Electrochemical Potentials
A. Factors responsible
1. ion concentration gradients on either side of the membrane
- maintained by active transport
Electrochemical Potentials
A. Factors responsible
2. selectively permeable ion channels
B. Gradients not just chemical, but electrical too
1. electromotive force can counterbalance diffusion gradient
2. electrochemical equilibrium
C. Establishes an equilibrium potential for a particular ion
based on Donnan equilibrium
Nernst equation
1. What membrane potential would exist at the true equilibrium
for a particular ion?
- What is the voltage that would balance diffusion gradients
with the force that would prevent net ion movement?
2. This theoretical equilibrium potential can be calculated (for a
particular ion).
ENa =
+]
RT
[Na
___ ln ___ out
+]
[Na
in
zF
R = Gas constant
T = Temp K
z = valence of X
F = Faraday’s constant
For K+ around -90mV
For Na+ around +60mV
Resting Membrane Potential
A. Vrest
1. represents potential difference at non-excited state
-normally around -70mV in neurons
2. not all ion species may have an ion channel
3. there is an unequal distribution of ions due to active pumping
mechanisms
- contributes to Donnan equilibrium
- creates chemical diffusion gradient that contributes to the
equilibrium potential
Resting Membrane Potential
B. Ion channels necessary for carrying charge across the membrane
1. the  the concentration gradient, the greater its
contribution to the membrane potential
2. K+ is the key to Vrest (due to increased permeability)
Resting Membrane Potential
C. Role of active transport
ENa is +55 mV in human muscle
Vm is -65-70 mV in human muscle
Action Potentials
large, transient change in Vm
depolarization followed by repolarization
propagated without decrement
consistent in individual axons
“all or none”
Action Potentials
A. Depends on
1. ion chemical gradients established by
active transport through channels
2. these electrochemical gradients
represent potential energy
3. flow of ion currents through “gated”
channels
- down electrochemical gradient
4. voltage-gated Na+ and K+ channels
Action Potentials
B. Properties
1. only in excitable cells
- muscle cells, neurons, some receptors, some secretory cells
Action Potentials
B. Properties
2. a cell will normally produce identical action potentials
(amplitude)
Action Potentials
B. Properties
3. depolarization to threshold
- or just local response (potential) if it does not reach threshold
- rapid depolarization
- results in reverse of polarity
Action Potentials
B. Properties
a. threshold current (around -55 mV)
b. AP regenerative after threshold (self-perpetuating)
Action Potentials
B. Properties
4. overshoot: period of positivity in ICF
5. repolarization
a. return to Vrest
b. after-hyperpolarization
Action Potentials
C. Refractory period
1. absolute
2. relative
a. strong enough stimulus can elicit
another AP
b. threshold is increased
Action Potentials
D. ∆ Ion conductance
- responsible for current flowing across the membrane
Action Potentials
D. ∆ Ion conductance
1. rising phase:  in gNa
overshoot approaches ENa
(ENa is about +60 mV)
2. falling phase:  in gNa and  in gK
3. after-hyperpolarization
continued  in gK
approaches EK
(EK is about -90 mV)
Gated Ion Channels
A. Voltage-gated Na+ channels
1. localization
a. voltage-gated
Gated Ion Channels
A. Voltage-gated Na+ channels
2. current flow
a. Na+ ions flow through channel at 6000/sec at emf of -100mV
b. number of open channels depends on time and Vm
Gated Ion Channels
A. Voltage-gated Na+ channels
3. opening of channel
a. gating molecule with a net charge
Gated Ion Channels
A. Voltage-gated Na+ channels
3. opening of channel
b. change in voltage causes gating molecule to undergo
conformational change
Gated Ion Channels
A. Voltage-gated Na+ channels
4. generation of AP dependent only on Na+
repolarization is required before another AP can occur
K+ efflux
Gated Ion Channels
A. Voltage-gated Na+ channels
5. positive feedback in upslope
a. countered by reduced emf for Na+ as Vm approaches ENa
b. Na+ channels close very quickly
after opening (independent of Vm)
Gated Ion Channels
B. Voltage-gated K+ channels
1. slower response to voltage changes than Na+ channels
2. gK increases at peak of AP
Gated Ion Channels
B. Voltage-gated K+ channels
3. high gK during falling phase
decreases as Vm returns to normal
channels close as repolarization
progresses
Gated Ion Channels
B. Voltage-gated K+ channels
4. hastens repolarization for generation of more action potentials
Does [Ion] Change During AP?
A. Relatively few ions needed to alter Vm
B. Large axons show negligible change in Na+ and K+
concentrations after an AP.