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
2
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
7
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.
1 2
3
4
5
6
7 8
9
30
29/7/2021
Prof Dr K M Padmavathy MBBS, MD
Physiology, Wollega University
9
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
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
13
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
16
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
17
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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