The document discusses the resting membrane potential and action potentials in nerve cells. At resting potential, the membrane is negatively charged inside due to higher potassium concentration inside driving diffusion out of the cell. Sodium-potassium pumps maintain the gradient. An action potential occurs when rapid sodium influx depolarizes the membrane, driven by opening of voltage-gated sodium channels. Then, voltage-gated potassium channels open, driving potassium out of the cell and repolarizing the membrane back to its resting potential. Local anesthetics and tetrodotoxin block sodium channels, preventing action potentials.

1. IONIC BASIS OF RESTING MEMBRANE
AND ACTION POTENTIALS
Dr. Hidaayah O.
Jimoh-Abdulghaffaar
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2. Outline
• Introduction
• Resting membrane potential
– Ionic basis
• Action potential
– History
– Ionic basis
– Initiation
– Components
– Stages
• Clinical correlates
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3. Introduction
• The membranes of almost all cells in the body
have electrical potentials.
• The membrane of some cells (nerve and muscle)
can generate rapidly-changing electrochemical
impulses. These impulses are used to transmit
signals.
• The functions of some cells (glandular cells,
ciliated cells and macrophages) are activated by
local changes in membrane potentials.
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4. Introduction Cont’d
• Membrane potentials are due to the diffusion
of ions down their concentration gradients,
the electric charge of the ion, and any
membrane pumps for that ion.
• Influx is the net movement of ions into the cell
from the ECF.
• Efflux is the net movement of ions out of the
cell to the ECF.
• Flux (the movement of charges) is always
measured in millivolts (mV).
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5. Introduction Cont’d
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6. Resting Membrane
Potential
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7. Resting (membrane) potential
• When the membrane potential of a cell can go
for a long period of time without changing
significantly, it is referred to as a resting
potential or resting voltage. This term is used
for the membrane potential of non-excitable
cells but, also for the membrane potential of
excitable cells in the absence of excitation.
• Calculated by the Goldman Equation:
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8. 4/10/2024 8
Fig. 1: Resting Membrane Potential

9. Ionic Basis
• Potassium concentration is great inside a nerve fiber
membrane but, very low outside the membrane.
• Assuming the membrane is permeable to K+ only, K+
diffuse outward through the membrane, making the
outside more electropositive and leaving the inside
electronegative.
• This leads to the development of a potential difference
between the inside and outside - diffusion potential.
• Diffusion potential is 94mV, negative inside the fiber.
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10. Fig. 2: Equilibrium Potential of Potassium
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11. Ionic Basis Cont’d
• There is a high concentration of Na+ outside the
membrane and a low concentration in side the cell.
• The membrane is highly permeable to Na+ but
impermeable to other ions.
• Na+ diffuse inside the cell, making the inside more
electropositive and the outside, electronegative.
• Diffusion potential is 61mV positive inside the cell.
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12. Fig. 3: Equilibrium Potential of Sodium
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13. 4/10/2024 13
Fig. 4: Resting Membrane Potential

14. Fig. 5: Diffusion Potentials of Sodium and Potassium
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15. Fig. 6: The Sodium-Potassium ATPase Pump
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16. RMP
• In nerves, -70mV when not transmitting nerve
signals.
• Cardiac muscle?
• Skeletal muscle?
• Salivary gland?
• Red blood cell?
• Other cells/tissues??
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17. Action Potential
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18. Fig. 7: The Squid Giant Axon
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22. Action Potential – history Cont’d
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23. Fig. 8: Graphical Representation of Action Potential
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24. Ionic Basis of Action Potential
• An action potential occurs when there is a reversal of
the normal resting potential, going from negative to
positive - depolarization.
• Depolarization occurs when a stimulus causes the
voltage-gated Na+ channels to open, allowing Na+ to
rapidly influx down its concentration gradient.
• The sudden in-rush of positive sodium ions reverses the
membrane potential for a few milliseconds.
• Then the voltage-gated K+ channels open, allowing K+
to rapidly efflux due to its concentration gradient. This
brings the membrane back to negative inside and is
called repolarization.
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25. Resting stage
• RMP before action potential begins.
• Membrane is said to be polarized at this stage.
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26. Depolarization stage
• Membrane suddenly becomes permeable to Na+…..
• This rapidly neutralizes the normal “polarized state” of
-90mV.
• In large nerve fibers, the great excess of Na+ causes the
membrane potential to actually overshoot beyond the zero
level.
• However, this doesn’t occur in smaller fibers.
• Activated by the voltage-gated Na+ channel.
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27. Repolarization stage
• After being highly permeable to Na+, within
milliseconds, the membrane Na+ channels begin
to close and the K+ channels open more than
normal.
• There is rapid diffusion of K+ to the exterior.
• This helps to re-establish the normal RMP.
• Activated by the voltage-gated K+ channel.
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28. Ionic Basis of Action Potential
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Sodium channels have 2 gates,
a normal voltage (activation)
gate (which is closed at rest)
and an inactivation gate (which
is open at rest). The rapid
opening of the voltage gate
lets Na+ rush in and
depolarizes the cell. This is
immediately followed by
closing of the inactivation gate
which stops the Na+ influx. At
the same time the K+ gate
opens letting K+ efflux
(repolarization).

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30. Ionic Basis of Action Potentials
0 mV
-80 mV
PK>>PNa
PNa>>PK
PK>>PNa
ENa
EK
Time 
1. At rest only K+
leak channels
open, PK>>PNa
2. With stimulus,
voltage-gated Na
channels open,
PNa>>PK
Na+ flows into
the cell carrying
positive charge
3. Delayed opening
of voltage-gated
K channels,
PK>>PNa
K+ flows out of
cell removing
positive charge
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31. Summary
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32. 4/10/2024 32

33. Action Potential
0 mV
-80 mV
Rising phase or
depolarization
Falling phase or
Repolarization
Resting membrane
potential
Threshold Potential
Undershoot or after-
hyperpolarization
Overshoot
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37. Clinical Correlates
• Local anesthetics e.g. procaine, lidocaine
– Prevent generation of action potential by blocking the
opening of voltage-gated Na+ channels thereby
preventing depolarization and transmission of pain
sensation to the brain.
• Tetrodotoxin
– Potent toxin produced by puffer fish which bind to
voltage-gated Na+ channels.
– Ingestion of improperly-prepared fish diet even in
small quantities can cause death.
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