The Basis of Cell Excitability Flashcards
What is the timescale for an action potential?
1-2msec
What causes depolarisation?
Na+ influx
What is depolarisation?
The cell becoming less negative
What does depolarisation do to PNa
It causes the opening of Na+ channels and PNa increases
What causes repolarisation?
K+ efflux
What is repolarisation?
The return of Em to resting potential
What does repolarisation do to Na+ gated channels?
Closes them
What is conductance of ions?
It is equivalent to permeability
Measured instead of permeability, membrane acts as an electrical resistor (R)
Conductance, g=1/R
Each ion has its own conductance
Conductance is proportional to what?
To the number of open ion channels
Change in gion will change what?
Em
What causes cells to further depolarise?
opening of voltage gated Na+ ion channels
What causes the initial depolarisation?
Synaptic activity Generator potential (sensory neurone)
What does Em approach in the depolarisation phase?
ENa
What happens to Na+ channels in repolarisation?
They inactivate
When do K+ channels open?
As the neurone is repolarising, they open at positive values of Em
What is hyperpolarisation?
Em has returned to its initial value but K+ channels are still open
Em approaches Ek
How is hyperpolarisation dealt with?
The K+ channels finally close
The leak channels restore Em to resting value
What is the all-or-nothing principle?
An action potential either happens or it doesn’t. They have no difference in size or power
What causes the all-or-nothing principle?
Existance of a threshold value
What is the threshold?
The point at which an action potential will fire
What is happening in the neurone at the threshold value?
Na+ influx is greater than K+ efflux
What is the absolute refractory period?
No further action potential by any stimulus regardless of size
What is the relative refractory period?
A stronger stimulus will open sufficient Na+ channels and overcome increased gK that makes the threshold greater
Explain absolute refractory period in terms of voltage-gated channels?
Na+ are mostly inactivated
K+ are mostly open
Explain relative refractory period in terms of voltage-gated channels?
Na+ are recovering from inactivation
K+ some are still open
Why do action potentials not travel backwards in axons (unidirectional)?
The membrane behind the action potential is in the refractory period
What is electrotonic spread?
The propagation of an action potential in an unmyelinated axon
How much faster is the speed of an impulse in a myelinated neurone?
1000x
What happens to the impulse in a myelinated neurone
Jumps between nodes of Ranvier
What does the myelin sheath do?
Increases the speed of action potential conduction
What is saltatory conduction?
The jumping of the impulse between gaps in the myelin sheath
Explain how an action potential is generated?
- Initial stimulus causes generator potential and starts initial depolarisation
- Opening of voltage gated na ion channels increases gNa (sodium conductance)
- Na influx into cells leads to further depolarisation and even greater gNa
- Na influx is greater than K efflux
- Em surpasses threshold value (-55mV) and approaches ENa
- Depolarisation peaks at +40mV where Na channels inactivate and Na influx stops despite stimulus
- Decreased gNa coincides with the opening of Voltage gated K channels.
- This opening is delayed to prevent Em from remailing equal
- K efflux leads to repolarisation and increased gK returns to rest.
- Em approaches Ek at hyperpolarisation
- Voltage gated K channels close
- Em returns to resting value (-65mV) via leak channels.
Why is there a delay between the peak of depolarisation and the opening of Voltage gated K channels?
To prevent the Em from having equal charge in/out of the cell at the peak of depolarisation.
What is caused when voltage gated ion channels are open?
Depolarisation
What is caused when voltage gated ion channels are closed?
Repolarisation
What affects the speed of conduction in an axon?
Diameter of the axon
smaller = more constricted and therefore the conduction speed is slower.
