action potentials Flashcards
Difference between action potentials and electrotonic potentials
size?
speed?
lasting?
graded?
Electrotonic potential: Small (1-15mV) Slow depolarisation Slow decay Graded
Action potential: Large (~100mV) Rapid depolarisation Rapid repolarisation All-or-nothing
Normal arrangement of Na+ channel before any potential arrives
the two gates?
activation gate = held closed at rest (voltage sensitive domain attracted to -ve charge)
Inactivation gate = help open at rest
What happens when electronic potential arrives?
the membrane will slightly depolarise -> could be due to excitatory synaptic input
as it is an electronic potential -> the activation gate is still stromg enough to hold the gate closed
If a big electronic potential arrives that can hit threshold
effect of this? why?
The force holding the gate closed isn’t strong enough hence the gate springs open
Na+ flows into the cell -> depolarises the cell
What makes an action potential all or nothing?
what does Na+ influx lead to?
the initila depolaristaion that reach threshold and caused the Na channel to open will lead to Na+ influx which causes further depolarisation and channels opening and more influx hence a positive feedback mechanism
Positive feedback ensures rapid activation of all available channels (PNa increases x100)
How is Na+ channel shut?
why can’t activation gate close?
what happens instead?
Activation gate cannot close - it is powerfully attracted to the outside of membrane
BUT inactivation gate is now pulled close because it is attracted to the outside hence prevents Na+ influx
Why is K+ channels opening delayed?
gives time for AP to hit peak with Na+ channels opening
absolute refractory period
what does this mean? why?
Na+ channels are inactive at this point and can’t reset themselves till threshold gets below threshold
Blocked by inactivation gate hence this keeps each AP a separate + individual event
relative refractory period
what does it take time to do?
what can take place but is harder?
cell is hypppolarising and takes time for the cell to get back to normal resting potential
takes a stronger stimulus to get it up to threshold so it can fire AP but less likely to do so
Important Principles of APs
what opens?
what does this lead to? effect of this?
what stops depolarisation?
what repolarises?
Voltage-gated Na+ channels are opened by depolarisation above a threshold value (~ -50Mv)
Positive feedback ensures that the permeability to Na+ is briefly far higher than to K+, ensuring very rapid, all-or-nothing depolarisation
To stop the depolarisation, the vgNa channels inactivate and while they are inactivated the cell cannot fire another AP
To repolarise, voltage gated K+ channels open and carry the excess positive charge back out of the cell
Why do we need Aps
Because nerve cells have to communicate over long distances
How do Aps travel distances?
what happens along axon?
Aps propagate hence replicate themselves
The electric current will spread further as the next bit of axon will hit the threshold and fire AP so the elextronic potentials are pushed further and further down the membrane
Why do Aps travel in one direction?
Ap moves in 1 direction as first AP goes into refractory period so the ap goes down the axon
How to increase the speed of AP propagation? (3)
larger diameter
low membrane permeability (good insulation) e.g. myelin sheath
Myelin sheath
what kind of conduction? how?
if damaged what happens?
The myelin sheath greatly increases the speed of action potential propagation
salvatory conduction as it jumps from node of ranvier
If myelin sheath is damaged, AP transmission may be delayed or completely blocked
