Action Potential: Stages, Types, Propagation

1. Dr. Rashid Mahmood
Action Potential & its Propagation
By Prof. Dr. Rashid Mahmood

2. Dr. Rashid Mahmood
Objectives
• Goal /Aim:
• By the end of this session students should be able to understand the mechanism of
production & propagation of electrical changes occurring in excitable tissues.
• Student should be able to:
• To know the types of Disturbances across the Cell Membrane include action
potential & local response
• Define Action Potential
• Describe the Typical Action Potential in Large Myelinated Nerve fiber
• Explain & draw the Stages of Action Potential
• Describe the Types of Channels involved in Various Excitable Tissues
• Tell the difference between Voltage Gated Na+ and K+ Channels
• Describe the Propagation (Transmission) of Action Potential
• Explain Saltatory Conduction
• Differentiate between different Types of Action Potential
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Lesson contents
• Types of Disturbances across the Cell Membrane
• Definition of Action Potential
• Typical Action Potential in Large Myelinated Nerve fiber
• Stages of Action Potential
• Types of Channels involved in Various Excitable Tissues
• Voltage Gated Na+ and K+ Channels
• Na+
and K+
Conductance
• Propagation (Transmission) of Action Potential
• Saltatory Conduction
• Direction of Propagation
• Types of Action Potential
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Types of Disturbances across the Cell
Membrane
• TWO Types:
• Non-Propagated Potentials:
– Synaptic
– Generator
• Propagated
• Action Potential
Action Potential

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ACTION POTENTIAL

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Action Potential
• Definition:
– Rapid changes in the membrane potential that spread
rapidly along cell membrane of excitable tissues
– Abrupt / sudden Change (reversal) in resting membrane
potential in response to a threshold stimulus.
– Sequence of events along the membrane of an Excitable
tissue in response to threshold stimuli
• Transmission of signals
• Sudden change, sudden end
• Transmission continues until fiber ends
• Transport of ions

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Assessment Q.1
• Define Action Potential
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Typical Action Potential in Large
Myelinated Nerve fiber
Exact

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Resting stage
Depolarization
Repolarization
1
2
3
4
5
6
7
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Stages of Action Potential

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Types of Channels involved in Various
Excitable Tissues
• Na+ Leak Channels
– From -90 (or-70) mv to -50 mv
Na+
• Voltage gated channels (fast)
– from -70 mv onwards K+
• Slow Ca++
- Na+
Channels
• Ligand Gated Channels
• Na+
- K+
Pump

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Voltage Gated Na+ Channels

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Voltage Gated Na+
Channels

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Voltage Gated K+
Channels

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Stages of Action Potential
1. Resting stage (RMP)
(Polarized state)
2. Latent Period
3. Threshold (Firing Level)
4. Depolarization
5. Overshoot
6. Repolarization
7. After-Depolarization
8. After-Hyperpolarization (Positive After-Potential)
(Recovery Stage)

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Assessment Q.2
• List the Stages of Action Potential
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Activation and Inactivation of Na+
and
K+
Channels

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Summary of mechanism of development of
different stages of Action Potential
1. Resting stage (RMP)
(Polarized state)
-90 mV
1. Depolarization
Stimulus → sudden ↑in
Na+ Permeability
Na+ Leak Channels
From -90 (or-70) mv to
-50 mv
Voltage gated channels
(fast)
from -70 mv onwards
3. Overshoot Potential
4. Repolarization
Closure of Na+ Channels
Inactivation of Na+
Channels (Same
stimulus but Delayed)
Opening of K+ Channels
(Activation by same
stimulus but Delayed)
4. Hyperpolarization:
K+ Channels remain open
Na+ Out
4. Recovery Stage
Na+ K+ ATPase Pump

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Na+
and K+
Conductance

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Stages of
Action
Potential vs
Na+ & K+
Conductance

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Video of Action Potential:
Roll of Sodium channels

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Assessment Q.3
Draw the figure of a Typical Action potential
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Summary
of events
in Action
Potential

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Local Response vs Propagated Action
Potential

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Propagation
(Transmission)
of Action
Potential
• Electrotonic
Conduction.
• Local circuit of
Current along both
sides of membrane

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Propagation of Action Potential-II
(Saltatory Conduction)

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Propagation of Action Potential-II
(Saltatory Conduction)
• Jumping
• Advantages:
1. ↑ velocity
Myelinated fibers: 100 m/s
Unmyelinated fibers: 0.25
m/s
1. Energy Conservation

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Direction of Propagation
• All directions
• Till end
• Orthodromic
• Antidromic

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Action Potential Video
• ..Videosaction-potential video. studentconsult.htm
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Types of Action Potential
1. Spike Potential
2. Action Potential with Plateau
3. Slow Rise & Slow Fall (No Plateau, No
Spike)
4. Compound Action Potential
5. Biphasic Action Potential

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1. Spike Potential
Action Potential in Self Excitatory Tissues (S-A Node)

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2. Action Potential with Plateau

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Action Potential with Plateau
(Cardiac Muscle)
Red curve shows rise and fall in tension of ventricular myocyte

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Action Potential in Smooth Muscle

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3. Slow Rise & Slow Fall
• No Plateau
• Only Slow Channels

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4. Compound Action Potential
• Mixed Nerves
• Multiple Peaks
• Families of fibers with different speeds of
conduction

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Compound Action Potential

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5. Biphasic Action Potential
(Both electrodes are outside the membrane)

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Recording of Membrane and
Action Potential

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Recording of Membrane and Action
Potential

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Action Potential is energy consuming
process

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Self-Study Topics
• All-or-nothing Principle
– “Safety factor” for propagation
• Rhythmicity
I don’t t each my st udent s, I j ust
provide guidance and environment
conduct ive t o learning.
Albert Einstein (1877-1955)

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Summary
• Types of Disturbances across the Cell Membrane include action potential &
local response
• Action Potential is defined as Rapid changes in the membrane potential that
spread rapidly along cell membrane of excitable tissues
• Main stages of Action Potential include Resting stage (Polarized state),
Depolarization, Overshoot, Repolarization & Recovery Stage
• Types of Channels involved in Various Excitable Tissues include Voltage Gated
Na+ and K+ Channels, Na+
-K+
leak channels, & in some cases Ca++
channels
• Propagation (Transmission) of Action Potential occurs in All directions till the
end of the fiber
• Saltatory Conduction is the Jumping of Action Potential from one Node of
Ranvier to next in Myelinated fibers
• The advantages of Saltatory Conduction include increase velocity of conduction
& Energy Conservation
• Main Types of Action Potential include Spike Potential, Action Potential with
Plateau, Compound Action Potential & Biphasic Action Potential
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Assessment Q.4
• What is meant by Saltatory conduction?
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Learning Resources
• Guyton and Hall (Text book of physiology), 13th Edition
• Ganong (Text book of physiology), 24th Edition
• Berne & Levy Principles of Physiology (Koeppen BM), 6th
Edition
• Human Physiology Stuart Ira Fox, 13th Edition
• Human Physiology : from cells to system Lauralee
Sherwood,9th
Edition
• Internet
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