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Science for Medicine > How Nerves Work 4 > Flashcards

How Nerves Work 4 Flashcards

1
Q

What is the threshold potential?

A

-55mV

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2
Q

What happens once the threshold potential has been reached?

A

Sudden massive depolarisation and overshoot to +30mV

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3
Q

In AP what causes depolarisation?

A

Voltage gated Na channels allowing Na to flood into the cell

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4
Q

In AP what causes hyperpolarisation?

A

Voltage gated K channels allowing K to flow out of the cell

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5
Q

What happens immediately after threshold is reached?

A
  • Na floods in and depolarises the cell.

- Massive increase in permeability causes decrease in resistance

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6
Q

What is an example of positive feedback

A

Opening of Na channels once threshold id reached

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7
Q

What increases K permeability?

A

The opening of more K channels

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8
Q

What does the opening of K channels cause?

A

Hyperpolarisation then repolarisation

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9
Q

What is the refractory period?

A

Time in which the AP cannot be fired immediately after channels close

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10
Q

What can block voltage dependent Na channels?

A
  • Local anaesthetics (procain/lidocaine)
  • Puffer fish toxin= tetrodotoxin
  • Shellfish toxin= saxitoxin
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11
Q

What are the properties of AP?

A
  • Have a threshold
  • All or none
  • Cannot encode stimulus intensity in amplitude, only frequency
  • Self-propagate
  • Voltage-gated channels
  • Travel slowly
  • Refractory period
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12
Q

What does increasing the stimulus of an AP do?

A

Increases the time the AP are fired for

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13
Q

What ensures that AP only travel in 1 direction?

A

Refractory period

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14
Q

What does self-propagation of AP do?

A

Depolarises the next bit of the axon and opens the voltage dependent Na channels there

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15
Q

Why do AP travel slowly?

A

Delay in the opening of the Na channels

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16
Q

What can improve conduction velocity?

A
  • Big axons

- Myelination

17
Q

Why does large axons increase conduction velocity?

A
  • Bigger axons=lower axial resistance
  • Depolarisation can spread further
  • Na channels can be spread out further
  • Less opening of voltage gated channels
18
Q

What is myelin?

A

Folds of membrane from

  • Schwann cells (PNS)
  • Ogliodendrocytes (CNS)
19
Q

What are the gaps between myelin sheaths called?

A

Nodes of Ranvier

20
Q

How does AP spread with little decrement in a myelinated axon?

A
  • Na channels are present in the nodes.

- Myelin increases resistance of membrane and allows AP to spread like a local current

21
Q

What kind of conduction takes place in myelinated axons?

A

Saltatory conduction

22
Q

What are the advantages of myelination?

A
  • Speeds up conduction
  • Metabolically good= ions only cross membrane at nodes
  • Saves space
23
Q

What do Schwann cells do?

A
  • Prevents leaking
  • Increases membrane resistance
  • Decreases membrane capacitance
24
Q

Why does the current in a myelinated axon spread further than in a non-myelinated axon?

A

There is less current wasted leaking out of the membrane or charging up the capacitance

25
Q

What conditions involve de-myelination?

A
  • Multiple sclerosis

- Guillain-Barre syndrome

26
Q

What is the effect of de-myelination?

A
  • Big local current decays quicker
  • Does not depolarise to next node to threshold
  • And conduction fails
  • It decays too quickly and there is not sufficient depolarisation to reach the threshold at the next node
27
Q

Why are compound action potentials evoked?

A
  • Mammals have small and large myelinated and unmyelinated axons.
  • Therefore extracellular recording from a bundle of axons (a nerve trunk) evokes a compound AP.
28
Q

What fibres are most sensitive to anoxia?

A

big ones

29
Q

What fibres are most sensitive to local anaesthetics?

A

Small ones

30
Q

What are the classes of fibres and what are they responsible for?

A
  • Aa: proprioception, motor neurones
  • AB: touch, pressure
  • Ay: motor neurones of muscle spindles
  • Ao: touch, cold, fast pain
  • B: preganglionic autonomic fibres
  • C: heat, slow pain

Science for Medicine (50 decks)

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