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Electrical excitability of cells > The action potential > Flashcards

The action potential Flashcards

1
Q

What is an AP?

A

Transient impulse triggered by a depolarisation beyond threshold

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2
Q

Length of AP?

A

1-2 Ms

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3
Q

What three things can trigger an AP?

A

Electrode
Synaptic activity
Propagation of AP

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4
Q

How much is the Em displaced in an action potential?

A

Up to 100mv

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5
Q

What underpins an action potential?

A

Changes in membrane conductance to different ions

VGIC

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6
Q

Describe how pd changes in AP?

A

RMP at -70mV

Depolarisation til threshold of -55mV

Rapid upstroke to 40mV

Hyperpolarisation below RMP (-80mv)

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7
Q

What happens in initial depolarisation of AP?

A

Gentle increase in Em

Cation (Na+) channels open by chemical/physical stimulus

Or current has spread from neighbouring already excited tissues

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8
Q

What happens in upstroke?

A

Threshold reached, VGNC open and Na+ goes down electrochemical gradient

Once some open, the depolarisation initiates opening of others causing propagation (+ve feedback)

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9
Q

What happens in repolarisation?

A

Na+ channels inactivated

VGKC open, leads to K+ efflux down their electrochemical gradient

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10
Q

When do VGKC get stimulated to open and when do they open, why?

A

Stimulated at the same time as VGNC, but slow to open so open later

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11
Q

Why does hyperpolarisation occur?

A

VGKC slow to close

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12
Q

What occurs in the hyperpolarisation stage?

A

Hyperpolarisation of the membrane - raised K+ permeability takes the Em closer to Ek.

The Na+ channels are restored from inactivated to closed.

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13
Q

How is the hyperpolarised membrane returned to RMP?

A

Mainly action of Na/KATPase

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14
Q

What are the K+ channels called?

A

Delayed rectifier channels

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15
Q

What is the difference between absolute and relative refractory period?

A

Absolute: no upstroke can be initiated regardless of depolarisation. VGNC recover from activation (time and voltage dependent)

Relative refractory period: large stimulus required to elicit AP. Slightly after closure of K+ channels

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16
Q

What is the refractory period?

A

Neurone cannot fire another action potential

When the voltage-gated sodium ion channels are in their inactivated state the

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17
Q

What can the cell membrane be imagined as and why (physics)?

A

A capacitor, it separates charge

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18
Q

How is AP propagated in unmyelinated neurones?

A

Na+ influx generates local depolarisation

Negative region draws +ve charge from areas behind and ahead

Depol in one region is stimulus for depol and opening of VGNC in the other

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19
Q

What does the refractory period provide?

A

Unidirectional flow

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20
Q

What is the term for one way AP propagation?

A

Orthrodromic

21
Q

How can nerve propagation be sped up (overall)?

A

Capacitance or resistance must be reduced, to decrease the time period of depolarisation.

22
Q

What are gaps between myelination called?

A

Nodes of ranvier

23
Q

Effect of myelination on length and time constant

A

Decrease capacitance - decrease time constant

Increase membrane resistance - increase length constant

24
Q

Describe AP propagation in myelinated neurones

A

Saltatory conduction

Depolarisation wave jumps from node to node as the Na+ channels are packed into the nodes.

25
Q

What effect does myelination have on speed of propagation?

A

Increase the speed from 1-2m/s to up to 120m/s.

26
Q

What neurones are myelinated?

A

Motor neurones

Preganglionic (ANS)

27
Q

Why does myelination increase the length constant?

A

Larger Rm

28
Q

Why does myelination decrease time constant?

A

Decreases capacitance (increases the distance between the cations on the outside of the axon and the Na⁺-ions that enter the axon at the nodes of Ranvier during an action potential)

29
Q

What is the time constant?

A

Time taken for voltage to fall to 1/e of its initial value

30
Q

Faster time constant…

A

Membrane ahead of impulse reaches threshold quicker so faster conduction velocity

31
Q

What is length constant?

A

Distance needed for voltage to drop to 1/e

32
Q

Long length constant means…

A

More distant areas of membrane ahead of impulse can be depolarised to threshold - faster conduction velocity

33
Q

Equation for length constant

A

Square root: membrane resistance/internal resistance

34
Q

Equation for time constant

A

Membrane resistance x capacitance

35
Q

What factors affect conduction velocity?

A

Myelination (increases)

Nerve diameter (larger, faster)

Temperature (higher, faster)

36
Q

How does increased nerve diameter affect length constant?

A

Decreased internal resistance so increased length constant

37
Q

Why does increased temperature increase conduction velocity?

A

High temp ions diffuse faster into neurone and along axon

38
Q

What is internal resistance inversely proportional to?

A

Cross sectional area

39
Q

Describe a alpha nerve fibres?

A

100m/s
Myelinated
15-20 micrometer
Innervate skelelal muscle

40
Q

Describe a gamma nerve fibres?

A

2-5 micrometer
Sensory (pain and temp)
Myelinated

41
Q

Describe b nerve fibres?

A

Moderately myelinated Conduction velocity is 3 to 14 m/s

General visceral afferent fibers and preganglionic nerve fibers of the autonomic nervous system.

42
Q

Describe c nerve fibres?

A

0.5-1 micrometer
1m/s
Unmyelinated sensory (transmit info e.g. pain temp to CNS)

43
Q

What is membrane resistance inversely dependent on?

A

Axon circumference

44
Q

Compare the stimulus needed to initiate AP in small vs large diameter axons

A

Stimulus needed is larger in small axons

45
Q

Is the Na+/K+ ATPase pump necessary to restore the membrane potential to its resting value at the end of an action potential?

A

No

46
Q

What four changes increase velocity of AP?

A

Decrease in external resistance

Decrease in internal resistance

Decrease in membrane capacitance

Increase in membrane resistance

47
Q

The initial (local) depolarization (immediately after point A) is mediated by…

A

Ligand-gated ion channels

48
Q

How would TTX affect the threshold of AP?

A

Make it higher, less negative

Electrical excitability of cells (9 decks)

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