Action Potential-Mammalian

Front 1

1. Define polarization.

Back 1

Charges are separated across the plasma membrane, so the membrane has potential to do work. Anytime the value of the membrane potential is not 0mV, in either positive or negative direction, the membrane is in a state of polarization.

Front 2

2. Define depolarization.

Back 2

A change in potential that makes the membrane less polarized (less negative) than at resting potential. Depolarization decreases membrane potential, moving it closer to 0 mV (going from -70mV to -60mV), fewer charges are separated than at resting potential.

Front 3

3. Define repolarization?

Back 3

The membrane returns to resting potential after having been depolarized.

Front 4

4. Define hyperpolarization.

Back 4

A change in potential that makes the membrane more polarized (more negative) than at resting potential. Hyperpolarization increases membrane potential moving it even farther from 0mV (from -70mV to -80mV), more charges are separated than at resting potential.

Front 5

5. How are changes in membrane potential brought about?

Back 5

They are brought about by changes in ion movement across the membrane. If the net inward flow of positively charged ions increases compared to the resting state, the membrane becomes depolarized (less negative inside).

Front 6

6. If the net outward flow of positively charged ions increases compared to the resting state, the membrane becomes hyperpolarized(more negative inside). True/False

Back 6

True.

Front 7

7. List 4 triggering events.

Back 7

1. A stimulus such as sound waves stimulating specialized neural endings in the ear.

2. A change in the electrical field in the vicinity of an ion channel within the membrane of an excitable cell.

3. An interaction of a chemical messenger with a surface receptor on a neuron or muscle cell membrane.

4. A spontaneous change of potential caused by inherent imbalances in the leak pump cycle.

Front 8

8. What are the two types of general channels found on the membrane?

Back 8

1. Leak channels- (nongated channels), these are open all the time.

2. Gated channels- which can be opened or closed in response to specific triggering events.

Front 9

9. Can it be said that triggering events alter membrane permeability?

Back 9

Yes. Consequently ionic flow occurs across the membrane by opening or closing the gates guarding particular ionic channels.

Front 10

10. Describe physically the gated channels.

Back 10

They have movable folds in the proteins that can alternately open, permitting ion passage through the channel, or closed preventing ion passage through the channel.

Like many proteins, these channels are inherently flexible molecules whose conformations can be altered in response to external factors.

Front 11

11. There are three or possibly four gated channels depending on the factor that induces the change in channel conformation. What are they?

Back 11

1. 1. Voltage gated ion channels, which open or close in response to changes in membrane potential.

2. Chemically gated channels, (ligand gated), which change conformation allosterically in response to the binding of a specific chemical messenger with a membrane receptor that is in close association with the channel(signal transduction channels)

3. Mechanically gated channels, which respond to stretching or other forms of mechanical deformation.

There may be a fourth channel which opens or closes when a phosphate group is covalently added by a kinase.

Front 12

12. There re 2 basic forms of electrical signals. What are they?

Back 12

1. Graded potentials, which are short distance signals

2. Action potentials, which signal over longer distances.

Front 13

13. What are graded potentials?

Back 13

These are local changes in membrane potential that occur in varying grades or degrees of magnitude or strength. membrane potential could change from -70mV to -60mV. This represents a 10mV change in membrane potential.

Front 14

14. Graded potentials are usually produced by a specific triggering event that causes gated ion channels to open in a specialized region of the excitable cell membrane. True/False

Back 14

True. Gated sodium channels open in response to the triggering event leading to the inward movement of sodium down its concentration and electrical gradient. This graded potential is confined to this small specialized region of the total plasma membrane.

Front 15

15. What is a membrane active area?

Back 15

When a graded potential occurs locally in a neuron or muscle cell membrane, the potential in this area differs from that in the remainder which is still at resting potential. this is the active area or region.

Front 16

16. Any flow of charges is called a current. True/False

Back 16

True. The direction of current flow is always designated by the direction in which the positive charges are moving.

Front 17

17. What is resistance?

Back 17

Resistance is the hindrance to electrical charge movement. The greater the difference in potential the greater the current flow.

Front 18

18. The plastic that surrounds electrical wires have high resistance, as do body lipids. True/False

Back 18

True. Current does not flow across the plasma membranes lipid bilayer. It is carried across only through channels.

Front 19

19. Current leaks out of an electrical wire unless the wire is covered with an insulating material. True/False

Back 19

True. Current likewise is lost across the cell membrane as charge carrying ions leak out through the uninsulated parts of the membrane, through open channels. Because of the current loss the magnitude of the local current slowly diminishes with increasing distance from the initial site of origin.

