Session 5(Electrical Excitability)

Question 1

How are Na+ channels closed during depolarisation?

Answer 1

A mechanism called inactivation

Question 2

What pump is not involved in repolarisation?

Answer 2

Na+/K+ pump

Question 3

What two things happens to cause repolarisation?

Answer 3

Na+ channels inactivate and K+ channels activated (opened)

Question 4

What are the two recovery periods called after an action potential?

Answer 4

Absolute refractory period (all Na+ channels inactivated)

Relative refractory period (some Na+ channels inactivated)

Question 6

Describe the basic structure of a Na+ channel

Answer 6

1 polypeptide, 4 repeats, each repeat has 6 alpha helices
The 4th alpha helix has a positive charge
There is an inactivation particle (between 3.6 and 4.1)

Question 7

Describe the basic structure of a K+ channel

Answer 7

4 polypeptides, each of 1 repeat. They each consist of 6 alpha helices
4th alpha helix is positively charged

Question 8

Give an example of a local anaesthetic that would bind to block Na+ channels

Answer 8

Procaine

Question 9

In what order are nerve fibres blocked when using local anaesthetics?

Answer 9

Small myelinated axons first
Non-myelinated axons next
Large myelinated axons last

Question 10

What type of axon have the fastest conduction?

Answer 10

Thick myelinated ones

Question 12

What kind of molecules are most local anaesthetics?

Answer 12

Weak bases

Question 13

How do local anaesthetics cross a membrane (in what form)?

Answer 13

Unionised form

Question 14

When can local anaesthetics block Na+ channels with the greatest ease?

Answer 14

When the channels are open

Question 15

What state of Na+ channels do local anaesthetics have the higher affinity for?

Answer 15

Those in the inactivated state

Question 16

What is a spread of current in an axon called?

Answer 16

Local current

Question 17

What increases if the local current spread further down the axon?

Answer 17

Conduction rate

Question 18

What is Rm?

Answer 18

Membrane resistance

Question 19

What does an increased Rm mean?

Answer 19

It means there will be a larger spread of depolarisation, therefore it is more likely that the region after the action potential will be depolarised past the threshold point.
Overall an increased Rm increases the rate of conduction

Question 20

What is capacitance?

Answer 20

The ability to store charge

Question 21

What does an increased capacitance mean?

Answer 21

A membrane takes more current to charge (or a longer supply of said charge) resulting in a slower conduction rate

Question 22

What does a low Rm mean?

Answer 22

More ion channels are open hence the slower conduction

Question 23

What does myelination do in terms of Cm and Rm and therefore conduction rates?

Answer 23

It decreases Cm but increased Rm which in turn increases conduction rates

Question 24

What two types of cell myelinate and where?

Answer 24

Schwann (in the PNS) and oligodendrocytes (CNS)

Question 25

What sort of conducting occurs as a result of myelination?

Answer 25

Saltatory conduction

Question 26

How are velocity and diameter related in myelinated and non-myelinated axon?

Answer 26

In myelinated axons= velocity is directly proportional to diameter
In non-myelinated axons= velocity is directly proportional to the square root of the diameter

Question 27

What condition causes a break down of myelin sheaths?

Answer 27

Multiple sclerosis

Question 28

Name a specific blocker for L type Ca2+ channels

Answer 28

Dihydropyridines (nifedipine)

Question 29

What is the junction between an axon and muscle called?

Answer 29

A neuromuscular junction

Question 30

What does an increased action potential frequency do to [Ca2+]?

Answer 30

It increases the concentration resulting in the release of more neurotransmitter

Question 31

How fast and why do Ca2+ channels inactivate?

Answer 31

Slower than Na+ channels | Because they're depend on Ca2+ levels within the cell

Question 32

What breaks down ACh?

Answer 32

Acetylcholine esterase

Question 33

How does Ca2+ cause release of neurotransmitter? (4)

Answer 33

Ca2+ binds to synaptotagmin Vesicle brought to membrane Snare complex makes fusion pore Transmitter released

Question 34

How many ACh molecules are needed to open a nAChR?

Answer 34

2

Question 35

What does a nAChR move?

Answer 35

Cations- NA+ / K+

Question 36

Name a competitive blocker of nAChr

Answer 36

Tubocurarine (d-TC)

Question 37

Name a depolarising blocker of nAChR?

Answer 37

Succinylcholine

Question 38

How do depolarising blockers work on nAChRs? (Eg succinylcholine)

Answer 38

``` Open nAChR Cause depolarisation (and maintain it) Na+ channels inactivate Na+ channels are unable to recover No more Action potentials are produced ```

Question 39

Name a disease in which nAChRs are targeted (loss of function)

Answer 39

Mayasthenia grans

Question 40

What does mayasthenia grans cause?

Answer 40

don't respond well to ACh therefore action potentials don't reach threshold therefore patients get weak

Question 41

What are mAChRs?

Answer 41

Muscarinic ACh receptors

Question 42

Where are mAChR found?

Answer 42

Parasympathetic neuromuscular junctions

Question 43

What speed of response do mAChR give and why?

Answer 43

Slow(er) | Linked to GPCR which trigger cascade events (no intrinsic ion channel)

Question 46

How are action potentials generated?

Answer 46

By an increase in permeability to Na+ (causing the membrane potential to move closer to the Ena+

Question 54

Why don't Na+ channels open again immediately?

Answer 54

Because they can only reactivate when the membrane potential has returned to its resting level