Action Potentials And Its Properties (4) Flashcards
What does an action potential depend on?
- Ionic gradient
- Relative permeability of the membrane
What is an action potential?
- A change in voltage across a 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
- Skeletal
- SAN
- Cardiac
- 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’?
- ~0.4%
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.
- Procaine
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?
- Diphasic
- Monophasic
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.
What does the spread of local current depend on?
- Membrane’s resistance
- Membrane’s capacitance
What does an increase in capacitance lead to?
Effect on Vmax and time
- Vmax is reached over a longer period of time.
What effect does an increase in membrane resistance lead to?
effect on how far local charge can spread
- Increases the distance local charge can spread to.
What is local currents useful for?
- Propagating other AP
- Prevents the reversal of AP.
What is used in myelination for CNS and Peripheral NS?
- CNS: Schwaan cells
- Peripheral NS: Oligodendrocytes
What does myelination do to the distribution of Na channels?
- Extremely high density of Na channels in Nodes of Ranvier.
How is the optimum conduction velocity caused in relation to myelin sheaths and axons?
- D: myelin sheath & axon
- d: axon only
- d/D = 0.7 is optimum
What is saltatory conduction and how does it occur?
- AP jump from node to node: faster conduction velocity
- Myelination is a good insulator, causing local circuit current to depolarise the next node to above threshold -> AP
- Maintains AP above threshold for a greater distance
What is conductance velocity proportional to in: myelinated and unmyelinated axons and their max speed?
- Myelinated: diameter of axon. ~120m/s
- Unmyelinated: square root of diameter of axon. ~20m/s
How does myelin sheaths improve conduction?
- 100 x increase in membrane resistance
- 100 x decrease in membrane capacitance
- Due to these = increase length constant
- Slight decrease in time constant
What is conduction velocity proportional to?
- Length constant / time constant
What effect does demyelination have on transmission of AP?
- Density of action current is reduced due to:
- Resistive and capacitance shunting
- So doesn’t reach threshold value and so no AP
