Ion Channels And Synapses Flashcards
What is the flow of ions across channels at Equilibrium (Eion)?
no ion movement
What is the driving force for ion movement across chanells?
the difference between the membrane potential (Vm) and the equilibrium potential (Eion)
- driving force = Vm - Eion
What determines the size of ion flow across channels (current)?
- driving force
2. no. of ion channels (conductance/ gion)
How can you calculate the Current for an ion?
- current = conductance x (driving force)
- driving force = membrane potential - equilibrium
Iion = gion x (Vm x Eion)
During an action potential (depolarisation), what happens to the conductance of the sodium ions?
- conductance increases -> sodium influx… occurs down sodium conc. gradient
- sodium channels open due to increasing MP
What is the conductance increase for potassium vs sodium?
- conductance increases more slowly and for longer
- occurs during re-polarisation/ hyper-polarisation phases
What acts as a stimulus for the opening of voltage-gated ion channels?
- the increasing membrane potential
- rate of channel opening depends on rate of membrane potential depolarisation
resting MP caused by?
leak K+ channels (Few Na+)
whats rising phase… and falling phase?
RISING
inward Na+ current (VG Na+ channels)
FALLING
outward K+ current (VG K+ channels)
how do Na, K, Ca cannels behave at negative MP?
pore (channel) closed…
open by depolarisation (inc in Vm stimulus)
What is the voltage clamp method.. shows what?
how ion conductance changes with membrane potential and time.
What is the importance of the voltage clamp method?
- changing MP changes amount of active VG channels open (changed driving force)
each can -> inc/dec in ion flow movement
so..Vm has to be fixed for stable measurement of channel activity
voltage clamp:
whats size of current affected by?
driving force and conductance, g.
for VG channel, gion is dependant on Vm
voltage clamp: what happens after 1. electrode controls intracellular Vm?
hows current measured (indiectly)
when ions flow through channels, equal + opposite current injected (via Im) to maintain Vm
if Im measured, we know the channel current!
What is the current clamp method- measures what?
intracellular voltage difference across cellular membrane while injecting constant positive ions into the cell.
What is the process of Current clamp method?
o Electrode placed inside membrane to measure MP. VG Na+ channels also placed in the membrane
o current (+ve ions) injected into the cell by electrode -> depolarization and is stimulus for the Na-channels to open
o Na+ now moves down conc. gradient into the cell –> further increases MP
o change in MP can be recorded in neurons during AP
What are the issues with the voltage clamp method?
- doesn’t identify which ion moves
- however ion flow determines whether current is positive or negative
– Positive (up) deflection = +ve ions leaving cell (efflux)
– Negative (down) deflection = +ve ions entering cell (influx)
. time dependant
. pharmacological separation
how do voltage dependant ion channels react to the method?
technique allows change in Vm to be applied to cell
voltage dependant channels will respond to a depolarisation
if depol large enough,
=> Na+ (INa) + K+ (IK) current (summed)
Which pharmacological agents can be used to identify individual currents?
o Tetrodotoxin (TTX) – blocks Na-channels (only K flow is evident)
o Tetraethylammonium (TEA) – blocks potassium channels (only Na flow is evident)
What can be measured after analysis of the voltage clamp experiments?
- the relationship between the flow of current and the membrane potential
(max current in/out measured at each Vm) - this can be plotted on a current vs voltage graph
show how size of current peak varies w Vm
What can be seen from a current vs voltage graph for sodium and voltage gated sodium channels?
- votage increase -> rapid influx of sodium (steep line)
- eventually these channels close
- Channels close as they are voltage gated – as more channels close the membrane potential returns to normal (negative)
down then up
What can be seen from a current vs voltage graph for potassium and voltage gated potassium channels?
- voltage increase -> efflux of potassium
- potassium current also increases - suggesting more K channels opening leads to K efflux
- this is a slower and prolonged process (flatter line)
p676
Why does current depend on membrane potential? (Vm)
incraesed membrane potential:
- opens more channels (increased stimulus) depolarisation
- leads to a change in driving force (Vm - Eion)
How does depolarisation effect the flow of current for potassium ions?
- the equilibrium for poatassium = -80 mV
- in depolarisation the driving force is increased as we (Vm) are moving away from the equilibrium
- this increases the opening of K channels and the outward flow of potassium
- current reaches plateau quickly
How does depolarisation effect the flow of current for sodium ions?
- equilibrium = +55mV
- during depolarisation driving force is decreased as we move closer to equilibrium
- this leads to less sodium channels open and slower influx of sodium
but when Vm > +55, (Vm - ENa) = +ve
What is the final current dependant on?
the combinations of the sodium and potassium currents
What is Ohm’s Law?
voltage is equal to the current multiplied by resistance
V = IR
How to calculate conductance using Current and Resistance?
- Current divided by resistance
- g = I / R
How to calculate voltage using Current and Conductance?
- Current divided by Conductance (g)
- V = I/ gI
g V
BUT ionic current depends on driving force, not just V
bc current = NOT 0 when Vm = 0!!!
thus for Na+…
INa = gNa x (Vm = ENa)
What are the different types of Calcium channels (voltage gated)
- low voltage Activated (LVA): T-type (CaV3)
- High voltage Activated (HVA)
