Flashcards in 4. Action Potentials & Electrical Excitability Deck (24)
How is an action potential generated?
Increasing membrane permeability to Na+ (bringing the membrane potential closer to ENa, +61mV)
What is meant by the all or nothing rule of action potentials?
If the threshold value is met, it will always give a complete response, otherwise no response is given.
Repolarisation involves what changes to which channels?
Inactivation of Voltage Gated Na+ channels
Opening of Voltage Gated K+ channels
What is the absolute refractory period and how does it affect the direction in which an impulse travels?
When Voltage Gated Na+ Channels are either all open or inactivated hence no matter how much stimulation is received, another action potential will not occur.
This means that an action potential will only travel in one direction along an axon.
What is the relative refractory period?
Where Voltage Gated Na+ channels are recovering and returning to the closed state from the inactivated state.
As the number of channels inactivated decreases, the excitability returns to normal.
What is the structure of Voltage Gated Na+/Ca2+ channels?
Four homologous repeats each with 6 transmembrane domains
One domain is voltage sensitive
Contains inactivation particle
What is the structure of Voltage Gated K+ channels?
6 transmembrane domains
One domain is voltage sensitive
Contains a p region that contributes to pore sensitivity
Name an anaesthetic that blocks Na+ channels to stop an action potential.
Name the order of nerve types and sizes in which procaine blocks conduction best in.
1 Small myelinated
2 Non myelinated
3 Large myelinated
Under which electrode does electrical stimulation occur?
Under which electrode is excitability reduced?
What is diphasic recording?
Where both electrodes are extracellularly recorded to measure conduction velocity
What is monophasic recording?
Where one electrode is purposely put in a damaged part hence only the other is used to record conduction velocity
How do you calculate conduction velocity?
Distance between stimulating and recording electrodes divided by time gap between stimulus and action potential being recorded
How is an action potential conducted?
Local current causes the change in membrane potential in one area to spread to adjacent areas of the axon, causing them to reach threshold and fire an action potential
What is the relationship between local current spread and conduction velocity?
The further down the axon it spreads, the higher the conduction velocity.
Name three factors that can increase conduction velocity.
High membrane resistance
Low membrane capacitance
Large axon diameter ( as this leads to low cytoplasmic resistance)
What does reducing the capacitance at the internodal region via myelination lead to?
Saltatory conduction, the local current induced at one node can travel far enough down to the next node to depolarise that one
Which is faster, local current spread or action potential spread?
Local current spread, hence why saltatory conduction is the quickest
What is multiple sclerosis?
Demyelination of axons in the CNS
What is Devic's disease?
Demyelination of optic and spinal axons
Name two diseases which involve demyelination of axons in the peripheral nervous system
Landry Gullain Barre syndrome
Charcot Marie Tooth disease
Immediately after demyelination of an axon why is there complete failure to conduct an impulse?
Increased membrane capacitance preventing the channels at the nodes being raised to threshold