Lecture 4 - Transporters - Channels Flashcards Preview

Neuroscience - Unit 5 > Lecture 4 - Transporters - Channels > Flashcards

Flashcards in Lecture 4 - Transporters - Channels Deck (99):
1

What did the voltage clamp experiments by Hodgkin and Huxley predict?

1) Separate Na and K channels 2) Voltage sensors in channels 3) high conductance of channels to specific ions

2

What helped form Hodgkin and Huxley’s predictions?

voltage clamp experiment indicating how Na+ and K+ currents change with increasing

3

What happens once the patch clamp voltage reaches +52 mV?

that the early inward Na+ current is missing

4

In the voltage clamp experiment, what happens at +65 mV?

it reverses to an outward flow.

5

What happens as the voltage becomes more and more positive?

the later outward K+ current increases in magnitude

6

What does the path-clamp technique allow for?

It allows for characterization of single channels

7

Patch-clamp is

a refinement of the voltage-clamp technique where voltage change activates channel openings.

8

Who developed the patch clamp?

developed by Sackman and Neher (Nobel Prize winners).

9

What is the patch-clamp technique?

Glass pipette is pressed against a cell membrane – slight suction is applied to generate a ‘gigaseal’ (low noise).

10

What is the purpose of the gigaseal?

All current flows through electrode and does not leak through the seal.

11

Describe the current in the patch clamp technique?

1) Macroscopic currents ~10-100 picoAmps (pAs) due to current flow through many channels 2) Microscopic current amplitude ~fraction of pA to several pAs due to current flow through one channel (lower right panel).

12

What is the macroscopic current flow due to?

Current flow through many channels

13

What is the microscopic current flow due to?

Current flow through one channel

14

Patch clamp recordings

it detects current flowing through single membrane channels due to depolarization

15

Describe the channels in the patch clamp experiment.

1) channels open and close in an all or none fashion 2) there is fast switch between open and close states 3) channels open and close in stochastic (random) manner

16

In the patch clamp experiment, what does gating refer to?

1) Gating is the transition between open and closed states 2) gating involves a temporary conformational change in the channels structure

17

In the patch clamp, what happens in response to the depolarizing effect from the pipette?

single channels open and close in an all or none fashion. Random or stochastic in nature

18

In patch clamp, what does the probability of opening depend on?

The stimulus; 1) voltage change or 2) ligand binding

19

What does the patch clamp measurements of ionic currents through single Na channels reveal?

1) voltage gated Na channels 2) depolarization increases the probability of a channel being open and hyperpolarizing decreases it

20

Depolarizing stimulus

increases the probability that the Na+ channel is opened.

21

The greater the depolarization

the higher the probability of channel opening.

22

For patch clamp looking at Na channels what happens to K+ channels?

they were blocked in this experiment to look at Na channels. Therapeutic drugs that act on ion channels are now being tested using this technique.

23

What is the patch clamp measurements of inward ionic currents through single Na channels vs the cell?

Macroscopic current arises from the aggregate effects of 1000s of microscopic currents (individual channels)

24

Stimulus (membrane potential depolarization of patch)

changes the probability that channel is open or closed.

25

Comparing the time course of the macroscopic current and the sum of many trials of the single ion channel show what?

close correlations of time courses of the macroscopic and microscopic currents

26

Is channel opening controlled in the patch clamp experiment?

Random or stochastic opening of channels

27

Probability of opening

increases with depolarization

28

Microscopic current

single channel

29

Macroscopic current

summed activity of 1000s of Na+ channels (K+ channels blocked).

30

Compare the Na and K channel data from the patch clamp experiment.

opposite current direction, longer latency for activation and long duration of activation for the K+ channel vs the Na+ channel properties.

31

The sum of many microscopic trials approximates what?

the time course of the macroscopic currents from the whole cell.

32

Sustained response (patch clamp)

on average the K+ channels tend to be an open state while the membrane is depolarized.

33

K+ channels diversity.

Multiple types of voltage gated K+ channels exist that have different properties and influence neuron firing.

34

Microscopic and macroscopic currents

Properties of microscopic currents (patch clamp) are the same as those of macroscopic currents

35

Na channels

1) opening is voltage dependant 2) opening near beginning of depolarization pulse 3) inactivate 4) current reverses at Na equilibrium potential 5) TTX blocks

36

K channels

1) opening is voltage-dependant 2) opens later 3) many do not inactivate, they just close 4) TEA or (Cs) blocks it

37

K channels in the CNS

most CNS neurons have multiple Potassium channels with different characteristics

38

K channel diversity as it pertains to voltage

voltage dependence of activation (low voltage versus high voltage activation)

39

K channel diversity as it pertains to rate?

Diversity in the rate of activation (How fast the population reaches maximum conductance)

40

K channel diversity as it pertains to inactivation

inactivation properties, some inactivate quickly, some inactivate slowly, some don’t inactivate, this produces a diversity of spike waveforms and spike patterns for different cells

41

Functional roles of the after hyperpolarization

1) Fast AHP 2) Medium AHP 3) Slow AHP

42

Fast AHP

1) (2-5 ms) shortens the AP by quickly repolarizing the membrane. 2) Only affects early spike frequency at very high frequencies 3) BK K channels, activation by Ca and depolarization and then rapid inactivation

43

Medium AHP

1) (10-100 ms) controls early interspike interval 2) contributes to early spike frequency adaptation, slowly activating by Ca entry 3) controls late spike frequency adaptation (IK and SK, K channels, non inactivating)

44

Slow AHP

(100ms – 3000ms) Limits firing frequency by an unknown channel

45

For the functional roles of AHP, which are rapid inactivation and which are non-inactivating?

