ND2: Action potentials Flashcards
What is an action potential and what are its properties?
The change in voltage across a membrane
- Depends of ionic gradients and relative permeability of the membrane
- Only occurs if a threshold level is reached
- All-or-nothing
- Propogated without loss of amplitude
What happens to the membrane potential if the conductance to any ion is increased?
The membrane potential will move closer to the equlibrium potential for that ion
Upon what is the conductance of the membrane to a particular ion dependent?
The number of channels for that ion which are open
How can it be experimentally shown that Na+ is responsible for action potential depolarisation?
Conducting experiments with changing concentrations of extracellular Na+
The peak of the action potential will change in a manner parallel to the changes in ENa
What does a voltage clamp enable?
Membrane currents to be measured at a set membrane potential
Explain these diagrams.
Diagram 1
The membrane potential is held at –70mV, then increased to –15, 0, and +15 mV in three separate experiments
Diagram 2
The influx of Na+ happens more quickly at more positive membrane potentials
The current of Na+ influx decreases towards 0 even though Vm is maintained
K+ channels take longer to close
Why does overshoot occur after an action potential has taken place?
K+ channels take longer to close
By how much does each action potential increase Na+ concentration in the axon?
40 μM
What is the axon hillock?
The part of the cell body (soma) of a neuron that connects to the axon
Where does depolarisation to threshold initiate an action potential?
At the axon hillock
Describe the positive feedback loop during an axonal action potential.
Describe channel activity during an axonal action potential.
What are the two types of refractory period after an action potential? What is the difference between the two?
Absolute refractory period (ARP): nearly all Na+ channels are in the inactivated state
Relative refractory period (RRP): Na+ channels are recovering from inactivation; the excitability returns towards normal as the number of channels in the inactivated state decreases
Do channels open and close in an ordered or random manner?
Random
What is the basic structure of a voltage-gated Na+ channel?
One α-subunit consiting of four similar repeats (I – IV), each containing six α-helix regions, and a pore at the centre
How does the action potential propagate along the axon?
- The action potential is initiated at the axon hillock by an EPSP
- It travels down the acon without loss of amplitude
- It goes in one direction only
- The propogation of an action potential depends on local current spread depolarising the membrane potential of adjacent sections of the axon above threshold
How do local currents result in a spread of depolarisation along the axon?
They repel any other positively charged ions and trap any negatively charged ions
What is the length constant (λ) ?
The distance taken for the potential to fall to 37% of its original value
What will be the effects of an injection of current into the axon?
The resulting charge will spread along the axon and cause an immediate local change in the membrane potential
How can propogation be maintained?
By local depolarisation ahead of the action potential above threshold initating the positive feedback loop
How can action potential conduction speed be increased?
Myelination
Why does myelination increase speed of action potential conduction?
The Na+ channels are concentrated at unmyelinated regions called the nodes of Ranvier, whereas unmyleinated axons have an even distribution of Na+ ions
What is saltatory conduction?
The jumping of action potential between nodes of Ranvier
The myelin sheath acts as a good insulator, causing the local currents to depolarise the next node above threshold and initiate an action potential
