Electrical Excitability Flashcards Preview

ESA 2- Membranes and Receptors > Electrical Excitability > Flashcards

Flashcards in Electrical Excitability Deck (70):
1

What is an action potential?

A change in voltage across a membrane

2

What does the action potential depend on?

Ionic gradients
The relative permeability of the membrane

3

What is meant by action potentials being ‘all or nothing’?

There is no half, or double, action potentials etc

4

What happens once an action potential has been generated?

It is propagated without loss of amplitude

5

What happens once the membrane has been depolarised to threshold voltage?

Voltage gated Na channels open

6

What is the effect of the opening of voltage gated Na channels?

It allows Na influx as Na ions attempt to move to their equilibrium potential

7

What is the equilibrium potential of Na ions?

+61mV

8

What does the influx of Na ions cause?

It depolarises the membrane further, causing more voltage gated Na channels to open and even more depolarisation

9

By what process does depolarisation cause more Na ion channels to open?

Positive feedback

10

What is the positive feedback causing further Na channels to open to basis of?

The all or nothing characteristic of the action potential

11

What happens in maintained depolarisation?

Na channels close by a mechanisms called inactivation 
Voltage gated K channels are opened

12

What happens when voltage gated K channels are opened?

There is K efflux, as K attempts to move towards its own equilibrium

13

What is the equilibrium potential of K?

-88mV

14

What does the combination of K efflux and Na channel inactivation cause?

Repolarisation of the membrane

15

What is happening in the upstoke of the action potenital?

Na channels open

16

What is the result of the Na channels that open to cause depolarisation being voltage gated?

As the moment potential becomes more positive, positive feedback means that more channels will open until they all are

17

Why can depolarisation not stop half way?

At any voltage above threshold, it will be at a voltage where more channels open, thus causing more depolarisation

18

What is true of Na channels in the absolute refractory period?

Nearly all N channels are in the inactive state

19

What level is excitability at in the absolute refractory period?

0

20

What is true of the Na channels in the relative refractory period?

They are recovering from inactivation

21

What happens to excitability in the relative refractory period?

It returns to normal as the number of channels in the inactivated state decreases

22

What happens as a stimulus gets longer?

A larger depolarisation is necessary to initiate an action potential

23

Why does a longer stimulus mean a larger depolarisation is required to initiate an action potential?

Because Na channels become inactivated during the stimulus

24

How are Na and Ca voltage gated channels similar?

In structure

25

Describe the structure of a Na/Ca voltage gated channel?

They main pore forming subunit is one peptide consisting of four homologous repeats. 
Each repeat consists of 4 transmembrane domains

26

What gives the Na/Ca channel its voltage sensitive properties?

One of the transmembrane domains is able to sense voltage across the membrane

27

What does a functional Na/Ca channel require?

One subunit

28

How do voltage gated K channels compare to Na/Ca channels?

They are similar in structure, except each repeat is actually a separate subunit.

29

Describe the structure of voltage gated K channels

Each subunit has 6 transmembrane domains, one of which is voltage sensitive.

30

What does a functional K channel require?

4 subunits

31

Give an example of a local anaesthetic?

Procaine

32

How do local anaesthetics act?

By binding to and blocking Na channels, thereby stopping action potential generation

33

In what order to local anaesthetics block conduction in nerve fibres?

Small myelinated axons
Non-myelinated axons
Large myelinated axons

34

What is the result of local anaesthetics blocking conduction in nerve fibres in the order that they do?

They tend to effect sensory before motor neurons

35

What kind of molecules are local anaesthetics?

Weak bases

36

How do localised anaesthetics cross the membrane?

In their unionised form

37

When do local anaesthetics block the Na channels?

When the channel is open

38

Do local anaesthetics have a high affinity to the activated or inactivated state of the Na channel?

Inactivated

39

What are electrodes used for in extracellular recording of action potentials?

To raise the membrane to threshold to generate an action potential

40

How can conduction velocity be calculated?

By recording changes in potential between the stimulating (cathode, -ve), and recording (anode, +ve) electrodes along an axon, and using the equation conduction velocity = distance / time

41

What does the depolarisation of a small region of membrane produce?

Transmembrane currents in neighbouring regions

42

What is the result of transmembrane currents being produced in neighbouring regions?

As Na channels are voltage gated, it opens more channels, causing propagation of the action potenital

43

What happens if the local current spreads further?

The conduction velocity of the axon is faster

44

What properties of an axon lead to a high conduction velocity?

A high membrane resistance 
A high axon diameter
A low membrane capacitance

45

Why does a high axon diameter lead to a high conduction velocity?

Has a low cytoplasmic resistance

46

What is Ohm’s Law?

V=IR

47

What does Ohm’s Law state?

The higher the resistance of the membrane, the higher the potential difference across it

48

What does more voltage across the membrane result in?

More voltage gated Na channels being open

49

What is the result of more voltage gated Na channels being opened?

It is easier to reach the threshold to fire an AP

50

What effect does an increase in number of Na channels have on conduction velocity?

It increases it

51

Why does large axon diameter increase conduction velocity?

Ohm’s Law states that the lower the resistance, the larger the current, and therefore the action potential will travel further, increasing action potential

52

What is capacitance?

The ability to store charge

53

What will a membrane with high capacitance do?

Take more current to charge, or a longer time for a given current

54

What happens to a the time taken to charge in membrane with a low capacitance, for a given current?

It is shorter, thus increasing conduction velocity

55

What happens to conduction velocity with myelination of axons?

It increases considerably

56

What kind of axons are myelinated?

Large diameter axons, such as motor neurones

57

What kind of axons are not myelinated?

Smaller ones, such as sensory neurones

58

Why does myelination increase conduction velocity?

It reduces capacitance and increases membrane resistance of the axon

59

What does myelination allow for?

Saltatory conduction

60

What is saltatory conduction?

Where the AP ‘jumps’ between Nodes of Ranvier

61

Why does saltatory conduction occur?

Because the myelin sheath acts as a good insulator, causing the local circuit current to depolarise the next node above threshold and generate an AP

62

What do Nodes of Ranvier have?

High density of voltage gated Na channels

63

How does Na channel density differ from Nodes of Ranvier in unmyelinated axons?

They have an even distribution of channels

64

Which cells myelinate peripheral axons?

Schwann cells

65

Which cells myelinate axons in the CNS?

Oligodendrocytes

66

How do cells myelinate axons?

They envelop axons in their plasmalemma

67

Give an example of a disease that strips areas of some axons of their myelin sheaths

Multiple Sclerosis

68

What type of disease is MS?

Autoimmune

69

What happens in MS?

Myeline is destroyed in certain areas of the CNS

70

What effects can demyelination have?

Can have dramatic effects on the ability of previously myelinated axons to conduct action potentials properly, leading to decreased conduction velocity, complete block or cases where only some action potentials are transmitted