Flashcards in Action Potentials And Its Properties (4) Deck (41)
What is an action potential?
- A change in voltage across a membrane
What does an action potential depend on?
- Ionic gradient
- Relative permeability of the membrane
What is the rule given to action potentials and why is this the case?
- All or nothing
- An action potential will only occur if the threshold value is met.
Give the values of the beginning of depolarisation and the beginning of repolarisation for the following:
- Axon: -70mv, +30mv
- Skeletal: -90mv, +40mv
- SAN: -60mv, +30mv
- Cardiac: -90mv, +30mv
With reference to sodium and potassium channels describe how depolarisation and repolarisation occur.
- Depolarisation: Na channels open to allow an influx of Na to increase membrane potential.
K channels remain closed.
- Repolarisation: Na channels become inactive no more Na influx.
K channels open to allow an efflux of K to decrease membrane potential.
How can Na channels reopen after repolarisation?
- A negative enough potential much be reached to allow the reformation of the closed Na channels.llll
What does the conductance for a particular ion depend on?
- The number of channels for the ion that are open.
What sort of percentage change in ion concentration is required for an action potential to 'fire'?
How can ionic currents be measured?
- Voltage clamping
- Patch clamping enables current flowing through individual ions channels to be measured
- Using different ionic concentrations allows contribution of specific ions to be calculated.
What is an axon hillock?
- Point on axon where AP is initiated.
How does depolarisation of an AP occur?
- Depolarisation to threshold: Na channels open
- Na influx
- Membrane depolarises
- Positive feedback loop
How does repolarisation of an AP occur?
- Depolarisation opens K channels and inactivates Na channels
- K channels open causes K efflux
- Inactivation of Na channels causes Na influx to stop
What does ARP stand for and what occurs during this period?
- Absolute refractory period
- Can't fire another AP
- Nearly all Na channels are inactivated
What does RRP stand for and what occurs during this period?
- Relative refractory period
- Na channels are recovering from inactivation, excitability returns towards normal as the number of channels in inactivated state decrease.
- Second AP can fire in this period if signal is strong enough.
Outline how Na channels move from closed to open to inactivated.
- Na channels have two voltage gates.
- When closed the top gate is close and the bottom gate is open and the top one is closed
- When the membrane potential is at -70mv the bottom gate opens to allow Na to influx causing the potential to increase
- When potential has reached +30mv the Na gates become inactive, this is when the top gate remains open but the bottom gate shuts allowing Na to enter then rebound out.
- When the potential is negative enough the bottom gate reopens and the top gate shuts, so closed.
What is hyperpolarisation's role with Na channels?
- Na channels moving from inactive form to closed form.
What is accommodation?
- Longer the stimulus, the larger the depolarisation necessary to initiate an AP
- Threshold becomes more positive
What effect does accommodation have on the threshold value and the possibility of an AP firing?
- Doesn't change AP
- With accommodation no AP may be fired even if slightly over threshold value.
To a subunit level what is a Na channel?
- 4 subunits turn in to face each other.
- The 'inactivation particle' between subunit 3 and 4 acts as a plug for the channel.
Give an example of an anaesthetic.
How do anaesthetics act on Na channels?
- Bind and block Na channels thereby stopping AP generation
In what order does anaesthetics act on axons?
- Small myelinated axons (responsible for pain)
- Non-myelinated axons
- Large myelinated axons
For what type of state of Na channel does anaesthetics have the greatest affinity?
- Inactivated as can fit into pore with greatest ease to block.
Which diseases are associated with AP?
- CNS: - Multiple sclerosis: all CNS nerves
- Devic's disease: optic and spinal chord nerves only
- Peripheral NS: - Landry-Guillian- Barre
- Charcot-Marie tooth disease
- Disease resulting from breakdown/damage to myelin sheaths.
What would a graph of the reading taken from an intact membrane during an AP look like?
What would one of a damaged membrane look like?
How can conduction velocity be calculated?
Axon length (x) / time (ms)
What is local current theory?
- Injection of current into an axon will cause the resulting change to spread along the axon and cause IMMEDIATE LOCAL CHANGE in membrane potential
What is capacitance?
- The ability to store charge.
What does membrane resistance depend on?
- Depends on number of ion channels open
- The lower the resistance the more ion channels that are open.