Front 20

20. The spread of a graded potential is decremental. True/False

Back 20

True. This form of electrical depolarization is termed passive conduction. This is typical of sound waves.

Front 21

21. What does Ohm's law state?

Back 21

This law states that the reduction in voltage across a membrane in response to a current flowing through the cell is directly related to the amount of current multiplied by the resistance of the membrane.

Front 22

22. The lipid bilayer is an effective capacitor because it can maintain the separation of charged ions across a relatively narrowed space. True/False

Back 22

True.

Front 23

23. List 5 types of graded potentials.

Back 23

1. Post-synaptic potentials

2. Receptor potentials

3. End plate potentials

4. Pacemaker potentials

5. Slow wave potentials.

Front 24

24. Define exactly an action potential.

Back 24

These are brief, rapid large, (100mV) changes in membrane potential during which the potential actually reverses so that the inside of the excitable cell transiently becomes more positive than the outside.

Front 25

25. How are graded potentials different from action potentials?

Back 25

Action potentials are conducted, or propogated throughout the membrane in nondecramental fashion. They do not diminish in strength as they travel through their site of initiation throughout the remainder of the cell membrane

Front 26

26. How is an action potential initiated?

Back 26

A triggering event causes the membrane to depolarize from the resting potential of - 70mV. Until threshold potential is reached, between -50 and -55mV or 10-15 mV above resting potential.

Front 27

27. How would a recording of the above action potential event appear?

Back 27

It would show a sharp upward deflection to +30mV as the potential rapidly moves towards 0mV, then reverses itself so that the inside of the cell becomes positive compared to the outside.

Front 28

28. What is hyperpolarization?

Back 28

The forces responsible for driving the membrane back to resting potential push it too far resulting in a transient hyperpolarization, during which the inside of the membrane briefly becomes even more negative than normal.

Front 29

29. The action potential is an electrical waveform composed of depolarization, repolarization and hyperpolarization. TRue/False

Back 29

True.

Front 30

30. About how long does an action potential last in a neuron?

Back 30

It lasts for about 1 msec. It lasts longer in muscle, with the duration varying depending on the myscle type.

Front 31

31. What is Overshoot?

Back 31

This is the portion of the action potential during which the potential is reversed(between 0mV and +30mV. An action potential is also called a spike

Front 32

32. Potassium contributes the most to establishing the resting potential because the membrane at rest is much more permeable to K+ than to Na+. True/False

Back 32

True.

Front 33

33. Why is it that during an action potential marked changes in membrane permeability take place?

Back 33

Marked changes in membrane permeability to sodium and potassium take place because of voltage gated channels, permitting rapid fluxes of these ions down their electrochemical gradients.

Front 34

34. Voltage gated channels consist of proteins that have a number of charged groups. True/False

Back 34

True. The electric field surrounding the channels can distort the channel structure as charged portions of the channel proteins are electrically attracted or repelled by charges in the fluids surrounding the membrane. Small distortions in channel shape induced by potential changes can cause them to alternate to another conformation.

Front 35

35. All vertebrates and those invertebrates evolved for speed have comparatively fast opening Na+ channels, in contrast, sluggish invertebrates such as sea slugs have Na+ channels that open at rates approx. 10X slower. True/False

Back 35

True.

Front 36

36. The voltage gated Na+ channels have two gates. What are they called?

Back 36

Activation gate

Inactivation gate

Front 37

37. What does the activation gate do?

Back 37

It guards the channel by opening and closing like a hinged door.

Front 38

38. Describe the inactivation gate.

Back 38

This consists of a ball and chain like sequence of amino acids. This gate is open when the ball is dangling free, and closed when the ball binds to its receptor located at the channel opening.

Front 39

39. Must both gates be open to permit the passage of sodium?

Back 39

Yes. The closure of either gate will inhibit this passage.

Front 40

40. The basic voltage gated K+ channelof excitable tissues is simpler. True/False

Back 40

True. It has only one gate which can be either open or closed.

Front 41

41. Passage of sodium and potassium is prevented through these channels at resting potential. True/False

Back 41

True. Because of the presence of many potassium leak channels the resting membrane is 50-75 times more permeable to potassium than to sodium.

Front 42

42. Explian the explosive positive feedback cycle.

Back 42

At the triggering event electrical gradients for sodium favor its movement into the cell, and Na+ starts to move in. The inward movement of positively charged sodium depolarizes the membrane further, opening adjacent voltage gated sodium channels and always allowing more sodium to enter.

At threshold potential, the positive feedback ensures an explosive increase in Na+ permeability which is symbolized as PNa+ as the membrane becomes 600 times more permeable to Na+ than to K+.

