SESSION 5 Flashcards
(46 cards)
The action potential is often described as all- or- nothing.
What does this mean?
Action potential is dependent on whether the threshold is reached
If the threshold is reached then he potential will definitely fire
If the threshold is not reached then the potential will not be fired
All- or- nothing- no transmission or transmission of the same amplitude
Amplitude never changes but increasing the frequency can effect the type of threshold reached
Which ion channels are involved in generating an action potential?
Voltage gated sodium and potassium channels
Briefly describe the molecular nature of the sodium and potassium channels
Voltage gated 6 transmembrane domains
Number 4 is positively charges
Change in membrane potential and depolarisation
due to charge repulsion there is a conformational change which allows the pore to open and allow sodium in and potassium out
Define activation in terms of voltage- activated ion channel
Activated means the opening and closing of the channel ion as a result of the electrical membrane potential
Define inactivation in terms of a voltage- activated ion channel
Inactivation means ion impermeable
The inactivation can be thought of as a plug
Closure of the inactivation gate causes sodium flow through the channel to stop, causing membrane potential to rise
It will reactivate when hyperpolarisation occurs
Describe how ions move as a consequence of channel opening during action potential
Membrane depolarised:
- Sodium ion channels open
- Sodium influx
Repolarisation of action potential is a two stage process:
Stage 1:
- Sodium ion channels inactivate
- Sodium influx stops
Stage 2:
- Potassium ion channels open
- Potassium efflux
Hyperpolarisation- due to delayed closing of the potassium ion channels
If the Na+/ K+ pump were to be blocked, what would be the consequence for nerve conduction of a single action potential in a nerve fibre?
No consequence- ion concentration is such a minor change
If the Na+/ K+ pump were to be blocked, what would be the consequence for nerve conduction of a train of 1000 action potentials?
There will be nothing to return it to normal, very high intracellular ion
e.g. No sodium gradient thus an action potential cannot be generated
Na+K+ATPase is not required for action potential re- setting mechanism
However the pump is required to re- set the membrane potential when lots have been fired
Define the term absolute refractory period
The time between initially opening and the initial closing of the Na+ channels
No matter how strong the stimulus nothing will initiate an action potential
Define the term relative refractory period
The time to recover back to the resting potential
Potential for action potential if strong stimulus applied
Explain how the properties of ion channels lead to the absolute and relative refractory periods
ARP- nearly all the sodium channels are in the inactive state
RRP- sodium channels are recovering from inactivation, the excitability returns toward normal as the number of channels in the inactive state decease
How might a drug act to block the production of action potentials?
Competitive block
Blocking of nicotinic receptors through competitive blocking
Indicate how local anaesthetics act to block action potentials of peripheral nerves
Procaine is an example of an anaesthetic
Two types of channel blockers:
Hydrophobic pathway- pass through the plasma membrane when pores are open and block entry of ions
Hydrophilic pathway- can’t pass through the plasma membrane so act as competitive blockers
The more channels that open, the more anaesthetic binds, remain bound during inactivation
Tetrodotoxin blocks voltage- gated Na+ channels
4- aminopyridine blocks voltage- gated K+ channels
What effect would you expect each to have on the action potential?
Tetrodotoxin: Sodium channel blocked Sodium cannot enter the cell Depolarisation will not occur Therefore a threshold potential wont be reached No action potential generated
4- aminopyridine
Blocks potassium channels
The membrane will repolarisation but take longer as the non- voltage gated dependent channels would open
ARP is increased therefore another action potential cannot be fired during this time
Explain how release of ACh from the motor nerve terminal causes an action potential in the skeletal muscle
When acetyl choline binds to the channel that will cause a non formational change in the nicotinic acetylcholine protein causing the pore to open
The pore is cation selective- allowing flow of both potassium and sodium at equal rebates
Normal membrane potential is close to potassium equilibrium potential- therefore the driving force for potassium ions to leave is less than it is for sodium to enter
Sodium influx therefore predominates- leading to depolarisation
It will try to reach reversal potential for this channel which is halfway between the Ek and Ena
Depolarisation past threshold will result in an action potential
What is the difference between the mechanisms of action of D-tubocurarine and succinylcholine at the neuromuscular junction?
DTC- a competitive blocker of nAChr
The blocker can be reversed by increasing the concentration of ACh
Since this is competitive inhibition the substance present at higher concentration is more likely to bind
SC is a depolarising blocker f nAChr
This activates the receptor
Thus keeps the membrane depolarised permanently
Explain equilibrium potentials for sodium, potassium, calcium and chloride
The conductance of the membrane to a particular ion is dependent on the number of channels for the ion that are open
If the conductance to any ion is increased the membrane potential will move closer to the equilibrium potential for that ion
How are membrane currents measured?
Voltage- clamp
The membrane potential is controlled and the currents flowing through the membrane is measured
Define refractory period
A period immediately following stimulation during which a nerve or muscle is unresponsive to further stimulation
Describe the basic structure of voltage- gated sodium and calcium channels
One alpha subunit to make a functional channel
Consisting of 4 homologous repeats
Each repeat consists of 6 transmembrane spanning domains
4th sub particle is positive- sensing the voltage field across the membrane
Changing a membrane potential changes the membrane field
Resulting in a conformational change within the channel
As a result the pore opens
When the pore is open it is susceptible to inactivation
An inactivation particle enters the pore
This inactivation particle acts like a plug, stopping the flow of sodium ions through the pore in the membrane
The plug can be removed from the membrane once it has hyperpolarised
Describe the basic structure of voltage gated potassium channels
Potassium ion channels have a similar verbal structure to that of sodium and calcium
Except that each ‘repeat’ of a K+ channel us up fact a separate subunit
Thus it is made of Four alpha subunits rather than one
List the order local anaesthetic block in
1) small unmyelinated axons
2) un- myelinated axons
3) large unmyelinated axons
What properties of the axon lead to a high conduction velocity?
1) axons diameter- wider diameter axons will conduct faster
2) the degree to which the axon is myelinated
3) high membrane resistance
4) low membrane capacitance
Explain how the action potential is conducted along an axon, with reference to the local current theory
A change in membrane potential in one part can spread to adjacent areas of the axon by local current spread
Conduction velocity is determined by how far along the axon these local currents can spread
When local current spread causes depolarisation of part of the axon to threshold than an action potential is initiated
The further the local current spreads down the axon the faster the conduction velocity
