1

5. Chemical basis of action potentials

a. Sodium hypothesis: (Hodgkin and Katz, 1949)

[Na+]e reduction affects a.p., not EM

Proposed Na+ hypothesis:

a.p due to Na+ influx through momentarily permeable

membrane

2

Principle:

When permeability of membrane increases for a specific ion, EM moves toward EION

resting: K+ permeable, EM close to the -91 mV of EK

+

a.p.: Na+ permeable: EM moves towards+65 (ENa

+)

3

b. Voltage clamp technique:

(1) Set potential for any EM and hold it

Can watch ion movements

(2) Measure ion conductance (g):

rate at which specific ion is crossing the membrane

4

c. Ion movements and permeabilities during an action potential

voltage clamp enables us to see how ions move, which reflects channel activity

5

d. Channel activity

(1) Resting membrane: low gNa+

(2) At threshold: 600 X increase in gNa+

membrane contains Na+ channels

6

Channel structure:

Protein-lined channels: 316 kd, 3 subunits, 12 nm diameter, pore=.3nm (Na+ = .1 nm)

Very specific: Na+ and Li+ only

molecular “gates” on inside to control ion flow

7

Resting: Na+ channels closed

Threshold: voltage at which channels snap open and pass Na+ ions

“voltage gated” channel opening called

“Na+ activation”

8

Na+ diffuses into cell

9

Na+ diffuses into cell

Na+

Na+

++++++++++

-----------------

-----------------

++++++++++

10

Na+ diffuses into cell

Na+

Na+

++++++++++

-----------------

-----------------

++++++++++

11

Na+ diffuses into cell

Na+

Na+

12

Na+ diffuses into cell

“inward Na+ current”

Na+

Na+

+ + + + +

+ + + + +

13

Na+ diffuses into cell

“inward Na+ current”

membrane moves toward ENa+

Na+

Na+

+ + + + +

+ + + + +

14

Na+ entry sustained by positive feedback loop: Hodgkin cycle

15

Na+ entry sustained by positive feedback loop: Hodgkin cycle

DEPOLARIZATION

16

Na+ entry sustained by positive feedback loop: Hodgkin cycle

DEPOLARIZATION

Na+ CHANNEL ACTIVATION

17

Na+ entry sustained by positive feedback loop: Hodgkin cycle

DEPOLARIZATION

Na+ CHANNEL ACTIVATION

INCREASED gNa+

18

Na+ entry sustained by positive feedback loop: Hodgkin cycle

DEPOLARIZATION

Na+ CHANNEL ACTIVATION

INCREASED gNa+

INWARD Na+

CURRENT

19

Na+ entry sustained by positive feedback loop: Hodgkin cycle

DEPOLARIZATION

Na+ CHANNEL ACTIVATION

INCREASED gNa+

INWARD Na+

CURRENT

20

Limit: as membrane depolarizes, a positive electromotive force increases inside the cell

This opposes further Na+ entry

RESULT: Membrane becomes positive inside, Na+ entry slows

21

Membrane polarity is reversed at location of a.p.

Na+ influx at threshold is five fold greater than threshold current

22

+ + + +- - - - - -

+ + + +- - - - - -

+++++++++++++++++++++++++-------------------------------------------

+++++++++++++++++++++++++-------------------------------------------

++++++

++++++

Membrane polarity is reversed at location of a.p.

Na+ influx at threshold is five fold greater than threshold current

23

+ + + +- - - - - -

+ + + +- - - - - -

+++++++++++++++++++++++++-------------------------------------------

+++++++++++++++++++++++++-------------------------------------------

Local current flow to adjacent membrane depolarizes it to threshold

++++++

++++++

Membrane polarity is reversed at location of a.p.

Na+ influx at threshold is five fold greater than threshold current

24

+ + + +- - - - - -

+ + + +- - - - - -

+++++++++++++++++++++++++-------------------------------------------

+++++++++++++++++++++++++-------------------------------------------

Local current flow to adjacent membrane depolarizes it to threshold

++++++

++++++

Membrane polarity is reversed at location of a.p.

Na+ influx at threshold is five fold greater than threshold current

25

+ + + +- - - - - -

+ + + +- - - - - -

+++++++++++++++++++++++++-------------------------------------------

+++++++++++++++++++++++++-------------------------------------------

Local current flow to adjacent membrane depolarizes it to threshold

++++++

++++++

Membrane polarity is reversed at location of a.p.

Na+ influx at threshold is five fold greater than threshold current

26

+ + + +- - - - - -

+ + + +- - - - - -

+++++++++++++++++

----------------------------

++++++

+++++++++++++++++++++++

----------------------------

Local current flow to adjacent membrane depolarizes it to threshold

New Na+ influx in adjacent membrane

Na+

Na+

+ + + + +

+ + + + +

Membrane polarity is reversed at location of a.p.

Na+ influx at threshold is five fold greater than threshold current

27

(3) At spike: 2 events

(a) Na+ channels close

“Na+ inactivation”

gNa+ returns to 0

Na+ channel can not be reactivated (opened) until reset at -80 mV

28

(b) Delayed increase in gK+

K+ channel opens

“delayed K+ activation”

positive emf in cell drives outward K+ current

29

(b) Delayed increase in gK+

K+ channel opens

“delayed K+ activation”

positive emf in cell drives outward K+ current

K+

++++++

++++++

++++++

++++++

30

(b) Delayed increase in gK+

K+ channel opens

“delayed K+ activation”

positive emf in cell drives outward K+ current

K+

++++++

++++++

++++++

++++++

31

(b) Delayed increase in gK+

K+ channel opens

“delayed K+ activation”

positive emf in cell drives outward K+ current

repolarization of membrane

K+

++++++

++++++

++++++

++++++

32

(4) K+ activation maintained briefly after membrane returns to resting

results in brief hyperpolarization to below resting

K+ channels then close while Na+ channels reset to original configuration

33

(5) Resting

Na+ channels reset

K+ channels closed

34

e. Recovery of membrane after action potential

(1) Displaced ions returned to original locations by ion pumps

Pumps constantly running

35

(2) Entire process occurs with minute ion movements

CALCULATE: squid a.p. requires just 160 Na+ ions/µm2 (6 ions/µsec)

changes [Na+]i by 0.0001%.

