two gates that respond at different voltage levels and at different rates; activation is faster but needs more depolarization; inactivation is slower but happens closer to the resting potential; contributes to refractory period of action potentials
time after repolarization from an action potential in which another action potential cannot be produced; must hyperpolarize the cell to reverse inactivation
only way to reverse inactivation of sodium?
experiments to study inactivation
depolarizing or hyperpolarizing prepulses prior to big stimulus; depol - reduced the final sodium current; hyper- increases the sodium current; one exp with no prepulse, one with depol, one with hyper => all led to the same Vm
small (subthreshold) depolarizations or hyperpolarizations prior to the big stimulus that have an effect on the amount of sodium current produced
do potassium channels involved in action potenials inactivate?
no, but others do
what turns Na channels off?
depolarization and time
what turns Na channels on?
what allows the Na channel to be turned on again?
hyperpolarization, refractory period
what turns the k channel on?
what turns the k channel off?
kinetics of the conductance dependence on voltage? (both Na and K)
sigmoid; s-shaped; implies cooperativity between subunits (activation of one increases likelihood of the activation of another); like hemoglobin and oxygen
function for the time course of GNa
GNa = GNa(max) x m^3 x h
m = entity that opens the channel; h = entity that closes the channel; m and h are variable between 0 and 1
function for time course of GK
GK = GK(max) x n^4
n = variable between 0 and 1
determination of n value for time course of GK
n = 1- e^(-t/taun)
taun = time constant, dependent on Vm, ranges from 4 msec (small depolarization) to 1 msec (large depolarizations) K channel opens when 4 particles move to the right place and n is the probability that each of these does move to place
determination of m for time course of GNa
m = 1-e^(-t/taum)
taum- time constant, depends on Vm, ranges from 0.6 m sec (small depol) to 0.2 msec (large depol)
determination of h for time course of GNa
h = e^(-t/tauh)
tauh - time constant, depends on Vm, ranges from 4 msec (small depol) to 1 msec (large depol)
interpretation of time course equation for GNa
3 particles need to be in place for channel to open; m is the probability that happens; if h moves the channel closes; h is the probability that happens; h particles are slower than m particles
equation for the total membrane current
I = [Cm(dV/Dt)] + [GK(max)](n^4)(Vm - EK) + [GNa(max)](m^3 x h)(Vm - ENa) + g1(Vm - E1)
g1 = leak conductance equation can predict action potentials
the lumped non voltage sensitive conductances (mostly Gk and GCl) that are present in the membrane
model for studying the v-gated sodium channel
electric eel sodium channel isolated and sequenced; 1820 amino acids, 4 homologous domains (I, II, III, IV); each domain contains 6 transmembrane sequences = alpha helices (S1-S6)
the Na channel protein contains no hydrophobic leader sequences meaning...
N-terminus is probably cytoplasmic, meaning the loops between the four domains are cytoplasmic
scorpion toxin binds domains I, II, and IV of Na channel meaning...
they are close to each other; toxin also binds on the outside
locations of carbohydrate side chains of the Na channel
location of phosphorylation sites of the Na channel
location of inactivation site for Na channel; proof?
inside; inject pronase into the cytoplasm (a protease) prevents inactivation, apply pronase to the outside, inactivation continues
the voltage sensor of Na channels?
S4 which has positive charges every three residues
pore lining for Na channels?
S5 and S6
K channel protein
single domain, 616 amino acids, 4 of these assembled into a tetramer
production of hybrid K channels
instead of 4 of the same domain assembling into a teramer, different domains from different types of K channels assemble to form a hybrid
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