1) Fast AHP = rapid inactivation 2) Medium AHP = non-inactivating

46

In some types of neurons what is the role of voltage gated Ca channels?

They result in bursts of APs that may last 100ms or longer

47

Channel timings

1) Na channels open 2) Na channels inactivate, Ka+ and Kdr+ channels open 3) Kbk+ channels open 4) Ca channels open 5) other known and unknown K+ channels open

48

What results in neurons with diverse electrical properties?

Larger number of ion channel genes

49

Voltage gated channels typically

allow only a single type of ion to pass through the channel although there are exceptions.

50

Ligand gated ion channels often

allow two or more types of ions to pass through the channel.

51

Ionic channels are organized based on

sequence homology

52

Voltage dependant ion channels differ in

their cellular expression and subcellular localization impacting their relative contribution to brain function

53

Kv4.1

these channels play a positive role in tumorigenic human mammary cells.

54

What does double immunofluorescence staining for Kv1.4 and Kv2.1 in the adult hippocampus show?

1) Staining for Kv1.4 is red and are axons 2) Staining for Kv2.1 is green and are soma proximal dendrites

55

In adult hippocampus, Kv1.4 staining?

In terminal fields of the medial perforant path in the middle molecular layer of the dentate gyrus and mossy fiber axons and terminal s. lucidum of CA-3

56

In adult hippocampus, Kv2.1 staining?

It is most prominent in the pyramidal cell CA-1 layer

57

Why are there so many genes encoding K+ channels?

So the genes can differ in: 1) activation 2) gating 3) inactivation

58

What does the diversity of K channels allow?

Influence the duration of AP and resting membrane potential

59

The Kv2.1 channels

show little inactivation and are related to channels involved in repolarization.

60

The Kv4.1 channels

inactivate rapidly to depolarization.

61

The inward rectifier channels

allow more current flow during hyperpolarization than during depolarization.

62

The Ca++ activated K+ channel

opens in response to increased intracellular Ca++ and sometimes to membrane depolarization.

63

Ion channels encoded by

large and diverse families of homologous genes

64

Ion channel differences

they differ widely in cellular expression and subcellular localization

65

Voltage gated channel differences

different voltage gated channels differ in functional properties (activation, inactivation and gating)

66

Ion channels contribute to

rich electrical responses

67

Ion channel diversity

is key to developing new therapeutics for central nervous system disorders

68

Channelopathies

they are genetic diseases resulting from mutations in channel genes

69

Channelopathies, voltage gated Ca channels

1) congenital stationary night blindness 2) familial hemiplegic membrane 3) episodic ataxia type 2

70

Channelopathies, Na channel defect

generalized epilepsy with febrile seizures

71

Channelopathies, K channel mutations

benign familial neonatal convulsion

72

Toxins target what sites on ion channels

extracellular domains and pore regions

73

Tetrodotoxin

(puffer fish) block Na channels

74

Saxitoxin

(red tide) a homologue of TTX

75

Alpha toxins

(scorpion) prolong duration of Na currents

76

Beta toxins

(scorpion) shift voltage activation of Na channels

77

Batrachotoxin

(frogs) inactivation of Na channels (used by South American indians)

78

Dentrotoxin

(wasps) K+ channel blockers

79

Amapin

(bees) ?

80

Omega – conotoxins

(cone snails) – N-type Ca channels

81

Omega – agatoxin

(spiders) P/Q – type Ca channels

82

Active ion transporters are

membrane proteins that create and maintain ion gradient

83

Active ion transporters that translocate what?

Translocate ions against their electrochemical gradient (consume energy)

84

Active ion transporters form what?

Form complex with ion they transport

85

Active transport; binding and unbinding

is slow (ms)

86

Active transport versus channels

ion translocation is slower in transporters than in channels (1000/sec)

87

ATPase pumps (Na/K, Ca)

acquire energy from hydrolysis of ATP

88

Ion exchangers and co-transporters

depend on the electrochemical gradient of other ions as other sources

89

Ion exchangers

trade an intracellular ion for an extracellular ion, e.g., Na/Ca, Na/H and do not use ATP as an energy source.

90

Ion co-transporters

transport two or more ions/molecules in the same direction across the membrane.

91

Ion channels regulate

the flow of ions across the membrane, influencing cell activities

92

Channels differ in

ion selectivity and in factors that control their gating

93

Ion selectivity

is achieved through interactions between the ion and the amino acids that line the walls of the channel pore (selectivity filter)

94

Patch clamp technique

measure current flow through single open channels

95

Gene cloning

determine the sequence of genes that encode channels

96

X-ray crystallography

provided detailed 3D structure of the bacterial K channel

97

Channels are targets of

blockers, toxins, and various diseases resulting from genetic mutations

98

Active ion transporters are

membrane proteins that create and maintain ion gradients using ATP as the energy source

99

Ion exchangers

use the electrochemical gradients of co-transported ions as an energy source to exchange ions