Florida International University

PCB 2099

Principles of Human Physiology, 5e (Stanfield) Chapter 7 Nerve Cells and Electrical Signaling

Multiple Choice Questions

1)Which of the following correctly lists the components of the central nervous system?

1. 1. brain and nerves
   2. brain and spinal cord
   3. spinal cord and nerves
   4. brain, spinal cord, and nerves
   5. sympathetic and parasympathetic nervous system

2. Which of the following is located in the autonomic nervous system?
   1. sympathetic nervous system only
   2. parasympathetic nervous system only
   3. somatic nervous system only
   4. both sympathetic and parasympathetic nervous systems
   5. both sympathetic and somatic nervous systems

3. Which of the following accurately describes afferent neurons?
   1. They transmit information from the periphery to the CNS.
   2. The cell body is located in the ventral horn of the spinal cord.
   3. They are the most abundant class of neurons.
   4. They transmit information from the CNS to the periphery.
   5. They are typically multipolar neurons.

4. What is the functional unit of the nervous system?
   1. neurons
   2. glial cells
   3. the central nervous system
   4. axons
   5. the brain

5. On what portion of the neuron do action potentials propagate?
   1. soma
   2. cell body
   3. dendrite
   4. axon
   5. nucleus

6. Information is transmitted from cell to cell across the chemical synapse via a(n)                                  .
   1. neurotransmitter
   2. action potential
   3. graded potential
   4. collateral potential
   5. neurosecretory hormone

7. An action potential originates at the                            and travels along the axon until it reaches the

                     .

1. 1. axon terminal : axon hillock
   2. dendrite : axon terminal
   3. axon hillock : dendrite
   4. dendrite : axon hillock
   5. axon hillock : axon terminal

8. What type of ion channels in the membrane of neurons allow ions to move across the membrane at rest and thereby contribute to resting membrane potential?
   1. voltage-gated channels
   2. resting channels
   3. potential-gated channels
   4. leak channels
   5. ligand-gated channels

9. What type of ion channels in the membrane of neurons open or close in response to a neurotransmitter binding to its receptor?
   1. voltage-gated channels
   2. synaptic channels
   3. potential-gated channels
   4. leak channels
   5. ligand-gated channels

10. In a neuron, where is the greatest concentration of voltage-gated sodium and voltage-gated potassium channels?
    1. dendrites
    2. soma
    3. axon hillock
    4. axon
    5. axon terminal

11. In a neuron, where are voltage-gated calcium channels located?
    1. dendrites
    2. soma
    3. axon hillock
    4. axon
    5. axon terminal

12. What is the structural classification of a neuron composed of a single axon and a number of dendritic projections from the nerve cell body?
    1. multipolar
    2. bipolar
    3. pseudo-unipolar
    4. unipolar
    5. polar

13. Which of the following is a functional classification of neurons that, for the most part, are bipolar in structure and carry information from the peripheral axon to the central axon?
    1. interneurons
    2. efferent neurons
    3. afferent neurons
    4. bipolar cells
    5. multipolar cells

14. Which of the following terms is NOT used to describe a bundle of axons in the central or peripheral nervous system?
    1. ganglia
    2. pathways
    3. tracts
    4. nerves
    5. commissures

15. Which of the following cells is NOT classified as a glial cell?
    1. astrocyte
    2. ependymal cell
    3. oligodendrocyte
    4. Schwann cell
    5. ventricular cell

16. What type of cell enhances the velocity of electrical transmission of an action potential along an axon in the central nervous system?
    1. oligodendrocyte
    2. Schwann cell
    3. astrocyte
    4. ependymal cell
    5. microglia

17. Which of the following best describes the function of the myelin sheath?
    1. decrease ion permeability in the nodes of Ranvier
    2. increase leakage of ions across the membrane
    3. decrease axonal conduction velocity
    4. increase a membrane's ion permeability
    5. reduce a membrane's ion permeability

18. Which of the following is the correct term for the movement of an electrical charge across a membrane?
    1. resistance
    2. current
    3. potential difference
    4. transistor
    5. capacitance

19. Which of the following determines the resistance to an ion's movement across a membrane?
    1. enzymes on the surface of the cell membrane
    2. ion channels within the membrane
    3. receptors on the cell membrane
    4. the ions present on either side of the membrane
    5. the resting membrane potential

