1 5. Chemical basis of action potentials a. Sodium hypothesis: (Hodgkin and Katz, 1949) [Na + ] e reduction affects a.p., not E M Proposed Na + hypothesis: a.p due to Na + influx through momentarily permeable membrane

2 Principle: When permeability of membrane increases for a specific ion, E M moves toward E ION resting: K + permeable, E M close to the -91 mV of E K + a.p.: Na + permeable: E M moves towards +65 (E Na + )

3 b. Voltage clamp technique: (1) Set potential for any E M 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

10 Na + diffuses into cell Na

11 Na + diffuses into cell Na +

12 Na + diffuses into cell “inward Na + current” Na

13 Na + diffuses into cell “inward Na + current” membrane moves toward E 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

Membrane polarity is reversed at location of a.p. Na + influx at threshold is five fold greater than threshold current

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

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

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

Local current flow to adjacent membrane depolarizes it to threshold New Na + influx in adjacent membrane 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+K

30 (b) Delayed increase in gK + K + channel opens “delayed K + activation” positive emf in cell drives outward K + current K+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+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/µm 2 (6 ions/µsec) changes [Na + ] i by %. Squid axon can transmit ,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

48 Na

Na + K+K

Na + K+K

Na + K+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 nodes A.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 TRANSMISSIONContinuousSaltatory SPEED 100 m/sec DIAMETER µm25 µm ATP CONSUMPTION high1/5000th

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. synaptic vesicles a. presynaptic cell axon terminal Generalized structure of chemical synapses

82 b. synaptic vesicles a. presynaptic cell axon terminal c. synaptic cleft Generalized structure of chemical synapses

83 b. synaptic vesicles a. presynaptic cell axon terminal c. synaptic cleft d. postsynaptic cell Generalized structure of chemical synapses

84 b. synaptic vesicles 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 ++

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