M&R The action potential

Question 1

How does an action potential come about?

Answer 1

Once the membrane has been depolarised to the threshold voltage, voltage gated Na+ channels open allowing influx of Na+.
The influx depolarises the membrane further causing even more voltage gated sodium channels to open.

Question 2

What happens during repolarisation?

Answer 2

Sodium channels close and become inactive.
Voltage gated potassium channels are opened by the depolarisation causing an efflux of K+.
These 2 events cause the membrane to repolarise

Question 3

What are the 2 types of refractory period?

Answer 3

Absolute refractory period:
Nearly all sodium channels are the inactivated state so it is impossible to excite the membrane

Relative refractory period:
Sodium channels are recovering from inactivation, it is now possible to excite the membrane however a larger stimulus is needed.

Question 4

Describe accommodation

Answer 4

The longer a stimulus is, the larger the depolarisation needed to initiate an action potential. The threshold potential gradually increases and the peak of the action potential decreases.

Question 5

What is the structure of :

• voltage gated Na+/Ca2+ channels
• voltage gated K+ channels

Answer 5

Voltage gated Na+/Ca2+ channels:
1 peptide with 4 homologous repeats
Each repeat has 6 transmembrane domains
1 domain is able to sense voltage across the membrane

Voltage gated K+ channels:
There are 4 subunits
Each subunit has 6 transmembrane domains
1 domain is able to sense voltage across the membrane

Question 6

How do local anaesthetics work?

What order of nerve fibres do they block?

Answer 6

Local anaesthetics e.g. procaine, bind to and block sodium channels, which stops AP generation. They block the channels when they are open, and have a higher affinity for the inactivated channels.

The first block small myelinated axons, then non-myelinated axons then large myelinated axons –> so they affect sensory before motor neurones.

Question 7

How is extracellular recording used to measure conductance velocity?

Answer 7

Electrodes are used to raise membrane potential to threshold to generate an AP
Changes in potential are measured between the stimulating cathode (-ve) and the recording anode (+ve) along an axon.
The conduction velocity is calculated by distance / time

Question 8

What is the local circuit theory of propagation?

Answer 8

The depolarisation of a small region of membrane produces TRANSMEMBRANE CURRENTS un neighbouring regions.
As sodium channels are gated, this opens more channels causing propagation of the action potential.

Question 9

What factors affect conduction velocity?

Answer 9

High conduction velocity:

• high membrane resistance
  Ohm’s law states the higher the resistance, the higher the potential difference across it. More voltage means more sodium channels open so easier to fire AP.
• high axon diameter
  Ohm’s law states the lower the resistance (lower cytoplasmic resistance from large diameter) the larger the current
• low membrane capacitance
  Capacitance is the ability to store charge, so a low capacitance takes less time to charge so faster conduction velocity

Question 10

What effect does myelination have on conductance velocity?

Answer 10

Myelination considerably increases conduction velocity.

Myelination:

• reduces capacitance
• increases membrane resistance
• allows for saltatory conduction

Question 11

What are the consequences of demyelination and how does it occur?

Answer 11

Multiple Sclerosis is an autoimmune disease which destroys myelin in areas of the CNS
This has dramatic effects on the axons to conduct APs

• speech problems
• dizziness
• spasms
• incontinence