20. What is the inverse of resistance?
    1. voltage
    2. current
    3. conductance
    4. impedance
    5. flux

21. Which of the following is NOT a factor involved in the determination of resting membrane potential?
    1. concentration of sodium
    2. concentration of potassium
    3. presence of sodium channels
    4. presence of potassium channels
    5. concentration of sodium receptors

22. Which of the following statements about sodium is FALSE?
    1. There is a chemical force driving sodium ions into the cell.
    2. At the resting membrane potential, there is an electrical force driving sodium ions into the cell.
    3. At the sodium equilibrium potential, there is an electrical force driving sodium ions out of the cell.
    4. At the potassium equilibrium potential, there is an electrical force driving sodium ions out of the cell.
    5. At the sodium equilibrium potential, the electrochemical force for sodium movement across the plasma membrane is zero.

23. Which of the following best describes the electrochemical forces acting on sodium and potassium ions at the resting membrane potential?
    1. Forces on both sodium and potassium ions are to move into the cell.
    2. Forces on both sodium and potassium ions are to move out of the cell.
    3. The force on sodium ions is to move into the cell, and the force on potassium ions is to move out of the cell.
    4. The force on sodium ions is to move out of the cell, and the force on potassium ions is to move into the cell.
    5. There is no force on either ion to move.

24. Given a cation with an equilibrium potential of -55 mV, if the plasma membrane of the cell is permeable only to this ion, then which of the following best describes the resting membrane potential?
    1. -55 mV

B) +55 mV

3. -70 mV
4. More negative than -55 mV
5. More positive than -55 mV

25. At the resting membrane potential, the membrane is most permeable to                           , which moves        the cell due to its electrochemical gradient.
    1. sodium : into
    2. potassium : into
    3. potassium : out of
    4. sodium : out of
    5. chloride : into

26. At the resting membrane potential, the electrochemical gradient for sodium across the membrane is such that the net flux for sodium movement is directed                   , thereby causing the cell's membrane potential to become more                                                                                            .
    1. inward : positive
    2. inward : negative
    3. outward : positive
    4. outward : negative
    5. at equilibrium : positive

27. The membrane potential at which there is no net flux of an ion across the membrane is called that ion's      .
    1. potential difference
    2. action potential
    3. resting membrane potential
    4. equilibrium potential
    5. graded potential

28. The presence of the                         prevents the dissipation of the concentration gradient for Na+.
    1. Na+/K+ pump
    2. action potential
    3. equilibrium potential
    4. Na+/Ca2+ exchanger
    5. Na+/H+ antiporter

29. The Na+/K+ pump is called an electrogenic pump because the imbalance between                                .
    1. Na+ in to K+ out leaves the inside of the cell with a net negative charge
    2. Na+ out to K+ in leaves the inside of the cell with a net positive charge
    3. Na+ in to K+ out leaves the inside of the cell with a net positive charge
    4. Na+ out to K+ in leaves the inside of the cell with a net negative charge
    5. ATP utilization inside the cell relative to the outside

30. Why is the electrical potential of a membrane at rest closest to potassium's equilibrium potential than to sodium's equilibrium potential?
    1. more sodium channels are open, allowing more sodium to move into the cell
    2. more sodium channels are open, allowing more sodium to move out of the cell
    3. more potassium channels are open, allowing more potassium to move out of the cell
    4. more potassium channels are open, allowing more potassium to move into the cell
    5. all potassium channels are open

31. If, under resting conditions, the membrane is much more permeable to sodium than potassium, what would happen to the resting membrane potential?
    1. be altered very little
    2. become more negative
    3. approach potassium's equilibrium potential
    4. approach sodium's equilibrium potential
    5. approach chloride's equilibrium potential

32. As a membrane's permeability to a particular ion increases, membrane potential will move

                      that ion's                       .

1. 1. away from : electrical gradient
   2. away from : electrochemical gradient
   3. away from : equilibrium potential
   4. closer to : electrochemical gradient
   5. closer to : equilibrium potential

33. What equation is used to calculate the membrane potential based on ion concentration gradients and permeabilities?
    1. GHK equation
    2. NAD equation
    3. Nernst equation
    4. Ficks equation
    5. Lotts equation

34. Membrane permeability is altered in the short term (milliseconds to seconds) by changes in the                     of ion channels.
    1. gating
    2. cleavage
    3. production
    4. formation
    5. degradation

35. The opening of an ion channel increases a cell membrane's                             , whereas                       

will decrease.

