M&R 4.1 - The Action Potential and its Properties

Question 1

What is an action potential?

Answer 1

The change in voltage across a membrane depending on both ionic gradients and relative permeability of the membrane

Question 2

Why is the ‘all or nothing’ principle of action potentials significant?

Answer 2

Ensures there are no half or double action potentials i.e. they are propagated without getting smaller

Question 3

What is the significance of positive feedback in opening VG Na+ channels during the action potential?

Answer 3

• Positive feedback causes more channels to open so causes more depolarisation
• Ensures depolarisation doesn’t stop half way through

Question 4

What is the absolute refractory period?

Answer 4

The period of time at which nearly all the Na+ channels are inactivated meaning the excitability is at 0

Question 5

What is the relative refractory period?

Answer 5

• The period of time at which the Na+ channels are recovering from inactivation so the excitability is returning to normal
• Number of inactivated channels decreases

Question 6

Describe what happens during repolarisation

Answer 6

• Inactivation of Na+ channels = potential returns to RMP

- VG K+ channels open = potential moves towards Ek

Question 7

Describe accommodation

Answer 7

• Longer stimulus = larger depolarisation needed to generate an action potential due to inactivation
• Threshold becomes more positive
• Decreases intensity as initial action potential doesn’t reach threshold as more Na+ channels are inactive

Question 8

Describe the structure of Na+/Ca2+ channels

Answer 8

• One peptide of FOUR homologous repeats
• Each repeat has SIX transmembrane domains
• One transmembrane domain is sensitive to changes in voltage (positive amino acid residue)
• ONE pore formed by a dipped Beta sheet in which the inactive particle sits

Question 9

Describe the structure of a VG K+ channel

Answer 9

• FOUR subunits
• Each subunit has SIX transmembrane domains
• One transmembrane domain is sensitive to changes in voltage
• Pore is formed by FOUR subunits

Question 10

Describe the effects of local anaesthetics e.g. Procaine

How are these effects caused?

Answer 10

• Bind to and block Na+ channels = stops AP generation
• Cross membrane in unionised form and block Na+ channels when open
• Have a higher affinity to the inactive state of Na+ channels

Question 11

In what order are local anaesthetics most likely to affect neurons? Why?

Answer 11

• Small myelinated axons
• Non-myelinated axons
• Large myelinated axons
• Tend to affect sensory neurons before motor neurons