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Physiology > Action Potential > Flashcards

Action Potential Flashcards

1
Q

What is the time constant?

A

The amount of time it takes for the voltage to change by a certain percentage (63%) of the eventual new steady state value

t=RC

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2
Q

What is the length constant?

A

The distance between the injection site and the point where the steady state transmembrane voltage change has decayed by 63% from its peak value.

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3
Q

What happens to the size of the action potential as it goes down an axon?

A

Stay the same

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4
Q

What is the “all or nothing” principle?

A

Size of action potential does not depend on the size of the triggering stimulus

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5
Q

What are the six stages of a membrane depolarization, starting with the cell at rest?

A
  1. Cell at rest; polarized-cell negative inside
  2. Depolarization phase - sometimes called rising phase
  3. Overshoot - variable
  4. Peak of action potential
  5. Repolarization phase - sometimes called falling phase
  6. Hyperpolarization; sometimes called undershoot phase
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6
Q

What is the ionic basis for the depolarization phase?

A

Opening if Na channels, causing Na to rush in

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7
Q

What is the ionic basis for the repolarization phase?

A

Na channels close (inactivation gate)

K channels open to allow K to rush outside the cell

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8
Q

What is the ionic basis for the hyperpolarization phase?

A

K channels remain open long enough to go past the resting membrane potential

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9
Q

If Na/K pumps are disable, say through oubain poisoning, how long are nerve fibers still competent?

A

Many thousands of nerve impulses

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10
Q

What are the two domains of the Na/K pump?

A

Activation gate and inactivation gate

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11
Q

When the Na channel is in its resting state, what is the state of the activation gate? Inactivation gate?

A

Activation gate= Closed

Inactivation gate = open

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12
Q

What causes the activation gate to swing open?

A

Rise in membrane potential

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13
Q

What is the inactivation state of the Na/K pump?

A

When the inactivation gate is closed

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14
Q

What closes the inactivation gate?

A

Same change in membrane potential as the activation gate, but it closes more slowly.

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15
Q

True or false: The Na+ channel cannot go directly from the inactivated to the activated state; it must first go back to the resting state.

A

True

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16
Q

What causes the change in the Na/K pump from the inactivation state to the resting state?

A

Change in membrane potential back to the resting potential

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17
Q

What activates the K channels (opens them)?

A

Depolarization, but takes longer to open

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18
Q

What are the K channels responsible for?

A

The re-polarization of the membrane

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19
Q

What is the meaning of a threshold (what chemically determines this)?

A

The value of membrane potential at which inward flow of Na+ exceeds the passive outward flow of K

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20
Q

Where is the threshold of an axon usually the lowest?

A

At the initial segment of the axon

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21
Q

Where is the “initial” segment of the axon?

A

Axon hillock to about 20-50 μm down the axon

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22
Q

What happens in hypocalcemia?

A

The probability that a Na channel will open is greatly increased (hyper sensitivity of nerves)

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23
Q

What is the clinical consequence of hypocalcemia and hypoparathyroidism?

A

Hypoparathyroidism leads to reduction in serum calcium levels and a tendency for muscles to begin twitching spontaneously.

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24
Q

What are the three types of electrical signals that occur in the body?

A
  1. Receptor signals
  2. Synaptic potentials
  3. Action potentials
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25
Q

The cell membrane can be modeled as a simple circuit. Which part of the membrane is a capacitor, and which part is a resistor?

A 
Capacitor = bilayer
Resistor = ion channels
26
Q

What happens to the permeability of Na/K as it goes from the resting state, to the depolarized state, to the repolarized state?

A

Resting, K»Na

Depolarizaed: K<>Na

27
Q

How do local anesthetics work?

A

Block Na channels through various routes

28
Q

What determines the voltage threshold of an action potential?

A

The minimal voltage needed to kick the Na channels into the fast positive feedback cycle

29
Q

What are the three factors that determine the AP threshold?

A

Na channel
K channel
[Ca]

30
Q

What happens in hypocalcemia to neurons?

A

Hyper sensitivity d/t lowering of threshold

31
Q

What are the four clinical manefistations of hypocalcemia?

