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Ahmad Gharaibeh

Doctros of 2016

Dr.M. Khatatbeh

Action Potential

The resting membrane potential of large nerve fibers, when not transmitting nerve signals, is about -90 mV (inside is 90 mV more negative than outside).In other books or sources you may find it -70 mV, it's all about the concept. This is

usually measured by a Voltmeter( Potentiometer )😃 😃

Just a quick revision for what have been already said:😀

*At RMP( Resting Membrane Potential)

* The high driving force for Na to move from OUT-->IN is generated by 2 forces:

- Chemical Force (Particles Concentration)

- Electrical Force (Particles Charges)

That's why Na is more permeable than K, because K has only 1 driving force meanwhile, Na has 2.

#YET!!!!! ‼ ‼ the membrane is not highly permeable for Na.

ACTUALLY, it has very low permeability for Na.

* An increase in K channel activity will increase the efflux of K+ ( IN!OUT)

which is called (HYPERRR-polarization). 🙂

*An increase of Na channel activity will increase the influx of Na(OUT!IN),

which will cause (DEEE-polarization).🙃

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Let's go to the exciting and most important part of the lecture.😬 😬

Action Potential Phases

1- Resting Stage: before the action potential begins, the voltage is -90 mV

therefore the membrane is called "Polarized". 😯

2- Depolarization Stage: the membrane suddenly becomes permeable to Na ions, allowing tremendous numbers of + Na ions to diffuse to the "axon". But, in order to the membrane to depolarize, it has to reach the THRESHOLD:( (العتبةdefines as the critical level to which a membrane

potential must be depolarized to initiate an action potential. 😦

All or none law(None or all principle) (مـــبدأ الـــكل أو الـــعدم): the strength by which a nerve or muscle fiber responds to a stimulus is independent of the strength of the stimulus. If that stimulus has caused changes in membrane potential (depolarization) to reach threshold potential, the nerve or muscle

fiber will give a complete response; otherwise, there is no response.😧

*note that K channels are not involved in depolarization, only Na channels.

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VERY VERY VERY Important NOTE:⚠ ⚠ ⚠ ⚠

With depolarization, the main goal of Na ions entering the cell trying to reach a membrane potential of +61 !!! YET, it will never reach +61

WHY???🤔 🤔 Because at some voltages, the voltage gated channels

become inactive at some voltages ( e.g. +10 mV, +20 mV). Then the repolarization starts.

3-Repolarization Stage :The Na channels start to close and the K channels open more than normal rate at resting. Then, rapid diffusion of K Ions to the

exterior re-establishes the normal negative Resting membrane potential. 😪

!

4- Refractory period (Undershoot):is the recovery time of an excitable membrane to be ready for a second stimulus once it returns to its resting state. "It is a more negative potential wave recorded after the action potential". Not mentioned in the lecture, and we will be talking about it later, sadly the (فــــتـرة

��.): we learned in high school is partially wrong (الجموح 😔

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A 4 minutes YouTube Video about AP: Nerve Impulse Molecular Mechanism [3D Animation]😏

https://www.youtube.com/watch?v=iBDXOt_uHTQ

*Lets summarize the whole thing and explain it all over again but with

graphs.😁

(The graphs show the relation between membrane voltage (Y axis in mV) and time (X axis in ms)

THIS GRAPH : tells you all about the whole subject, but why not giving some

notes :😌

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*Threshold is the voltage at which the action potential is triggered (activating

Na channels) which forms what is known as the "FIRING STAGE" 🔥

* The rate of activation of Na channels is much higher than the rate of

activation of K channels😛

*Falling phase is the same as Repolarization "تعددت األسماء و المسمى واحد"😅 😅

*Positive after-potential(after hyper-polarization): a spontaneous or inducible increase in trans-membrane potential of a muscle or nerve cell

following the completion of repolarization.😎

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Overshot : when inside is positive compared to out.

**** In repolarization, the voltage will NEVER EVER exceed -94mV when K+ ions are leaving the cell WHY??? Because at -94mV the cell would reach Potassium

chemical equilibrium. 😕

****Please note that during the whole process and phases no ATP is consumed.

If you calculate the potential regarding outside to inside u will-

.get everything in the reverse way😬

** Na-K pump is needed after action potential to restore high concentration of K inside, and Na out to contribute in the start of a new action potential.

Chord(wire) conductance equation:

An alternative method of approximating the membrane resting potential is by this equation. The Chord Conductance Equation is useful to determine resting potential when the ions conductance's and ion distributions and equilibrium

potentials are unknown * semi -😈 😈 😈 laugh*

Note that G is the conductance of that specific ion.

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Patch Clamp Technique :

The patch clamp technique is a laboratory technique in electrophysiology that allows the study of the potential of a membrane DURING action potential (not

at RMP)😈 😈

*Don't waste your time on it, just know the definition

Propagation of Action Potential: ( the movement of the current on the same neuron)

The action potential generated at the axon hillock propagates as a wave along

the axon😶

-This happens by Na ions moving from the area of Action Potential to the adjacent area, then the K ions leave the cell to the ECF.

***Note that the function of the Axon is moving currents😛

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General Information :

The pumps and the in the cells are conformation channels ( we learned -1

about conformational isomers in Organic AAAAH😂 😂 )😍 😍 😍

Cells are divided into 2 kinds regarding excitation: Excitable cells (e.g. -2 muscles, Cardiac cells, and neurons ) and non excitable cells (any cell that has (a POSITIVE potential, in other words all cells except the excitable ones.

Cardiac's AP differs from neuron's AP because in Cardiac we have Ca++ also -3 moving from outside to inside during repolarization, and this happens to delay .the next pulse, in order to make sure that the heart is empty of blood.

With Great Love ❤

Ahmad Gharaibeh

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