Action potential Flashcards
Describe the properties of a neuronal action potential
0.5ms duration
Change in voltage across a membrane with a set threshold in which an all or nothing response occurs from a stimulus.
Action potentials propagate along nerves without losing amplitude. -75–>+25 if fires.
Explain which ions do what in inaction potential
Depolarisation opens voltage gated sodium channels.
Sodium influxes on the upstroke.
Voltage gated potassium channels open and sodium channels inactivate due to the depolarisation giving hyper polarising effect on the down stroke.
If a membrane increase permeability to sodium what happens?
The MP will move toward the Ek for that ion.
What happens to the resting membrane potential in hyperkalaemia?
Hyperkalaemia makes the RMP more positive and so close to the threshold. This opens some sodium channels which are then prone to deactivation which then raises the threshold for a potential to fire.
Describe the structure of a potassium channel.
6 transmembrane helix structure.
Pore region between 5 and 6 helix.
N and C termini are intracellular.
Four a functional channel you need four of these 6 transmembrane alpha units.
Describe the structure of a sodium channel.
4 repeating units all connected.
Each subunit has 6 transmembrane helices. The 4th helix is the voltage señor. The pore region is formed between the 5th and 6th helices.
Between helix 6 on the third subunit and helix 1 on the fourth there is an inactivation particle that can plug the pore.
How does procaine work?
Give the order of nerves that are affected.
Local anaesthetics block sodium channels.
1Small myelinated
2Unmyleinated
3Large myelinated
How can conduction velocity be measured?
Voltage clamps at different positions on an axon.
What is the local circuit theory of propagation?
Sodium influx will repel any positive ions near it which causes a local circuit. Immediate local depolarisation reached the threshold and propagates the AP. High resistance and low capacitance assist the spread.
In myelinated nerves saltatory conduction occurs.
Myelination means what for conduction velocity?
Conduction velocity is faster because the time content is reduced and the length constant is increased.
How does myelination improve conductance?
Mylein sheath is a good insulator so the local circuit must jump to the next Node of Ranvier to propgate the AP.
Increase resistance
Decrease capacitance
increase the length constant (how far the AP goes without losing amp)
Decrease time constant
What conditions are associated with demyelination.
CNS: MS and Devics disease
PNS: Landry-Guillain-Barre syndrome and Charcot-Marie-Tooth disease
How does demylienation cause problems?
Poor local transmission in areas where mylein has been lost (High resistance and high capacitance) this means the action potential loses amplitutde and may not reach the threshold potential at the next node of Ranvier.
How does demylienation cause problems?
Poor local transmission in areas where mylein has been lost (low resistance and high capacitance) this means the action potential loses amplitutde and may not reach the threshold potential at the next node of Ranvier.
