ channel closes at + 35 mv
( the potential does not reach to the Nernst potential
for Na ions ). So no Na+ ions enters from ECF to ICF.
B .At the same time in response to the original stimulus
the voltage gated potassium K+ channels starts
opening.

27.  In the initial part of the repolarisation the K+ efflux or exit
is rapid so there is sudden drop in the potential. This is
responsible for initial 70 % of rapid repolarization.
 Latter on there is slow drop in the potential leading to after
depolarization.
 It is due to slow K+ efflux and also some of the voltage
gated K+ channels closes.

28. Hyperpolarisation
 It is increase in the intracellular negativity more than the
RMP.
 The potential drops below RMP because some of the voltage
gated K+ channels are still open and K+ efflux continues
taking the potential to the Nernst Potential of K+ i.e. -95 mv.
 After some time the K+ channels closes and the potential
comes back to its original resting value (RMP) .

29. Significance of AP
1. It controls the cell functions.
2. Signals are transmitted from nerve to nerve or nerve to muscle in
the form of AP.
3. It is the language of excitation in the excitable cells so it is
coupled with response known as excitation contraction coupling.
4.ECG ,EMG, ERG, EEG etc are basically action potentials
5.Local anaesthetics blocks the opening of sodium channels so the
generation of action potential is prevented.

30. Characteristics of AP
1.It requires threshold stimulus.
2.AP obeys All or None Law.

31. 3.AP has Refractory
Period.
After stimulation there
is a brief period of
interval during which
tissue cannot be
stimulated again.
The Refractory period
is of two types Absolute
and Relative .Therefore
two AP can never be
summated.

32. 4. self propagated Non
decrementally
1.local currents.
2.positive feed back
mechanism

33. Classification of AP
Depending on Waveform
 Spike Potential :-Seen in nerves, skeletal muscles, smooth muscles
 Plateau Potential :-In cardiac muscle and some smooth muscles

36. References
1. Text book of Medical Physiology
-Guyton & Hall, 12th edition.
2. Ganong’s review of Medical Physiology
- 23rd edition.
3. Text book of Medical Physiology
- 2nd edition
4. Net sources ( Acknowledge for all online source)
5. Text book of Medical Physiology
- Prof. A.K.Jain