Front 43

43. As sodium rushes in, the potential reaches +30mV which is close to Na+ equilibrium potential. True/False

Back 43

True. The potential does not become any more positive, because at the peak of then action potential, the sodium channels close to the inactivated state and PNa+ falls to its low resting value.

Front 44

44. What causes the PK+ to become 300X more permeable than the PNa+?

Back 44

The return to resting potential is hastened by the opening of K+ gates at the peak of the action potential.Opening of the voltage gated K+ channels greatly increases the K+ permeability(*PK+) to about 300 times the resting PNa+.

Front 45

45. What is tetrodotoxin?

Back 45

This toxin has porovided evidence for the specificity of the voltage dependent ion channels and their role in the propagation of an action potential. It is found in the Japanese pufferfish. It also has been identified in some crabs, tunicates, snails and algae.

Front 46

46. Pufferfish have no genes related to TTX synthesis. True/False

Back 46

True. They cannot make it if grown in isolated aquaria. Perhaps symbiotic bacteria manufacture the toxin for their hosts. The blue ringed octopyus of Australia harbor these bacteria. Perhaps a peaceful, beneficial coexistance has evolved between these bacteria andf their hosts.

Front 47

47. The TTX made by the bacteria is used by some of these organisms as defense(algae and pufferfish) and by others to kill prey. TRue/False

Back 47

True.

Front 48

48. TTX is one of the most potent blockers of animal sodium channels known. True/False

Back 48

True.

Front 49

49. How do the host animals survive their bacterial partners?

Back 49

Genetic analysis reveals that the pufferfish has a single point mutation in its sodium channel gene, rendering the channel protein resistant to TTX binding. Garter snakes trhat eat TTX bearing newts have been found to have these as well.

Front 50

50. There are app. 8 descriptive phases of action potential in summary. Describe phase 1.

Back 50

At resting potential all voltage gated Na+ and K+ channels are closed. Only unbalanced charges separated across the membrane contribute to potential.

Front 51

51. Describe phase two of AP.

Back 51

After a depolarizing triggering event brings the membrane to the threshold potential of -50mV, the sodium activation gates open.

Front 52

52. Describe phase three of the AP.

Back 52

The resultant movement of sodium inward neutralizes negative charges inside the cell(by attaching to them). This inward movement of Na+ leaves behind on the outside the negative charges(primarily Cl-) with which sodium had been paired.

Front 53

53. Regarding phase three, these negative charges neutralize positive charges that had been contributing to membrane potential, making the outside progressively less positive. True/False

Back 53

True.

Front 54

54. Describe phase 4 of the AP.

Back 54

The continued inward movement of sodium progressively reduces the potential( the inside becoming less negative and the outside less positive) until 0 mV is reached.

Front 55

55. Describe phase 5 of the AP.

Back 55

Continued inward movement of sodium reverses the potential with the inside becoming positive and the outside becoming negative as the action potential peaks.

Front 56

56. Regarding phase 5, what other changes occur at the peak of the action potential?

Back 56

The sodium inactivation gates begin to close and the potassium gates open. The entry of sodium stops, and K+ starts to exit the cell.

Front 57

57. Describe phase six of the AP.

Back 57

The outward movement of K+ leaves behind negative charges(A-) inside the cell and neutralizes negative charges outside. As a consequence, the inside becomes progressively less positive and the outside less negative until 0mV is reached.

Front 58

58. Describe phase seven of the AP.

Back 58

Continued outward movement of K+ restores the resting membrane potential, with the potential reversing back, so that the inside is once again negative and the outside positive. Further outward movement of potassium through the still open potassium gates briefly hyperpolarizes the membrane.

Front 59

59. Describe phase eight of the AP.

Back 59

After hyperpolarization is complete, there is a return to resting potential. The potassium gates close, and the membrane returns to resting potential.

Front 60

60. A single nerve cell or neuron consists of three basic parts; what are they?

Back 60

The cell body

Dendrites

Axon

Front 61

61. Dendrites carry signals towards the cell body. True/False

Back 61

True. In most neurons the plasma membrane of the cell body and the dendrites have protein receptors for binding chemical messengers(neurotransmitters) from other neurones. The dendrites and the cell body are part of the input zone.

Front 62

62. what is the function of the axon?

Back 62

The axon or nerve fiber, is a single elongated tubular extension that conducts action potentials away from the cell body and terminates at other cells.

Front 63

63. What is a nerve as opposed to a neuron?

Back 63

It is a bundle of axons outside the CNS , whereas a fiber tract is a bundle of axons inside the CNS.

Front 64

64. What is the axon hillock?

Back 64

The very first portion of the axon plus the region of the cell body from which the axon leaves is called the axon hillock. This is the neuron's trigger zone.

