The Action Potential

Question 1

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

Answer 1

A rapid change in the membrane potential

0.5ms in an axon

Question 2

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

Answer 2

Only occurs if a threshold level is reached

Question 3

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

Answer 3

At the axon hillock

Question 4

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

Answer 4

Towards

Question 5

What causes an action potential to start?

Answer 5

A large increase in permeability to Na+ ions

Question 6

What two channels are involved in an action potential?

Answer 6

Voltage dependent Na+ channels

Voltage dependent K+ channel

Question 7

What does depolarisation do to voltage gated channels?

Answer 7

Open them

Question 8

What does hyperpolarisation do to voltage dependent channels?

Answer 8

Close them

Question 9

What happens to Na+ channels after they open?

Answer 9

They inactivate

Question 10

Do K+ channels inactivate?

Answer 10

No

Question 11

K+ channels are ______ in their closing causing ______

Answer 11

Delayed

Hyperpolarisation

Question 12

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

Answer 12

A small amount

Question 13

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

Answer 13

Voltage clamps

Question 14

Depolarisation is caused by…

Answer 14

An influx of sodium ions

Question 15

Repolarisation is caused by….

Answer 15

Inactivation of sodium channels

(Slower) opening of K+ channels

Question 16

Hyperpolarisation is caused by…

Answer 16

Increased K+ conductance

Delayed closing of K+ channels

Question 17

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

Answer 17

ARP - Absolute refractory period (1ms)

RRP - Relative refractory period (4ms)

Question 18

What happens during the ARP?

Answer 18

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

Question 19

What happens during RRP?

Answer 19

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

Question 20

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

Answer 20

1 peptide —-> 1 alpha subunit

Question 21

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

Answer 21

4 alpha subunits

Question 22

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

Answer 22

4

Question 23

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

Answer 23

1

Question 24

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

Answer 24

Many positives amino acids, acts as a voltage sensor

Question 25

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

Answer 25

An inactivation particle between repeats 3 and 4

Question 26

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

Answer 26

The pore region

Question 27

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

Answer 27

Na+ channels Procaine

Question 28

Local anaesthetics block different axons in which order?

Answer 28

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

Question 29

What is the local current theory?

Answer 29

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

Question 30

What is the length constant?

Answer 30

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

Question 31

_______ membrane resistance = increased conduction velocity/spread of charge

Answer 31

Increased

Question 32

________ membrane capacitance = increased conduction velocity/spread of charge

Answer 32

Decreased

Question 33

What is capacitance?

Answer 33

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

Question 34

What does resistance of a membrane depend upon?

Answer 34

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

Question 35

Why does high capacitance result in decreased conduction velocity?

Answer 35

Voltage changes more slowly in response to current injection

Question 36

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

Answer 36

Spreads further along the axon

Question 37

What is responsible for the propagation of the action potential?

Answer 37

Local currents

Question 38

How does myelination increase the conduction velocity?

Answer 38

Increases membrane resistance | Decreases membrane capacitance

Question 39

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

Answer 39

Schwann Cells

Question 40

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

Answer 40

Oligodendricytes

Question 41

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

Answer 41

Na+ channels

Question 42

How are Na+ channels distributed along an unmyelinated axon?

Answer 42

Evenly

Question 43

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

Answer 43

The jumping of the local circuit current from node to node Increased conduction velocity

Question 44

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

Answer 44

Proportional

Question 45

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

Answer 45

Square root

Question 46

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

Answer 46

Multiple sclerosis Affects all cns nerves Poorer transmission of action potentials (doesn't reach threshold)

Question 47

What is a neuromuscular junction?

Answer 47

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

Question 48

What channels are present at NERVE TERMINALS?

Answer 48

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

Question 49

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

Answer 49

Calcium into the cell | Release of neurotransmitter

Question 50

What does an increased frequency of action potentials result in?

Answer 50

Increases calcium entry at nerve terminals More transmitter released Bigger response

Question 51

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

Answer 51

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

Question 52

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

Answer 52

Blocks L type calcium channels High blood pressure

Question 53

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

Answer 53

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

Question 54

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

Answer 54

More slowly At more positive membrane potentials Inactivate more slowly as well

Question 55

What is voltage gated calcium channel inactivation dependent on?

Answer 55

The intracellular concentration of calcium

Question 56

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

Answer 56

Synaptotagmin Vesicle is brought close to the membrane?

Question 57

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

Answer 57

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

Question 58

How are nicotinic acetylcholine receptors activated?

Answer 58

Binding of two molecules of ACh ---> conformational change Channels opens to Na+ and K+ ions

Question 59

How does crurare cause paralysis?

Answer 59

Blocks transmission between nerve and muscle

Question 60

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

Answer 60

Competitive blocker (tubocurarine) Depolarising blocker (succinylcholine)

Question 61

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

Answer 61

Blocking of ligand binding sites ---> channel cannot open Increasing [ACh]

Question 62

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

Answer 62

Binds to ligand binding sites activating the receptors and causing depolarisation Stays bound, maintained depolarisation will not activate Na+ channels as they are inactivated

Question 63

What is mayasthenia gravis? What are its symptoms?

Answer 63

Autoimmune disease targeting nAChR Muscle weakness and fatigue