The document discusses action potentials, defined as rapid changes in the membrane potential of neurons that transmit nerve signals, detailing its five phases: resting membrane potential, threshold, depolarization, repolarization, and hyperpolarization. It explains how a neuron must reach a threshold of -55mV to initiate an action potential, followed by rapid inflow of Na+ ions and subsequent K+ outflow for repolarization. Additionally, it mentions the refractory period during which a neuron cannot generate another action potential, highlighting the role of the Na+/K+ pump in restoring the resting membrane potential.

1. Pt. Ravishankar Shukla University
" Action potential"
Presented by:-
Diksha Chandrakar msc
biochemistry 2nd sem.
Guided by:-
Dr. Aarti parganiha
Dr. Gyanchandra sahu

2. Action potential:-
** Nerve signals are transmitted by Action potential.
**Action potential is defined as a sudden , fast , transitory and propagating change of
the resting membrane potential.
** Action potential being with a sudden change from normal resting negative
membrane potential to a positive potential and end with an almost equally rapid change
back to negative potential.
** The duration of nerve action potential is 0.3 ms.
**The action potential arises at the trigger zone ( here , at the junction of the axon
hillock and the initial segment and then propagates along the axon to the axon
terminal.
( action potential will Change the membrane polarity)

3. 5 steps of action potential:-
1. Resting membrane potential ( resting stage)
2. Threshold ( early de polarization)
3. De polarization phage
4. Re polarization phage
5. Hyperpolarization phage

4. 1. Resting membrane potential ( resting phage):-
**Resting membrane potential is an electrical potential difference ( voltage) that exists across
the plasma membrane of an excitable cells under resting condition.
**In neuron the resting membrane potential range from -40 to -90 mv. A typical value is - 70
mv . the minus sign is indicate that the inside of the cell is negative related to the outside .
.
**the cell that exhibit a membrane
potential is said to be polarized, most of
the body cells is polarized.

5. 1.
3.
2.
** three factors that contribute to the resting
membrane potential:-
BECAUSE the plasma membrane has more k+ leaky channels than Na+ leaky channels
, the number of k+ ion leave the cell is greater then the number of Na+ ion that enter
the cell.
As more and more k+ ions leave the cell, the inside of the membrane become
increasingly negative (-) and the outside of the membrane become increasingly
positive (+)..
Trapped anion cannot follow k+ out of the cell because they are attached to the
nondiffusible molecules such as ATP and large protein.
The electrogenic Na+ k+ ATPase ( pump) expels 3Na+ ion fir every 2k+ ion
imported.

6. Threshold:-
** An action potential occur in the membrane of the axon of neuron when depolarization
reached a certain level termed the threshold ( about - 55mv ) .
**when the depolarization reaches about -55mv a neuron will fire an action potential . This is
the threshold . If the neuron does not reache this critical threshold level then no action
potential will starts
** in this step some voltage gated Na+ channel open which cause Na+ to move through the
channel into the cell due to stimulus. Remember Na+ has +ve charge , so the inside of the
neuron becomes more positive and out side of the membrane becomes more negative

7. ** Depolarization phage:-
** when membrane potential of axon reached threshold . Voltage gated Na+ channel
open rapidly.
**the inflow of Na+ change the membrane potential from -55mv to +30 mv.
**at the peak of action potential the inside of the membrane is 30mv more positive then the
outside.
** each voltage gated Na+ channel has two seperate gates an activation gate and an
inactivation gate
**in the resting sate of voltage gated Na+ channel , the inactivation gate is open but
activation gate is closed.

8. ** as a result Na+ cannot move into the cell throught this channel.
** At the threshold voltages gated Na+ channel are activated .
** In the activated stae of voltage gated Na+ channel both the activation and inactivation
gate are open and Na+ inflow begins.
** As more channel open inflow increase the membrane depolarize further .
** So the inside of neuron becomes more +ve and becomes depolarized.

9. Repolarization phage:-
**after the activation gates. Of the voltage gated Na+ channel open, the inactivation gates
is close. Now the voltage gated Na+ channel is in an inactivated state.
** in addition to opening voltage gated Na+ channel , threshold level depolarization also open
voltage gated k+ channel.
** voltage gated k+ channel open slowly , their opening occur at about the same time
the voltage gated Na channel are closed.
**
**The membrane start to become repolarized as some k+ Ion leave the neuron . So the
outside of the neuron becomes more positive and becomes repolarized.

10. ** cl- moves from outside to inside that turn into negative.
** Na channel activation gate closed and inactivation gate open . Returning to
resting sate when k+ gate closed.
** K+ outflow causes the membrane potential to change from +30 mv to - 70 mv
.

11. Hyperpolarization phage :-
** the voltage gated k+ channel stay open a little longer outflow of k+ maybe Large
enough to cause a hyperpolarization.
**the membrane potential becomes more negative -90mv.
**As the voltage gated k+ channel close , the membrane potential return to the resting
level of -70 mv.

12. Refractory period :-
** the period of time after an action potential begins during which an excitable cells cannot
generate another action potential in response to a normal threshold stimulus is called the -
Refractory period.
** During refractory period interestingly Na+/k+ pump are activated by the ATP molecules.
**in Na+/k+ pump , 2k+ move inside the cell in exchange of 3k+ outside. That results into
reaching resting membrane potential.

14. Reference:-
** Tortora ,G. J. And Derrickson B. (2014) , principal of anatomy and
physiology. 14th edition.

15. Thank you for listening to