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Physiology I: Cellular Neuro > Lectures 7 & 8: Action Potentials > Flashcards

Lectures 7 & 8: Action Potentials Flashcards

1
Q

Action potential

A
  • Rapid change in membrane potential of a cell

- Followed by a return to the resting Vm

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2
Q

Action potential characteristics

A
  • Produced by excitable cells; definition of excitability
  • Size and shape vary from one excitable cell to another
  • Propagated/conducted from one part of a cell to another, unchanged (non- decremental)
  • Are all-or-none
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3
Q

Action potentials are produced and conducted by

A
  • The plasma membrane of cells

- Used for communication over long distances

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4
Q

Chronaxie is useful as index of

A
  • Membrane excitability

- The larger the chronaxie, the less excitable the preparation

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5
Q

After depolarization, Vm does not return to rest immediately, but stays depolarized to some degree

A
  • For 1-2 msec
  • Seen in muscle usually, not nerve
  • May be due to K accumulation in T- tubules
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6
Q

Post spike hyperpolarization

A
  • 3 to 5 msec in duration
  • Until increased PK turns off
  • Amplitude around 5 mV
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7
Q

Negative afterpotential

A
  • Around 30 msec in duration
  • Due to K accumulation outside membrane
  • Stays until K can diffuse away
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8
Q

Positive afterpotential

A
  • Around 200 msec
  • 2 mV in amplitude
  • Due to stimulation of Na pump by K or Na
  • Pump is electrogenic and creates a hyperpolarization
  • Seen in skeletal muscle
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9
Q

Activation/inactivation

A
  • Na current turns on

- This is followed by Na current turning off

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10
Q

Same time as Na current turns off,

A
  • K current turns on (activation)

- Will be maintained as long as membrane is clamped

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11
Q

Hodgkin cycle

A
  • The self-sustaining entry and circular role of Na during depolarization
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12
Q

Blockers of sodium channels

A
  • TTX

- Saxitoxin

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13
Q

Cardiac muscle ion currents during action potential

A
  • Fast Na channels carry fast inward current (depolarization)
  • Slow inward Ca current produces plateau along with turning on of delayed K current
  • Leads to repolarization
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14
Q

Smooth muscle ion currents during action potential

A
  • Lacks fast Na channels
  • Have slow Na and Ca channels (L - type calcium channels) for depolarization
  • For repolarization: K activation, along with inactivation of slow channels
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15
Q

Voltage inactivation

A
  • Once Na channels are inactivated, membrane must be repolarized toward normal
  • Resting Vm before they can be reopened
  • If membrane is too depolarized (high Ko or low Ki or use of depolarizing drugs) a considerable number of Na channels are inactivated and enough can’t open to produce an action potential
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16
Q

Accommodation

A
  • Slow depolarization

- Threshold is passed without an action potential being fired

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17
Q

Accommodation is due to

A
  • Inactivation of a significant number of Na channels before threshold potential reached
  • Sodium channels have undergone voltage inactivation and will not open again unless the membrane is significantly repolarized
  • If depolarization is slow enough, the critical number of Na channels needed to produce an action potential may not be achieved
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18
Q

K channels open in response to

A
  • Depolarization

- Make the membrane more refractory to depolarization

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19
Q

Because of accommodation, a weak stimulus

A
  • Will not elicit an action potential no matter how long applied
  • Hence, the rheobase on strength-duration curve
20
Q

Depolarizing drugs such as potassium cause nerve blocks or muscle paralysis due to

A
  • Accommodation

- Voltage inactivation

21
Q

Conduction velocity is determined by

A
  • Cm

- Electrical resistance to current flow

22
Q

Typical value of Cm

A
  • Around 10 farads/cm2 membrane
23
Q

To depolarize membrane from -100 mV to 0 mV,

A
  • 10 coulombs of charge must flow across each square centimeter of membrane
24
Q

Cm determines

A
  • How much charge must flow to depolarize membrane

- The greater the Cm, the greater the charge that must flow and the slower the rate of electrotonic spread

25
Q

Resistance also determines how rapidly charge can flow (two components)

A
  • Rm: membrane resistance

- Rin: the resistance to longitudinal current flow in cytoplasm

26
Q

Length constant

A
  • √(Rm/Rin): distance over which an electrotonically conducted signal falls to 37% (l/e) of its initial voltage
  • Typical mammalian is 2-3 mm
  • Also determined by capacitance and resistance
27
Q

The larger the length constant,

A
  • The further the electronic conduction spreads
28
Q

Effect of fiber size

A
  • The larger the fiber diameter, the greater the conduction velocity
  • Diameter affects resistances and capacitances in manner so this relationship holds
29
Q

Myelin increases the conduction velocity by

A
  • Decreasing Cm of axon and by allowing action potential to be generated only at nodes of Ranvier
  • It effectively decreases the decremental loss of potential with distance (effectively increases the length constant)
30
Q

Saltatory conduction occurs

A
  • Impulses (action potentials) occur only at nodal membranes
31
Q

The Na pump is more metabolically efficient because

A
  • Ionic currents are restricted to nodal membrane
  • Less Na enters and K leaves per unit area of membrane
  • Requiring less pumping to maintain Na and K gradients
32
Q

With certain “demyelinating diseases” conduction velocities may be sufficiently reduced to cause

A
  • Significant impairment of sensory or motor function

- As part of patient assessment measurements of nerve conduction velocity may be conducted

33
Q

Increasing [Ko] or decreasing [Ki] causes

A
  • Depolarization

- Mainly affects the resting membrane potential

34
Q

Changing [Nao]/[Nai] mainly affects

A
  • Peak of the action potentia

- If there were no overshoot then [Nao] = [Nai]

35
Q

Metabolic poisons inhibit

A
  • Sodium/potassium pump

- Neurons can still conduct millions of action potentials before failure

36
Q

Graded potential features (also called passive or electrotonic potentials)

A
  • All are sub-threshold
  • Magnitude varies with stimulus
  • Propagated with decrement
  • Persist only as long as stimulus is present (sometimes fail to occur)
37
Q

Propagated with decremen

A
  • Diminish in size as it moves away from site of stimulation
38
Q

Action potential features

A
  • Produced by depolarizing excitable cells to threshold Vm (supra-threshold)
  • Magnitude same regardless of stimulus
  • At peak, Vm usually reverses with regard to exterior
  • Propagated non-decrementally
  • All-or-None
  • Persist long after stimulus ends
39
Q

Propagated non-decrementally

A
  • Size unchanged as you move away from site of stimulation
40
Q

Refractory behavior

A
  • Occurs immediately following the firing of an AP
  • Period of time during which additional AP cannot occur
  • Varies from one excitable cell to another
  • Incorporates two periods of time (absolute and relative)
41
Q

Absolute refractory period

A
  • No AP possible
42
Q

Relative refractory period

A
  • Attenuated AP possible upon greater than normal stimulation
43
Q

During accommodation, action potentials fail to occur due to

A
  • Voltage inactivation of Na channels because of slow depolarization of membrane
  • Opening of K channels
44
Q

Electronic spread is dependent upon

A
  • Charge storage capacity of membrane, Cm

- Ratio of Membrane resistance (Rm) to internal resistance (Rin) to current flow

45
Q

Small Cm and large (Rm/Rin)^1/2 means

A
  • Greater spread of current

- Higher AP conduction velocity

46
Q

Myelination increases conduction velocity

A
  • Reduces Cm

- Increases Rm/Rin

Physiology I: Cellular Neuro (8 decks)

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