D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MEMBRANE PHYSIOLOGY
Lecture 5: Electrical Properties of Membranes, Action Potential (chapter 5)
1.
Membrane Potentials:
• diffusion potentials: two particular ions display concentration gradients
across cell membrane of excitable cells: sodium is 10x more concentrated outside
than inside; potassium is 30 - 35x more concentrated inside than outside (chloride is
10 - 25x more concentrated outside) (fig. 4 – 1 & ppt. 1)
- these disparities are largely maintained by Na+/K+ pump & result in
leakages – potassium outward, sodium (& chloride) inward; leakages produce
diffusion potentials (electrical gradients that oppose chemical gradients produce
electrochemical equilibria); potentials can be measured with minute electrodes & a
sensitive voltmeter (fig. 5 – 2 & ppt. 2)
- studies of these leakages (using membrane patches – fig. 4 – 6 & ppt. 3)
have ascertained diffusion potentials for various ions
• Nernst equation: defines the diffusion potential (Nernst potential) for a
given ion that exactly opposes its chemical gradient at equilibrium = log Ci /Co
(ppt. 4)
- for sodium: +61 mv.; for potassium : -94 mv. (fig. 5 – 1 & ppt. 5); when the
diffusing ion is negative, the + sign is used, when it’s positive, the minus sign is used
• Goldman equation: describes collective contributions to membrane
potential of potassium, sodium & chloride, accounting for relative permeabilities of
each:
(ppt. 6)
- 61 log CNa+iP Na+
+ CK+iP K+ + CCl-oP ClCNa+oP Na+ + CK+oP K+ + CCl-iP Cl-
Bio 659
2.
- p. 2 -
Resting Potential:
• polarized membrane: inside more negatively charged than outside (inside
large nerve fibers , -70 to -90 mv.)
• significance of sodium-potassium pump: electrogenic, since 3 sodium ions
are pumped out for every 2 potassium ions pumped in (fig. 5 – 4 & ppt 7)
Bio 659
- p. 3 -
• sodium & potassium diffusion (K+/Na+ leak channels): permeability for
potassium is about 100x that for sodium
• relative contributions (fig. 5 – 5 & ppt. 8): potassium leakage (Nernst
potential = -94 mv.) accounts for majority of membrane potential because of high
chemical gradient and high permeability
- sodium leakage (Nernst potential = +61 mv.) has smaller chemical
gradient and has lower permeability constant; sodium leakage combined with that of
potassium, results in membrane potential of -86 mv.
- chloride leakage evidently has negligible influence on membrane potential
- Na+/K+ pump (electrogenic) contributes -4 mv.; resultant -90 mv.
3.
Action Potential (AP):
• ‘spike’ potential lasting only a few tenths of a millisecond (fig. 5 – 6 & ppt.
9)
• threshold: initiation of depolarization is a response to a stimulus (event that
causes slight increase in sodium permeability); once voltage level known as
threshold is reached, positive feedback mechanism – increasing sodium
permeability – causes even greater increase as more and more gated channels open,
causing AP
• depolarization: sudden increase in sodium permeability produces surge in
membrane potential to more positive values (reversal potential of +35 in large nerve
fibers); attributable to sodium influx (facilitated by open voltage-gated sodium
channels)
• repolarization: sudden decrease in sodium permeability followed by
increase in potassium permeability produces return of membrane potential to
resting value (sometimes to more negative value = hyperpolarization or positive
afterpotential); attributable to potassium efflux (due to closure of voltage-gated
sodium channels & opening of voltage-gated potassium channels)
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- p. 4 -
• gated channels (fig. 5 – 7 & ppt. 10): using a voltage clamp technique (fig. 5 –
8 & ppt. 11), Nobel prize winners, Hodgkin & Huxley determined that permeability
changes were attributable to voltage-gated sodium channels, which opened & then
closed abruptly & voltage-gated potassium channels, which opened as sodium
channels closed (fig. 5 – 9 & ppt. 12)
• graded potentials: many stimuli may cause insufficient depolarization to
reach threshold, resulting in transient graded potentials (subthreshold potentials that
last a millisecond or longer, but decay before spreading very far (fig. 5 – 18 & ppt. 13)
• refractory period: insensitivity of depolarized membrane to further
stimulation results from inactivation of voltage-gated sodium channels; further
stimulation elicits new AP only after repolarization (about 0.4 ms)
• all-or-none principle: as long as membrane’s threshold is reached, action
potential of same duration and amplitude results; this ensures propagation of AP
without decrement
• propagation of AP: (= nerve or muscle impulse); depolarization of AP
excites adjacent membrane to reach threshold and fire additional APs (fig. 5 – 11 &
ppt. 14); myelinization of axons (extensive wrapping of Schwann cell membranes)
promotes more rapid and energy-efficient propagation (saltatory conduction via
nodes of Ranvier) (figs. 5 – 16, 5 – 17 & ppts. 15 & 16)