1. The document discusses the action potential, which is a brief reversal of the membrane potential that occurs when an excitable cell such as a neuron or muscle cell is sufficiently stimulated. 2. When the membrane potential is depolarized to the firing threshold by an stimulus, voltage-gated sodium channels open, allowing sodium ions to rush in and cause rapid depolarization. Then, voltage-gated potassium channels open, causing repolarization. 3. Key features of the action potential include initiation, propagation along the cell membrane via local circuits of current, and refractory periods during which the cell cannot be re-excited.

1. Lecture 8
Action Potential

2. Action Potential (1 hr)
• Initiation of AP
• Behavior of ionic channels in AP
• Stimulus parameters
• All -or- none principle & refractoriness
• Propagation of AP
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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3. • Action potential is developed when an excitable cell (nerve or
muscle) get excited.
• Excitability: the ability of a tissue to respond to a stimulus.
• Stimulus: a change of environment (electrical or chemical)
which elicit or tends to elicit a response.
• Threshold stimulus: when the strength of the stimulus is just
adequate to elicit a response.
– When a stimulus is applied whose strength is threshold or
above the tissue responds by developing an action potential.
Action Potential
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
3

4. • Polarized state
– is resting condition, when cell maintains a potential difference
across the membrane, the interior being negative as
compared to exterior.
• Depolarization
– when potential difference across the membrane is reduced,
i.e. interior becomes less negative.
• Repolarization
– after depolarization, the membrane returns to its polarized
state.
• Hyperpolarization
– when the interior of the cell becomes more negative.
• Cells become more excitable (get excited by sub threshold
stimulus) during depolarization.
• Cells become less excitable (can be excited by only supra
threshold stimulus) during hyperpolarization.
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
4

5. Initiation of the Action Potential
• Positive-feedback vicious cycle opens more sodium channels
• First, as long as the membrane of the nerve fibre remains
undisturbed, no action potential occurs.
• Any event causes enough initial rise in membrane potential
(from –90 millivolts toward zero level), causes beginning of opening
many voltage-gated sodium channels.
– This allows rapid inflow of sodium ions, which causes a further
rise in the membrane potential, thus opening still more voltage-
gated sodium channels and allowing more streaming of sodium
ions to the interior
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
5

6. 29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
6
Depolarization
Opening of more
Na channels –
Positive feedback
Opening of K channels during
repolarization &
Closure (Negative feedback)
returns to Polarized state

7. Threshold for Initiation of the Action Potential
• This occurs when the number of Na+ ions entering the fiber
becomes greater than the number of K+ ions leaving the fibre.
• A sudden rise in membrane potential of 15 to 30 millivolts
(firing level ) usually is required.
• Sudden increase in the membrane potential in a large nerve
fibre from –90 millivolts up to about –65 millivolts usually
causes the explosive development of an action potential.
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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8. Action Potential
• Action potential is temporary reversal in the membrane
potential that is transmitted along the nerve / muscle
(membrane and tubule).
• Action potentials are obtained when a nerve or a muscle gets
excited by electrical, mechanical, thermal or chemical
stimulus.
• The strength of stimulus must be threshold level or above
(so that the depolarization reaches firing level) to set off an
action potential.
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
8

9. 1. RMP
2. Depolarizing stimulus.
3. Depolarizes to firing level.
4. Rapid Na+ entry cause rapid
depolarization.
5. Na+ channels close, K+ channels
open.
6. Repolarization K+ comes out
7. Hyperpolarization: Additional
K+ leaves the cell
8. K+ channels close
9. RMP established.
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29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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10. Action Potential
29/7/2021
• Stimulus depolarizes membrane potential to firing level.
• Opening of all voltage gated (activation gates) Na+ channels
(Fast channels).
– Na+ rush inside the cell towards the concentration and
electrical gradient.
– Rapid depolarizing phase of action potential is due to rapid
entry of Na+ and membrane potential comes to + 30 mV.
• At this point (+ 30 mV):
– Na+ Channels close. Prevent further entry of Na+ halts
depolarization.
– K+ channels open, K+ start coming out of cell causing
repolarization.
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
10

11. Action Potential
29/7/2021
• After repolarization, reaches RMP, the ionic
gates of Na+ channels are restored.
• But K+ channels are slow to close. Additional
amount of K+ leave cell causing
hyperpolarization.
• Eventually the slow K+ channels close and
membrane potential comes back to resting
level.
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
11

12. 29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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13. All or none law
All or none law
• When a stimulus is applied to an excitable tissue either it will
not respond at all or it will respond maximally.
• This means if the strength of stimulus is less than that of
threshold there will be no response (no action potential).
• If the strength of stimulus is at threshold level, the tissue
responds maximally .
• Further increase in strength of stimulus (no matter how much)
beyond threshold does not increase the response. i.e.,
amplitude of action potential does not increase.
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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14. All- or- none law
Action potential in an average
neurone
• The amplitude and
duration (3 milli seconds )
remain same in spite of
wide change of intensity of
the stimulus
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
14

15. Refractory Period
• After an excitable tissue is excited, for a brief period of time it
does not respond to a second stimulus. This period is called
refractory period.
It is divided into two parts :
• Absolute refractory period : the tissue remain completely
unexcitable
• Relative refractory period: the tissue responds to a strong
(supra threshold) stimulus.
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
15

16. •In an excitable tissue, a new action
potential can only be formed when
the preceding action potential is over
and potential has returned to
the original RMP.
•T he reason is the Na+ channel
remain closed till most of the
repolarization has occurred.
• During the hyperpolarized state
(relative refractory period) the
membrane is less excitable and
therefore needs strong stimulus for
excitation.
Refractory Period
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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17. Propagation of the Action Potential
• Action potential elicited at any one point on an excitable
membrane usually excites adjacent portions of membrane,
resulting in propagation of the action potential along membrane.
• Nerve or muscle impulse : Transmission of depolarization along a
nerve or muscle fibre
• When nerve fibre has been excited,
– permeability to sodium increases  “local circuit” of current
flow from depolarized areas of membrane to adjacent resting
membrane areas.
– i.e., positive electrical charges are carried by inward-diffusing
sodium ions through the depolarized membrane, for several
millimeters in both directions along the core of the axon.
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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18. Conduction of action potential
in both directions along the
conducting nerve
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
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