lecture on resting membrane potential by dr roomi.ppt

7/30/2019 Lecture on Resting membrane potential by Dr Roomi.ppt

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Resting Membrane Potential (RMP)

Dr. Mudassar Ali Roomi (MBBS, M. Phil.)


2/18

Important channels or carriers involved in the

electrical activity of the cell

Na+, K+ Leak channels: these are open all the

time. Involved in generation of resting

membrane potential (RMP).

Gated channels: these channels open and close

at a specific time when needed. Not open all the

time. All of the gated channels are closed at RMP1. Voltage gated channels e.g. voltage gated

sodium, potassium, calcium channels.

2. Ligand gated channels e.g. Acetylcholine

gated channels at neuromuscular

junctions.

Sodium-potassium ATPase pump: it works all

the time. Also contributes to generation of

resting membrane potential.

NUTSHELL: Both leak channels and Na+/K+

pump are active at rest ***

17 January 2013 2


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4/18

Resting Membrane Potential

Definition: it is the potentialdifference across cell membraneat rest:

it is negative inside with respectto outside.

Potential values of RMP vary in various

excitable tissues:

In nerve fiber: -90 mV

In skeletal muscle: -90 mV

In cardiac muscle: -85 mV SA node: -55 mV

In nerve cell body: -70 mV

In smooth muscle: -55 mV to -60mV


5/18

Mechanism of

Generation of RMP

1. Contribution of thePotassium DiffusionPotential (-94 mVolts)

2. Net Contribution ofSodium DiffusionThrough the NerveMembrane (+8 mVolts)

3. Contribution of theNa+-K+ Pump (-4mVolts)


6/18

Mechanism of Generation of RMP


7/18


8/18

Mechanism of regulation of RMP

Another factor is Na+-K+ pump (active all the time).

Pumps out 3 Na+ ions &

Pumps in 2 K+ ions.

So more electro-positive ions are pumped out &

contributes to RMP.

As membrane is more permeable to K+ at rest, &

much ionic diffusion, so large amount of K+ shouldmove out, but net movement is not much. Why???


9/18

Mechanism of regulation of RMP

This is because when K+ moves out, anions(non-diffusible: proteins, organic phosphate &organic sulfate anions) attract them back.

But this movement is sufficient to causerelative electro-negativity inside themembrane.

Out of -90mV, -86mV is due to K+

outflux &-4 mV is due to Na+ K+ pump.

What is the evidence???


10/18

Evidence of major contribution to RMP by

K+ ion:

If we change, [K+] in ECF RMP is disturbed.

If there is hypo-kalemia in ECF RMP will becomemore negative due to more potential difference

hyper-polarization (more K+ leak out than restinglevel).

If there is hyper-kalemia RMP becomes lessnegative hypo-polarization.

But changes in [Na+] in ECF has no significant effecton RMP.

We can also prove this by Nernst Equation.


11/18

Nernst Equation

EMF (mV) = +/- 61 log conc inside / conc outside

Assume that at rest, cell membrane is highlypermeable to Na+, then:

-61 log 14/140

(- for Na+ & K+ & + for Cl-)

= -61 log 0.1

= -61 x -1

= + 61 mVSo potential difference across cell membrane should be+61mV, which is wrong.


12/18

Assume that at rest, cell membrane is highly

permeable to K+, then:

- 61 log 140 / 4

= -61 log 35

= -61 x 1.54

= -94 mV So this assumption is correct that at rest, cell

membrane is highly permeable to K+.


13/18

Goldman Hodgkin-Katz Equation

EMF (mV) =

- 61 log C Na+ i P Na+ + C K+ i P K+ + C Cl-o P Cl-

C Na+ o P Na+ + C K+ o P K+ + C Cl-i P Cl

-

WITH GOLDMAN EQUATION SUMMATED POTENTIAL = -86 mV

(C = CONCENTRATION, P= PERMEABILITY)


14/18


15/18

K+ Na+ leak channels are highly permeable to

K+ but there is some inward movement of Na+

+ 8 mV potential is generated.

-94 mV is due to K+ efflux

+ 8 mV is due to Na+ influx

-94 + 8 = -86 mV = Goldman potential

-4 mV is due to Na+ K+ pump

-86 4 = -90 mV = RMP.


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UNITS OF EXCITABILITY

1. RHEOBASE

2. UTILIZATION TIME3. CHRONAXIE


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RHEOBASE, UTILIZATION TIME, CHRONAXIE

RHEOBASE: Voltage /strength of stimulusrequired just to excite thetissue, e.g., 1 mV.

UTILIZATION TIME: The time

for which Rheobase mustbe applied to excite thetissue.

CHRONAXIE: A time forwhich a stimulus, double

the rheobase when applied,just excites the tissue, e.g.,2 mV. (Chronological is fromtime).


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CLINICAL APPLICATION / SIGNIFICANCE OF

CHRONAXIE

A particular value of it for a particular tissue.

Type A nerve fiber has minimum value of chronaxie,

i-e., they are more excitable as compared to cardiacmuscle. (less chronaxiemore excitability)

In nerve injury repair procedureWe assess therecovery by finding chronaxie of nerve effected &muscle effected.

It improves with recovery. Damage to nerve fiber is determined by measuring

chronaxie.

lecture on resting membrane potential by dr roomi.ppt

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