L/D 2: Membrane & Action Potentials

Question 1

what does a negative membrane potential signify? which direction is the force on cations? anions?

Answer 1

inside is lower potential than outside
force on cations is inward
force on anions is outward

Question 2

what does a positive membrane potential signify? which direction is the force on cations? anions?

Answer 2

inside is higher potential than outside
force on cations is outward
force on anions is inward

Question 3

[K+] in = 100 mEq/L
[K+] out = 10 mEq/L
and K+ is at equilibrium distribution,
what is Vm?

Answer 3

Vm = Ek = -61log(100/10)

= -61 mV

Question 4

If you know Vm and various in vs out ion concentrations of a cell membrane, how do you determine if the net force on an ion is inward, outward, or equilibrium?

Answer 4

2 possible ways:
-calc net driving force for the ion (Vm-Ei)
Ei = -(61/z)log(in/out)
-calc electrochemical potential of ion
∆μ = 61log(in/out) + zVm

if DF or ∆μ is negative, cats in ans out
if DF or ∆μ is positive, cats out ans in
if DF or ∆μ is 0, no net movement
*DF and ∆μ always have same sign...
...they differ by factor of z

Question 5

at 20 degrees C, equilibrium potential of an ion =

Answer 5

Ei = -58/z log(in/out)

Question 6

at 37 degrees C, equilibrium potential of an ion =

Answer 6

Ei = -61/z log(in/out)

Question 7

in physiological temperature, equilibrium potential of an ion =

Answer 7

Ei = -61/z log(in/out)

Question 8

at (British) room temperature, equilibrium potential of an ion =

Answer 8

Ei = -58/z log(in/out)

Question 9

how is the VGNa+ channel a-gate opened?

Answer 9

depolarization (+ charges on inner membrane) force + residues of the a-gate (on the inner side of the gate) apart

Question 10

is the VGNa+ channel a-gate nearer the internal or external side of the membrane?

Answer 10

internal

Question 11

when does the VGNa+ channel i-gate bind the channel?

Answer 11

after a delay, the i-gate binds the open a-gate

Question 12

when does the VGNa+ channel i-gate release the channel?

Answer 12

only after repolarization closes the a-gate

Question 13

how does the VGNa+ channel i-gate interact with the a-gate?

Answer 13

binds a-gate when open

releases a-gate when closed

Question 14

how is the peak of an AP affected when external [Na+] is reduced? why?

Answer 14

peak decreases
because Ena = -61/z log(in/out) is decreased,
so DF = Vm-Ena is decreased,
and less driving force drives less Na+ through VGNa+ channels

Question 15

how is the peak of an AP affected when internal [Na+] is increased?

Answer 15

peak decreases
because Ena = -61/z log(in/out) is decreased,
so DF = Vm-Ena is decreased
and less driving force drives less Na+ through VGNa+ channels

Question 16

when extracellular Na+ is decreased, how does this affect AP shape?

Answer 16

• slower rate of depolarization
• lower peak
• same repolarization rate
• same undershoot

Question 17

when extracellular Na+ is decreased, how does this affect AP repolarization and undershoot?

Answer 17

it doesn’t

these depend on VGK+ channels

Question 18

how does increased extracellular K+ affect AP shape?

Answer 18

• same depolarization rate
• slightly higher peak
• slower repolarization rate
• less undershoot

Question 19

why doesn’t V resting change when we experimentally alter K+ or Na+ concentrations on either side of the cell membrane?

Answer 19

because we alter Cl- concentrations as well – to maintain macroscopic electroneutrality (otherwise Vm changes would be wild)

Question 20

how does decreased extracellular K+ affect AP shape?

Answer 20

• same depolarization rate
• slightly lower peak
• faster repolarization rate
• deeper undershoot

Question 21

how does TEA affect action potential shape?

Answer 21

TEA blocks K+ channels

• same depolarization rate
• slightly higher peak
• very slow repolarization rate (leak channels)
• no undershoot

Question 22

when stimulated with maximum frequency, how is the afterpotential for an axon with glial cells different from that of the same neuron without glial cells?

Answer 22

glial cells surround axon and keep ejected K+ from diffusing away, so:

• so with glial cells, ECF [K+] gets progressively higher with each AP, so Ek and Vm increase slightly and afterpotential increases
• without glial cels, K+ diffuses away into ECF and cell is easier to repolarize back to normal afterpotential

Question 23

on which ion does afterpotential depend?

Answer 23

K+

Question 24

what accounts for the post-tetanic extra-hyperpolarization of a small axon?

Answer 24

post-tetanic increased ECF K+ concentration and increased ICF Na+ concentration stimulates the Na-K-ATPase pump to higher activity, which hyperpolarizes the membrane extra

Question 25

why are post-tetanic extra-hyperpolarizations not seen as commonly in large axons?

Answer 25

because the post-tetanic extra-hyperpolarization effect depends on high ECF K+ concentration and high ICF Na+ concentration that stimulate KK-NaNaNa pump to higher activity and hyperpolarizes the membrane extra
-in a large fiber, the K+ and Na+ concentration changes are not as significant due to smaller surface area / volume ratio.

Question 26

what does oubain do to membrane transport?

Answer 26

inhibits Na-K-ATPase pump

Question 27

how does oubain affect post-tetanic extra-hyperpolarization?

Answer 27

limits it because oubain inhibits Na-K-ATPase pumps
(post-tetanic increased ECF K+ concentration and increased ICF Na+ concentration stimulates the Na-K-ATPase pump to higher activity, which hyperpolarizes the membrane extra)