L3: resting membrane potential - Levy Flashcards
membrane resting potential in most human cells is near…
-70 mV
a reduction in resting potential is called ___
depolarization
a “graded” response is…
proportional to the stimulus
an action potential is initiated when the local potential reaches…
threshold depolarization
roughly how big is the gap in a synapse
~20 nm
T/F an action potential propagates without decrement
true
T/F a local potential propagates without decrement
false - a local potential diminishes over distance
why is an action potential said to be a “stereotyped electrical signal” ?
the amplitude and shape is always the same
how does the nervous system code the strength of a stimulus?
frequency of impulses
magnitude is always the same regardless of strength
how does the nervous system code the duration of a stimulus?
length of action potential train
(repeated action potentials over time.. the length of a single action potential is always the same regardless of duration)
one way nervous system codes strength of stimulus is frequency of impulses, what is another way?
activation of more sensory fibers
I = zFJ
current carried by an ion = valence x faraday x flux
cell resting potential depends upon the __ and __of different ions across the membrane
- concentration gradients
- permeabilities
ECF Cytoplasm (mEq/L) K+ Na+ Cl- Ca2+
ECF Cytoplasm (mEq/L) K+ 4 150 Na+ 145 12 Cl- 112 10 Ca2+ 2 10^-4
@ physiologic temperature equilibrium potential =
E = 61/z log (Cout/Cin)
if the nerve cell were only permeable to K+, one would expect a membrane potential of…
equilibrium potential for K+ = -96 mV
EK+ = 61 log (4/150) = -96 mV
how do voltage gated channels work?
+ and - charged amino acid residues on the channel change conformation when a voltage is applied
what kind of channel gate is “mechano receptive” ?
stretch-gated channels
driving force of K+ across a membrane =
DF = Vm - Ek+
I = g (V - E)
ion current = conductance x driving force
at equilibrium:
(V - E) for Na+ > (V - E) for K+
yet I K+ = I Na+, how?
I K+ = I Na+
g (V-E) = g (V-E)
for Na+, (V-E) is higher but g is lower
oubain (or digitalis) is a drug that blocks…
blocks the Na+ K+ ATPase
so depolarization can be observed
T/F the Na+ K+ ATPase causes the resting membrane potential
false - the resting membrane potential is caused mostly by K+ leak channels, modified by Na+ leak channels, and then maintained slightly more polarized by the Na+ K+ ATPase. to explain: concentrations of cations and anions on the same sides of the membrane are macroscopically electroneutral. a small outward leak of K+ (most permeable) causes the equilibrium potential near -96 mV. inward Na+ leak channels (less permeable) depolarize slightly (say ~ - 60 mV), and the Na+ K+ ATPase repolarizes slightly to typical -70 mV resting potential.
at rest, is a nerve cell more permeable to K+ or Na+ ?
10x more permeable to K+
at rest
T/F the resting membrane potential is a weighted sum of the equilibrium potentials, and will lie closest to the equilibrium potential for the ion with the greatest conductance
true
what is the math expression for resting membrane potential as a weighted sum of equilibrium potentials
Vm = sum (gi Ei) / sum gi Vm = (g1E1 + g2E2 + ...) / (g1 + g2 + ...)
ion current across membrane is related to…
conductance (g) and driving force (V-E)
at electrochemical equilibrium, how are ion currents, conductances, and driving forces related to each other
I1 + I2 + I3 +… = sum I = 0
g1(Vm-E1) + g2(Vm-E2) + g3 (Vm-E3) +..= sum I = 0
Vm = (g1E1 + g2E2 + …) / (g1 + g2 + …)
Vm = sum (gi Ei) / sum gi
how is the Na+ K+ ATPase pump “electrogenic”
K+K+ in
Na+Na+Na+ out
generates an electric potential
what substance is a specific blocker of the Na+ K+ ATPase pump
oubain
accounting for both passive ion flux and active pump current, how is membrane potential calculated
Vm = [ sum (gi Ei) / sum gi ] + (Ia / sum gi)
where Ia = active pump current
accounting for both passive ion flux and active pump current, at electrochemical equilibrium, how are ion currents, conductances, and driving forces related to each other
Ia + I1 + I2 + I3 +… = Ia + sum I = 0
I a + g1(Vm-E1) + g2(Vm-E2) + … = Ia + sum I = 0
Vm = (Ia + g1E1 + g2E2 + …) / (g1 + g2 + …)
Vm = [ Ia + sum (gi Ei) ] / sum gi
where Ia = active pump current
does Vm hyperpolarize or depolarize if K+ is added to ECF?
depolarizes (ususally, unless gK increased)
- must add Cl- along with extracellular K+ to maintain macroscopic electroneutrality. thus concentration gradient down, less flux across K+ leak channels, smaller polarization and Veq
- -if raising K+ increases gK+ by opening more K+ channels, then cell may hyperpolarize instead
Which of the following statements is correct?
A. Increasing the extracellular K+ concentration will cause the membrane to hyperpolarize.
B. Increasing the Na+ conductance of the leak channels will cause the membrane potential to hyperpolarize.
C. If Cl- is at equilibrium, increasing gCl will cause an increase in the net movement of ions through the membrane leak channels.
D. Increasing the intracellular Na+ concentration will increase the Na-K ATPase activity.
E. Blockage of the Na-K pump with ouabain will cause the membrane potential to hyperpolarize
D. Increasing the intracellular Na+ concentration will increase the Na-K ATPase activity
