6. Electrical Excitability - the Action Potential and its Properties Flashcards
What percentage increase of Na+ ions is needed to generate an action potential if resting [Na+] is 10mM?
0.4% increase only! 40uM increase.
How can the mechanism of action potential generation be investigated?
Voltage-clamping so the membrane potential is controlled and ionic currents can be measured.
Using different ionic concentrations so the contribution of various ions can be assessed.
Patch-clamping enable currents flowing through individual ion channels to be measured.
How does voltage-clamping work to measure membrane current at a constant volume?
Glass is used as the clamp so membrane current can’t affect membrane voltage. The membrane voltage is controlled by the investigator and can be altered to measure the effect on the current of ions.
What happens during depolarisation of the action potential?
Depolarisation to the threshold means Na+ channels open so Na+ enters the cell and causes depolarisations, causing more Na+ channels to open and more Na+ to enter so the depolarisation is rapid and extreme.
What happens in the downstroke of the action potential?
Na+ are inactivated after a time so can’t pass through a current so the Na+ influx stops. After a time, K+ channels are opened so K+ is effluxed. These two together cause depolarisation.
What is the difference between absolute refractory period and relative refractory period?
ARP - nearly all the Na+ channels are inactivated so don’t allow Na+ influx or an action potential to be fired.
RRP - Na+ channels are recovering from inactivation, the excitability returns towards normal as the number of channels in the inactivated state decrease so there can be some Na+ influx.
What is accommodation?
The effect of a slow stimulus. The number of Na+ channels open increases but this means more can inactivate so the new threshold isn’t met. The longer the stimulus, the larger the depolarisation necessary to initiate an action potential.
What is the basic structure of voltage gated Na+ channels?
One peptide chain of four subunits. They have an inactivation particle between the 3rd and 4th subunits. Each subunit has a voltage sensor, so one subunit can make a functioning channel.
What is the structure of voltage gated K+ channels?
4 subunits are needed for it to function. 6 transmembraneous domains. The 4th has a voltage sensor.
How do local anaesthetics work?
By blocking Na+ channels. The drug interacts with the pore forming part of the channel. They work in a use dependent man or, the more a wound is worked on, the more action potentials fired so the more effective the anaesthetic becomes.
In what order do local anaesthetics block axons?
Small myelinated axons, unmyelinated axons and then large myelinated axons.
What do action potentials depend on?
Ionic gradients and relative permeability of the membrane.
