lecture 7 voltage gated ion channels

Question 1

what did hodgkin’s and huxley’s experiment do

Answer 1

used giant squid axon to fit multiple electrodes and study ion channels

Question 2

why does gna go up and then down on an action potential graph

Answer 2

VGna channels open and then deactivate after too much depolarization

Question 3

why is there an undershoot

Answer 3

delayed rectifier channels for potassium take a bit longer to close so it gets into too much hyperpolarization territory

Question 4

what is the structure of the VGna channel

Answer 4

one long polypeptide with an n and c terminus that weaves in and out of the membrane
has domains 1-4 and each domain has 6 transmembrane alpha helices

Question 5

what is the voltage sensor that opens up the VGna channel

Answer 5

s4 is positively charged and twists away from the inside of the cell when the cell is depolarized from -65 to around -40mv

Question 6

in resting phase VGna channel is…

Answer 6

closed but not inactivated

Question 7

what causes the na channels to rapidly open

Answer 7

depolarization from a graded potential

Question 8

when is VGna channel inactivated

Answer 8

rising phase/peak of vm

Question 9

when is the VGna channel deinactivated

Answer 9

when the action potential graph shows repolarization back to rest (undershoot to steady state)

Question 10

which has more conductance a closed or inactivated VGna channel

Answer 10

neither have conductance

Question 11

what is the time until deinactivation called

Answer 11

absolute refractory period

Question 12

difference between absolute refractory and relative refractory

Answer 12

absolute refractory is when channel is inactivated- no amount of depolarization will get it to open
relative refractory is when an action potential can happen but the stimulus will need to be a bit larger than normal (more closed na channels than normal, slowly deinactivating so the threshold will be higher)

Question 13

difference between VGna and VGk

Answer 13

VGk opens slower following depolarization because of delayed rectifier channels
do not inactivate with depolarization
open in rising phase and begin to close in falling

Question 14

which ions get the action potential out of the undershoot and back to rest

Answer 14

sodium and chloride (k is useless, close to it’s eq so it’s chilling)

Question 15

what toxin can block VGna channels

Answer 15

tetrodoxin, lidocaine, novocaine, saxitoxin, scorpion toxin

Question 16

what does ttx do

Answer 16

blocks na channel from outside so depolarization cant happen (fugu dish with pufferfish causes this because of toxin in liver, if cooked wrongly can be lethal because it eliminates action potentials)

Question 17

what is a channelopathy

Answer 17

congenital brain diseases because of mutations in ion selective channel genes and can cause epilepsy, migraines, and hearing disorders

Question 18

what is a channelopathy

Answer 18

congenital brain diseases because of mutations in ion selective channel genes and can cause epilepsy, migraines, and hearing disorders

Question 19

Ohm’s Law

Answer 19

relationship of current and voltage through any conducting medium. Traditionally
V = IR, however it can be rearranged as follows (see notes from Lecture 6 and remember
conductance is the inverse of resistance g = 1/R)

Question 20

Voltage-gated ion channels

Answer 20

channels whose conductance (ability to pass ions) is dependent on membrane voltage. These channels will open once the cell is depolarized above the threshold voltage. Typically a local depolarization (e.g. synaptic potential, receptor potential, etc.) is what causes the neuron to be depolarized above threshold

Question 21

About 1 ms after the action potential threshold has been reached and an action potential is
triggered, what two things happen

Answer 21

the voltage-gated sodium channels inactivate and the voltage gated potassium channels open.

Question 22

Inactivation

Answer 22

property of some depolarization-gated ion channels whose conductance
goes to zero even though the membrane is depolarized

Question 23

The membrane must (hyperpolarize or depolarize) for the channels to de-inactivate and return to their closed conformation before they can open again

Answer 23

hyperpolarize to below the threshold voltage

Question 24

Describe the ionic events (which channels are involved, direction and magnitude of ion
flow) that lead to:
i. rapid depolarization of an action potential (rising phase)
ii. rapid hyperpolarization (falling phase)
iii. after-hyperpolarization (undershoot)

Question 25

Define and describe the biological mechanism for each of the following:
i. threshold
ii. all-or-none property of action potentials
iii. absolute refractory period

Question 26

General familiarity with the various ways toxins affect voltage-gated sodium channels