Electrochemical Potentials A. Factors responsible 1. ion
concentration gradients on either side of the membrane - maintained
by active transport Slide 2 Electrochemical Potentials A. Factors
responsible 2. selectively permeable ion channels Slide 3 Slide 4
B. Gradients not just chemical, but electrical too 1. electromotive
force can counterbalance diffusion gradient 2. electrochemical
equilibrium Slide 5 C. Establishes an equilibrium potential for a
particular ion based on Donnan equilibrium Slide 6 Slide 7 Slide 8
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). RT [Na + ] out [Na + ] in E Na =
zF ln ___ R = Gas constant T = Temp K z = valence of X F = Faradays
constant For K + around -90mV For Na + around +60mV Slide 9 Resting
Membrane Potential A. V rest 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 Slide 10 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 V rest (due to increased permeability) Resting
Membrane Potential Slide 11 C. Role of active transport E Na is +55
mV in human muscle V m is -65-70 mV in human muscle Resting
Membrane Potential Slide 12 Action Potentials large, transient
change in V m depolarization followed by repolarization propagated
without decrement consistent in individual axons all or none Slide
13 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 Slide 14 Action Potentials B.
Properties 1. only in excitable cells - muscle cells, neurons, some
receptors, some secretory cells Slide 15 Action Potentials B.
Properties 2. a cell will normally produce identical action
potentials (amplitude) Slide 16 Action Potentials B. Properties 3.
depolarization to threshold - rapid depolarization - results in
reverse of polarity - or just local response (potential) if it does
not reach threshold Slide 17 Action Potentials B. Properties a.
threshold current (around -55 mV) b. AP regenerative after
threshold (self-perpetuating) Slide 18 Action Potentials B.
Properties 4. overshoot: period of positivity in ICF 5.
repolarization a. return to V rest b. after-hyperpolarization Slide
19 Action Potentials C. Refractory period 1. absolute 2. relative
a. strong enough stimulus can elicit another AP b. threshold is
increased Slide 20 Slide 21 Action Potentials D. Ion conductance -
responsible for current flowing across the membrane Slide 22 Action
Potentials D. Ion conductance 1. rising phase: in g Na overshoot
approaches E Na (E Na is about +60 mV) 2. falling phase: in g Na
and in g K 3. after-hyperpolarization continued in g K approaches E
K (E K is about -90 mV) Slide 23 Gated Ion Channels A.
Voltage-gated Na + channels 1. localization a. voltage-gated Slide
24 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 V m Slide 25 Gated
Ion Channels A. Voltage-gated Na + channels 3. opening of channel
a. gating molecule with a net charge Slide 26 Gated Ion Channels A.
Voltage-gated Na + channels 3. opening of channel b. change in
voltage causes gating molecule to undergo conformational change
Slide 27 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 Slide 28 Gated Ion Channels
A. Voltage-gated Na + channels 5. positive feedback in upslope a.
countered by reduced emf for Na + as V m approaches E Na b. Na +
channels close very quickly after opening (independent of V m )
Slide 29 Gated Ion Channels B. Voltage-gated K + channels 1. slower
response to voltage changes than Na + channels 2. g K increases at
peak of AP Slide 30 Gated Ion Channels B. Voltage-gated K +
channels 3. high g K during falling phase decreases as V m returns
to normal channels close as repolarization progresses Slide 31
Gated Ion Channels B. Voltage-gated K + channels 4. hastens
repolarization for generation of more action potentials Slide 32
Does [Ion] Change During AP? A. Relatively few ions needed to alter
V m B. Large axons show negligible change in Na + and K +
concentrations after an AP.