Front 65

65. The axon is the conducting zone of the neuron, whereas the axon terminals constitute the output zone. True/False

Back 65

True. The axon of the nerve cell innervating a mammalian foot must travel the distance between the origin of its cell body within the posterior spinal cord all the way down the leg to the foot.

Front 66

66. How does cocaine exert its effect?

Back 66

Cocaine blocks reuptake of the neurotransmitter dopamine at presynaptic terminals by competitively binding with the dopamine reuptake transporter, a protein molecule that picks up released dopamine from the synaptic cleft and shuttles it back to the axon terminal.

Front 67

67. With cocaine occupying the dopamine transporter, dopamine remains in the synaptic cleft longer. True/False

Back 67

True. As a result it continues to react with its post-synaptic receptor sites. These pathways play a role in emotional responses, especially feelings of reward. The neural switches in the neural pathway are locked on the "on" position.

Front 68

68. Cocaine causes permanent molecular changes of the involved neurons. True/False

Back 68

True. As a result they cannot transmit normally across synapses without increasingly higher doses.

Front 69

69. What occurs after the neurons have been incessantly stimulated for an extended time?

Back 69

A transcription factor called FosB is activated by unknown means. this activates the gene for an enzyme called cyclin dependent kinase 5 or Cdk5. Cdk5 reduces sensitivity for cocaine.

Front 70

70. What happens when cocaine is no longer available?

Back 70

The sense of reward cannot be reached, because the normal level of dopamine release is no longer sufficient to stimulate the post-synaptic neuron.

Front 71

71. What is the basic nerve mechanism of strychnine?

Back 71

S. competes with an inhibitory neurotransmitter, glycine, at the post synaptic receptor site. The antagonist combines with the receptor but does not directly alter the potential of the synaptic cell.

Front 72

72. Strychnine blocks the receptor so that glycine cannot bind when released from the inhibitory presynaptic ending. True/False

Back 72

True. Post synaptic inhibition is abolished in nerve pathways that use glycine as an inhibitory transmitter. This can lead to convulsions and death.

Front 73

73. How does tetanus toxin work?

Back 73

This prevents the release of another inhibitory transmitter, Gamma amino butyric acid(GABA) from presynaptic inputs terminating on neurons that supply skeletal muscles. This results in uncontrolled muscle spasms.

Front 74

74. Where does the term "lockjaw" come from?

Back 74

These uncontrolled muscle spasms occur especially in the jaw muscles early in the disease. Eventually respiratory muscles will be involved and death can follow.

Front 75

75. Can any pollutants inhibit neural signalling?

Back 75

Yes. Lead is an important environmental toxicant, because it competes with calcium for entry into the terminal button.

Front 76

76. How does black widow venom exert its effect?

Back 76

It usually kills small animals, not humans. It causes an explosive release of Ach from the storage vesicles, not only at neuromuscular junctions, but at all cholinergic sites. The toxin is an agonist.

All cholinergic sites undergo prolonged depolarization. Respiratory failure is a common sequela.

Front 77

77. In black widow spider venom, want is depolarization block?

Back 77

The voltage gated sodium channels are trapped in their inactivated state, prohibiting the initiation of new action potentials and resultant contraction of the diaphragm.

Front 78

78. Where is curare obtained from, and what is its mechanism of action?

Back 78

C. is obtained from the skin of certain frogs in South America. It reversibly binds to the Ach receptor sites on the motor end plate. When receptor sites are occupied by curare, ACh cannot combine with these sites to open the channels that would permit the ionic movement, responsable for the EPP. Paralysis ensues.

Front 79

79. Curare has been used medically and in surgical procedures to help achieve more complete skeletal muscle relaxation. True/False

Back 79

True.

Front 80

80. Prozac selectively inhibits reuptake of sodium. True/False

Back 80

True. It is also known as 5 hydroxytryptamine.

Front 81

81. Has Prozac been used to treat cribbing in horses?

Back 81

Yes. Cribbing is forced swallowing gulps of air.

Front 82

82. What are tricyclic antidepressants?

Back 82

So called because of their 3 ringed chemical structure. They block the reuptake of serotonin and catecholamines from synapses. This promotes a state of calmness. They have been used to urine spraying in cats, urine marking in dogs as well as separation anxiety.

Front 83

83. What are stereotypies?

Back 83

These are repetitive behaviors that have no valuable function, such as pacing in zoo animals.

Front 84

84. Neuroleptics represent a family of drugs whose mode of action is the antagonism of dopamine. True/False

Back 84

True. They are used in the treatment of phobias and anxieties.

Front 85

85. Neuroleptics are used for flank biting in horses. True/False

Back 85

True. They have also been employed for feather picking in birds.