A
  1. Convulsion
  2. Arrhythmias
  3. Tetany
  4. Spasms and stridor

CATS

32
Q

What is chvostek’s sign?

A

Contraction of the muscles of the eye, mouth, or nose elicited by tapping over the facial nerve in front of the ear. Seen sometimes in hypocalcemia

33
Q

What is trousseau’s sign?

A

Muscles spasms that results after application of a BP cuff that is raised to higher than SBP

34
Q

What is the physiological effect of hypercalcemia on the action potential threshold?

A

Raises it (the probability that a Na+ channels is open at a certain voltage is decreased )

35
Q

What are the clinical manifestations of hypercalcemia?

A

Decrease in neuromuscular irritability

36
Q

What is a condition that results in hypercalcemia?

A

Hyperparathyroidism

37
Q

Does [Ca] change the resting potential of a membrane? Why or why not?

A

No, the cell membrane is not permeable to it

38
Q

How does Ca change the threshold potential?

A

Changing the Na channel opening probability

39
Q

At low [Ca] is it easier or more difficult to open Na channels?

A

easier

40
Q

How does extracellular [Na] affect the rise time and shape of an action potential?

A

Lowering the extracellular [Na] reduces both the rate and rise of the action potential, and its peak amplitude

41
Q

What accounts for the plateau seen in cardiac muscle cell action potential?

A

[Ca]

42
Q

What is the refractory period?

A

Period of time after the absolute refractory period during which you’d need a stronger than normal stimulus to elicit a new action potential

43
Q

What is the absolute refractory period? How long is it usually?

A

The period of time when an action potential cannot be generated regardless of conditions

Usually just the duration of the actual action potential

44
Q

What is the chemical basis for the absolute refractory period?

A

The Na channels are stuck in an inactivated state

45
Q

What is the chemical basis for the relative refractory period?

A

The delayed K channel opening and hyperpolarization

K+ ions leaving the cell oppose the depolarizing effect of opening Na+ channels

46
Q

What sets the upper limit of firing frequency?

A

The relative refractory period

47
Q

What creates the phenomenon of propagation?

A

When one patch of a neuron is depolarized, it creates a current of ion flow in/out in neighboring patches, which do likewise (but at a lesser degree)

48
Q

What will happen if an action potential is set up in the middle of a neuron?

A

The action potential will flow in both directions

49
Q

What ensure that an action potential only flows in one direction?

A

The axon hillock’s lower action potential threshold

50
Q

How does the axon diameter affect the conduction velocity? How?

A

Larger = faster

Since the electrical resistance decreases faster than the capacitance increases, increasing axon diameter increases the speed of conduction.

51
Q

When the diameter of an axon increases, what happens to its resistance?

A

decreases in proportion to the square of the radius of the axon (area of the axon)

52
Q

When the diameter of an axon increases, what happens to its capacitance?

A

Increases in direct proportion to the radius of the axon (C = 2πr)

53
Q

How does the mylenation of axons contribute to the velocity of the action potential?

A

Greatly decreases the axon membrane capacitance, and increases the membrane resistance

54
Q

What is mylein composed of?

A

Lipids, cholesterol, other non-conducting proteins

55
Q

What is the myelinating cells in the PNS?

A

Schwann cells

56
Q

What is the myelinating cells in the CNS?

A

Oligodendrocytes

57
Q

What is the effect of schwann cells?

A

Allows nerves to only form circuits at the nodes of ranvier, since these are the only places that Na channels are

58
Q

Where can action potentials be generated in a myelinated nerve?

A

Only at the nodes of ranvier

59
Q

Why is conduction speed increased with myleination (chemially)?

A

Capacitance is lower, and do not waste time generating small action potentials everywhere

60
Q

Is myleinating nerves energy efficient or inefficient? Why?

A

Efficient, because there are fewer spots that have action potentials generated, meaning fewer Na/K pumps

61
Q

What is the cause of MS?

A

Antibodies attacking oligodendrocytes, causing demyelination of the CNS

62
Q

What is Guillain-Barre Syndrome, and what is is caused by?

A

Antibodies mistakenly attack the PNS Schwann cells, causing demyelination. This is usually reversible.

Physiology (75 decks)

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