Module 2 Lecture 2 Flashcards
sodium concentration outside cell vs inside cell
145 mM; 5-15 mM
potassium concentration outside cell vs inside cell
5 mM; 140 mM
chlorine concentration outside cell vs inside cell
110 mM; 4 - 30 mM
calcium concentration outside cell vs inside cell
1 - 2 mM; 0.0001 mM
protein concentration outside cell vs inside cell
few; many
what does “rest” mean for a neuron
when it is not firing action potentials or being stimulated by other neurons
what is the resting potential of most neurons
about -55 to -80 mV (inside of cell is more negatively charged than the outside)
when does electrochemical equilibrium occur
when the driving force is zero
when can the driving force be zero
when 1 of 2 things are true:
1. the electrical and chemical forces are both zero
2. the electrical and chemical forces are equal and opposite
what happens when an ion reaches electrochemical equilibrium
there will be no net flow of that ion across the membrane anymore, and the membrane voltage will remain a constant value
what is the equilibrium potential
the stable membrane voltage (abbreviated E_ion)
potassium equilibrium potential
-75 mV
sodium equilibrium potential
+55 mV
chloride equilibrium potential
-41 mV
calcium equilibrium potential
+145 mV
Nernst simplified equation
E_x = (58/z) log([Xout]/[Xin])
GHK equation
V_m = 58 log([total ion out]/[total ion in])
what are the majority of the ions crossing the membrane at rest
92.5% K+, 7.5% Na+, so Vrest = -69.5 mV
what happens to the membrane voltage if the membrane is permeable to only one ion
the membrane voltage will converge to that ion’s equilibrium potential
what happens to the membrane voltage if the membrane is permeable to two ions
the membrane will converge to the average of the equilibrium potential of the two ions, weighted by the relative permeability of each ion
what drives the cell’s resting membrane potential the most strongly
K+ channels being open almost all the time, causing K+ conductance to be high
- equal flow of K into and out of the cell at rest
sodium potassium pump exchange rate
3 Na+ for every 2 K+
Ohm’s law
V = IR
conductance formula
g = 1/R
what are the only conditions that GHK equation works under
- membrane is homogenous
- the electric field is constant throughout the membrane
- the ions move through a membrane the same way they do as in solution
- the ions act independently
- you don’t get weird stuff at high or low concentrations
what is the Nernst potential
55 mv
what happens to potassium when concentration force = electric force
potassium stops flowing
what is the reversal potential
the point at which the current in the channel reverses from inside to out (no net flow)
what causes the shape and magnitude changes in the voltage
the opening and closing of specific ion channels for Na+ and K+
what causes the resting potential in humans
the presence of open K+ channels at rest
first stage of action potential
open K+ channels create the resting potential
second stage of action potential
any depolarizing force will bring the membrane potential closer to threshold
third stage of AP
at threshold, voltage-gated Na+ channels open, causing a rapid change of polarity – the AP
fourth stage of Ap
Na+ channels are inactivated; gated K+ channels open, repolarizing and even hyperpolarizing the cell (afterpotential)
fifth stage of AP
all gated channels close; the cell returns to its resting potential
when does the fast positive cycle occur
early
when does the slow negative cycle occur
late
what does a depolarized membrane potential cause in the fast positive cycle
opening of Na+ channels
what does opening of Na+ channels cause in the fast positive cycle
an increase in Na+ current
what does an increase in Na+ current cause in the fast positive cycle
more depolarizing
what does a depolarized membrane potential cause in the slow negative cycle
opening of K+ channels
what does the opening of K+ channels cause in the slow negative cycle
increase in K+ current
what does an increase in K+ current cause in the slow negative cycle
hyperpolarizing
what happens to the Na driving force and the probability of the channel being open in the cell as it gets more positive
driving force goes down, and the probability goes up
characteristics of NALCN selectivity
non-selective, but pass Na+ better than K+
what happens in absence of K+ and equivalent Na+ on both sides in NALCN
reversal potential is 0
how is biology compartamentalized
- boundaries are formed by (usually semi-permeable) membranes
- membranes have areas with distinct lipids and/or proteins
how can molecules move across membranes
via diffusion or transport (passive/active)
what does stochastic mean for channels
at every point in time the conformation can switch
- at any moment, there is a probability to find the molecule in a given state
what would happen if we only had leaky Na and K channels
the ions would flow through the membrane until they reached equilibrium
how many subunits does the Na+/K+ ATPase have
3 subunits
what is the P-domain in the Na+/K+ ATPase
phosphorylation domain
- once ATP is hydrolyzed at the N-domain it phosphorylates the P-domain
what is the N-domain in the Na+/K+ ATPase
nucleotide-binding domain
- ATP binds here; when it is hydrolyzed, it phosphorylates the P-domain
what is the A-domain in Na+/K+ ATPase
actuator domain; Na+ & K+ bind and are shuttled through
what is the alpha subunit in the Na+/K+ ATPase responsible for
ion translocation (binding & moving ions through the membrane)
- well suited for diving into the plasma membrane
- 10 transmembrane domains; bulk of rest of alpha subunit found cytoplasmically
- 1000 aa
what do ATP hydrolysis and phosphorylation drive
conformational changes
what happens when ATP binds the N domain of Na+/K+ ATPase
it gets hydrolyzed and phosphorylates the P domain
characteristics of beta subunit in the Na+/K+ ATPase
ancillary, helps with structure and stability
- extracellular
- ancillary
- 400 aa
what role does the FXYD subunit have in Na+/K+ ATPase
modulatory role
- 60 aa
what does the Na - K pump result in
Na ions leaving the cell & K ions coming in against their concentration gradients
