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Membrane potential

Potential difference (voltage) across the cell
membrane.

In all cells of the body (excitable and nonexcitable).

Caused by ion concentration differences between
intracellular and extracellular fluid.
Membrane potential caused by diffusion of
ions
142 mM
4 mM
140 mM
14 mM
Nernst potential

Nernst equation (37°C, for univalent ions):
± 61 log
Concentration inside
(mV)
Concentration outside

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.
Diffusion potential


The membrane is permeable to several different
ions at the same time!
Goldman equation:
Em = - 61 log
CNaiPNa + CKiPK + CCliPCl
(mV)
CNaoCNa + CKoPK + CCloPCl
(C) Concentration
(P) Membrane permeability
Em =
PK
Ptot
EeqK+
PNa
Ptot
PCl
EeqNa+
Ptot
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.
2.
3.
Voltage gated K+ channels
K+ leak channels
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 fibers
Myelin 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.