GRADED POTENTIAL
&
ACTION POTENTIAL
DR.ZAHOOR ALI SHAIKH
LECTURE ---8
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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.
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GRADED POTENTIAL
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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.
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Graded Potential
 Occurs in small, specialized region of excitable cell
membranes
 Magnitude of graded potential varies directly with the
magnitude of the triggering event
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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.
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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
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ACTION POTENTIAL
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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.
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ACTION POTENTIAL (cont)
How action potential occurs ?
 When stimulus is applied, RMP which is
-70mV changes to Threshold potential which is -50 or -55
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).
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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.
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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.
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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.
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Action Potentials
 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
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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.
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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.
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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
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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.
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Absolute and Relative Refractory Period
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Comparison of Graded
Potentials and Action Potentials
Graded Potential
1. Stimulus does not reach
threshold level.
2.
Stimulus causes local
change in membrane
potential e.g. -70 to -60mv
It dies down over short
distance.
4. Can be summated.
3.
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Action Potential
1. Stimulus reaches threshold
level therefore causes AP.
2.
Stimulus causes
depolarization and change
up to +30mV.
3.
It is propagated.
4.
Can not be summated.
Comparison of Graded Potentials
and Action Potentials (cont)
Graded Potential
Does not obey all or
none law.
6. It has no refractory
period.
7. It decreases with
distance.
5.
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Action Potential
5. Obeys all or none law.
6. It has absolute and relative
refractory period.
7. Propagated in
undiminished fashion.
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
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THANK YOU
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