S3: Membrane Potentials and Action Potentials

Membrane potential Potential difference (voltage) across the

cell membrane. In all cells of the body (excitable and non-

excitable). Caused by ion concentration differences

between intracellular and extracellular fluid.

Membrane potential caused by diffusion of ions

4 mM 140 mM

142 mM

14 mM

Nernst potential

For each ion proportional to ratio of concentrations inside and outside the cell.

Always expressed as extracellular fluid has potential zero, and Nernst potential that from inside the cell.

± 61 log

Concentration insideConcentration outside(mV)

Nernst equation (37°C, for univalent ions):

Diffusion potential The membrane is permeable to several

different ions at the same time! Goldman equation:

Em = - 61 logCNaiPNa + CKiPK + CCliPCl

(mV)CNaoCNa + CKoPK + CCloPCl

(C) Concentration(P) Membrane permeability

Em =PK PNa PCl

Ptot Ptot Ptot

EeqK+ EeqNa+ EeqCl

Membrane permeability for K+ and Na+ (resting state) In resting nerve cells – open potassium

”leak” channels (“tandem pore domain”). 100x more permeable for K+ than Na+.

outside

Origin of Resting Membrane Potential

Contribution of Na+/K+ pump

Maintenance of concentration gradients for K+ and Na+ across cell membranes.

Electrogenic: creates additional negativity ~4 mV.

Measurement of membrane potential

Nerve Action Potential

Voltage-gated Na+ channels

1. Voltage gated K+ channels2. K+ leak channels3. Na+/K+ pump

Voltage-gated Na+ and K+ channels

Action Potential

Role of Ca2+

c(Cai)=10-7 mol/l

c(Cao)= 10-3 mol/l Strong concentration gradient (10 000-

fold concentration difference) In resting state, permeability for Ca2+

negligable. In heart cells, voltage-gated Ca2+

channels participate in action potential (plateau).

Action potential with plateau (heart)

Initiation of action potentials Action potentials will not discharge until

there is appropriate stimulus – depolarization. Exception – spontaneous rhythmicity.

Stimulus can be mechanical (mechanoreceptors), chemical (neurotransmitters) or electrical (heart muscle).

Positive feedback opens more and more Na+ channels.

Initiation of action potentials “Acute local potentials” must reach

threshold for eliciting AP “all or nothing” phenomenon.

Refractory Period Period of decreased excitability (relative r.p.)

or complete inexcitability (absolute r.p.)during and after action potential.

mV

Rhythmicity of Excitable Tissues Repeated spontaneous rhythmical

discharges (no outside stimulus). Heart (SA-node rhythmic activity),

intestinal smooth muscle (perystalsis) i CNS (breathing pace-maker).

Other excitable tissues can spontaneously discharge if threshold is lowered.

Spontaneous rhythmicity Resting membrane

potential -60 do -70 mV (close to threshold) activation Na + and Ca2+ channels.

Depolarizationa activates slow K+ channels repolarization i hyperpolarization.

Propagation of action potentials

Myelinated nerve fibersMyelin sheath: Insulation Decreases membrane

capacity every 1-2 mm along axon

myelin sheath is interrupted prekid mijelinske ovojnice Ranvier nodes 2-3 μm in length.

Saltatory conduction

Action potential are generated only in nodes of Ranvier energy saving and faster conduction (100 m/s).

Non-myelinated fibers conduction velocity 0,25 m/s.