Action Potential Flashcards
Describe events of an action potential
Membrane is depolarised to threshold
Voltage gated sodium channels open ➡️ sodium influx
Further depolarisation towards the Ena
Na channels close once a certain voltage has been reached
Voltage gated potassium channels are opened
Potassium efflux
Repolarisation of the membrane
List some properties of the action potential
All or nothing
Propogated without the loss of amplitude
Depends on ionic gradients and relative permeability
Only occurs if threshold level is reached
What is the absolute refractory period?
When nearly all sodium channels are in their inactive state so excitability is at 0.
What is the relative refractory period?
Sodium channels are recovering from inactivation. Excitability returns towards normal as the number of deactivated channels decreases. Requires a larger stimulus than usual to trigger an action potential
How many subunits/polypeptide chains are required for the function of a sodium channel?
1
What is an action potential?
A change in voltage across a membrane
Explain the ‘all or nothing’ principle G
The sodium channels are voltage gated.
As the membrane potential becomes more positive, positive feedback means that more channels will open until they all are.
Depolarisation cannot stop halfway, as this voltage is the voltage that wold open more sodium channels.
Describe accommodation
The longer the stimulus is, the larger the depolarisation necessary to initiate an action potential. This is because sodium channels become inactivated.
Describe structure of sodium and calcium channels
A pore forming subunit is made up of:
One polypeptide chain containing
Four homologous repeats
Each repeat consists of six transmembrane domains
In a sodium or calcium channel, how many of its domains are voltage sensitive?
1
Structure of a potassium channel?
Four peptides
Six transmembrane domains
How many subunits does a functional potassium channel require?
4
How do local anaesthetics act?
By binding to and blocking sodium channels
In which order to local anaesthetics block conduction?
Small myelinated axons
Non-myelinated axons
Large myelinated axons
Do anaesthetics tend to affect sensory or motor neurones first?
Sensory
What state of sodium channel do anaesthetics have a highest affinity form
Sodium channels in their inactivated state
How can an action potential be measured?
Negatively charged cathode used to stimulate an axon (or group of axons) to threshold and initiate an action potential.
Record changes in potential between the cathode and anode along an axon.
How do you work out conduction velocity?
Measure the distance between the stimulating electrode and the recording electrode; and the time gap between the stimulus and the AP being registered by the recording electrode
CV = distance ➗ time
Explain local circuit theory of propagation
Depolarisation of a small region of the membrane produces transmembrane currents in neighbouring regions.
Na channels are voltage gated, so this opens more channels, propagating the action potential.
What is the relationship between the distance the local current spreads and the speed of the conduction velocity of the axon?
The further the local current spreads down the axon, the faster the conduction velocity of the axon
What factors contribute to a higher conduction velocity?
High membrane resistance
Low membrane capacitance
Large axon diameter as this leads to low cytoplasmic resistance
What does Ohm’s Law state?
The higher the resistance of the membrane, the higher the potential difference across it.
What does Ohm’s Law mean for the effect of high membrane resistance?
More voltage across the membrane means more sodium channels are open. Easier to reach the threshold for an AP. Conduction velocity is increased.
What does Ohm’s law mean for axon diameter?
States that the lower the resistance, the larger the current, so action potential will travel further. Conduction velocity is therefore increased with a larger diameter.
What is capacitance?
The ability to store charge
What effect does capacitance have on conduction velocity?
A high capacitance reduces conduction velocity, because it takes more current to charge it.
What is the effect of myelination on conduction velocity?
Increases it
Which axons are myelinated?
Large diameter ones such as motor neurones.
What is salutatory conduction?
AP jumps between nodes of Ranvier.
The myelin sheaths acts as an insulator, causing local currents to depolarise the next node
What is the distribution of sodium channels in a myelinated axon?
High concentration of sodium channels at nodes.
What are peripheral axons and CNS axons myelinated by?
Peripheral - Schwann cells
CNS - oligodendrocytes
What is a condition that strips axons of their myelin?
Multiple sclerosis
What can MS lead to?
Affects the ability of previously myelinated axons to conduct action potentials
Decreased conduction velocity
Complete block or cases where only some action potentials are transmitted.
When does myelination occur?
4 months after foetal development
What is the effect of the myelin sheath on capacitance of a nerve fibre? Why?
Decreases capacitance.
Reduces dissipation of the local current within the fibre
What can be done to treat de myelinated nerves?
Potassium channel blockers
How does accommodation occur?
The progressive depolarisation gives time for some sodium channels to inactivate and for some potassium channels to open.
Why, at small diameters, do non-myelinated neurones conduct faster than myelinated?
Smaller the diameter -> slower the conductance
At small diameters, the myelin sheath is included in the diameter so fractional space occupied by the myelin is high
