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630: Medical Physiology > Cellular Physiology (Part 2) > Flashcards

Cellular Physiology (Part 2) Flashcards

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USMLE® Step 1 flashcards
1
Q

What is resting membrane potential?

A

The potential difference that exists across the membrane of excitable cells, such as nerve and muscle, in the period between action potentials

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2
Q

The resting membrane potential is established by what?

A

diffusion potentials, which result from the concentration differences for various ions across the cell membrane

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3
Q

Which ions will make the greatest contributions to the resting membrane potential?

A

Ions with the highest permeabilities at rest

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4
Q

Resting membrane potential of excitable cells falls in the range of −__ to −__mV

A

−70 to −80mV

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5
Q

What is the resting membrane potential close to?

A

The equilibrium potentials for K+ and Cl- because the permeability to these ions at rest is high

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6
Q

What is the resting membrane potential far from?

A

The equilibrium potentials for Na+ and Ca2+ because the permeability to these ions at rest is low

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7
Q

What role does the Na+ - K+ APTase play in creating the resting membrane potential?

A

It creates and maintains the K+ concentration gradient, which establishes the resting membrane potential

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8
Q

What is an action potential?

A

A phenomenon of excitable cells, such as nerve and muscle, and consists of a rapid depolarization (upstroke) followed by repolarization of the membrane potential

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9
Q

What is the basic mechanism for transmission of information in the nervous system and in all types of muscle?

A

an action potential

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10
Q

What is depolarization?

A

the process of making membrane potential less negative

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11
Q

What is repolarization?

A

The process by which the membrane potential returns the membrane potential to a negative value after the depolarization phase of an action potential

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12
Q

What is hyperpolarization?

A

the process of making the membrane potential more negative

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13
Q

What is inward current?

A

The flow of positive ions from the ECF into the ICF

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14
Q

What is an example of inward current?

A

The flow of sodium into the cell during the upstroke of the action potential

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15
Q

What is outward current?

A

The flow of positive ions from the ICF to the ECF

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16
Q

What is an example of outward current?

A

The flow of potassium out of the cell during the repolarization phase of the action potential

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17
Q

Inward currents _____ the membrane potential and outward currents ______ the membrane potential.

A

depolarize

hyperpolarize

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18
Q

What is threshold potential?

A

the membrane potential at which occurrence of the action potential s inevitable

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19
Q

An ____ current is required to depolarize the membrane potential to threshold

A

inward

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20
Q

The portion of the action potential where the membrane potential is positive is called what?

A

overshoot

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21
Q

The portion of the action potential, following repolarization, where the membrane potential is actually more negative than it is at rest is called what?

A

undershoot

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22
Q

What is the refractory period?

A

a period during which another normal action potential cannot be elicited in an excitable cell

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23
Q

Each normal action potential for a given cell type looks ____, depolarizes to the same potential, and repolarizes back to the ___ resting potential.

A

identical

same

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24
Q

Describe the propagation of an action potential

A

An action potential at one site causes depolarization at adjacent sites, bringing those adjacent sites to threshold.

  • This propagation is nondecremental
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25
Q

What channel is responsible for the upstroke of the action potential in nerve and skeletal muscle?

A

the voltage-gated Na+ channel

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26
Q

What are the 2 types of gates on the Na+ channel?

A

activation and inactivation gates

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27
Q

In order for Na+ to move through the Na+ channel what gate must be open?

A

both

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28
Q

How do the gates on the Na+ channel respond throughout the course of an action potential?

A

1) during rest the activation gate is closed and the inactivation gate is open
2) during the upstroke of the action potential, the activation gate opens quickly (the inactivation gate is still open) allowing Na+ to flow through the channel
3) at the peak of the action potential the inactivation gate closes, stopping the passage on Na+ and repolarization begins

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29
Q

What are the 2 types of refractory periods?

A

Absolute or Relative

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30
Q

The absolute refractory period overlaps with how much of the action potential?

A

Almost the entire duration

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31
Q

What is the basis of the absolute refractory period?

A

Closure of the inactivation gates of the Na+ channel in response to depolarization

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32
Q

How long are the inactivation gates in the closed position?

A

Until the cell is repolarized back to its resting membrane potential

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33
Q

When does the relative refractory period begin?

A

It begins at the end of the absolute refractory period and overlaps primarily with the period of hyperpolarization afterpotential

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34
Q

Can an action potential be elicited during the relative refractory period?

A

Yes, but only if a greater than usual depolarizing (inward) current is applied

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35
Q

What is the basis of the relative refractory period?

A

The higher K+ conductance than is present at rest

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36
Q

Why is more inward current needed to bring the membrane to threshold for the next action potential to be initiated after the relative refractory period?

A

Because the membrane potential is closer to the K+ equilibrium potential

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37
Q

Describe the concept of accommodation

A

When a nerve or muscle cell is depolarized slowly or is held at a depolarized level, the usual threshold potential may pass without an action potential having been fired.

