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ESA 2- Membranes and Receptors > Electrical Excitability > Flashcards

Electrical Excitability Flashcards

(70 cards)

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1
Q

What is an action potential?

A

A change in voltage across a membrane

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2
Q

What does the action potential depend on?

A

Ionic gradients

The relative permeability of the membrane

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3
Q

What is meant by action potentials being ‘all or nothing’?

A

There is no half, or double, action potentials etc

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4
Q

What happens once an action potential has been generated?

A

It is propagated without loss of amplitude

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5
Q

What happens once the membrane has been depolarised to threshold voltage?

A

Voltage gated Na channels open

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6
Q

What is the effect of the opening of voltage gated Na channels?

A

It allows Na influx as Na ions attempt to move to their equilibrium potential

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7
Q

What is the equilibrium potential of Na ions?

A

+61mV

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8
Q

What does the influx of Na ions cause?

A

It depolarises the membrane further, causing more voltage gated Na channels to open and even more depolarisation

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9
Q

By what process does depolarisation cause more Na ion channels to open?

A

Positive feedback

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10
Q

What is the positive feedback causing further Na channels to open to basis of?

A

The all or nothing characteristic of the action potential

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11
Q

What happens in maintained depolarisation?

A

Na channels close by a mechanisms called inactivation

Voltage gated K channels are opened

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12
Q

What happens when voltage gated K channels are opened?

A

There is K efflux, as K attempts to move towards its own equilibrium

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13
Q

What is the equilibrium potential of K?

A

-88mV

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14
Q

What does the combination of K efflux and Na channel inactivation cause?

A

Repolarisation of the membrane

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15
Q

What is happening in the upstoke of the action potenital?

A

Na channels open

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16
Q

What is the result of the Na channels that open to cause depolarisation being voltage gated?

A

As the moment potential becomes more positive, positive feedback means that more channels will open until they all are

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17
Q

Why can depolarisation not stop half way?

A

At any voltage above threshold, it will be at a voltage where more channels open, thus causing more depolarisation

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18
Q

What is true of Na channels in the absolute refractory period?

A

Nearly all N channels are in the inactive state

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19
Q

What level is excitability at in the absolute refractory period?

A

0

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20
Q

What is true of the Na channels in the relative refractory period?

A

They are recovering from inactivation

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21
Q

What happens to excitability in the relative refractory period?

A

It returns to normal as the number of channels in the inactivated state decreases

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22
Q

What happens as a stimulus gets longer?

A

A larger depolarisation is necessary to initiate an action potential

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23
Q

Why does a longer stimulus mean a larger depolarisation is required to initiate an action potential?

A

Because Na channels become inactivated during the stimulus

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24
Q

How are Na and Ca voltage gated channels similar?

A

In structure

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25
Describe the structure of a Na/Ca voltage gated channel?
They main pore forming subunit is one peptide consisting of four homologous repeats.  Each repeat consists of 4 transmembrane domains
26
What gives the Na/Ca channel its voltage sensitive properties?
One of the transmembrane domains is able to sense voltage across the membrane
27
What does a functional Na/Ca channel require?
One subunit
28
How do voltage gated K channels compare to Na/Ca channels?
They are similar in structure, except each repeat is actually a separate subunit.
29
Describe the structure of voltage gated K channels
Each subunit has 6 transmembrane domains, one of which is voltage sensitive.
30
What does a functional K channel require?
4 subunits
31
Give an example of a local anaesthetic?
Procaine
32
How do local anaesthetics act?
By binding to and blocking Na channels, thereby stopping action potential generation
33
In what order to local anaesthetics block conduction in nerve fibres?
Small myelinated axons Non-myelinated axons Large myelinated axons
34
What is the result of local anaesthetics blocking conduction in nerve fibres in the order that they do?
They tend to effect sensory before motor neurons
35
What kind of molecules are local anaesthetics?
Weak bases
36
How do localised anaesthetics cross the membrane?
In their unionised form
37
When do local anaesthetics block the Na channels?
When the channel is open
38
Do local anaesthetics have a high affinity to the activated or inactivated state of the Na channel?
Inactivated
39
What are electrodes used for in extracellular recording of action potentials?
To raise the membrane to threshold to generate an action potential
40
How can conduction velocity be calculated?
By recording changes in potential between the stimulating (cathode, -ve), and recording (anode, +ve) electrodes along an axon, and using the equation conduction velocity = distance / time
41
What does the depolarisation of a small region of membrane produce?
Transmembrane currents in neighbouring regions
42
What is the result of transmembrane currents being produced in neighbouring regions?
As Na channels are voltage gated, it opens more channels, causing propagation of the action potenital
43
What happens if the local current spreads further?
The conduction velocity of the axon is faster
44
What properties of an axon lead to a high conduction velocity?
A high membrane resistance  A high axon diameter A low membrane capacitance
45
Why does a high axon diameter lead to a high conduction velocity?
Has a low cytoplasmic resistance
46
What is Ohm’s Law?
V=IR
47
What does Ohm’s Law state?
The higher the resistance of the membrane, the higher the potential difference across it
48
What does more voltage across the membrane result in?
More voltage gated Na channels being open
49
What is the result of more voltage gated Na channels being opened?
It is easier to reach the threshold to fire an AP
50
What effect does an increase in number of Na channels have on conduction velocity?
It increases it
51
Why does large axon diameter increase conduction velocity?
Ohm’s Law states that the lower the resistance, the larger the current, and therefore the action potential will travel further, increasing action potential
52
What is capacitance?
The ability to store charge
53
What will a membrane with high capacitance do?
Take more current to charge, or a longer time for a given current
54
What happens to a the time taken to charge in membrane with a low capacitance, for a given current?
It is shorter, thus increasing conduction velocity
55
What happens to conduction velocity with myelination of axons?
It increases considerably
56
What kind of axons are myelinated?
Large diameter axons, such as motor neurones
57
What kind of axons are not myelinated?
Smaller ones, such as sensory neurones
58
Why does myelination increase conduction velocity?
It reduces capacitance and increases membrane resistance of the axon
59
What does myelination allow for?
Saltatory conduction
60
What is saltatory conduction?
Where the AP ‘jumps’ between Nodes of Ranvier
61
Why does saltatory conduction occur?
Because the myelin sheath acts as a good insulator, causing the local circuit current to depolarise the next node above threshold and generate an AP
62
What do Nodes of Ranvier have?
High density of voltage gated Na channels
63
How does Na channel density differ from Nodes of Ranvier in unmyelinated axons?
They have an even distribution of channels
64
Which cells myelinate peripheral axons?
Schwann cells
65
Which cells myelinate axons in the CNS?
Oligodendrocytes
66
How do cells myelinate axons?
They envelop axons in their plasmalemma
67
Give an example of a disease that strips areas of some axons of their myelin sheaths
Multiple Sclerosis
68
What type of disease is MS?
Autoimmune
69
What happens in MS?
Myeline is destroyed in certain areas of the CNS
70
What effects can demyelination have?
Can have dramatic effects on the ability of previously myelinated axons to conduct action potentials properly, leading to decreased conduction velocity, complete block or cases where only some action potentials are transmitted

ESA 2- Membranes and Receptors (13 decks)

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