1. 1. resistance : permeability
   2. permeability : conductance
   3. permeability : electrical current
   4. resistance : conductance
   5. conductance : resistance

36. The fact that a cell has an electrical potential difference across its membrane makes that cell

                     .

1. 1. depolarized
   2. hyperpolarized
   3. repolarized
   4. polarized
   5. polar

37. A change in a cell's membrane potential, such that it becomes more positive, is referred to as a        .
    1. depolarization
    2. hyperpolarization
    3. hypopolarization
    4. polarization
    5. repolarization

38. A change in a cell's membrane potential, such that it becomes more negative, is referred to as a      .
    1. depolarization
    2. hyperpolarization
    3. hypopolarization
    4. polarization
    5. repolarization

39. A                         is a subthreshold change in membrane potential within the cell body that decays as it travels away from its point of origin.
    1. polarization
    2. hyperpolarization
    3. depolarization
    4. action potential
    5. graded potential

40. The direction of change in membrane potential, in response to a stimulus that initiates a graded potential, is dependent upon                                       .
    1. that membrane's threshold potential
    2. the gating of potassium channels only
    3. the gating of sodium channels only
    4. the changes in ion concentration across the membrane
    5. the ion channels that are opened or closed

41. Which of the following does NOT produce graded potentials?
    1. the release of a neurotransmitter onto a cell body
    2. the release of a neurotransmitter onto a dendrite
    3. arrival of a suprathreshold stimulus at the axon hillock
    4. light impinging on a photoreceptor
    5. touching a sensory receptor

42. Which of the following statements is FALSE?
    1. Graded potentials can sum over time but action potentials cannot.
    2. Graded potentials do not have refractory periods, but action potentials do.
    3. Graded potentials and action potentials are all-or-none.
    4. Graded potentials and action potentials are caused by ions moving through channels.
    5. Graded potentials and action potentials can change the membrane potential of adjacent areas of the membrane through electrotonic conduction.

43. The                        in graded potential that occurs as current spreads along the membrane happens as a consequence of the                                                    of current across the membrane.
    1. elevation : leakage
    2. decrement : leakage
    3. decrement : blockade
    4. elevation : blockade
    5. elevation : generation

44. Which of the following is an example of spatial summation?
    1. Two stimuli from two sources produce graded potentials on the same neuron at the same time such that the two potentials sum.
    2. Two rapid stimuli from the same source produce graded potentials on the neuron that sum.
    3. An action potential occurs at the same time as a graded potential, and they sum.
    4. A neuron sends out information through collaterals to several target cells.
    5. Two action potentials occur at the same time and sum.

45. Which of the following changes in membrane potential is considered excitatory?
    1. hyperpolarization only
    2. depolarization only
    3. repolarization only
    4. both hyperpolarization and depolarization
    5. both hyperpolarization and repolarization

46. The spread of voltage by passive charge movement is called                              .
    1. diffusion
    2. integration
    3. propagation
    4. electrotonic conduction
    5. saltatory conduction

47. If the graded potential remains above threshold once it reaches the                           , an action potential will be generated.
    1. cell body
    2. dendrite
    3. axon
    4. axon hillock
    5. nucleus

48. Which of the following characteristics does NOT describe an action potential?
    1. rapid increase in potassium permeability
    2. rapid reversal of membrane potential
    3. it does not decay over long distances
    4. large depolarization
    5. rapid increase in sodium permeability

49. The opening of sodium channels causes a rapid                              of sodium that                         the neuron's membrane.
    1. efflux : hyperpolarizes
    2. influx : hyperpolarizes
    3. efflux : depolarizes
    4. influx : depolarizes
    5. influx : repolarizes

50. The depolarization phase of the action potential is generated by a rapid                              .
    1. opening of sodium channels
    2. closure of potassium channels
    3. closure of sodium channels
    4. opening of potassium channels
    5. opening of chloride channels

51. The repolarization phase of the action potential in a neuron is driven by the                               .
    1. closure of potassium channels
    2. opening of calcium channels
    3. opening of sodium channels
    4. opening of sodium channels and closure of potassium channels
    5. closure of sodium channels and opening of potassium channels

52. The patterns of change in ion channel permeability that occur during an action potential are due to        gating of voltage-sensitive potassium and sodium channels.
    1. mechanically-induced
    2. ligand-driven
    3. time-dependent
    4. temperature-induced
    5. light-sensitive

