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NEUROSCIENCE 2 > LECTURE 2 - voltage gated ion channels > Flashcards

LECTURE 2 - voltage gated ion channels Flashcards

1
Q

What are voltage gated ion channels?

A

Na+ and K+ channels

  • closed at -ve Vm (membrane potential)
  • opens by depolarisation (increased Vm = stimulus)
  • controlled Vm = ability to study channel properties
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2
Q

What is voltage clamp?

A

A method to record the flow of ions (current) through channels

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3
Q

Why does Vm need to be fixed during voltage clamp?

A

To enable a stable measurement of channel activity, the flow of ions leads to a change of Vm

  1. change in number of active V-gated channels and
  2. change in driving force
    can both lead to change in current
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4
Q

What is driving force?

A

The difference between membrane potential and the equilibrium potential for an ion
Vm-Eion

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5
Q

What is the voltage clamp methodology?

A
  1. Electrode measures and controls Vm (some Na channels are opened by voltage change, Na enters)
  2. Im (current of membrane) injects current to keep voltage constant (at -30mv)
    * an influx of 2 +ve charges requires injection of 2 -ve charges
  3. Measuring Im allows us to see how much current was required to keep voltage constant because that = how much voltage has flown through

Equal and opposite current is injected (via Im) to maintain Vm

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6
Q

What does a voltage clamp recording of current show?

A

2 depolarisations are essentially occurring

  1. Small - leak current only
  2. Larger - sodium and potassium current summed
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7
Q

What does the voltage clamp experiment show?

A

Shows that the channels are only opening in response to a change in voltage and nothing else

HOWEVER the experiment does not identify which ion is involved but direction of flow determines if current is +ve or -ve –> gives a good clue

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8
Q

What does the direction of ion flow mean in terms of ion movement?

A

Positive (up) = +ve ions leaving cell or -ve ions entering cell
OUTWARD

Negative (down) = +ve ions entering cell or -ve ions leaving cell
INWARD

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9
Q

Describe the voltage clamp recording of current

A

There are 2 depolarisations
Small: leak current only, at resting membrane potential (-60mV) there is no NET FLOW, at -50mV there is NET FLOW

Large: total ionic current, summed current of V-gated K and Na channels

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10
Q

How can voltage gated K+ channels be pharmacologically isolated?

A

TTX = tetrodotoxin

  • blocks Na+ channels leaving only K+ channels remain
  • K+ channels are slower opening and do not inactivate
  • graph shows sigmoidal curve with IK moving outward (up) as there is K+ efflux
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11
Q

How can voltage gated Na+ channels be pharmacologically isolated?

A

TEA = tetraethylammonium

  • blocks K+ channels leaving just Na+ channels
  • Na+ channels open much faster (= sharp start to graph) and then inactivate (= levels off at 0)
  • graph shows sharp inward movement of ions (down), then gradually increases to 0
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12
Q

Describe the current vs voltage graph for V-gated K+ current

A

(Each data point is the max. current at that Vm)
x axis = voltage
y axis = current

  • slightly curved upwards line starting at -50mV
  • more depolarisation = bigger current and more channels opening –> increased driving force
  • driving force = the difference between voltage and equilibrium potential for an ion
  • Ek ~ -80mV, moving to the right of graph (upwards) = moving away from Ek therefore there is a bigger driving force
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13
Q

Describe the current vs voltage graph for V-gated Na+ current

A

(upside down bell curve)
x axis = voltage
y axis = current
- starts decreasing at -50mv
- inwards movement of ions (therefore down on graph)
- initially there is a bigger current but it gets smaller
- above +60mV (Vm) there is an outward current because more channels are opening but you are moving closer to ENa
- ENa = line crosses axis

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14
Q

Why does Vm influence the size of current (current voltage graphs)?

A
  1. Increased Vm (increased depolarisation) = more channels opened as they are V-gated
2. Increased Vm = changes in driving force (Vm-Eion)
For Ik (Ek =-80mV)
- depolarisation moves AWAY from Ek = bigger driving force = increased Ik
For INa (ENa = +55mV)
- depolarisation moves CLOSER to ENa = smaller driving force = decreased INa

Final current amplitude depends on combination of 1 and 2

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15
Q

What does the size of IK and INa depend on (current)?

A
  1. Number of open channels - indicated by conductance

2. Driving force

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16
Q

What is conductance?

A

A direct measure of channel activity

Ohms law: V = R x I
Conductance: g=1/R
so I = g x V

calculate conductance:
gK = Ik/ (Vm -Ek)

17
Q

How can you get conductance vs time graph from current?

A

Divide the current by the driving force at each time point

18
Q

Explain the conductance vs time graph

A
  • gNa is positive because both Ina and (Vm-ENa) are -ve (-ve/-ve = +ve)
  • increased depolarisation = more channels open and they open faster
  • K+ channel opening is delayed
  • Na+ channels: activate then inactivate
19
Q

Explain the conductance vs voltage graph

A

both gK and gNa sigmoidal curve with gK finishing slightly higher than gNa

  • shows how the activity of channels varies according to Vm
  • x axis = vm
  • y axis = conductance (g)
  • calculated from current-voltage data
  • maximal g = all channels open
  • at +60mV, gNa is large but Ina is 0 because Vm-ENa= 0
  • conductance is calculated from current and driving force but is NOT explained by either
  • slope of line reflects sensitivity to Vm, NOT SPEED OF OPENING
20
Q

What are A-type K+ channels (IA) and what are they important for?

A
  • another type of V-gated K+ channels
  • the previous ones were DELAYED RECTIFIER K+ CHANNELS (IK)
  • important for controlling onset and frequency of firing APs
21
Q

What are the properties of A-type K+ channels?

A
  • conduct K+ efflux - REDUCE excitability of neurones
  • VOLTAGE-GATED = opened by depolarisation
  • activate quickly (unlike delayed rectifier)
  • inactivate (like Ina, unlike DR) - proportion of channels that are inactivated depends on Vm
  • mostly inactivated at resting membrane potential (unlike INa which inactivates at more depolarised Vm)
  • inactivation removed by hyper polarisation before channels are opened by depolarisation
22
Q

Explain the experimental recording of A-type K+ channels starting at both -40 and -80mV and ending at -5mV

A

Starting at -40mV:
- result only due to IK (delayed rectifier) ALONE

Starting at -80mV:
- result due to BOTH delayed rectifier and A-type as -80mV is a hyper polarisation voltage so A-type get activated

23
Q

How does IA delay initiation of AP firing?

A
  • hyperpolarisation removes the inactivation of IA
  • depolarisation opens IA channels
  • K+ efflux decreases cell excitability which means there is a delay in reaching threshold

NEUROSCIENCE 2 (32 decks)

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