L5 Excitable cell Part - 2 Flashcards
What is the IDF proportional to?
It’s proportional to the difference between Vm and E(ion)
What does IDF stand for?
Ionic driving force
What are the standard units for action potential?
mV
What are the 5 phases of action potential?
- Resting potential
- Rising phase
- Overshoot
- Falling phase
- Undershoot
What happens during resting potential of an action potential?
Inside of the neuron is negatively charged compared to the outside (-70mV) due to 3Na+ out and 2K+ in
What happens during rising phase ( depolarisation)?
- Membrane potential at the axon hillock or somewhere else reaches threshold potential
- Driven by influx of sodium ions into the cell through voltage-gated sodium channels
- Inside of the cell becomes increasingly positive and closer to 0
What happens during overshoot?
- Where membrane potential is above zero
What happens during the falling phase (repolarisation)?
- Shortly after the sodium channels open, they become inactivated.
- Simultaneously, voltage-gated potassium channels open, allowing positively charged potassium ions to flow out of the cell.
- This outflow of positive charge begins to restore the negative charge inside the cell, bringing the membrane potential back towards the resting potential.
What happens during undershoot (hyperpolarisation)?
- This is a brief period where the membrane potential dips below the resting potential
- Due to continued efflux of K+, as K+ channels remain open slightly longer
(Also called refractory period)
What is an action potential?
A transient, rapid, and reversible change in membrane potential from negative to positive
Are all action potentials the same?
No. Different types of excitable cells may have different types of action potentials
What usually triggers an action potential in a neuron?
An increase in sodium permeability
What are the characteristics of action potentials (spikes) generated by a cell?
- All are of the same size and duration
- They do not decrease as they are conducted down the axon
What are the changes in membrane permeability that underlie an action potential?
- The sodium ion channel open so permeability for sodium ions increase (depolarisation to 0 and above) inside the cell
- Na+ channels shuts and permeability to K+ dominates again (repolarisation)
When does the sodium channels open?
They open when the membrane is depolarised.
What is the structure of the voltage - gated Na+ channels? (4)
- 6 transmembrane domains
- Has a pore loop
- Has a selectivity filter
- Has a voltage sensor gate
How fast does channel inactivation occur?
1ms
What poison blocks K+ channels ?
Tetraethylammonium, TEA
Used as anaesthetic.
What poison blocks Na+ channels?
Tetrodotoxin, TTX
Pufferfish
How does an action potential conduction travel in one direction?
Action potential occurs by spread of charged particles (the Na+ ions) although they spread in both directions, Na channels behind inactivated, so only Na channels ahead available to open…hence why action potential travels in one direction from point of initiation.
What is Cv?
Conducting velocity
What are the factors that influence conducting velocity?
- The diameter of the axon
- Physical properties of the axon
- Permeability of membrane
Why does the diameter of the axon influence Cv?
A wider axon provides less resistance to the flow of ions along the inside of the axon
- Resistance to current flow is inversely proportional to cross-sectional area of the axon