53. For the sodium channel to open and allow sodium into the cell,                              .
    1. the activation and inactivation gates must both be open
    2. only the activation gate must be open
    3. only the inactivation gate must be open
    4. the inactivation gate must open shortly before the activation gate opens
    5. the activation and inactivation gates must both be closed

54. Which of the structures below lacks voltage-gated ion channels responsible for the production of action potentials?
    1. axon hillock
    2. axon
    3. nodes of Ranvier
    4. muscle cell membrane
    5. epithelial cell membrane

55. Which of the following events is fastest?
    1. opening sodium activation gates
    2. closing sodium inactivation gates
    3. opening voltage-gated potassium channels
    4. closing voltage-gated potassium channels
    5. closing sodium activation gates

56. Stimuli A and B are both suprathreshold stimuli that last for one second, but stimulus A is stronger. Which of the following statements is TRUE?
    1. Only stimulus A can produce an action potential.
    2. The action potential produced by stimulus A will be larger than that produced by stimulus B.
    3. The action potential produced by stimulus A will be of longer duration than that produced by stimulus B.
    4. Stimulus A will cause a higher frequency of action potentials.
    5. A single action potential will be produced by both stimulus A and stimulus B. The action potentials produced from each stimulus will be identical in size and duration.

57. The repolarization phase of action potentials in neurons is due primarily to                                .
    1. increased activity of the Na+/K+ pump
    2. sodium flow out of the cell
    3. sodium flow into the cell
    4. potassium flow into the cell
    5. potassium flow out of the cell

58. During which of the following states are the majority of voltage-gated sodium channels closed and incapable of opening?
    1. at the resting membrane potential
    2. during depolarization
    3. during the absolute refractory period
    4. during the relative refractory period
    5. during the after-hyperpolarization

59. The regenerative nature of a sodium channel's activation gate refers to the situation where an activation gate opens causing a depolarization that triggers which of the following?
    1. closure of other sodium channels' activation gates
    2. opening of the inactivation gate of the same sodium channel
    3. opening of other sodium channels' activation gates
    4. closing of other sodium channels' inactivation gate
    5. opening of a potassium channels

60. The stimulation of an inadequate number of sodium channels for the generation of a positive sodium channel feedback loop is considered a                                                              stimulus.
    1. threshold
    2. degenerative
    3. suprathreshold
    4. subthreshold
    5. regenerative

61. In order to generate an action potential, the magnitude of the inward sodium current must be large enough to overcome which of the following?
    1. outward potassium current
    2. inward potassium current
    3. outward sodium current
    4. inward chloride current
    5. outward calcium current

62. What terminates the positive feedback loop that is involved in the generation of an action potential?
    1. opening of activation gates on sodium channels
    2. closure of activation gates on sodium channels
    3. opening of inactivation gates on sodium channels
    4. closure of inactivation gates on sodium channels
    5. closure of potassium channels

63. Why will the magnitude of an action potential never reach sodium's equilibrium potential?
    1. the inactivation gate closes the sodium channel so rapidly
    2. the potassium channel opens slowly
    3. the sodium channel only opens for a short time
    4. the inward movement of sodium is countered by the outward movement of potassium
    5. the outward movement of sodium is countered by the inward movement of potassium

64. The all-or-none principle, associated with the action potential, states that                              .
    1. the positive feedback loop for the sodium channel is terminated by the inactivation gate
    2. there is a positive feedback loop for sodium channels that results in a rapid membrane depolarization
    3. all of the action potentials will be generated from the axon hillock
    4. once membrane potential reaches threshold, an action potential will be generated and that action potential will always be the same magnitude
    5. following an action potential, the membrane will be repolarized by the opening of a potassium channel

65. In order for a neuron to move from the absolute to the relative refractory period, a majority of that neuron's sodium channels must have their                                         .
    1. inactivation gates open
    2. activation gates closed
    3. inactivation gates closed
    4. activation gates opened and inactivation gates closed
    5. inactivation gates closed and activation gates closed

66. Toward the end of the relative refractory period, the continued decrease in stimulus intensity required to initiate an action potential is caused by                                                    .
    1. decreased sodium permeability
    2. increased potassium permeability
    3. closure of the sodium activation gate
    4. decreased potassium permeability
    5. the number of sodium channels whose inactivation gate has not opened

