Action Potential Flashcards
What is an action potential?
Change in voltage across a membrane
What does an action potential depend on?
Ionic gradients and relative permeability of the membrane to different ions
An action potential only occurs when…
…a threshold voltage level is reached
Action potential in axon values:
1) Length
2) RMP
3) Depolarisation peak
1) 0.5ms
2) RMP = -70mV
3) Depolarisation peak = +30mV
Depolarisation peak: +30mV
Action potential skeletal muscle values:
1) Length
2) RMP
3) Depolarisation peak
1) 0.5ms
2) RMP = -90mV
3) Depolarisation peak = +40mV
Depolarisation peak: +40mV
Sino-atrial node action potential values:
1) Length
2) RMP
3) Depolarisation peak
1) 100ms
2) RMP = -60mV
3) Depolarisation peak = +30mV
Depolarisation peak: +30mV
Cardiac ventricle action potential values:
1) Length
2) RMP
3) Depolarisation peak
1) 100ms
2) RMP= -90mV
3) Depolarisation Peak = +30mV
Depolarisation peak: +30
How long does action potential last in axon/skeletal muscle?
0.5ms
How long does action potential last in SA node/cardiac ventricle?
100ms
What does the conductance (g) of a membrane to an ion dependent on?
How do you increase the conductance of sodium?
- The number of channels for that ion that are open
- E.g. to increase the conductance of sodium, you need to increase the number of open sodium channels in the membrane
higher conductance = more channels open
What happens to the membrane potenital if you increase conductance of any ion?
Membrane potential moves towards the equilibrium potential for that ion
What amount of ions need to move to produce a relatively large change in the membrane potential?
How does axon diameter affect this?
- A very small amount of ions
- The larger the axon diameter, the smaller the amount of ions required to move to produce a large change in MP
Membrane potential during action potential
V gated Na+ channels open Na+ channels inactivate V gated K+ channels open Hyperpolarisation Na+ channels become reactivated but remain closed V gated K+ channels close
What occurs when V gated channels of Na+ open?
Membrane gets depolarised as Na+ enters cells
How are V gated Na+ channels reactivated?
After membrane gets hyperpolarised
How can we show experimentally that Na+ is responsible for AP depolarisation?
If you decrease extracellular Na+, peak of action potentials get smaller
How can membrane currents be measured over time at a set membrane potential?
Voltage clamp
K+ causes hyperpolarisation, Na+ causes depolarisation
How does conductance change over action potential?
Na+ channels conductance changes rapidly - quickly inactivate in response to depolarisation
K+ channels are slower at closing so have a shallower curve
How does hyperpolarisation occur?
Non voltage gated K+ channels are naturally open
When you pair this with open v gated k+ channels membrane gets more negative than RMP
What causes membrane to go back RMP after hyperpolarisation?
Closure of V gated K+ channels
What triggers an action potential?
Depolarisation to threshold at axon hillock
What does depolarisation to threshold trigger?
Positive feedback to open Na+ channels and allow influx of Na+
What does depolarisation trigger?
Inactivation of V gated Na+ channels
Voltage gated K+ channels open
Is the Na+K+ATPase involved in repolarisation?
NOOOOOOOOO (just sets up gradients)
What is absolute refractory period (ARP)?
Na+ channels are inactivated
Depolarisation cannot re-occur no matter stimulus
No other action potential can occur during this
What is relative refractory period?
Na+ channels are closed (not inactivated)
If stimulus is strong, can cause opening of channels and depolarisation
V gated K+ channels are closing
structure of V gated Na+ channel
1 alpha subunit
four similar sections
voltage sensor (in S4)
pore region (P)
what is important about the S4 voltage sensor?
Contains positively charged amino acid residues
senses changes
conformational change
What is located between 3rd and 4th sections of alpha subunit on Na+ channel?
Inactivation particle
Enters pore when membrane is depolarised
Inactivates channel so no Na+ can enter
structure of v gated K+ channels
4 individual a subunits
voltage sensor (s4)
Pore region
NO INACTIVATION
What does pore region contribute to?
Selectibility - only tries to allow K+ ions through
differences V gated K+ channel and V gated Na+ channel
No inactivation in K+ channel
K+ is made of 4 subunits, Na+ only 1
Phases of V gated Na+ channel
Closed, Open, Inactivated
What form of anesthetic is permeable to the membrane?
Unprotonated
hydrophobic/lipophilic
example of anaesthetic
Lidocaine
How does lidocaine work?
Hydrophilic pathway (open channel block) Hydrophobic pathway (inactivated)
(block Na+ channels)
Hydrophilic/open channel pathway
Channel must be open Drug crosses membrane (uncharged hydrophobic) Gets protonated inside cell Enters Na+ channel Blocks
Hydrophobic pathway
Drug crosses membrane (uncharged hydrophobic)
Moves into Na+ channel within membrane (across)
Gets protonated
Blocks Na+ channel
STRONGEST
When is block in strongest form?
When the Na+ channel is in inactivated state
How is unblocking occur?
Channel must be closed
Drug gets deprotonated
Local anaethetics blocking order
small myelinated axons
unmyelinated axons
large myelinated axons
why is important Na+ channels become inactivated?
So action potentials occur in one direction
What is the consequence of delayed closing of V gated K+ channels?
Membrane gets hyperpolarised
Na+ channels can be reactivated
