Lecture 10 - The Action Potential Part 2: Ionic permeability and channel conductance

What happens to membrane potential when we inject negative current?

• Set up an experiment with 2 probes that are inserted into an axon:

  • The first injects current and measures the effect on Vm

  • The second measures Vm a certain distance awa

When inject negative current at a constant rate, see the membrane potential becomes more negative at point a (the membrane is hyperpolarized)

Why don’t we detect change at point b?

• Because was not enough of a stimulus to induce action potential

• Also, because membrane is leaky to charge, no current arrives to change Em at point b

What happens to membrane potential when we inject positive current?

• If the injected positive current is small, we see slight depolarization, but the same decay in current with distance

• As a result, see no change in membrane potential at point b

• YET! If depolarization is large enough current (that reaches a threshold membrane potential), then we see an action potential at both points a and b that occurs as a result of the change in ion conductance across the cell membrane

• There is a time delay between the action potential at point a and its arrival at point b

Why do we have changes in ionic conductance during AP?

• An action potential is the result of the rapid depolarization of the cell membrane. Following this:

  • During rising phase of action potential, see an increase in gNa

  • At peak of action potential, action potential fails to reach ENa

• At action potential peak, gK is substantial, and so, K+ carries out most charge brought in by Na+.

  • Vm never reaches ENa

  • current based on conductance AND driving force (how far membrane is from ion eqm potential)

What causes the changes in ionic current across the cell membrane during an action potential?

• Eion does not change magnitude of current —> because small number of ions lined up across membrane

• Vm is needed for change in membrane potential

• gion permeability is what causes change ??

What are permeabilities for resting cells?

*

What is the relation between sodium permeability and Vm?

• Sodium is much less permeable across the cell membrane when compared to potassium in a cell at rest

• This is why Vm in a cell at rest is closer to EK rather than ENa

• At the start of an action potential PNa increases substantially

What can influence ion permeability?

• Permeability of a channel is dependent on Vm

• Eg: in this voltage-gated sodium channel

  • Monomeric protein

  • Organized into 4 homologous transmembrane domains (motifs)

  • Non-helical P loops between alpha helices 5&6 in each domain line the pore and make up the selectivity filter

  • Transmembrane alpha helix 4 in each domain serves as the voltage sensor

  • See similar architecture in other voltage-gated ion channel

Voltage-gated cation channels have homologous domain

• Voltage-gated sodium and calcium channels consist of one large pore-forming subunit containing 4 domains

• Voltage-gated potassium channels are made up of four pore-forming subunits

• S4 segment is the voltage sensor that plays an important role in the voltage-gating of the channel

• This is the region that transduces the depolarization of the cell membrane into a conformational change and opening of the ion channel

When do ion channels open and close?

• they open and close when charge distribution changes within the ion channel

• The red cylinder represents the positive charges in the S4 channel segments- this is the gating charge region

• Depolarization of the cell membrane causes the gating charge region to move outward towards the extracellular side of the cell, and thus changes conformational state of ion channel

Conductance of ion channels is dependent on Vm, so explain Na+ channel conductance

• Na+ channel conductance

  • Voltage-gated sodium channels have two gates

  • An activation gate (m-gate)

  • An inactivation gate (h-gate)

• At resting Vm, m-gate is closed and h- gate is open

• Both gates respond to depolarization, but with different speeds and in opposite directions

• m-gates open immediately after depolarization

• The delayed h-gates then close a short time later (still during depolarization, therefore= sodium channel inactivation)

Does sodium entry use positive or negative feedback loop during depolarization?

1. When the cell membrane is depolarized, PNa increases, allowing Na+ into the cell….

2. .... this depolarizes the cell further, causing a greater increase in PNa

3. This process continues until all sodium channels in the membrane are open and the membrane potential is closer to E 19

Explain the conductance of K+ channels.

• Voltage-gated potassium channels are ALSO closed at resting Vm

• These channels have an n-gate which also responds to changes in depolarization of membrane potential

• In contrast to m-gate of sodium channels, there is a much slower response to depolarization, which causes PK to increase with a delay following depolarization

• The timing of n-gate opening is pretty much at the same time as Na+channel h-gate closure

• n-gates stay open until they respond to hyrepolarization

What are the voltage-gated ion channel changes during AP?

*

Explain the sequence of ion channel changes during AP.

• Depolarization causes voltage-gated Na+ channel m-gates to open, setting off a cascade of Na+ channels to open and attempt to bring Vm towards ENa

• Delayed voltage-gated Na+ channel h- gates close together with opening of n- gates of voltage-gated K+ channels, which brings Vm more towards EK

• Repolarization causes h-gates to open and n-gates to close