67. The stimulus intensity required to initiate an action potential is                                through the course of the relative refractory period.
    1. stable
    2. increased
    3. unaltered
    4. progressively reduced
    5. progressively increased

68. Which of the following characteristics of an action potential does NOT result directly from the refractory period?
    1. the lack of summation of action potentials
    2. the all-or-none principle of action potentials
    3. the peak level of depolarization reached
    4. the frequency of action potentials
    5. the unidirectional propagation of action potentials

69. The time between action potentials is directly determined by the                             the graded potential at the axon hillock.
    1. distance traveled by
    2. duration of
    3. frequency of
    4. amplitude of
    5. source of

70. For an unmyelinated axon, conduction velocity is primarily determined by the                                 .
    1. type of potassium channel activated
    2. type of sodium channel activated
    3. diameter of the axon
    4. permeability of the axonal membrane
    5. number of ion channels present on the membrane

71. As an action potential is propagated away from the axon hillock, why does propagation continue in one direction?
    1. the region just behind the action potential is in the absolute refractory period
    2. the region just in front of the action potential is in the absolute refractory period
    3. the region just behind the action potential is in the relative refractory period
    4. the region just in front of the action potential is in the relative refractory period
    5. they will travel the path of least resistance

72. In myelinated nerve fibers, where do action potentials occur?
    1. cell body
    2. nodes of Ranvier
    3. Schwann cell
    4. underlying myelin sheath
    5. oligodendrocyte

73. The jumping of an action potential from node-to-node is called                              .
    1. nodal conduction
    2. propagation
    3. electrotonic conduction
    4. saltatory conduction
    5. nodal propagation

74. Which of the following axons would have the fastest conduction velocity?
    1. diameter = 5 microns, myelinated
    2. diameter = 5 microns, unmyelinated
    3. diameter = 20 microns, myelinated
    4. diameter = 20 microns, unmyelinated
    5. diameter = 1 micron, myelinated

75. What percentage of people with diabetes develop peripheral neuropathy?
    1. 5
    2. 10
    3. 20
    4. 30
    5. 50

76. What percentage of people with neuropathy have it secondary to diabetes?
    1. 5
    2. 10
    3. 20
    4. 30
    5. 50

77. Why do the distributions of sodium and potassium ions across the plasma membrane of neurons not change appreciably, even following hundreds of action potentials?
    1. The movement of sodium and potassium ions that occurs during an action potential is countered by the passive leak of these ions when a neuron is at rest.
    2. The movement of sodium and potassium ions that occurs during an action potential is countered by the active transport of these ions by the Na+/K+ pump.
    3. The movement of sodium and potassium ions that occurs during an action potential is countered by the passive movement of these ions during the repolarization phase.
    4. The movement of sodium and potassium ions that occurs during an action potential is countered by the passive movement of these ions during the after-hyperpolarization.
    5. The movement of sodium and potassium ions that occurs during an action potential is countered by counter-transport of potassium with sodium during rest.

1. 1. True/False Questions

1. Effector organs act as receptors that detect information about the external environment and transmit that information to the central nervous system.

2. Excitable cells are capable of producing action potentials.

3. Axons can branch.

4. Leak channels are most concentrated in the soma of neurons.

5. Afferent neurons are generally bipolar neurons.

6. Oligodendrocytes are located in the peripheral nervous system, providing the myelin sheath that forms the nodes of Ranvier.

7. Schwann cells are the only glial cells in the peripheral nervous system.

8. The number of ions whose movement across the membrane creates the resting membrane potential are so few that their movement does not affect that ion's concentration gradient.

9. The membrane potential of a cell is determined exclusively by that cell's sodium and potassium permeability.

10. At the resting membrane potential, a cell is at equilibrium.

11. The Na+/K+ pump is electrogenic.

12. An ion's net electrochemical force will tend to move that ion across the membrane in a direction that will cause membrane potential to move toward that ion's equilibrium potential.

13. The Nernst equation is used to calculate the resting membrane potential.

14. The GHK equation is used to calculate the equilibrium potential for a specific ion.

15. In temporal summation, stimuli from different sources are applied at the same time such that they overlap and sum.

16. Excitatory graded potentials are those where the stimulus initiates a hyperpolarization of the cell.

17. Under resting conditions, the sodium channel responsible for generating an action potential is closed and incapable of opening.

18. Both activation and inactivation gates of a sodium channel are stimulated at the same time by a depolarization with the inactivation gate acting more slowly than the activation gate, thereby allowing sodium to enter the cell.

