Action Potentials Flashcards

1
Q

Four types of excitable cells (cells that can experience APs) & what makes them excitable

A

Neurons

Skeletal muscle

Cardiac muscle

Smooth muscle

Must have enough voltage-gated Na+ or Ca2+ channels

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2
Q

Action potentials are produced by ion diffusion across cell membranes as a result of..

A

selective and brief changes in membrane permeability to those ions.

(Remember, if membrane permeability to a given ion is increased, then the transmembrane potential will shift twoard the Nernst potential of that ion)

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3
Q

conductance (g)

A

The ease with which ions can flow; an indirect measurement of permeability

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4
Q

Rapid depolarization

A

Na+ channels open, allowing rapid incrase of gNa+ and Na+ influx.

Hodgkin cycle: initial depolarization produces more dpeolarization in a regenerative, positive feedback

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5
Q

Overshoot

A

gNa+ now exceeds gK+ and the action potential overshoots 0mV –> rapidly terminated by Na inactivation

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6
Q

Repolarization

A
  • Na inactivation causes gNa+ to fall back to normal
  • Delayed increase in gK+ causes Em to move towards Ek as K+ efflux
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7
Q

Undershoot/Hyperpolarizing afterpotential

A

Prevailing elevation of gK+ ->K+ efflux causes Em to remain somewhat hyperpolarized after gNa+ returns to normal level

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8
Q

Do K+ channels have inactivation gates like Na+ channels do?

A

K+ channels do not have inactivation gates, just activation gates - thus, gK does not inactivate during sustained depolarization

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9
Q

States of voltage-gated ion channels during resting state, depolarizing phase, repolarizing phase, and undershoot

A

Note how the activation gate of both K and Na open in response to depolarization, but the K+ activation gate is delayed and there is no inactivation gate for K

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10
Q

Transient vs Steady state inactivation of Na+ channels

A

Transient inactivation: this is what you see during the action potential; a very sudden depolarization triggers inactivation

Steady state inactivation: the maintained depolarization seen between action potentials, AKA the resting membrane potential

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11
Q

Threshold can change - what plays a key role in determining the threshold of depolarization AKA excitability?

A

Activation & inactivation, esp of Na channels

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12
Q

A critical number of Na channels must be recruited to generate a propagating action potential. What is “recruitment”?

A

As the membrane is progressively depolarized, mroe and mroe Na channels reach threshold and are activated

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13
Q

Moderate levels of depolarization that are short lasting will have what effect on excitability?

A

They increase excitability (lower threshold) by recruiting/activating more Na channels

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14
Q

Larger, maintained levels of depolarization have what effect on excitability?

A

Transiently increases excitability then drops it by elevating the threshold and causing depolarizing blockade

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15
Q

What is a depolarizing blockade?

A

If the membrane is depolarized for a while, more Na channels will be in the inactive state- refractory to another stimulus –> not enough channels available to be recruited for an AP, so firing an AP is impossible no matter how intense the depolarization.

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16
Q

Absolute vs relative refractory period

A

Absolute refractory period: impossible to generate another AP no matter how strong the stimulus because the critical number of Na channels is not available due to closure of their inactivation gates

Relative refractory period follows- a second action potential can be evoked by an especially strong depolarization; occurs because

  • Some gates are still closed
  • Must overcome the hyperpolarizing afterpotential
17
Q

Where is the absolute and relative refractory period during an action potential?

A
18
Q

Why is there a small fluctuation of the total current across a cell membrane?

A

Channels are always either opened or closed, but they can do so randomly even when transmembrane potential is subthreshold

19
Q

The frequency and duration of channel openings is a function of what 2 things?

A

The degree of depolarization

Concentraiton of Ca++

20
Q

Various excitable cells have APs of different shapes and sizes depending on the

A

type and properties of voltage-gated channels in that cell membrane

21
Q

Autorhythmicity

A

Firing at roughly a constant rate

Ex) APs from cells in the SA node

22
Q

Ion channel function can also be modulated by G proteins, neuroransmitters, and intracellular signaling molecules

A