M&R - Action Potential Flashcards
What are the three states voltage gated channels pass through on depolarisation?
Closed, open and inactivated
Why can a channel in the inactivated state not reopen?
What does this prevent?
It needs to be re-primed by repolarisation of the membrane.
Once inactivated, Na+ channel cannot be reopened during an AP
Irreversible depolarisation of the membrane, permits directionality to nerve impulse conduction and allows information to be coded with respect to the frequency in which APs are fired.
How does a wave of depolarisation progress over a cell membrane?
By the self reinforcing way Na+ channels open. The depolarising phase of an AP at one point on the cell membrane, by local current flow, will raise the membrane potential in adjacent regions of the membrane sufficiently to exceed the threshold for AP initiation in that region.
What occurs directly after the onset of an action potential?
What occurs after that?
Absolute refractory period - the membrane cannot be further excited
Relative refractory period - becomes progressively easier to elicit a further action potential as the Na+ channels recover from inactivation
What do action potentials do to the voltage across the membrane? And what does it depend on?
When do action potentials occur?
Change the voltage across membranes which depends on ionic gradients and relative permeability
If a threshold level is reached
How are action potentials generated?
By an increase in permeability to Na+
Bringing the permeability close to the Na+ equilibrium ENa
How can you measure the effect of changes of membrane potential on sodium and potassium currents?
Describe.
Technique called voltage clamp
Membrane potential is controlled and the currents flowing through membrane are measured.
Much clearer measurement of the effect of voltage on the number of Na+ and K+ channels open at different membrane potentials.
Both channels are voltage gated –> depolarisation will cause them to open
What is inactivation of Na+ channels and when does it occur?
During maintained depolarisation
Na+ channels close by inactivation
Describe what happens during the upstroke (depolarisation) of AP
What type of feedback is this?
Depolarisation to threshold
Na+ channels open
Na+ enters the cell
Membrane depolarises
Positive feedback
What channels are involved in repolarisation and what happens to them?
Na+ channels - inactivation
K+ channels activated
What is important about the Na+/K+ pump in action potentials?
It is NOT involved in repolarisation
It sets the concentration gradients
When can Na+ channels recover?
When the membrane potential has returned to its resting level
What happens to the Na+ channels in the absolute refractory period (ARP) and the Relative Refractory Period (RRP)?
ARP - nearly all Na+ channels are in the inactivated state
RRP - Na+ channels are recovering from inactivation, the excitability returns towards normal as the number of channels in the inactivated state decreases
Describe the molecular nature of Na+ and Ca2+ channels
How many transmembrane spanning domains are in one repeat?
Their main pore forming subunit is one peptide consisting of 4 homologous repeats.
Each repeat consists of 6 transmembrane spanning domains with one of these domains being able to sense the voltage field across the membrane.
Give an example of a local anaesthetic and describe the way it stops an action potential being generated?
Procaine
Binds to and blocks Na+ channels –> stops action potential generation
In what order to local anaesthetics block conduction in nerve fibres?
How do they work?
Small myelinated axons
Non myelinated axons
Large myelinated axons
They are weak bases and cross the membrane in their unionised form.
They block Na+ channels easier when the channel is open and have a higher affinity to the inactivated state of the Na+ channel
If the conductance ion is increased, what happens to the membrane potential?
It will move closer to the equilibrium potential
