4.1 The action potential & its properties Flashcards
What is relative permeability in action potential?
relative permeability - relative to other ion channels - which one dominates
opening of ion channels in AP - to reach threshold (Na+ and Ca2+ dominate)
Describe the ionic basis of an action potential and the changes in membrane ionic permeability
increase permeability of Na+ channels (influx)
bringing membrane potential closer to E(Na)
Na+ channels open in depolarisation
Na+ channels inactivated and K+ channels open in repolarisation
What is maintained depolarisation? What happens during maintained depolarisation?
should depolarisation continue on for longer than normal, then Na+ will remain in inactivated state throughout
How do Na+ channels return to normal activated state? Describe the course of Na+ channels from open to depolarisation to cycling back to open state
open –> depolarised –> inactivated –> hyperpolarised (move -ve) –> closed –> opened
What is ARP and what happens during it (in terms of changes in permeability)
absolute refractory period
after Na+ channels have been depolarised, they become inactivated, when there is NO excitability of the Na+ channels at all
(see page 63 workbook graph)
What is RRP and what happens during it? (in terms of changes in permeability)
relative refractory period
when Na+ channels recover from inactivation, excitability returns begins to normal with more and more Na+ channels becoming in the open state
When do Na+ channels return to normal?
when the cell membrane is returned to resting state (not depolarised (open –> inactivated) nor hyperpolarised (between inactivated and closed))
Describe the basis of all or nothing law in terms of changes in permeability
once Na+ channels open, it causes Na+ to enter the cell, causing membrane to depolarise (less neg), opening even more Na+ channels: positive feedback of all or nothing effect
All or nothing: has to reach threshold, otherwise there will be NO depolarisation
What is accommodation?
the longer the stimulus, the larger the depolarisation necessary - the threshold also becomes more positive
the peak of the AP is also less positive as more Na+ channels will be in the inactivated state due too prolonged stimulus (depolarisation)
until stimulus above threshold, so still can’t fire AP
Describe the key molecular properties of ion channels:
structures of voltage-gated channel and examples
voltage-gated channel e.g. Na+ / Ca2+’s structures
main pore forming subunit: 1 peptide with 6 homologous repeats, each repeat has 6 transmembrane spanning domains
the 4th domain sense the change in membrane voltage
(p,64 of workbook)
Describe the key molecular properties of ion channels:
K+ channel
each repeat is a subunit - total of 4 subunits
all 4 come together to form ‘pore’ for K+ to exit via
Outline the action of local anaesthetics using an example and the order in which the nerve fibres are blocked
e.g. procaine: bind and block Na+ channels, stop AP generating
block conduction of nerve fibres in order of:
1. small myelinated axon
2. unmyelinated axon
3. large myelinated axon
When do local anaesthetics block channels easiest?
when channels are in inactivated state: technically ‘open’ but has no Na+ influx
What are local anaesthetics and how do they cross the membrane?
they are weak bases (high pH) and easiest to cross membrane when local anaesthetic in unionised form
Describe the properties of an action potential and associated membrane ionic permeability
through change in voltage of cell membrane
happens through ionic gradients and opening of ion channels
Occurs when threshold reached (all or nothing), so AP is propagated without loss in amplitude
