Action Potentials II Flashcards
Driving force of an ion =
- |membrane potential (Vm) – Equilibrium potential (Ex)|
(Absolute value)
How do voltage-gated Na+ channels and voltage-gated K+ channels differ?
- They are both activated during depolarization of the membrane.
- Na+ channels quickly inactivate.
- K+ channels remain open as long as the membrane is depolarized.
Toxins can affect ion channel function: Tetrodotoxin (TTX) blocks Na+ channels
What effect does TTX have on action potentials?
What effect does TTX have on the resting membrane potential?
- Found in the puffer fish (fugu)
What effect does TTX have on action potentials? - No depolarization
What effect does TTX have on the resting membrane potential? - No effect, it would stay at rest
Tetraethlammonium (TEA)
Blocks voltage-gated K+ channels
What effect does TEA have on action potentials?
What effect does TEA have on the resting membrane potential?
What effect does TEA have on action potentials?
- Rapid depolarization, longer-lasting (no undershoot or relative refractory)
What effect does TEA have on the resting membrane potential?
- No effect on resting membrane potential
What happens to the resting potential and shape of the action potential if you lower the external concentration of Na+?
- It is slower
- Eq of Na is lowered,driving force is less than it used to be. The peak of the graph will also be lwer because it is the peak of the new ENa.
How does an action potential propagate down an axon?
An action potential can be represented by an influx of positive charge
- What happens immediately in front of this
region (AP)?
-> positive current spreading carries the next portion to depolarization (opens voltage-gated Na+ channels) - Can action potentials “turn back” on themselves?
-> K+ ions are repolarizing the membrane, and Na+ channels are inactive. AKA: one direction
What happens to the resting potential and shape of the action potential ifyou increase the external concentration of K+?
- Ek moves up because there is now more K outside of the cell. Driving force is lower and will only bring K+ down to the new Ek
- Undershoot doesn’t become more negative than Ek.
Action potentials depend on two types of current flow:
Passive and Active
Passive current flow:
AP’s decay with distance
- conduction by neurons in the absence of action potentials. It is like conduction of electricity in a wire.
1) Inject current through an electrode.
2) Measure the membrane potential at
various points along the axon.
3) As the distance from the electrode
increases, the amplitude of the
potential change diminishes.
4) Current leaks out of the membrane
What happens if the current injection is above threshold?
- No decay, and yes to an AP
Action potential propagation also depends on …
- active current flow through ion channels
- Yet, need both active and passive current to depolarize voltage-gated Na+ and propogate.
Differences between Active and Passive Currents
- Passive current flow: decays over distances.
Depolarizing current passively flows down the axon away from the site where the action potential is occurring. - Active current flow: current flow through ion channels. When the depolarization reaches threshold, Na+ channels open and an action potential is initiated.
- An action potential requires the coordinated action of these two types of current.
- This explains how action potentials can travel long distances.
Conduction velocity:
- The speed at which action potentials are propagated down an axon.
Ways to increase conduction velocity (1):
- Increasing the diameter of the axon
decreases the resistance to passive
current flow.
This is why giant axons have evolved in
invertebrates, such as the squid.
Rapidly conducting axons in all animals
tend to be larger than slowly conducting
ones.
Large axons are less leaky
Ways to increase conduction velocity:
(2)
- oligodendrocytes in CNS, Schwann cells in PNS
- Reduces leak of current across membrane (insulation)
- Present only in vertebrates
Decrease leaks
