The Action Potential Flashcards
Describe the changes in membrane ionic permeability associated with the action potential
Depolarisation: to threshold causes voltage gated Na channels to open, Na influx depolarises membrane further towards Na equilibrium potential, 61mV.
Repolarisation: Na channels inactivated by prolonged depolarisation. Voltage gated K channels open, K efflux moves potential towards K equilibrium potential -88mV.
Describe the properties of the action potential and its ionic basis
A change in voltage across the membrane.
Depends on ionic gradients and the relative permeability of the membrane.
They are ‘all or nothing’ and propagated without loss of amplitude.
What is the positive feedback loop?
Depolarisation to threshold cause voltage gated channels to open and an influx of Na, this causes further depolarisation and so more voltage gated Na channels open and bigger Na influx etc
If threshold isn’t reached some Na channels open but not enough to enter positive feedback loop.
Describe the basis of the ‘all or nothing’ law
Once threshold is reached the positive feedback loop is entered so more and more voltage gated Na channels open until they all are, meaning that depolarisation cannot stop half way.
Describe the basis of the refractory periods
Absolute refractory period: All Na channels inactivated, excitability is 0 so AP cannot be fired with any size stimulus
Relative refractory period: Na channels are recovering from inactivation, excitability returns to normal as more are recovered. AP can be fired only with large stimulus
How are Na channels are returned from the inactivated state to the closed state?
Through hyperpolarisation, the more hyperpolarised the membrane the quicker the recovery of Na channels
Describe accommodation
The longer the stimulus the larger the depolarisation necessary to initiate an AP, as threshold becomes more positive.
Because Na channels become inactivated even when threshold isn’t reached, so fewer are available to open when threshold is reached.
Give a physiological example of accomodation
Occurs at synapses: if inputs arrive to synapses at different times threshold increases and no AP is generated.
Describe the structure of voltage-gated Na/Ca Channels
1 peptide with 4 homologous repeats consisting of 6 transmembrane spanning domains each.
1 domain is voltage sensitive, function requires 1 subunit
Describe the structure of voltage-gated K channels
4 peptides with 6 transmembrane domains each.
1 domain is voltage sensitive, function requires 4 subunits
Describe the actions of local anaesthetics
Local anaesthetics eg Procaine, act by binding to and blocking Na channels, stopping action potential generation
Describe the differences between the hydrophobic and hydrophilic pathways of local anaesthetics
Hydrophobic: anaesthetic enters membrane, moves into Na channel and blocks it, no use dependence
Hydrophilic: anaesthetic becomes protonated, blocks open Na channels and have a higher affinity to Na channels in the inactivated state. Means that as more channels open, more become blocked = use dependent.
In what order do local anaesthetics block conduction?
- Small myelinated axons
- Non-myelinated axons
- Large myelinated axons
This means they tend to affect sensory before motor neurones
How can conduction velocity be measured?
Use electrodes to raise the membrane potential to threshold to generate an AP
Record changes in potential between the stimulating electrode (cathode) and recording electrode (anode) along an axon.
Conduction velocity= distance/time
Explain the local circuit theory of propagation
The depolarisation of a small region of membrane produces transmembrane currents in neighbouring regions, this opens more VG Na channels causing depolarisation to threshold and an AP to fire in that location.
The further the local current spreads the faster the conduction velocity