19. The magnitude of the action potential is dependent upon the extent to which the change in membrane potential is above threshold.

20. During the relative refractory period, the stimulus intensity required to initiate an action potential is elevated.

21. Once an action potential is generated, it will always depolarize the neighboring membrane above threshold, ensuring the action potential will travel along the axon without interruption.

22. Tingling can be a sign of diabetic neuropathy.

23. Diabetic neuropathy can affect nerves of the autonomic nervous system.

1. 2. Matching Questions

Match the following branches of the nervous system with its correct description. Choose the BEST single answer.

1. enteric nervous system
2. somatic nervous system
3. autonomic nervous system
4. afferent nervous system
5. peripheral nervous system
6. central nervous system
7. efferent nervous system

1. 1. The portion of the peripheral nervous system that communicates to effector organs.

1. 2. The portion of the peripheral nervous system that transmits information from sensory receptors to the central nervous system.

1. 3. The portion of the efferent branch of the nervous system that communicates to glands and cardiac muscle.

1. 4. The brain and spinal cord.

1. 5. Portion of the efferent nervous system that communicates with skeletal muscle.

1. 6. Provides communication between peripheral organs and the brain and spinal cord.

1. 7. Nervous system of the intestinal tract.

For each of the following, indicate whether the property corresponds to graded potentials, action potentials, both, or neither.

1. both graded potentials and action potentials
2. neither graded potentials nor action potentials
3. action potentials
4. graded potentials

1. 8. Can sum.

1. 9. Are affected by refractory periods.

1. 10. Are a result of opening or closing of ion channels.

1. 11. Dissipate in size as the potential moves away from the site of initiation.

1. 12. All-or-none.

1. 13. Can reach or exceed the sodium equilibrium potential.

1. 3. Essay Questions

1. Describe the organization of the nervous system, including a description of the different branches.

2. Describe the structure of a neuron and the important consequences of that arrangement.

3. Every cell within the body has a potential difference across its membrane. Discuss the role of potassium ions in creating that potential difference across the cell membrane at rest. Include in your discussion a description of equilibrium potential.

4. Several ions are responsible for the resting membrane potential. Describe the forces that determine resting membrane potential.

5. Graded potentials develop in the cell body of neurons as well as in sensory receptor cells. In order for sensory information to reach the central nervous system, that graded potential must be converted into an action potential. How are graded potentials created, and how are they different from action potentials?

6. Describe the voltage gating of ion channels and how this plays a role in an action potential.

7. Once an action potential is generated, there is a delay before another action potential can be generated. Name and describe the basis for the two refractory periods.

8. Describe how an action potential, originating at the axon hillock, is propagated along the axon. Include those factors that can alter conduction velocity.

9. Describe the types of ion channels that are found in a neuron and how those channels are gated.

1. 4. Short Answer Questions

Using Figure 7.1, answer the following questions:

1. Describe the event C indicated in the figure above and how that event is initiated.

2. What ion channel(s) is/are responsible for event A in the figure above?

3. What ion channel(s) is/are responsible for event B in the figure above?

4. Identify the type of summation that is occurring in the figure above, where S refers to a stimulus from one source measured in the postsynaptic membrane.

5. In the figure above, if the direction of event C was reversed (hyperpolarization), how would this affect the ability of the postsynaptic membrane to generate an action potential?

6. In the figure above, if S2 indicated a stimulus from a different source, and S1 occurred coincident with S2, what type of summation has been generated?

7. Movement of what ion is responsible for event D in the figure above?

8. Information gathered about our internal environment (i.e., fullness of the stomach, blood pressure, etc.) is called                                                         information.

9. What is the most common neuronal cell type?

10. What is the only glial cell found outside of the central nervous system?

11. Each                         provides the myelin sheath for many axons in the central nervous system.

12. Once a membrane potential has been developed, the force that drives a particular ion across the membrane is its                                                        .

13. When they are not at equilbrium, an ion will move across the membrane in a direction that moves membrane potential toward that ion's                                        .

14. What is the passive spread of current along a membrane called?

15. Following the opening of the activation gate of the sodium channel, the                                gate closes shortly thereafter.

16. The inactivation gate is opened by                           .

17. The fact that the opening of some sodium channels can induce several other sodium channels to open describes the                                         property of these channels.

18. What is the level of membrane depolarization required to induce the sodium channel's positive feedback loop called?