1. The action potential involves changes in membrane potential that occur following excitation, beginning with sodium channels opening to cause rapid depolarization, followed by potassium channels opening to cause repolarization. 2. During the resting state, the membrane is polarized due to a higher permeability to potassium ions, maintaining a negative interior. Upon stimulation, sodium channels open briefly, causing rapid depolarization. 3. Potassium channels then open more slowly, causing repolarization and restoring the resting potential, after which the sodium-potassium pump activates to restore ion gradients.

1. Lecture 41
Electrical properties of cell membrane III
Dr Shamshad
Majamaah University
KSA

2. Objectives
1. Define action potential (AP)
2. List the changes occurring during it
3. Discuss the Ionic basis of AP
4. Illustrate monophasic AP
5. Discuss briefly the basis for the conduction of AP.

3. Action Potential & changes occurring during it
Action Potential (AP):-
The brief sequence of changes which
occur in the membrane potential following excitation.
Types:-
Biphasic AP
Compound AP
Monophasic AP

4. Phases of AP
1. Resting potential
2. Latent period
3. Firing level
4. Threshold
5. Depolarization
6. Spike
7. Repolarization
8. Hyperpolarization

5. Resting potential/Polarized State/RMP
Cell Negative Inside & Positive Outside
Resting Membrane Potential/RMP
Neuron:-60 to-70mV
Skeletal muscle: -95mV
Smooth muscle:-60mV
Cardiac muscle:-90mV.

6. Latent period
 Application of stimulus leads to brief irregular
deflection called stimulus artifact
 Marks point of stimulus
 Followed by latent period
Defn:-The time taken for the impulse to travel along
the recording electrode.
 Inversely proportional to speed of conduction of the axon.

7. Firing level/Threshold excitation
 After an initial of 15mV of depolarization the rate of
depolarization increases
 Defn:-
This point at which change in rate occurs is called Firing
level/Threshold excitation.

8. Depolarization/Reverse polarization
 Reduction in the membrane potential from its Negative value
towards Zero.
 The tracing rises and overshoot to appox. +35mV.

9. Repolarization
 It then reverses & falls rapidly towards the resting level
 After 70% repolarization the rate of repolarization
decreases & tracing approaches the resting level more
slowly.
 Sharp rise & fall of the potential is
called spike potential.

10. Hyperpolarization & after hyperpolarization
 After reaching the previous resting potential the tracing
become more Negative
 This prolonged increase in membrane potential is called after
hyperpolarization.
 It represents recovery process in the neurons.

11.  Depolarization decreases the stability of the
membrane
 Hyperpolarization increases the stability of
membrane.

13. 1. RMP: Inside cell -ve.
K+ Channel maintains RMP
K+ ions permeability > than Na+ ions
2. Depolarization: At point of stimulation slight in RMP
 Due to :Passive redistribution of ions
a. K+ & Cl- ions influx
b. Then after 7mV Na+ channel activate through m gates
c. As firing level is reached influx of Na+ along its concentration &
electrical gradient is

14. Na+ permeability is short lived
Due to :-
 Inactivation of Na+ Channels through h gates
 Direction of electrical gradient for Na+ ions is reversed during
overshoot as MP is reversed
 Opening of K+ Channels n gates.

15. 3. Repolarization:
K+ efflux (opening of K+ channels)
& Na+ influx
But opening of K+ Channels is slower & prolonged than the Na+
Channels.
Both K+ efflux & Na+ influx leads to net transfer to +ve ions
& completes repolarization.

16. After depolarization:
At the end of spike potential K+ ions
conduction is much slowed.
 Thus few msec of delay in restoring MP.
After hyperpolarization: Though RMP is achieved but ionic
status is not achieved
 Thus leads to after hyperpolarization.
Active elctrogenic Na+-K+ pump
transports 3Na+ out & 2K+ inside cells
Finally restores –vty of cells [RMP].

17. Ionic changes during different phases of AP

18. Properties of the Action potential
1] Threshold stimulus
2] All or none response
3] Refractory period
3A] Absolute refractory period
3B] Relative refractory period
4] Conductivity /Wave of
depolarization
5] Accommodation
6] Graded potential
OR
Electrotonic potentials

20. Summary of the sequential events
1:During the resting state, the conductance for K+ ions is 50 -100
times as great as the conductance for Na+ ions.
Due to leakage of K+ ions than Na+ ions through the leaky
channels.
2: Onset of the AP, the Na+ channels instantaneously become
activated & allow up to a 5000-fold increase in Na+ conductance.

21. 3:The inactivation process then closes the Na+ channels within
fraction of a millisecond.
Voltage gating of the K+ channels, begin opening more slowly a
fraction of a millisecond after the Na+ channels open.
4:End of the AP, the return of the membrane potential to the
negative state causes the K+ channels to close back to their
original status, after millisecond or more.

22. Roles of Other Ions During the Action Potential
 Negative anions like protein , organic phosphate compds., sulfate
compds. & Ca2
+ ions.
 Ca2+ pump transports Ca2
+ ions from the interior to the exterior of
the cell membrane creating a Ca2
+ion gradient of about 10,000-fold.
 There are voltage-gated calcium channels.
 Diffusion gradient for passive flow of calcium ions into the cells
occurs.
 permeability to calcium far >sodium under normal physiological
conditions.

23.  Voltage-gated calcium ion channels contributes to the
depolarizing phase on the AP in some cells.
 Calcium channels are slow channels
 Sodium channels are fast channels
 Opening of calcium channels provides sustained
depolarization,
 Sodium channels initiate action potentials.
 Calcium channels are numerous in both cardiac muscle &
smooth muscle.

24. • Defn: Monophasic action potentials (MAPs) are
extracellularly recorded wave forms that, under optimal
conditions, can reproduce the repolarization time course of
transmembrane action potentials (TAPs) with high fidelity.

25. IIustrate monophasic AP:-
It can be recorded by placing both the recording
electrodes either in the ECF or ICF.

27. Clinical uses
 When the electrical activity of excitable tissues must be
monitored
 EEG: used to help in diagnosis of brain diseases
 ECG: to detect damage to the cardiac muscles
 EMG: record skeletal muscles to help diagnosis of
neuropathies and myopathies.