7 - Action Potential

Question 1

Draw the graph for an action potential.

Answer 1

• All or nothing if threshhold reach
• Same aplitude if threshold reached
• Membrane potential trying to get to E_Na

Question 2

What are the differences in action potentials in the:

• axon
• skeletal muscle
• SAN
• Cardiac ventricle

Answer 2

Question 3

How is an action potential generated?

Answer 3

• If ENa is altered by change in membrane permeability, peak of action potential will change

Question 4

In an axon, how does a neurotransmitter cause an action potential?

Answer 4

Question 5

What is conductance (g) of a membrane?

Answer 5

• Depends on the number of channels for an ion that are open, higher conductance means more permeable
• Increase in conductance for an ion, increase in no of channels open, leads to membrane potential moving closer to that ions Ek

Question 6

How many sodium ions have to move in order to result in an action potential?

Answer 6

Very small amount

40 um!!!

Larger diameter, even smaller conc change needed

Question 7

What is voltage clamp?

Answer 7

A technique used to measure the voltage current at a set membrane potential

Can see how ions flow through voltage gated K+ and Na+ channels at different membrane potentials

Question 8

What are the conductance changes that occur for Na and K during an action potential?

Answer 8

Question 9

Explain the positive feedback of an action potential.

Answer 9

Repolarisation is two stages, inactivation of Na and opening of K

Question 10

What are refractory periods?

Answer 10

ARP - All Na channels inactivate. No A.P can be fired whatsoever. Time taken from initial opening and initial inactivation of Na channels (1ms)

RRP - Na channels recovering as K channels open. If strong enough stimulus an A.P can be initiated. (4ms)

Question 11

What is the structure of an Na and K channel?

Answer 11

• Remember how many subunits to be functional
• Ca channel similar to Na

Question 12

How do individual channels open?

Answer 12

• Open in random manner
• Once open they are prone to inactivation and inactivate at random times

Question 13

How does a voltage sensor work?

Answer 13

• Depolarised membrane means more positive charge in
• +ve charge in cell repels +ve sensor
• Conformatonal change, opens pore

Question 14

How do local anaesthetics work?

Answer 14

• Lidocaine can be protonated (hydrophilic) or unprotonated (lipophilic)
• In body mainly protonated due to pKa

Question 15

What fibres do local anaesthetics block?

Answer 15

In order:

1. Small myelinated axons
2. Unmyelinated axons
3. Large myelinated axons

Question 16

What happens when you change the diameter of an axon?

Answer 16

• Increase diameter
• Increase conduction velocity

Question 17

How do you measure conduction velocity?

Answer 17

• Take nerve fibre with several different sized axons
• Stimulate an action potential with electric field
• Measure time taken from stimulus to electrode
• Velocity = Distance from stimulus to electrode / Time taken

Question 18

What is the local current theory?

Answer 18

• Current flows into axon causing a local change in membrane potential, which spreads along the whole axon but decrease in amplitude as it does so

Question 19

What properties does an axon need to have a high conduction velocity?

Answer 19

Capacitance = ability to store charge

Resistance = no of ion channels open, more channels open lower resistance

High resistance = voltage spreads further along axon

High capaticance = voltage changes more slowly

Question 20

Draw graphs to represent increasing C_m and R_m

Answer 20

Question 21

Where does myelination occur and what does it do to the properties of axons?

Answer 21

• Large axons myelinated (motor) but small (sensory) not
• Decreases capacitance, increases resistance and therefore increases length constant
• Allows saltatory conduction and therefore increased conduction velocity

Question 22

How does myelin allow saltatory conduction?

Answer 22

• Good insulator
• Allows local current to depolarise next node above threshold and initiate an action potential
• Jumps from node to node

Question 23

Draw a graph to show relationship between fibre diameter and conduction velocity

Answer 23

Question 24

What are some diseases caused by demyelination?

Answer 24

• No myelin, length constant reduced, threshold not reached so conductance is blocked
• Stops saltatory conduction

Question 25

What carries the local current in an axon?

Answer 25

Na, K, Cl

Question 26

Why is there faster conduction velocity in unmyelinated axons below 1 um?

Answer 26

Resistance is limiting factor in myelinated as less axoplasm so smaller SA and more resistance d/D = 0.7

Question 27

How long does it take for an impulse to jump from node to node?

Answer 27

20 us at 37 degrees

Question 28

Do nerve cells regenerate?

Answer 28

Yes in PNS as Scwhann cells, 1mm a day

Question 29

When does myelination occur?

Answer 29

4 month fetus to 1 year old

Question 30

Where are K channels in myelinated axons?

Answer 30

Paranodal

Question 31

What is MS?

Answer 31

Autoimmune CNS disease. Antibodies attack proteins that maintain integrity of myelin so demyelination. No myelin, no insulation, less conductance

Question 32

What is the internodal delay?

Answer 32

0.5 ms, refractory period

Question 33

What happens in a demyelinated axon when you add a voltage K+ channel blocker?

Answer 33

Prolongs the action potential, increases the chance that the node can excite the next resting node, overcomes demyelination damage