Action potential Flashcards
What does permeability of membrane depend on?
conformational state of ion channels
- open by depolarisation
- inactivated by sustained depolarisation
- closed by membrane hyperpolarisation/repolarisation
movement of ions changes the membrane potential toward the equilibrium potential for that ion
changes in membrane potential during AP not due to ion pumps
- generated by K moving out of cells due to [] gradient
- pump only generated these [] gradients
Phase 1 of AP?
Pk > PNa
membrane close to equilibrium potential of K
Phase 2 of AP?
DEPOLARISING STIMULUS
stimulus applied to cell to produce small graded response (depolarisation)
membrane potential becomes more positive to -55mV (threshold)
Phase 3 of AP?
UPSTROKE
starts at threshold potential
PNa rapidly increase at VGSCs open quickly and enter cell down electrochemical gradient
PK increases as VGKCs open slowly and leave cell down gradient
membrane potential moved toward Na equilibrium potential
Phase 4 of AP?
REPOLARISATION
at +30mV PNa rapidly decreases at channels inactivate and Na entry stops
PK increases as more K channels open and K leaves cell
membrane potential moves back toward K equilibrium potential
How do Na channels inactivate?
What is the absolute refractory period?
inactivation protein of Na channel responds to +30mV voltage change and blocks pore so Na cannot exit despite gate still being open
Na channel inactivation gates are closed so new AP cannot be triggered even with strong stimulus
can only be stimulated if channel is repolarised
What is Phase 5 of AP?
AFTER-HYPERPOLARISATION
K channels still open - large population so close slowly and overshoot
K continues to leave cell down gradient and membrane potential moves closer to K equilibrium
some VGKCs close
What is the relative refractory period ?
inactivation gate open - pore open, gate closed
larger than normal stimulus can still trigger an AP
Ion exchange speed?
pump - slowly, seconds/milliseconds USE ATP
channels - micro/milliseconds
How are impulses propagated?
Graded responses/potential change will decline along action
local current flow depolarises adjacent region to threshold to form active area at peak of AP
local current then depolarises new adjacent region with new active area at peak of AP and adjacent areas at resting potential
cannot pass back as area of prior activation is refractory (uni-directional)
How is impulse propagated quickly?
[] of Na channels high at nodes of Ranvier with no insulation
impulse jumps from each node via saltatory conduction
AP travels quickly (myelinated = 120m/s, non-myelinated = 1m/s)
What influences conduction velocity?
axon diameter
- smaller axons have high internal resistance to local current flow which decreases velocity of conduction
myelination
- no myelin - no saltatory conduction as APs only at nodes of Ranvier
conduction slowed by cold, anoxia, compression and drugs (anaesthetics)
IDEAL = MYELINATED THICK