main theme of Post-Albers cycle
ATP hydrolysis and phosphorylation drive conformational changes –> conformational changes drive structural and functional changes
what are the two major conformational changes
E1 and E2; can be phosphorylated or not
what is E1 conformation
generally ‘open to the cytoplasm’
what is E2 conformation
generally ‘open to the extracellular space’
E1 3NA * ATP –>
E1 (3Na) * P
E1 (3Na) * P –> ? and characteristics
E2 3Na * P
- ATP hydrolyzed when Na+ binds, and phosphorylates the pump (ATP –> ADP), resulting in a conformational shift to E2 (open to extracellular space)
E2 3Na * P –> ? and characteristics
E2 * P
- alpha subunit has high affinity to 2 K+ ions
- open to extracellular space and phosphorylated
E2 * P –> ? and characteristics
E2 2K * P
- when K+ binds, pump is dephosphorylated
E2 2K * P –>
E2 (2K)
- dephosphorylation of the pump results in a conformational shift back to E1 (open to extracellular space)
E2 (2K) –>
E1 2K * ATP
- ATP binding facilitates the conformational shift back to E1
E1 2K * ATP –>
E1 * ATP
- K+ ions now diffuse into the cell
- Na+ ions bind to the now-high-affinity E1 pore, and the cycle starts over
characteristics of E1 * ATP phase
alpha subunit has high affinity to 3 Na+ ions
- open to the cytoplasm
what is the end result of the Post-Albers cycle
one molecule of ATP hydrolyzed for 3 Na+ pumped outside and 2 K+ ions pumped inside
what does the Post-Albers cycle do to the membrane potential
makes it more negative
result of Gorman & Marmor (1970 paper)
GHK equation only made good predictions at cold temperatures
- oubain (blocks pump) abolishes discrepancy with GHK equation
why does the GHK equation make good predictions at 4 C and not 17 C
at 17 C, Na-K pump is working overtime; at 4 C, Na-K pump is not working at all
how did the Gorman & Marmor experiment work
clamped onto mollusc model neuron at 11 C, measured membrane potential at different temps and measured voltage at different external K + concentrations
what did the Gorman & Marmor paper confirm about the Na-K pump
that it is electrogenic, and makes the membrane more negative
- 1st example of a GHK equation flaw
how does Na-K pump affect membrane potential
makes the membrane potential much more negative than expected
what was the goal of the Thomas experiment
to pump ions into the neuron without changing the voltage
interbarrel iontophoresis
a way to pass ions across what should be a stable membrane without causing current changes
how did Thomas set up interbarrel iontophoresis
- 2 extra electrodes were set up so 1 electrode had sodium acetate & the other had lithium acetate; if they passed current between each other, it would cause ion changes, but no current change (bc no net current)
what happened in the Thomas paper when injecting positive current from the potassium acetate probe
increased internal K+ concentration, but not voltage
how do K+ and Li+ affect membrane potential
have little/no effect
how do Na+ injections affect membrane potential
significantly hyperpolarize
what can block effects of increasing [Na+]i
oubain and by removing external K+
what happened after the Ouabain bath in the Thomas paper
membrane potential became a little more positive
- injections of Na+ did not have as much of an effect
what happened after the removal of K+ in the Thomas paper
adding Na+ with K+ removed doesn’t do anything; reintroducing K+ = quick rebound as a result of the pump turning back on
what did Pulver & Griffith (2009) show
AHP may have a role in information processing/’short term memory’ for rhythmic behaviors
how do plasma membrane transporters use Na+ gradient
- send Na_ in the cell down its gradient –> use energy from that to transport something else in the cell that’s going up its gradient
examples of antiporters
Na+/Ca+ exchanger, Na+/H+ exchanger
examples of co-transporters
Na+/K+/Cl- co-transporter, K+/Cl- co-transporter, Na+/neurotransmitter co-transporter
what is the ultimate energy source for plasma membrane transporters
ATP
what gradients do vesicular neurotransmitter transporters use
either the pH gradient or the voltage gradient established by the vesicular proton pump
how to pump neurotransmitter from the axon terminal (low concentration) to a high concentration?
spend a lot of ATP to make the vesicles more acidic
characteristics of VGLUT & EAAT 1,2
require ATP to fuel the pump that creates the Na+ gradient they rely on
SAT function
cotransports glutamine and Na+
EEAT function
cotransports glutamate and Na+
VGLUT function
antiports glutamate and H+
SN function
transports glutamine back out of astrocytes
what do SAT, EEAT, VGLUT, and SN all have in common
require ATP at some stages
why is there no K+ flow below -50 mV
channels are not open
TTX function
blocks early inward Na current
TEA function
blocks early inward K current
- only when applied from the inside
what does normal current look like over time
fast inward current from sodium, followed by a delayed outward current from potassium
how did tail current get its name
it occurs right at the tail of a voltage pulse; right as the voltage “steps” back down
what direction is K+ tail current if membrane potential is more negative than the equilibrium potential
inward
why do we see a tail current?
bc there is a bit of time between when the voltage steps down and the channel closes
if you step from 0 mV to -73 mV, do you see a tail current?
no, bc that’s K+ equilibrium potential
if you step from 0 mV to -60 mV, do you see a tail current?
yes, a positive one
what is the technique for observing a tail current
open all the channels up and step down to some voltage where the channel would usually be closed, and create an instantaneous current
first theory about how voltage dependency might work
as voltage gets higher, channels gets slowly wider and starts to increase conductance
what would be observed if the first theory of voltage dependency was true
we’d see changing slope over time while holding voltage constant
second theory of voltage dependency
conformational states (digital) that either allow potassium to flow, or do not
