W4- The Action Potential And Its Properties

Question 1

What does all or nothing mean?

Answer 1

That you either get an action potential or you do not

Question 2

Is an a.p propagated without a loss of amplitude?

Answer 2

Yes

Question 3

What is the range for an a.p in an axon e.g from -mV to +mV? And the time?

Answer 3

• 70mV to +30mV

0. 5ms

Question 4

What is the range for an a.p in skeletal muscle e.g from -mV to +mV? And the time?

Answer 4

• 90mV to +40mV

0. 5ms

Question 5

What is the range for an a.p in SAN e.g from -mV to +mV? And the time?

Answer 5

-60mV to +30mV

100ms

Question 6

What is the range for an a.p in a cardiac ventricle e.g from -mV to +mV? And the time?

Answer 6

-90mV to +30mV

100ms

Question 7

What is the conductance of the membrane to a particular ion dependent on?

Answer 7

The number of channels open for that ion

Question 8

What ion is responsible for the generation of an a.p?

Answer 8

A.p generated by increase in membrane permeability to Na+ so membrane moves closer to ENa.

Question 9

Why does the a.p peak not reach ENa?

Answer 9

Because the membrane is not only permeable to Na+ ions e.g K+ too.

Question 10

How do you make ENa more negative?

Answer 10

Reduce EC Na+ conc

Question 11

By how much does each a.p increase the Na+ conc in an axon?

Answer 11

By 40uM

0.4% increase if resting Na+ conc is 10mM

Question 12

What does a voltage clamp exp do?

Answer 12

Controls the membrane potential so that the ionic current flowing through the membrane can be measured

Question 13

What is a voltage clamp exp useful to see about ion channels?

Answer 13

Gives a good measurement of the effect of voltage on the number of Na+ and K+ channels open at different membrane potentials

Question 14

What does a patch clamp exp enable you to do?

Answer 14

Measure the current flowing through an individual ion channel

Question 15

What is the sodium hypothesis?

How does positive feedback occur?

Answer 15

Once the membrane has been depolarised to the threshold value, VGNa+ channels open allowing Na+ influx as Na+ ions attempt to move towards ENa of +61mV.

Influx depolarises membrane leading to further VGNa+ channel opening (positive feedback)

Question 16

What tow mechanisms cause repolarisation of the membrane?

Answer 16

VGK+ channels open leading to K+ efflux as they move towards EK of -88mV

Na+ channels close by inactivation

Question 17

Which channels open quicker- Na+ or K+?

Answer 17

Na+

K+ are delayed rectifiers

Question 18

At what part of the axon is the a.p initiated from?

Answer 18

The axon hillock

Question 19

What type of feedback is the basis of the all or nothing response?

Answer 19

The positive feedback that leads to the opening of more and more Na+ channels during depolarisation

Question 20

Is the Na+/K+ pump involved in the repolarization of the a.p?

Answer 20

No it plays no role

Question 21

What is the ARP?

Why can you not conduct an a.p?

Answer 21

The absolute refractory period

No a.p is fired even if the threshold is met because nearly all Na+ channels are in the inactivated state.

Question 22

What is the RRP?

Answer 22

The relative refractory period

Can have some slow depol as Na+ channels are recovering from inactivation and the excitability returns closer to normal

Question 23

Describe accommodation?

Answer 23

They longer a stimulus is, the larger the depol necessary to initiate an a.p because Na+ channels become inactivated.

Question 24

What happens to the threshold during accommodation?

Answer 24

It becomes more positive

Question 25

Describe the structure of Na+ channels and the 2 key TMDs

Answer 25

4 homologous repeats each containing 6 TMDs. S4 is Voltage sensitive (+ve aa's) S5-6 is pore forming region One subunit required to be functional

Question 26

What is the role of the inactivation particle?

Answer 26

It can swing into an open pore of a VGNa+ channel and block the flux of Na+ ions

Question 27

Describe the key features of the K+ channels

Answer 27

Each subunit has 6 TMDs S4 is voltage sensitive (+ve aa's) S5-6 is pore forming 4 subunits require to make functional domain

Question 28

How do local anaesthetics work? Give an example

Answer 28

Bind to and block Na+ channels when open stopping a.p generation. Weak bases, cross p.m in unionised form. Have higher affinity for inactivated state of Na+ channel E.g procaine

Question 29

List the order of nerve fibres blocked by LA form first to last Do they effect sensory or motor first?

Answer 29

Small myelinated axons Non-myelinated axons Large myelinated axons (Effect sensory before motor)

Question 30

What two things do a.p's depend on?

Answer 30

Ionic gradients and relative permeability of the membrane

Question 31

Name 2 diseases that can effect the CNS and 2 for the PNS

Answer 31

MS- all CNS nerve, Devic's disease- optic and spinal cord Landry-Guillian-Barre syndrome- PNS, Carcot-Marie-Tooth disease- PNS

Question 32

How do you do an EC recording of an a.p?

Answer 32

Use electrodes to raise the membrane potential to threshold and generate a.p. Record changes in potential between cathode (stimulating) and anode (recording) electrodes along axon. Calculate conduction velocity (distance X time)

Question 33

Explain the local current theory of propagation

Answer 33

The depol of a small region of membrane produces TM currents in neighbouring regions. Na+ channels open and a.p propagated. The further the local current spreads the aster the conduction velocity of the axon

Question 34

What is the length constant? What does it indicate?

Answer 34

The distance it takes for the potential to fall to 37% of its original value Indicates how far local depol spreads

Question 35

What is capacitance and what does membrane resistance depend on?

Answer 35

Capacitance= the ability to store charge Membrane resistance depends on number of ion channels open (lower resistance, more open)

Question 36

What 3 properties of a membrane, in terms of R,C and diameter lead to a high conduction velocity?

Answer 36

High membrane resistance and axon diameter, low membrane capacitance

Question 37

What is the eq for Ohm's Law?

Answer 37

V=IR

Question 38

Are sensory and motor neurones myelinated?

Answer 38

Motor are- larger, sensory are not- smaller

Question 39

How does myelination increase conduction velocity?

Answer 39

Reduces capacitance (x100) and increases membrane resistance (x100)

Question 40

What role does myelination play in saltatory conduction?

Answer 40

Myelin sheath acts as a goo insulator allowing local circuit currents to depol next node above threshold and generate a.p

Question 41

What VG channel is found in the Nodes of Ranvier? How does this contrast to unmyelinated axons?

Answer 41

VGNa+ channels- 10,000/channel Unmyelinated e.g C sensory axons have even distribution of channels along axon

Question 42

What cells myelinated in the CNS and PNS?

Answer 42

CNS- oligodendrocytes- wrap round many axons, PNS- Schwann cells- one axon

Question 43

What is the consequence of demyelination? Give an example of a disease where this happens

Answer 43

Decreased conduction velocity, complete block or only few a.p transmitted E.g MS- AI against myelin of the CNS