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Electrical excitability of cells > The action potential > Flashcards

Flashcards in The action potential Deck (48)
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1

What is an AP?

Transient impulse triggered by a depolarisation beyond threshold

2

Length of AP?

1-2 Ms

3

What three things can trigger an AP?

Electrode
Synaptic activity
Propagation of AP

4

How much is the Em displaced in an action potential?

Up to 100mv

5

What underpins an action potential?

Changes in membrane conductance to different ions

VGIC

6

Describe how pd changes in AP?

RMP at -70mV

Depolarisation til threshold of -55mV

Rapid upstroke to 40mV

Hyperpolarisation below RMP (-80mv)

7

What happens in initial depolarisation of AP?

Gentle increase in Em

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

Or current has spread from neighbouring already excited tissues

8

What happens in upstroke?

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

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

9

What happens in repolarisation?

Na+ channels inactivated

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

10

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

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

11

Why does hyperpolarisation occur?

VGKC slow to close

12

What occurs in the hyperpolarisation stage?

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

The Na+ channels are restored from inactivated to closed.

13

How is the hyperpolarised membrane returned to RMP?

Mainly action of Na/KATPase

14

What are the K+ channels called?

Delayed rectifier channels

15

What is the difference between absolute and relative refractory period?

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

16

What is the refractory period?

Neurone cannot fire another action potential

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

17

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

A capacitor, it separates charge

18

How is AP propagated in unmyelinated neurones?

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

19

What does the refractory period provide?

Unidirectional flow

20

What is the term for one way AP propagation?

Orthrodromic

21

How can nerve propagation be sped up (overall)?

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

22

What are gaps between myelination called?

Nodes of ranvier

23

Effect of myelination on length and time constant

Decrease capacitance - decrease time constant

Increase membrane resistance - increase length constant

24

Describe AP propagation in myelinated neurones

Saltatory conduction

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

25

What effect does myelination have on speed of propagation?

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

26

What neurones are myelinated?

Motor neurones
Preganglionic (ANS)

27

Why does myelination increase the length constant?

Larger Rm

28

Why does myelination decrease time constant?

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

What is the time constant?

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

30

Faster time constant...

Membrane ahead of impulse reaches threshold quicker so faster conduction velocity