Lecture 2 - Chapter 2/4: Action Potentials are Generated by Ion Currents Flashcards
What is electric current?
Flow of electrical charge, i.e. flow of potassium and sodium (ampere = coulomb/second)
What is membrane resistance?
A measure of how much the membrane opposes the passage of electrical charge (through ion channels)
What is membrane conductance?
The permeability of the cell membrane to those ions
What is voltage?
Difference in charge between two points (i.e. membrane gradient). It creates energy that pushes charge to move.
What is the membrane potential?
The voltage difference between the inner and outer surface of the cell membrane.
What is meant by the fact that membranes are capacitors?
The membrane, composed of a lipid bilayer, is impermeable to ions. It can therefore store charges.
What technique is able to control the membrane potential and simultaneously measure their underlying permeability changes? And what animal was used to study electrical signals?
A voltage clamp, first used in squids. Squids have enormous axons of 1 mm in diameter compared to 1-2 um in mammals.
How is voltage clamp measurement achieved?
The device is able to control membrane potentials at any level desired.
- Here, the membrane potential is measured by an electrode placed inside the cell, e.g. on an axon. This electrode is connected to the voltage clamp amplifier.
- The amplifier compares this membrane potential with the desired (command) potential.
- When Vm/membrane potential is different from the command potential, the clamp amplifier injects current into the axon through a second electrode. This way the membrane potential becomes the same as the command potential.
- The current flowing back into the axon, and thus across its membrane, can be measured here.
What is a capacitive current?
The capacitive current determines how fast the cell membrane potential responds to the flow of ion channel currents. It is the flow in membrane current due to a changing potential of the electrode which charges or discharges the membrane (i.e. the capacitor) → thus only flows when the membrane potential is changing.
What happens when the membrane potential is hyperpolarized from the resting level (-65 mV) to -130 mV?
There’s redistribution of charge across the axonal membrane. The capacitive current is a nearly instanteneous. Besides this, very little current flows when the membrane is hyperpolarized.
What happens when the membrane potential is depolarized from -65 mV to 0 mV?
First, there is a outward capacitive current. After this, a transient inward current is rapidly produced, where positive charges enters the cell. This results in a more slowly rising delayed outward current.
What ions are (probably) responisble for the transient inward and delayed outward current?
- Transient inward current → sodium
- Delayed outward current → potassium
Name three ways how it can be tested what ions are involved e.g. depolarization or hyperpolarization.
- By determining at what membrane potential the current reverses and comparing it to the electrochemical equilibrium potential of ions in solution
- By changing the external concentrations of certain ions and testing if the current is altered or reversed.
- By blocking certain channels and testing if this also block the current.
So it’s most likely sodium is responsible for the transient inward current during depolarization. Already mentioned is that this can be tested by blocking sodium channels. How is this achieved?
By blocking the voltage-gated Na+ channels with tetrodotoxine (TTX) (but this wasn’t possible in the earlier days so they used something else)
So it’s most likely sodium is responsible for the transient inward current during depolarization. Already mentioned is that this can be tested by changing the external/extracellular concentration of sodium ions. What happens to the inward current when sodium is removed from the extracellular space?
The inward current changes to an outward current. When sodium is recovered extracellularly, the inward current is also recovered. This shows that for this inward current, extracellular sodium is needed.
(This can also be tested for potassium, where potassium is removed intracellularly, which removes the delayed outward current).