Action potentials

Dr.Mohammed Sharique Ahmed Quadri
Assistant prof. Physiology
Al Maarefa College
• Define graded potential and action potential.
• Describe characteristics of graded potential
• Compare between the different phases of excitability
during a nerve action potential( relative refractory
period, absolute refractory period)
• step-by-step, explain how an action potential is
• Describe the characteristics of action potential.
Two major regulatory systems of the body: that
ensure survival of the body:
The Nervous System: Neural Communication
Accomplished by Nerve Cells( signals)
The Endocrine System: Hormonal
Communication Accomplished by Hormones
Neural Communication
• Nerve and muscle are excitable tissues
• Can undergo rapid changes in their membrane
• Can change their resting potentials into electrical
– Electrical signals are critical to the function of the
nervous system and all muscles
Neural Communication
• Electrical Signals are produced due to changes in ions
movement across the membrane.
• Two kinds of potential change( electrical signals)
– Graded potentials
• Serve as short-distance signals
– Action potentials
• Serve as long-distance signals
• Graded Potential is local change in the membrane
E.g. RMP changes from -70 mv to -60 mv (a 10mv
• Graded Potential is due to Na+ entry at the small
specialized region of plasma membrane.
• Graded Potential can be summated by giving
stronger stimulus.
Current Flow During a Graded Potential
Graded Potentials
• The Stronger a triggering event is, The larger
the resultant graded Potential
• Graded Potential spread by passive Current
• Graded potentials die over short distances
Graded Potential
• Occurs in small, specialized region of excitable cell
• Magnitude of graded potential varies directly with
the magnitude of the triggering event
‘Important Points’
• It is localized.
• It can be summated.
• Longer the stimulus – longer the duration of graded
• Graded Potential die down over short distance.
• Example of Graded Potential:
- Receptor Potential, Pace-maker Potential, end plate
Action Potentials
• Brief, rapid, large (100mV) changes in membrane
potential during which potential actually reverses
• Involves only a small portion of the total excitable
cell membrane
• Do not decrease in strength as they travel from their
site of initiation throughout remainder of cell
Changes in Membrane Potential
During an Action Potential
• 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.
Channels & Local Potentials
• The ionic basis of the action potential
membrane permeability
ion channels
Voltage gated Na+ and K+ Channels
Action Potentials
Action Potentials
Permeability Changes and Ion Fluxes During an Action Potential
Action Potentials
The Na+/K+ pump gradually restores the
concentration gradients disrupted by action
– Sodium is pumped into the ECF
– Potassium is pumped into the ICF
Action Potentials
• Additional characteristics
– As the action potential develops at one point in
the plasma membrane, it regenerates an identical
action potential at the next point in the
– Therefore, it travels along the plasma membrane
• 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 at all .
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
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
Refractory Period
The action potential (AP)
An action potential is: A regenerating depolarization of membrane
potential that propagates along an excitable membrane.
• Action potentials:
are all-or-none events
need to reach threshold
have constant amplitude
do not summate
are initiated by depolarization
involve changes in permeability
rely on voltage-gated ion channels
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
3. It dies down over short
4. Can be summated.
5. Does not obey all or none
Action Potential
1. Stimulus reaches threshold
level therefore causes AP.
2. Stimulus causes
depolarization to threshold
3. It is propagated.
4. Can not be summated.
5. Obeys all or none law.
• Human physiology by Lauralee Sherwood, seventh
• Text book physiology by Guyton &Hall,11th edition
• Text book of physiology by Linda .s contanzo,third

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