The document discusses action potentials and their propagation in excitable tissues. It begins by stating the objectives of understanding the mechanisms of action potential production and propagation. It then lists the main contents that will be covered, including the definition of action potential, the stages of a typical action potential, and the types of ion channels involved. It goes on to describe key aspects of action potentials in more detail, such as the stages of an action potential, how they propagate through saltatory conduction, and different types of action potentials that can occur. It emphasizes that action potentials are rapid changes in membrane potential that involve the coordinated opening and closing of sodium and potassium ion channels.

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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