Flashcards in Session 4 Deck (14):
What happens when a membrane is depolarised to threshold voltage?
Voltage gated Na+ channels open, allowing Na+ influx and depolarising the membrane further. More Na+ channels open leading to positive feedback. Na+ channels close by inactivation.
How are the effects of membrane potential on Na+ and K+ currents measured?
Using a voltage clamp. The membrane potential is controlled and currents flowing through the membrane are measured. The number of Na+&K+ channels open at different membrane potentials is clear.
What happens to Na+ channels after an action potential?
Most are inactivated. They can only re open again once the membrane potential has hyper polarised to its resting potential.
What is the absolute and relative refractory period?
Absolute - nearly all Na+ channels are in the inactivated state. Excitability is zero.
Relative - Na+ channels are recovering from inactivation. Excitability returns to normal
What is accommodation?
The longer the stimulus the larger the depolarisation necessary to initiate an AP due to a large number of Na+ channels being inactivated.
What is the structure of voltage gated Na+, Ca2+ and K+ channels?
Na+ and Ca2+: alpha subunit of one peptide containing four homologous repeats. Each repeat consists of 6 transmembrane domains, one of which senses voltage across the membrane.
K+: similar but each repeat is a separate subunit (4 alpha subunits)
How does the anaesthetic procaine work?
Blocks Na+ channels therefore stopping AP generation. They tend to affect sensory neurones before motor neurones (small axons before large ones).
They are weak bases and cross the membrane in their unionised form. They have a higher affinity to the inactive form.
How are conduction velocities measured?
An action potential is stimulated using a cathode. The time and distance between a stimulating and recording electrode is measured.
What determines the conduction velocity of an action potential along an axon?
How far along the axon local currents spread to adjacent areas. A high membrane resistance, a low membrane capacitance and a large axon diameter (leads to a low cytoplasmic resistance) all increase conduction velocity.
What is the length constant?
(Symbol=lambda) the distance it takes for the potential to fall to 37% it's original value.
What is the capacitance of a lipid bilayer?
Ability to store charge. A high capacitance takes more current to charge it (or longer charge for a given current) and can cause a decrease in the spread of the local current.
Why doesn't an action potential travel backwards?
The area of axon that has just fired is refractory
How does myelin increase the conduction velocity?
It reduces capacitance and increases membrane resistance.
The local current induced at an AP at a node of Ranvier spreads further down the membrane to depolarise the next node, without firing an AP in the internodal region.