lecture 7 voltage gated ion channels Flashcards
what did hodgkin’s and huxley’s experiment do
used giant squid axon to fit multiple electrodes and study ion channels
why does gna go up and then down on an action potential graph
VGna channels open and then deactivate after too much depolarization
why is there an undershoot
delayed rectifier channels for potassium take a bit longer to close so it gets into too much hyperpolarization territory
what is the structure of the VGna channel
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
what is the voltage sensor that opens up the VGna channel
s4 is positively charged and twists away from the inside of the cell when the cell is depolarized from -65 to around -40mv
in resting phase VGna channel is…
closed but not inactivated
what causes the na channels to rapidly open
depolarization from a graded potential
when is VGna channel inactivated
rising phase/peak of vm
when is the VGna channel deinactivated
when the action potential graph shows repolarization back to rest (undershoot to steady state)
which has more conductance a closed or inactivated VGna channel
neither have conductance
what is the time until deinactivation called
absolute refractory period
difference between absolute refractory and relative refractory
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)
difference between VGna and VGk
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
which ions get the action potential out of the undershoot and back to rest
sodium and chloride (k is useless, close to it’s eq so it’s chilling)
what toxin can block VGna channels
tetrodoxin, lidocaine, novocaine, saxitoxin, scorpion toxin
what does ttx do
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)
what is a channelopathy
congenital brain diseases because of mutations in ion selective channel genes and can cause epilepsy, migraines, and hearing disorders
what is a channelopathy
congenital brain diseases because of mutations in ion selective channel genes and can cause epilepsy, migraines, and hearing disorders
Ohm’s Law
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)
Voltage-gated ion channels
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
About 1 ms after the action potential threshold has been reached and an action potential is
triggered, what two things happen
the voltage-gated sodium channels inactivate and the voltage gated potassium channels open.
Inactivation
property of some depolarization-gated ion channels whose conductance
goes to zero even though the membrane is depolarized
The membrane must (hyperpolarize or depolarize) for the channels to de-inactivate and return to their closed conformation before they can open again
hyperpolarize to below the threshold voltage
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)
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
General familiarity with the various ways toxins affect voltage-gated sodium channels