Squid axon can transmit 50-100,000 action potentials before ion concentration differences are detectable

36

(3) Ion movement through channels is fast but via pumps is relatively slow

K+ activation enables membrane to repolarize immediately without having to wait for ATPase to return ions

37

f. Maximum frequency of action potentials

(1) Frequency of a.p.s determined by ability of Na+ channel to reset

While Na+ channel is being reset (1-2 msec), membrane is “refractory”

Can’t be activated

(2) Prevents fusion of action potentials

Each remains discrete all-or-none event

38

6. Drugs which alter channel function often poison nervous system

39

a. Prevent Na+ activation

“channel blockers”

(1) Tetrodotoxin from pufferfish (fugu)

40

a. Prevent Na+ activation

“channel blockers”

(1) Tetrodotoxin from pufferfish (fugu)

41

(2) Snail/frog toxins: histrionicotoxin

(3) Anesthetics: procaine, cocaine

42

b. Prevent Na+ inactivation leading to persistent depolarization

(1) African scorpion charybdotoxin

(2) Sea anemone toxins

43

b. Prevent Na+ inactivation leading to persistent depolarization

(1) African scorpion charybdotoxin

(2) Sea anemone toxins

44

c. Prevent K+ activation leading to persistent depolarization

(1) Batrachotoxin: blow dart frogs

45

c. Prevent K+ activation leading to persistent depolarization

(1) Batrachotoxin: blow dart frogs

46

7. Propagation of action potentials

47

7. Propagation of action potentials

Na+ influx at threshold is five fold greater than threshold current

Na+

Na+

++++++++++++++++++

+++++++++++++++++++

------------------------------

-------------------------------+

48

Na+

Na+

+++++++++++++

++++++++++++++

----------------------

-----------------------+ +

++++++++

++++ ++++

49

++++++++++++++++

++++++++++++++++

---------------------------

---------------------------

Na+

Na+

K+ + +++++++++

++++ ++++

50

++++++++++++

++++++++++++

--------------------

--------------------

Na+

Na+

K+

++++

++++

------

------+ +

++++++++

++++ ++++

51

++++++++++++

++++++++++++

--------------------

--------------------

Na+

Na+

K+

++++

++++

------

------+ +

Membrane has no inherent directionality, but once a.p. is started, never reverses

+++++++++

++++ ++++

52

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

53

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

54

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

A.P

55

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

56

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

57

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

58

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

59

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

60

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

61

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

62

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

63

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

64

8. Mechanisms to speed propagation of action potentials

a. Velocity increases as function of square root of axon diameter

Giant axons

continuous conduction

65

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

66

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

67

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodesA.P.

68

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

69

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

70

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

71

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

72

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

73

b. Velocity increases with insulation

Myelin

Compressed glial cells

Nodes of Ranvier have access to ECF

and high density of Na+ channels

Local current flow jumps between nodes

saltatory conduction

74

c. Problems with damage to myelin

No regeneration inside CNS

multiple sclerosis

Possible regeneration outside CNS

75

Comparison of saltatory and continuous transmission

GIANTMYELIN

TRANSMISSION Continuous Saltatory

SPEED <50 m/sec >100 m/sec

DIAMETER 50-100 µm 25 µm

ATP CONSUMPTION high 1/5000th

76

E. Communication Between Neurons

1. Sources of neuronal activation

a. environmental energy (sensory)

b. spontaneous depolarization (pacemakers)

c. other neurons (synapses)

77

2. Types of synapses

a. electrical: local current flow

b. chemical: neurotransmitters

Chemicals released which transmit information between neurons

78

Generalized structure of chemical synapses

(e.g. neuromuscular junction)

79

Generalized structure of chemical synapses

80

a. presynaptic cell axon terminal

Generalized structure of chemical synapses

81

b. synapticvesicles

a. presynaptic cell axon terminal

Generalized structure of chemical synapses

82

b. synapticvesicles

a. presynaptic cell axon terminal

c. synaptic cleft

Generalized structure of chemical synapses

83

b. synapticvesicles

a. presynaptic cell axon terminal

c. synaptic cleft

d. postsynaptic cell

Generalized structure of chemical synapses

84

b. synapticvesicles

a. presynaptic cell axon terminal

c. synaptic cleft

d. postsynaptic cell

e. subsynaptic membrane

Generalized structure of chemical synapses

85

3. Transmission across synapses

a. Depolarization of presynaptic cell

86

3. Transmission across synapses

a. Depolarization of presynaptic cell

87

3. Transmission across synapses

b. Increase in inward gCa++ via voltage gated Ca++ channels

Ca++ Ca++

88

3. Transmission across synapses

c. Vesicle migration and exocytosis of neurotransmitters

89

Neurotransmitters

(1) very small amounts

(2) rapidly synthesized and degraded

(3) small, simple molecules

(4) enzymatically synthesized in presynaptic cell

(5) released to synaptic cleft with stimulation

90

d. NT diffusion across cleft

91

e. NT binding and activation of receptors