action potential

Dr. Rashid Mahmood

Action Potential & its Propagation

By Prof. Dr. Rashid Mahmood

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

Dr. Rashid Mahmood

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Dr. Rashid Mahmood

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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Dr. Rashid Mahmood Dr. Rashid Mahmood 4

Types of Disturbances across the Cell Membrane

• TWO Types:• Non-Propagated Potentials:

– Synaptic– Generator

• Propagated• Action Potential

Action Potential


ACTION POTENTIAL


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

Dr. Rashid Mahmood

Assessment Q.1• Define Action Potential



Typical Action Potential in Large Myelinated Nerve fiber

Exact

Dr. Rashid Mahmood 10Resting stage

Depolarization Repolarization

1 2

3

4

5

6

7

8

Stages of Action Potential


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


Voltage Gated Na+ Channels


Voltage Gated K+ Channels


Stages of Action Potential

1. Resting stage (RMP) (Polarized state)

2. Latent Period3. Threshold (Firing Level)

4. Depolarization5. Overshoot

6. Repolarization7. After-Depolarization8. After-Hyperpolarization (Positive After-Potential)

(Recovery Stage)

Dr. Rashid Mahmood

Assessment Q.2

• List the Stages of Action Potential



Activation and Inactivation of Na+ and K+ Channels


Summary of mechanism of development of different stages of Action Potential

1. Resting stage (RMP) (Polarized state)

-90 mV2. 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. RepolarizationClosure of Na+ Channels

Inactivation of Na+ Channels (Same stimulus but Delayed)

Opening of K+ Channels (Activation by same stimulus but Delayed)

5. Hyperpolarization: K+ Channels remain openNa+ Out

6. Recovery StageNa+ K+ ATPase Pump


Na+ and K+ Conductance


Stages of Action

Potential vs Na+ & K+

Conductance


Video of Action Potential:Roll of Sodium channels

Dr. Rashid Mahmood

Assessment Q.3

Draw the figure of a Typical Action potential



Summary of events in Action Potential


Local Response vs Propagated Action Potential


Propagation (Transmission)

of Action Potential

• Electrotonic Conduction.

• Local circuit of Current along both sides of membrane


Propagation of Action Potential-II(Saltatory Conduction)


Propagation of Action Potential-II(Saltatory Conduction)

• Jumping• Advantages:1. ↑ velocity

Myelinated fibers: 100 m/sUnmyelinated fibers: 0.25

m/s

2. Energy Conservation


Direction of Propagation

• All directions• Till end• Orthodromic• Antidromic

Dr. Rashid Mahmood

Action Potential Video

• ..\Videos\action-potential video. studentconsult.htm

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Types of Action Potential

1. Spike Potential2. Action Potential with Plateau3. Slow Rise & Slow Fall (No Plateau, No

Spike)4. Compound Action Potential5. Biphasic Action Potential


1. Spike Potential

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


2. Action Potential with Plateau


Action Potential with Plateau (Cardiac Muscle)

Red curve shows rise and fall in tension of ventricular myocyte


Action Potential in Smooth Muscle


3. Slow Rise & Slow Fall

• No Plateau• Only Slow Channels


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

conduction


Compound Action Potential


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


Recording of Membrane and Action Potential


Action Potential is energy consuming process


Self-Study Topics

• All-or-nothing Principle– “Safety factor” for propagation

• Rhythmicity I don’t teach my students, I just provide guidance and environment conductive to learning.

Albert Einstein (1877-1955)

Dr. Rashid Mahmood

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


Dr. Rashid Mahmood

Assessment Q.4

• What is meant by Saltatory conduction?


Dr. Rashid Mahmood

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

© Prof. Dr. Rashid Mahmood 52

action potential

Health & Medicine