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38
Q

Why does accommodation occur?

A

Because depolarization closes inactivation gates on the Na+ channels, and if this depolarization occurs slowly enough the Na+ channels close and remain closed. So the upstroke of the action potential cannot occur because there are not enough Na+ channels available to carry inward current.

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39
Q

Accommodation is seen in what type of people?

A

Persons who have an elevated serum K+ concentration, or hyperkalemia

40
Q

Propagation of action potentials down a nerve or muscle fiber occurs by the spread of ____ currents from active regions to adjacent inactive regions

A

local

41
Q

As the initial segment of the nerve axon is depolarized as the result of an inward Na+ current what happens to the polarity of the membrane potential?

A

It is reversed and the cell interior becomes positive

42
Q

Describe how propagation of the local current down the axon

A

At the negative site, positive charges inside the cell flow toward negative charges at the adjacent inactive site. This current flow causes the adjacent region to depolarize to threshold. With the transmission of each sequential action potential down the axon, the original active region is restored to its inside-negative polarity.

43
Q

What is the time constant?

A

The amount of time it takes following the injection of current for the potential to change to 63% of its final value.

44
Q

What are the 2 factors that affect the time constant?

A
  • membrane resistance

- membrane capacitance

45
Q

When the membrane resistance is high the time constant _____.

A

increases

46
Q

What is membrane capacitance?

A

The ability of the cell membrane to store charge

47
Q

When the membrane capacitance is high, the time constant _____.

A

increases

48
Q

What are the 2 mechanisms that increase conduction velocity?

A
  • increasing the size of the nerve diameter

- myelinating the nerve fiber

49
Q

Myelin increases membrane _____ and decreases membrane _______ which results in increased conduction velocity

A

resistance

capacitance

50
Q

What would occur if the entire nerve were coated with lipid myelin?

A

No action potentials could occur because there would be no low resistance breaks in the membrane across which depolarizing current could flow.

51
Q

Nodes of Ranvier occur at intervals of _ to _ mm along the axon

A

-2 mm

52
Q

What are the two ways in which information can be transmitted from one cell to another across a synapse?

A

either electrically or via chemical transmitter

53
Q

Electrical synapses allow current to flow from one excitable cell to the next via what?

A

low resistance pathways between the cells called gap junctions

54
Q

In what types of tissue are electrical synapses found?

A

in cardiac muscle and some types of smooth muscle

55
Q

In chemical synapses there us a gap between the presynaptic cell membrane and the postsynaptic cell membrane known as what?

A

the synaptic cleft

56
Q

Information is transmitted across the synaptic cleft via what?

A

Neurotransmitters

57
Q

Describe the sequence of events that occur at a chemical synapse

A

1) an action potential travels down the motor neuron to the presynaptic terminal
2) depolarization of the presynaptic terminal opens Ca2+ channels, and Ca2+ flows into the terminal
3) an influx of Ca2+ into the presynaptic terminal causes the neurotransmitter, which is stored in the synaptic vesicles, to be released by exocytosis
4) The NT diffuses across the synaptic cleft, binds to receptors on the postsynaptic membrane, and produces a change in the membrane potential on the postsynaptic cell

58
Q

If the neurotransmitter that crosses the synaptic cleft and binds to the postsynaptic membrane is excitatory what will happen?

A

It causes depolarization of the postsynaptic cell

59
Q

If the neurotransmitter that crosses the synaptic cleft and binds to the postsynaptic membrane is inhibitory what will happen?

A

It causes hyperpolarization of the postsynaptic cell

60
Q

In contrast to electrical synapses, neurotransmission across the chemical synapse is ___directional

A

unidirectional

61
Q

What is the synaptic delay?

A

The time required for the multiple steps in the chemical neurotransmission to occur

62
Q

What are motoneurons?

A

the nerves that innervate muscle fibers

63
Q

What comprises a motor unit?

A

A single motor neuron and the muscle fibers it innervates

64
Q

____ motor units are involved in fine motor activities and ____ motor units are involved in large motor activities.

A

Small

Large

65
Q

The synapse between a motor neuron and a muscle fibers is called what?

A

the neuromuscular junction

66
Q

Describe the sequence of events in neuromuscular transmission

A

1) 1) an action potential travels down the motor neuron to the presynaptic terminal
2) depolarization of the presynaptic terminal opens Ca2+ channels, and Ca2+ flows into the terminal
3) ACh is extruded into the synapse by exocytotsis
4) ACh binds to its receptor on the motor end plate
5) channels for Na+ and K+ are opened in the motor end plate
6) depolarization of the motor end plate causes action potentials to be generated in the adjacent muscle tissue
7) ACh is degraded to choline and acetate by acetylcholinesterase and choline is taken back up into the presynaptic terminal on the Na+ - choline cotransporter

67
Q

How is acetylcholine formed?

A

From ecetyl conenzyme A (acetyl CoA) and by choline by the action of the enzyme choline acetyltransferase

68
Q

What are 4 agents that alter neuromuscular function?

