ICPP 5 Flashcards
What determines whether an action potential is generated in an axon hillock?
Whether summation of EPSP and IPSP reaches threshold potential.
What happens to the membrane potential of an ion if the conductance is increased?
The membrane potential will move closer to the equilibrium potential.
What happens during the upstroke of an action potential?
Na+ channels open and Na influx depolarises cell to threshold level which causes more Na+ channels to open…
What happens during the downstroke of an action potential?
Na+ channels close and become inactivated.
K+ channels open, K+ efflux returns membrane potential to resting.
What is hyperpolarisation?
When the membrane potential is lower than resting level due to more K+ channels being open.
What causes hyperpolarisation?
Slow closing of the K+ channels
How do sodium channels become re-activated?
Inactivation pore inactivates them, this is only removed when the membrane becomes hyperpolarised and the channel then closes.
What is the absolute refractory period?
When the sodium channels are all inactivated. No matter how strong a stimulus, an AP cannot be generated.
How is the relative refractory period different to the absolute refractory period?
In RRP, Na+ channels are recovering from inactivation, so if a stimulus is large enough then an AP may be generated.
How many polypeptides are there in 1 functional Na+ channel?
1
Outline the structure of a voltage gated Na+ channel.
1 polypeptide with 4 domains.
Each domain has voltage sensor in transmembrane spanning region 4.
Between the 3rd and 4th domain there is an inactivation particle.
There is a p region which dictates which ion channel flows through.
How do voltage gated K+ channels differ?
4 polypeptides make up 1 functional channel.
No inactivation particle.
What is the mechanism of action of local anaesthetics?
They block Na+ channels, stopping action potential generation in pain fibres.
What order do local anaesthetics block axons?
- Small myelinated
- unmyelinated
- Large myelinated
Local anaesthetics work in a use-dependent manner, explain this.
They block the ion channel more easily when it is open
True or false: smaller axon diameter will increase conduction velocity.
False.
Larger diameter leads to increased velocit.
How is AP conducted along an axon?
Change in MP causes a local current spread, which causes an immediate local MP change. If this threshold potential then an AP will fire.
What 3 properties lead to a high conduction velocity of an axon?
- High membrane resistance
- Low membrane capacitance
- Large diameterq
What does low resistance suggest?
More channels are open, more local current lost.
What is capacitance?
The ability to store charge
High resistance means…
Fewer ion channels open, so less charge lost..
Myelination of axons increases conduction velocity. How?
Reduces capacitance and increases resistance
What 2 special cells form myelin?
- Oligodendrocytes in CNS
2. Schwann cells in PNS
What is saltatory conduction?
No AP needs to be generated in insulated, intermodal space, so it jumps between nodes of ranvier which have high density of Na+ channels.
Give an example of a demyelinating disease.
Multiple Sclerosis.
Auto-immune disease which destroys myelin..decreasing conduction velocity or completely blocking AP transmission.
What is a neuromuscular junction?
Synapse between a nerve and a skeletal muscle fibre.
What channels are present at the nerve terminal?
Na+ channels, K+ channels and Ca2+ channels.
What does depolarisation in the nerve terminal cause?
voltage gated calcium channels open, influx of calcium causes neurotransmitter release.
How can you increase the amount of neurotransmitter released?
Increased frequency of action potentials. All the same size as all or nothing.
Is the structure of the voltage gated calcium channel most similar to a Na+ channel or K+ channel?
Na+ channel. 1 polypeptide and 4 domains.
What is MOA of dihydropyridines?
L-type calcium channel blocker. Decrease blood pressure.
Which proteins are involved in the exocytosis of neurotransmitters?
Synaptotagmin - binds to the calcium and brings vesicle to membrane.
Snare complex- forms a fusion pore at membrane.
Name a competitive nAChR blocker?
Tubocurarine
name a depolarising nAChR blocker and outline its MOA.
Succinylcholine. It binds to the receptor and causes depolarisation, however it doesn’t dissociate, so the membrane cannot hyperpolarise again which means the Na+ channels cannot activate.
What is myasthenia gravis?
Autoimmune disease targeting nACh receptors.
