The basis of 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
Depolarisation opens which voltage-gated channels?
Na+ 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 value of Ek?
-80mV
What is the value of ENa?
+62mV
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?
The existence of a threshold
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
