M&R 4 - Electrical Excitability Flashcards
What are the features of an action potential?
All or nothing
Propagated without loss of amplitude
Depend on ionic gradient and membrane permeability
What is an action potential?
A change in the voltage across a membrane
How long does an AP in an axon take?
0.5ms
How long does an AP in skeletal muscle last?
0.5ms
How long does an AP in the SAN last
200ms
How long does an AP in cardiac ventricle take?
280-300ms
How are APs generated?
By an increase in sodium permeability and this aims to bring the membrane potential closer to 61mV (equilibrium sodium)
What is the sequence of AP generation?
Membrane depolarised to threshold Sodium channels open Sodium influx Further depolarisation Positive feedback - more sodium
How does repolarisation come about?
This occurs during maintained depolarisation when sodium channels become inactivated and voltage gated potassium channels open causing a potassium efflux aiming to move towards the equilibrium of -88mV
What are the two refractory periods called?
Absolute refractory
Relative refractory
What happens during the absolute refractory period?
This is when all the sodium channels are inactivated
A second action potential cannot be initiated
Excitation is at 0
What happens during the relative refractory period?
A stronger than normal stimulus is needed to elicit an AP
Sodium channels are recovering from inactivation
Excitability returns to normal
What is accommodation?
This is when the longer the stimulus lasts the larger the depolarisation threshold needed to initiate an action potential is.
The sodium channels become inactivated
What are the properties of a sodium/calcium gated channel?
The active pore is one subunit
One peptide
4 homologous repeats of
6 transmembrane domains (1 of which is voltage sensitive)
What are the properties of a voltage gated potassium channel?
This is when the functional pore requires 4 subunits -
4 peptides
6 transmembranous domains ( 1 of which is voltage sensitive)
How do local anaesthetics work?
These act by binding to and blocking Na+ channels
They stop AP preventing nervous transmission
They cross the membrane unionised and block Na+ when open
In what order to local anaesthetics block conductance?
1 small myelinated
2 large unmyelinated
3 large myelinated
How is conductance velocity measured?
Electrodes are used to raise the membrane potential to generate AP.
Then the changes in potential between the stimulating cathode and the recording anode are measured
What is conductance velocity (HINT - equation)
CV = distance/time
What is the transport of AP called?
Propagation
How does propagation occur?
The depolarisation of a small region of membrane produces transmembrane currents in neighbouring regions this leads to more Na+ channels opening
What are three properties of axons that lead to high conductance?
High membrane resistance
Large axon diameter
Low membrane capacitance (ability to store charge)
Why does high membrane resistance increase CV ?
The higher the resistance of membrane = higher the pd across it
More voltage across membrane = more Na+ channels open
Larger current = travels further
Why does a large axon diameter lead to high CV?
Lower cytoplasmic resistance = larger current
