Nerve Impulses: Movement Of Action Potential Flashcards

1
Q

Nerve impulse

A

Movement of the action potential (pos charge inside axon relative to outside a neurone due to Na+ ions) along the whole length of an axon

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2
Q

In a non myelinated neurone, what is the first step to move the action potential towards the axon terminal end?

A

Within axon, Na+ (that had entered using voltage gated Na+ channel proteins) will diffuse sideways down electrochemical gradient to an area with low pos charge INSIDE the axon

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3
Q

Local current

A

Diffusion of Na+ ions sideways down electrochemical gradient within an axon

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4
Q

Once a local current has been generated, what are we left with in the axon?

A

Accumulation of Na+ ions thus pos charge in an adjacent patch in the neurone under neurone membrane
Taking this patch to threshold potential

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5
Q

What happens when the adjacent patch in the neurone under the neurone membrane is taken to threshold potential?

A

Voltage gated Na+ channel proteins open allowing more Na+ ions to diffuse in to neurone and increase pos charge causing further depolarisation over the neurone membrane
Generates a new local current

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6
Q

Local currents can move within a neurone…

A

Backwards and forwards: diffusion of Na+ ions down electrochemical grad toward cell body and toward axon terminal end

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7
Q

Why can the action potential only move forward?

A

Due to the refractory period

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8
Q

What is the refractory period?

A

The patch of neurone which just had the area of pos charge (but Na+ions diffused to next patch in the local current) is now repolarising from pos action potential back to resting neg potential
At this point voltage gated Na+ channel proteins SHUT

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9
Q

Why does the refractory period prevent action potential flowing backwards?

A

Because action potential cannot be generated backward in the neurone as the voltage gated Na+ channel proteins SHUT so Na+ ions cannot flow into neurone at this point

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10
Q

When can a new action potential be generated again?

A

After refractory period once restored back to resting potential, however the action potential would have moved forward at this point so therefore it cannot flow backward

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11
Q

‘Discrete’

A

Resting potential must be restored in a patch of neurone under the neurone membrane before generating a new action potential
So the action potentials cannot merge

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12
Q

What does discrete nature of neurone membranes ensure?

A

There is a limit to the number of action potentials transmitted in a neurone (limit to frequency)
So this places a limit on the strength of the stimulus that can be perceived

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13
Q

Passage of the local current (thus generated action potentials) in a myelinated neurone only occurs at…

A

Nodes of Ranvier
Patches on the neurone that are not covered by Schwann cells thus no myelin

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14
Q

Why does myelin maintain the strength of a local current?

A

Because it physically blocks leak of Na+ ions out of neurone as the Na+ diffuses down electrochemical gradient sideways within the neurone
Maintains strength of local current therefore so can travel a longer distance without needing to generate a new action potential

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15
Q

Why can the generation of a new action potential only occur at nodes of ranvier?

A

No myelin surrounds neurone membrane for exchange of ions to surrounding fluid:
Na+ ions into neurone
K+ ions leave neurone

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16
Q

When is a new action potential generated?

A

When the local current weakens due to leak of Na+ ion through leak channel proteins

17
Q

Why are myelinated neurones better at sending an impulse?

A

Local current travels longer before weakening due to leak of Na+ by leak channel proteins
Occurring only at nodes of ranvier means impulses are quicker

18
Q

Why is passage of an action potential in unmyelinated neurone quite slow?

A

the whole length of the neurone membrane needs to be progressively depolarised to pass an impulse from one end of an axon to another

19
Q

Saltatory conduction

A

Depolarisation only occurs at the nodes of Ranvier and so the nerve impulse essentially jumps from node to node so transmission of an action potential is FASTER

20
Q

Does saltatory conduction maintain strength of local current?

A

No, while both are due to myelination of neuron:
-Myelin insulates neurone so strength of local current maintained
-Myelin causes saltatory conduction so action potential is FASTER

21
Q

How does axon diameter effect speed of transmission?

A

Wider the axon, the faster transmission of impulse

22
Q

Why do wider axons have faster speeds of transmission in terms of resistance)

A

Wider axons have more alternative pathways for transmission of action potential = less resistance to flow of local current (Na+ diffusion)
Resistance caused by organelles

23
Q

Why do wider axons have faster speeds of transmission (in terms of surface area to volume ratio)

A

Wider axons have lower surface area to volume ratios
This reduces the leakage of local current ions Na+ per unit volume of axons and therefore strengthens these currents = can travel longer before weakening

24
Q

What are the 2 reasons for why wider axons have faster transmission of action potential?

A

Less resistance due to more alternative pathways for transmission of action potential
Wider axons have lower surface area to volume ratio

25
Q

Why does temperature increase transmission of impulse?

A

Higher temp = higher kinetic energy of Na+/K+ ions
So INCREASES SPEED OF ION DIFFUSION

26
Q

Effect of excessively high temperatures on transmission of action potential?

A

Causes voltage gated Na+/K+ channel proteins and Na+/K+ pump carrier proteins (resting potential) to denature so no transport of K+/Na+ over neurone membrane
No more generation of an action potential

27
Q

Factors affecting action potential speed

A

Myelination: saltatory conduction increases speed
Higher temperatures increase speed
Wider axon diameter increases speed

28
Q

What must be included when discussing maintaining the strength of action potential??

A

Electrical insulation due to myelination
Blocks leak of Na+ ions by leak channel proteins
SO MAINTAINS STRENGTH OF LOCAL CURRENT
Can travel longer length of axon before generating another action potential

29
Q

What must be included when discussing speed of impulse in terms of myelination?

A

Electrical insulation by myelin
So saltatory conduction: generation of action potentials only occurs at nodes of ranvier
So this is faster than progressive depolarisation over whole length of axon in unmyelinated neurone