MR 4 Flashcards
Voltage sensitive Na channels are either open, closed or inactivated.
Describe the sequence that occurs, in relation to their conformational state.
- closed
- then open
- then inactivated
- then close.
the channels cannot go from open to close, they must be in the inactivated state before they can close and then open again
What is meant by ‘all or nothing’
This is in regards to action potentials. Once a cell begins conducting an action potential, it cannot stop half way. It will conduct it fully.
- as the voltage approaches threshold, some of the Voltage sensitive Na channels open, which causes inward movement of the Na.
- inward flow of the Na causes more and more VONaC to open.
- positive feedback.
Describe refractoriness.
ARP
- the period during which there can be no AP conducted as all the Na are in the inactivated state.
RRP
- membrane excitability returns to normal as more and more Na channels have recovered from inactivation. Not all of the Na are in the closed state so a significantly larger stimulus will be required to conduct an AP.
Discuss the process of repolarisation.
This is the ‘downstroke’ of the action potential graph. The membrane potential approaches its RMP level.
- Occur due to the efflux of K+ through the opening of the voltage sensitive K+ channels.
- Inactivation of the voltage sensitive Na channel therefore there is less / no Na being influxed.
Describe the structure of the Voltage sensitive Na channels.
- 1 alpha subunit
- 6 transmembrane proteins (S1-S6)
- S4 is sensitive to voltage and changes shape according to voltage.
- S5-S6 is the pore region, region through which the ion travels.
- inactivation of the channel is done by physically blocking pore region.
What is accommodation?
The longer a stimulus is, the larger the depolarisation that is needed to initiate an action potential. The threshold becomes more positive. Eventually, the threshold will become so positive that there will be no action potential despite the stimulus.
Outline the action of local anaesthetics
Anaesthetics such as procaine work by blocking Na channels, so no AP can be fired. They are weak bases so they cross the membrane relatively easily. They bind to the channels when they are inactivated with a high affinity.
In terms of procaine and other local anaesthetics, what is their order of blocking?
- Small myelinated axons
- Non-myelinated axons
- Large unmyelinated axons.
Describe extracellular cording and how this can be used to measure conduction velocity of an AP.
Electrodes are used to bring the membrane to threshold and fire an AP.
We can measure changes in membrane potential between the stimulating electrode (cathode) and recording electrode (anode).
The velocity can be measured using
Distance/time = velocity (conduction velocity).
Explain the local circuit theory of propagation
When a small region of membrane is depolarised, this produces transmembrane currents in the neighbouring regions.
- Na channels are voltage gated so these open in response to the change in voltage. This then initiates further transmembrane currents in neighbouring regions which then keeps occurring along the membrane, causing propagation.
Explain how fibre diameter is linked to conduction velocity of APs.
Ohms law
I= V/R where I is current, V is potential difference and R is resistance.
So if there is lower resistance, then I is bigger.
Large fibre diameter = lower cytoplasmic resistance.
Large fibre diameter = larger current
This means the AP will be conducted further and velocity increases.
Describe the 3 properties that influence high velocity conduction
- High axon diameter. This reduces cytoplasmic resistance which increases the velocity of conduction.
- Low membrane capacitance - takes less current to charge so conduction velocity increases. If it was a high capacity then the velocity would be lower.
- High membrane resistance - means that more of the ion channels are closed so less of the propagated signal is lost. If it was low resistance, then more of the signal would have been lost and therefore the velocity would be lower.
Explain the implications of myelination for conduction
Myelination will decrease capacitance but increase membrane resistance. This will increase the velocity.
Explain what saltatory conduction is.
Myelination allows for saltatory conduction.
This is where the AP ‘jumps’ from one node of ranvier to the next.
This occurs because the myelin acts as a good insulator and causes the local circuit current to depolarise the next node of ranvier above threshold and fire an AP.
There is a high concentration of Na channels at the nodes.
Describe certain consequences of demyelination.
Certain diseases such as multiple sclerosis (autoimmune) can attack the myelin of myelinated nerves.
Myelinated nerves conduct APs a lot quicker than unmyelinated (above certain diameter).
Demyelination leads to decreased conduction, complete block or on some AP transmission.
