4. Action Potentials & Electrical Excitability

Question 0

What is meant by the all or nothing rule of action potentials?

Answer 0

If the threshold value is met, it will always give a complete response, otherwise no response is given.

Question 1

How is an action potential generated?

Answer 1

Increasing membrane permeability to Na+ (bringing the membrane potential closer to ENa, +61mV)

Question 2

Repolarisation involves what changes to which channels?

Answer 2

Inactivation of Voltage Gated Na+ channels

Opening of Voltage Gated K+ channels

Question 3

What is the absolute refractory period and how does it affect the direction in which an impulse travels?

Answer 3

When Voltage Gated Na+ Channels are either all open or inactivated hence no matter how much stimulation is received, another action potential will not occur.
This means that an action potential will only travel in one direction along an axon.

Question 4

What is the relative refractory period?

Answer 4

Where Voltage Gated Na+ channels are recovering and returning to the closed state from the inactivated state.
As the number of channels inactivated decreases, the excitability returns to normal.

Question 5

What is the structure of Voltage Gated Na+/Ca2+ channels?

Answer 5

One peptide
Four homologous repeats each with 6 transmembrane domains
One domain is voltage sensitive
Contains inactivation particle

Question 6

What is the structure of Voltage Gated K+ channels?

Answer 6

Four peptides
6 transmembrane domains
One domain is voltage sensitive
Contains a p region that contributes to pore sensitivity

Question 7

Name an anaesthetic that blocks Na+ channels to stop an action potential.

Answer 7

Procaine

Question 8

Name the order of nerve types and sizes in which procaine blocks conduction best in.

Answer 8

1 Small myelinated
2 Non myelinated
3 Large myelinated

Question 9

Under which electrode does electrical stimulation occur?

Answer 9

Cathode (negative)

Question 10

Under which electrode is excitability reduced?

Answer 10

Anode (positive)

Question 11

What is diphasic recording?

Answer 11

Where both electrodes are extracellularly recorded to measure conduction velocity

Question 12

What is monophasic recording?

Answer 12

Where one electrode is purposely put in a damaged part hence only the other is used to record conduction velocity

Question 13

How do you calculate conduction velocity?

Answer 13

Distance between stimulating and recording electrodes divided by time gap between stimulus and action potential being recorded

Question 14

How is an action potential conducted?

Answer 14

Local current causes the change in membrane potential in one area to spread to adjacent areas of the axon, causing them to reach threshold and fire an action potential

Question 15

What is the relationship between local current spread and conduction velocity?

Answer 15

The further down the axon it spreads, the higher the conduction velocity.

Question 16

Name three factors that can increase conduction velocity.

Answer 16

High membrane resistance
Low membrane capacitance
Large axon diameter ( as this leads to low cytoplasmic resistance)

Question 17

What does reducing the capacitance at the internodal region via myelination lead to?

Answer 17

Saltatory conduction, the local current induced at one node can travel far enough down to the next node to depolarise that one

Question 18

Which is faster, local current spread or action potential spread?

Answer 18

Local current spread, hence why saltatory conduction is the quickest

Question 19

What is multiple sclerosis?

Answer 19

Demyelination of axons in the CNS

Question 20

What is Devic’s disease?

Answer 20

Demyelination of optic and spinal axons

Question 21

Name two diseases which involve demyelination of axons in the peripheral nervous system

Answer 21

Landry Gullain Barre syndrome

Charcot Marie Tooth disease

Question 22

Immediately after demyelination of an axon why is there complete failure to conduct an impulse?

Answer 22

Increased membrane capacitance preventing the channels at the nodes being raised to threshold

Question 23

Why after a period of recovery after demyelination does the ability to conduct an impulse return?

Answer 23

Redistribution of nodal ion channels (effectively becomes an unmyelinated nerve)