A
  • botulinus toxin
  • curare
  • neostigmine
  • hemicholinium
69
Q

What does botulinus toxin do?

A

It blocks the release of ACh from presynaptic terminals, causing total blockade of neuromuscular transmission, paralysis of skeletal muscle, and eventually death from respiratory failure

70
Q

What does curare do?

A

It competes with ACh for the noctinic receptors on the motor end plate, decreasing the size of the end plate potential (EPP) which can cause paralysis and death when administered in maximal doses

71
Q

What does neostigmine do?

A

It prevents degradation of ACh in the synaptic cleft, which prolongs and enhances the action of ACh at the motor end plate

72
Q

Neostigmine can be used in the treatment of what disease?

A

myasthenia gravis, which is characterized by skeletal muscle weakness and fatigability, in which ACh receptors are blocked by antibodies

73
Q

What does hemicholinium do?

A

It blocks the reuptake into presynaptic terminals, thus deleting choline stores from the motor neuron terminal and decreasing the synthesis of ACh

74
Q

What are the 3 types of relationships between the input to a synapse and the output?

A

one-to-one
one-to-many
many-to-one

75
Q

Describe one-to-one synapses

A

A single action potential in the presynaptic cell causes a single action potential in the postsynaptic cell

76
Q

One-to-one synapses are illustrated by what?

A

the neuromuscular junction

77
Q

Describe one-to-many synapses

A

An action potential in the presynaptic cell causes a burst of action potentials in the postsynaptic cells

78
Q

One-to-many synapses are illustrated by what?

A

motor neurons on Renshaw cells of the spinal cord

79
Q

Describe many-to-one synapses

A

An action potential in the presynaptic cell is insufficient to produce an action potential in the postsynaptic cell, so many presynaptic cells converge on the postsynaptic cell, these pulses summate, and the sum of the inputs determines whether the postsynaptic cell will fire an action potential

80
Q

Many-to-one synapses are illustrated by what?

A

Nervous system cells

81
Q

Excitatory postsynaptic potentials are synaptic inputs that ___ the postsynaptic cell, beinging the membrane closer to threshold and closer to firing an action potential

A

depolarizes

82
Q

What produces an EPSP?

A

Opening of Na+ and K+ channels

83
Q

What are some examples of excitatory NTs?

A
  • Ach
  • norepinephrine
  • epinephrine
  • dopamine
  • glutamate
  • serotonin
84
Q

Inhibitory postsynaptic potentials are synaptic inputs that ___ the postsynaptic cell, taking the membrane potential away from threshold and farther from firing an action potential

A

hyperpolarize

85
Q

What are some examples of inhibitory NTs?

A
  • gamma-aminobutyric acid (GABA)

- glycine

86
Q

What are the 2 ways in which presynaptic information arrives at the synapse?

A
  • Spatially

- Temporally

87
Q

When does spatial summation occur?

A

When two or more presynaptic inputs arrive at a postsynaptic cell simultaneously

88
Q

When does temporal summation occur?

A

When two presynaptic inputs arrive at a postsynaptic cell in rapid succession

89
Q

What are 3 other phenomena that alter synaptic activity?

A
  • Facilitation/Augmentation/Post tetanic potentiation
  • Long-term potentiation
  • Synaptic fatigue
90
Q

Describe facilitation, augmentation and post tetanic potentiation

A

An increased release of NT into the synapse causes the response of the postsynaptic cell to be greater than expected

91
Q

Describe long-term potentiation

A

Occurs in the storage of memories and involves both the release of NT from presynaptic terminals and increased sensitivity of postsynaptic membranes to the transmitter

92
Q

Describe synaptic fatigue

A

Occurs where repeated stimulation produces a smaller than expected response in the postsynaptic cell, possibly resulting from the depletion of NT stores from the presynaptic terminal

93
Q

3 criteria used to formally designate a substance as a neurotransmitter

A
  • The substance must be synthesized in the presynaptic cell
  • The substance must be released by the presynaptic cell upon stimulation
  • If the substance is applied exogenously to the postsynaptic membrane at physiologic concentration, the response of the postsynaptic cell must mimic the in vivo response.
94
Q

What are the 3 groupings of neurotransmitter substances?

A
  • acetylcholine
  • biogenic amines (norepinephrine, epinephrine, and dopamine)
  • amino acids
  • neuropeptides
95
Q

3 functions of ACh

A
  • only NT that is utilized at the neuromuscular junction
  • released from all preganglionlic and most postgnglionic in the PNS
  • released from presynaptic neurons in the adrenal medulla
96
Q

What is the common precursor of the biogenic amines (norepinephrine, epinephrine, and dopamine)?

A

tryosine

97
Q

What are the 2 enzymes that degrade norepinephrine, epinephrine, and dopamine?,

A
  • catechol-O-methyltransferase (COMT)

- monoamine oxidase (MAO)

630: Medical Physiology (24 decks)

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