Session 4

Question 1

What is an action potential?

Answer 1

A change in voltage across a membrane.

Question 2

What does an action potential depend on?

Answer 2

On ionic gradients and the relative permeability of the membrane.

Question 3

What is the sodium hypothesis?

Answer 3

Once the membrane reaches a threshold voltage the Na+ channels open. Allows Na+ in. This depolarises the membrane, has a snowball effect.

Question 4

What happens to Na+ channels during repolarisation?

Answer 4

They close by a mechanism called inactivation.

Question 5

What other channels open during repolarisation?

Answer 5

Voltage gated K+ channels, so there is a K+ efflux.

Question 6

Why does K+ flow out of the cell during repolarisation?

Answer 6

Because it wants to move towards its equilibrium potential.

Question 7

Hiw do Na+ channels ensure an all or nothing response?

Answer 7

They are voltage gated with positive feedback. Therefore once some are activated they cause Na+ influx so it becomes more positive which causes more to open, causing more depolarisation.

Question 8

What is the Absolute refractory period?

Answer 8

All Na+ channels are in the inactivated state, excitability is at 0.

Question 9

What is the Relative refractory period?

Answer 9

Na+ channels are recovering from inactivation. Excitability returns to normal as the number of channels in the inactivated state decreases.

Question 10

What is accommodation?

Answer 10

The longer a stimulus is, the larger the depolarisation necessary to initiate an action potential. This is because Na+ channels become inactivated.

Question 11

What is the molecular similarity between voltage gated Na+ and Ca2+ channels?

Answer 11

The main pore is one peptide consisting of four homologous repeats. Each repeat has 6 transmembrane domains (one is voltage sensored) Function requires one subunit.

Question 12

What is the molecular structure of K+ channels?

Answer 12

It has 4 peptides and 6 transmembrane domains. One is voltage sensitive and function requires 4 subunits.

Question 13

How do local anaesthetics work?

Answer 13

They bind and block the Na+ channel (when it is open) so no action potential can be generated. Also have a high affinity for the inactivated state of the Na+ channel.

Question 14

What is the order that local anaesthetics block conduction in nerve fibres?

Answer 14

Small myelinated axons
Non myelinated axons
Large myelinated axons
(Means they tend to effect sensory before motor)

Question 15

How can conduction velocity be calculated?

Answer 15

Recording changes between the stimulating and recording electrodes on an axon and using an equation. (Velocity = Distance / Time)

Question 16

What is the local circuit theory of propagation?

Answer 16

Depolarisation of a small region causes transmembrane currents in neighboring regions. Because Na+ channels are voltage gated, more open casing propagation of the action potential. The further the current spreads, the faster the conduction velocity of the axon.

Question 17

What properties of an axon lead to high conduction velocity?

Answer 17

High membrane resistance
High axon diameter
Low cytoplasmic resistance
Low membrane capacitance.

Question 18

Why would the conduction velocity be higher with a high membrane resistance?

Answer 18

Ohms law states that higher resistance of the membrane means higher potential difference so more voltage across the membrane means more voltage gated Na+ channels are open.

Question 19

Why would conduction velocity be increased with a large axon diameter?

Answer 19

Ohms law states that the lower resistance, the larger the current so the action potential will travel further.

Question 20

Why would conduction velocity be increased with a low membrane capacitance?

Answer 20

Because capacitance is the ability to store charge, a low one means less time will be needed to charge it.

Question 21

What increases the conduction velocity?

Answer 21

Myelination of axons.

Question 22

What does myelination do?

Answer 22

Reduces capacitance
Increases membrane resistance
Therefore increases conduction velocity

Question 23

What is saltatory conduction?

Answer 23

Where the action potential jumps between Nodes of Ranvier.

Question 24

How does saltatory conduction work?

Answer 24

Because the myelin sheath is a good insulator and causes a local circuit current to depolarise to the next node above the threshold and generate an action potential.

Question 25

How does the distribution of sodium channels vary along a myelinated axon?

Answer 25

There are many sodium channels in the Nodes of Ranvier and very few in the myelinated areas.

Question 26

What cells form the myelin in peripheral axons?

Answer 26

Schwann cells.

Question 27

What cells form the myelin in CNS axons?

Answer 27

Oligodendrocytes.

Question 28

What disease destroys the CNS axon’s myelin?

Answer 28

Multiple Sclerosis - Autoimmune disease

Question 29

What does Multiple Sclerosis cause?

Answer 29

The action potentials cannot be conducted properly because the myelin has been broken down. There is decreased conduction velocity, complete blockage or partial blockage.