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ESA2 > M&R 4 - Electrical Excitability > Flashcards

M&R 4 - Electrical Excitability Flashcards

0
Q

What are the features of an action potential?

A

All or nothing
Propagated without loss of amplitude
Depend on ionic gradient and membrane permeability

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1
Q

What is an action potential?

A

A change in the voltage across a membrane

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2
Q

How long does an AP in an axon take?

A

0.5ms

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3
Q

How long does an AP in skeletal muscle last?

A

0.5ms

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4
Q

How long does an AP in the SAN last

A

200ms

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5
Q

How long does an AP in cardiac ventricle take?

A

280-300ms

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6
Q

How are APs generated?

A

By an increase in sodium permeability and this aims to bring the membrane potential closer to 61mV (equilibrium sodium)

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7
Q

What is the sequence of AP generation?

A 
Membrane depolarised to threshold 
Sodium channels open 
Sodium influx
Further depolarisation 
Positive feedback - more sodium
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8
Q

How does repolarisation come about?

A

This occurs during maintained depolarisation when sodium channels become inactivated and voltage gated potassium channels open causing a potassium efflux aiming to move towards the equilibrium of -88mV

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9
Q

What are the two refractory periods called?

A

Absolute refractory

Relative refractory

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10
Q

What happens during the absolute refractory period?

A

This is when all the sodium channels are inactivated
A second action potential cannot be initiated
Excitation is at 0

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11
Q

What happens during the relative refractory period?

A

A stronger than normal stimulus is needed to elicit an AP
Sodium channels are recovering from inactivation
Excitability returns to normal

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12
Q

What is accommodation?

A

This is when the longer the stimulus lasts the larger the depolarisation threshold needed to initiate an action potential is.
The sodium channels become inactivated

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13
Q

What are the properties of a sodium/calcium gated channel?

A

The active pore is one subunit
One peptide
4 homologous repeats of
6 transmembrane domains (1 of which is voltage sensitive)

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14
Q

What are the properties of a voltage gated potassium channel?

A

This is when the functional pore requires 4 subunits -
4 peptides
6 transmembranous domains ( 1 of which is voltage sensitive)

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15
Q

How do local anaesthetics work?

A

These act by binding to and blocking Na+ channels
They stop AP preventing nervous transmission
They cross the membrane unionised and block Na+ when open

16
Q

In what order to local anaesthetics block conductance?

A

1 small myelinated
2 large unmyelinated
3 large myelinated

17
Q

How is conductance velocity measured?

A

Electrodes are used to raise the membrane potential to generate AP.
Then the changes in potential between the stimulating cathode and the recording anode are measured

18
Q

What is conductance velocity (HINT - equation)

A

CV = distance/time

19
Q

What is the transport of AP called?

A

Propagation

20
Q

How does propagation occur?

A

The depolarisation of a small region of membrane produces transmembrane currents in neighbouring regions this leads to more Na+ channels opening

21
Q

What are three properties of axons that lead to high conductance?

A

High membrane resistance
Large axon diameter
Low membrane capacitance (ability to store charge)

22
Q

Why does high membrane resistance increase CV ?

A

The higher the resistance of membrane = higher the pd across it
More voltage across membrane = more Na+ channels open
Larger current = travels further

23
Q

Why does a large axon diameter lead to high CV?

A

Lower cytoplasmic resistance = larger current

24
Q

How does low membrane capacitance lead to high CV.

A

Low capacitance = less time to charge

25
Q

What is myelin?

A

Is an electrically insulating fatty sheath around axons

26
Q

How does myelination affect CV?

A

Conductance velocity increases with myelination

27
Q

How does myelin increase CV?

A

Reduces capacitance

Increases membrane resistance

28
Q

What is saltatory conduction?

A

AP jumps between the nodes of ranvier as myelin is highly insulating.
Myelin causes the local circuit to depolarise the next node (generate AP)

29
Q

Where is the highest density of Na+ channels in a myelinated axon?

A

In the nodes

30
Q

What forms myelin in the PNS?

A

Schwann cells

31
Q

What forms myelin in the CNS?

A

Oligodendrocytes

32
Q

What is the relationship between conductance velocity and fibre diameter for myelinated and unmyelinated fibres?

A

Myelinated - velocity (dp) diameter

Unmyelinated - velocity (dp) /diameter

33
Q

What is demyelination?

A

This is when some areas of axon lose their sheath

34
Q

What is multiple sclerosis?

A

The autoimmune destruction of myelin in CNS fails to conduct AP

ESA2 (38 decks)

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