1. GRADED POTENTIAL & ACTION POTENTIAL
DR.ZAHOOR ALI SHAIKH
LECTURE ---8

2. Different Stimuli Cause Electrical Signals
Electrical Signals are produced due to changes in ions movement across the membrane.
Electrical Signals produce:
1. Graded Potential or Local Potential
2. Action Potential – It sends signal over long distance.

3. GRADED POTENTIAL

4. GRADED POTENTIAL
Graded Potential is local change in the membrane potential.
E.g. RMP changes from -70 mv to -60 mv (a 10mv change).
Graded Potential is due to Na+ entry at the small region of plasma membrane.
It is localized change in the membrane.
Graded Potential can be summated by giving stronger stimulus.

6. Graded Potential
Occurs in small, specialized region of excitable cell membranes
Magnitude of graded potential varies directly with the magnitude of the triggering event

7. GRADED POTENTIAL ‘Important Points’
It is localized.
It can be summated.
Longer the stimulus – longer the duration of graded potential.
Graded Potential die down over short distance.

8. GRADED POTENTIAL ‘Important Points’ (cont)
Graded Potential does not reach the threshold.
It has no refractory period.
Example of Graded Potential:
- Receptor Potential, Pace-maker Potential

9. ACTION POTENTIAL

10. ACTION POTENTIAL
Action Potential is brief, rapid, large, about 100 mV change in Membrane potential in which inside of excitable cell becomes more positive than outside, for short time.
Action Potential are conducted or propagated.

11. ACTION POTENTIAL (cont)
How action potential occurs ?
When stimulus is applied, RMP which is
-70mV changes to Threshold potential which is -50 or mV.
At threshold potential, rapid depolarization takes place and inside membrane becomes more positive.
DEFINITION
Action Potential is process of depolarization and Repolarization when stimulus is given.
Action Potential leads to electrical impulse (nerve impulse).

12. ACTION POTENTIAL (cont)
What is the cause of Depolarization?
Depolarization is due to Na+ influx (going inside).
Peak potential is usually at +30 mV.
After depolarization, membrane Repolarizes quickly, dropping back to Resting Membrane Potential -70 mV.

13. ACTION POTENTIAL (cont)
What is the cause of Repolarization ?
Repolarization is due to k+ efflux (going outside) .
If more K+ goes out, it causes more Negativity inside e.g. -80 mV and it is called HYPERPOLARIZATION.
Time for AP in neuron is 1 msec.

14. ACTION POTENTIAL (cont)
AP is referred as Spike potential because it appearance looks like spike.
When excitable membrane produces AP, it is said it is Firing.
Therefore Action potential, Spike, Firing all refer to same thing.

18. Action Potentials Na+ is pumped out into the ECF
During Action Potential (depolarization Na+ in and Repolarization K+ out) electrical balance is there, but ionic balance is not there. It is achieved by
Na+/K+ pump which gradually restores the concentration gradients disrupted by action potentials.
Na+ is pumped out into the ECF
K+ is pumped in into the ICF

19. ACTION POTENTIAL ‘Important Points’
If initial stimulus (triggered depolarization) does not reach threshold stimulus – NO AP takes place.
Therefore Threshold level is ALL or NONE point.

20. ACTION POTENTIAL (cont)
Action Potential follows All or None Law.
It means excitable membrane either responds to a stimulus with a maximal action potential or it does not respond with an action potential.

21. Refractory period during AP
Refractory period is that period ,during which no new action potential can be initiated.
Refractory Period – Two Types:
1– Absolute Refractory period
2– Relative Refractory

22. Refractory Period Absolute Refractory Period
It is that period of action potential during which no new action potential can be initiated even by strong stimulus.
Relative Refractory period
It is that period during which second action potential can be produced by very strong stimulus.

23. Absolute and Relative Refractory Period

24. Comparison of Graded Potentials and Action Potentials
Stimulus does not reach threshold level.
Stimulus causes local change in membrane potential e.g. -70 to -60mv
It dies down over short distance.
Can be summated.
Stimulus reaches threshold level therefore causes AP.
Stimulus causes depolarization and change up to +30mV.
It is propagated.
Can not be summated.

25. Comparison of Graded Potentials and Action Potentials (cont)
Does not obey all or none law.
It has no refractory period.
It decreases with distance.
5. Obeys all or none law. 6. It has absolute and relative refractory period. 7. Propagated in undiminished fashion.

26. What You Should Know From This Lecture
Graded potential ( local potential ) and its characteristics
Threshold level
Action potential (AP)
-- Depolarization, Repolarization, Hyperpolarization.
Absolute refractory period during AP
Relative refractory period AP
Difference between Graded potential and Action potential

27. THANK YOU