Action potential By Dr. Mrs. Padmaja R Desai

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

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

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

8. 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.

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

10. 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.

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

12. 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

13. 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.

14. 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.  

15. Ionic Basis

16. 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

17. 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 .    

18. 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 .

19. 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.

20. 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) .

21. 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.  

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

23. 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.

24. 4 . self propagated Non decrementally 1.local currents . 2.positive feed back mechanism

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

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