LECTURE TARGETS

• Concept of membrane potential.

• Resting membrane potential.

• Contribution of sodium potassium pump in the development of membrane potential.

• Contribution of sodium and potassium ions in the development of membrane potential.

All plasma membranes All plasma membranes HaveHave

A membrane potentialA membrane potential((polarized electricallypolarized electrically))

i.e., chargedi.e., charged

DueDue to a separation to a separation

of charges across theof charges across the membrane membrane

membrane potentialmembrane potentialIs measured in Is measured in

mVmV

Membrane

Membrane has no potentialMembrane has no potential

ECF ICF

Membrane

Membrane has potentialMembrane has potential

ECF ICF

Membrane

Separated chargesresponsible for

potential

Remainder offluid electrically

neutral

Remainder offluid electrically

neutral

ECF ICF

Plasma membrane

A resting cell

MECHANISM OF DEVELOPING RMP

• Role of sodium potassium pump

• Role of sodium ions alone

• Role of potassium ions alone

• Combined effects of sodium and potassium ions

Effects of sodium-potassium pump on Effects of sodium-potassium pump on membrane potential.membrane potential.

Direct effectDirect effect indirect effectindirect effect

Coupling is 3kCoupling is 3k++ to 2Na to 2Na ++ Establish Establish kk++ to Na to Na ++

Concentrations across Concentrations across

membrane membrane

Membrane moreMembrane morePermeable toPermeable to KK++

(80% of RMP)

Separates chargeSeparates charge(20% of RMP)(20% of RMP)

Ionic composition (differences in differences in

the the concentration)concentration)

Pla

sma

mem

bra

ne

Extracellularfluid

Intracellularfluid

655

15

150

150

ICF

ECF

(Passive)Na+–K+

pump (Active)

(Active)(Passive)K+ channelNa+ channel

Figure 3.29 Page 92

• If membrane permeable to K+ only:– What are the forces

that act on K+?– When would diffusion

of K+ stops?– When diffusion stops

that is equilibrium potential

The concept of equilibrium potential

Nernst equation for calculation of equilibrium potential (E) of any

particular ion in isolation

• E =equilibrium potential for ion in mV• Co = the concentration of the ion outside the cell in mM• CI= the concentration of the ion inside the cell in mM

E = 61 logCo

CI

Nernst equation for calculation of equilibrium potential of K+ in

isolation

• EK =equilibrium potential for K+ in mV

• = -90 mV

E = 61 log5

150

E = 61 log 0.033

E = 61x-1.477

Plasma membrane

ECF ICF

Concentrationgradient for Na+

Electricalgradient for Na+

ENa+ = +60 mV

If the membrane is permeable to Na+ only

150 mM /l

15 mM/l

Nernst equation for calculation of equilibrium potential of Na+ in

isolation

• ENa =equilibrium potential for Na+ in mV

• = +60 mV

E = 61 log150

15

E = 61 log 10

E = 61x1

Plasma membrane

ECF ICF Relatively large netRelatively large netdiffusion of Kdiffusion of K++

outward tend to establishoutward tend to establishan Ean EKK

++ of –90 mV of –90 mV

No diffusion of A– No diffusion of A– across membraneacross membrane

Relatively small netRelatively small netdiffusion of Nadiffusion of Na++

inward neutralizesinward neutralizessome of thesome of thepotential created bypotential created byKK++ alone alone

Resting membrane potential = –70 mV

(A– = Large intracellular anionic proteins)

At rest neither K+ nor Na+ are at equilibrium.

There is continuous leakage of K+ to outside and of Na+ to inside,

But the concentration gradient is maintained

through continuous activity of Na+ - K+ pump which exactly counterbalances the effect of diffusion of ions.

RMP remains constant: passive forces = active forces

At resting membrane potential

ICF

ECF

(Passive)Na+–K+

pump (Active)

(Active)(Passive)K+ channelNa+ channel

Figure 3.29 Page 92

Mechanisms of RMP

• Diffusion of K+ from inside to outside

• Na + - K+ pump

• Negatively charged proteins inside

Page numbers 75 to 82 Sherwood physiology 7th edition

Message of the day

Actions speak louder than words