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Flashcards in The Action Potential Deck (63):
1

What is an action potential? How long does it usually take?

A rapid change in the membrane potential

0.5ms in an axon

2

What is meant by the 'all or nothing' rule of action potentials?

Only occurs if a threshold level is reached

3

If sufficient depolarisation takes place and action potential will be initiated. Where do this occur?

At the axon hillock

4

If the conductance of an ion is increased, the membrane potential will move ________ the equilibrium potential for that ion

Towards

5

What causes an action potential to start?

A large increase in permeability to Na+ ions

6

What two channels are involved in an action potential?

Voltage dependent Na+ channels
Voltage dependent K+ channel

7

What does depolarisation do to voltage gated channels?

Open them

8

What does hyperpolarisation do to voltage dependent channels?

Close them

9

What happens to Na+ channels after they open?

They inactivate

10

Do K+ channels inactivate?

No

11

K+ channels are ______ in their closing causing ______

Delayed

Hyperpolarisation

12

How many ions need to flow to cause an action potential?

A small amount

13

What can be used to measure membrane currents at a set membrane potential?

Voltage clamps

14

Depolarisation is caused by...

An influx of sodium ions

15

Repolarisation is caused by....

Inactivation of sodium channels
(Slower) opening of K+ channels

16

Hyperpolarisation is caused by...

Increased K+ conductance
Delayed closing of K+ channels

17

What are the two periods of recovery for Na+ channels after an action potential? How long does each period last?

ARP - Absolute refractory period (1ms)
RRP - Relative refractory period (4ms)

18

What happens during the ARP?

Na+ channels inactivated
0 membrane excitability
No further action potentials can be sent

19

What happens during RRP?

Na+ channels recovering from inactivation
Recover once mp= -ve
Strong stimulus may result in action potential

20

What forms the functional part of a voltage gated Na+ channel?

1 peptide ----> 1 alpha subunit

21

What forms the functional part of a voltage gated K+ channel?

4 alpha subunits

22

How many repeats are found in a voltage gated Na+ channel?

4

23

How many repeats are found in a voltage gated K+ channel?

1

24

The 4th transmembrane region of each repeat on both voltage gated sodium and potassium channels contains...

Many positives amino acids, acts as a voltage sensor

25

What does a voltage gated Na+ channel contain to allow it to become inactivated?

An inactivation particle between repeats 3 and 4

26

Which region on both voltage gated sodium and potassium channels allows ions through?

The pore region

27

Many local anaesthetics work by blocking the action of which channels? Give an example?

Na+ channels

Procaine

28

Local anaesthetics block different axons in which order?

First... small myelinated axons ---> unmyelinated axons ----> large myelinated axons

29

What is the local current theory?

Injection of a current to one part of axon, will result in a spread of charge and therefore immediate changes and depolarisation of adjacent parts of the axon

30

What is the length constant?

The distance it takes for the potential (of an injection of current) to drop to 37% of its original value

31

_______ membrane resistance = increased conduction velocity/spread of charge

Increased

32

________ membrane capacitance = increased conduction velocity/spread of charge

Decreased

33

What is capacitance?

A property of the lipid bilayer (ability to store charge)

34

What does resistance of a membrane depend upon?

The number of open channels
E.g. High resistance = less channels open

35

Why does high capacitance result in decreased conduction velocity?

Voltage changes more slowly in response to current injection

36

What affect does high resistance have on the spread of charge?

Spreads further along the axon

37

What is responsible for the propagation of the action potential?

Local currents

38

How does myelination increase the conduction velocity?

Increases membrane resistance
Decreases membrane capacitance

39

The myelin sheath is produced by which cells in the PNS?

Schwann Cells

40

The myelin sheath is produced by which cells in the CNS?

Oligodendricytes

41

There is a high density of _______ at the nodes of Ranvier of myelinated axons

Na+ channels

42

How are Na+ channels distributed along an unmyelinated axon?

Evenly

43

What does saltatory conduction describe? What does it result in?

The jumping of the local circuit current from node to node

Increased conduction velocity

44

In a myelinated axon, the diameter is ______ to the conduction velocity

Proportional

45

In an unmyelinated axon, the conduction velocity is proportional to ______ of the diameter

Square root

46

What is an example of a demyelinating disease? Which nerves does it affect? What does it result in?

Multiple sclerosis

Affects all cns nerves

Poorer transmission of action potentials (doesn't reach threshold)

47

What is a neuromuscular junction?

The synapse between a nerve and skeletal muscle cell (fibre)

48

What channels are present at NERVE TERMINALS?

Voltage gated Na+ channels
Voltage gated K+ channels
A high density of voltage gated Ca2+ channels

49

What does the opening of voltage gated calcium channels at nerve terminals result in?

Calcium into the cell
Release of neurotransmitter

50

What does an increased frequency of action potentials result in?

Increases calcium entry at nerve terminals

More transmitter released

Bigger response

51

What is the structure of a voltage gated Ca2+ channel?

Similar to structure of Na+ channel
1 alpha subunit makes a function channel

52

What effect does nifedipine have on calcium channels? What is used to treat?

Blocks L type calcium channels

High blood pressure

53

A pore forming alpha subunit is necessary for a functional channel, what is the function of other subunits?

They regulate the activity of the channel through phosphorylation/glycosylation of parts of the sub unit

54

How do calcium channels activate/inactivate in comparison to sodium channels?

More slowly
At more positive membrane potentials

Inactivate more slowly as well

55

What is voltage gated calcium channel inactivation dependent on?

The intracellular concentration of calcium

56

During neurotransmitter release what does calcium bind to after its entry into the cell? What does this result in?

Synaptotagmin

Vesicle is brought close to the membrane?

57

What happens for a vesicle of neurotransmitter to be released at a membrane?

Snare complex makes a fusion pore at the membrane
Transmitter released through the pore

58

How are nicotinic acetylcholine receptors activated?

Binding of two molecules of ACh ---> conformational change

Channels opens to Na+ and K+ ions

59

How does crurare cause paralysis?

Blocks transmission between nerve and muscle

60

Nicotinic ACh receptors can be blocked by which two methods, give an example of a drug that acts by each method?

Competitive blocker (tubocurarine)

Depolarising blocker (succinylcholine)

61

How does competitive blocking of nicotinic ACh receptors by drugs such as tubocurarine work? How can it be overcome?

Blocking of ligand binding sites ---> channel cannot open

Increasing [ACh]

62

How does depolarising blocking of nicotinic ACh receptors by drugs such as succinylcholine work?

Binds to ligand binding sites activating the receptors and causing depolarisation

Stays bound, maintained depolarisation will not activate Na+ channels as they are inactivated

63

What is mayasthenia gravis? What are its symptoms?

Autoimmune disease targeting nAChR

Muscle weakness and fatigue