Lecture 4.2

Question 0

What can extracellular recording tell us?

Answer 0

Information on the conduction velocity under various conditions

Question 1

How can electrical stimulation of a membrane be achieved?

Answer 1

Using a cathode and anode - electrical stimulation occurs under the cathode and excitability is reduced by the anode. This method can be used to stimulate an axon to threshold to generate an AP.

Question 2

How is conduction velocity measured?

Answer 2

Distance over time
Distance between stimulating electrode and recording electrode
Time between the stimulus and the AP being recorded by the recording electrode
The longer the distance, the quicker the conduction velocity

Question 3

How is an AP conducted along an axon?

Answer 3

1. Changes in membrane potential in one part of an axon can spread to adjacent parts of the axon
2. Due to local current which propagates the AP along the axon
3. Conduction velocity depends on how far the local currents can spread down an axon - the further the spread, the quicker the conduction velocity
4. When a local current spread causes depolarisation of part of the axon to threshold, then an AP is generated

Question 4

In what direction do local currents spread?

Answer 4

Can spread in either direction but the only way an AP is propagated in one direction is due to the previously depolarised part of the membrane being in the refractory period and cannot be stimulated again until it has recovered from the refractory period. In this way, an AP cannot be propagated backwards.

Question 5

State 3 properties of an axon that allows it to have a high conduction velocity

Answer 5

1. High membrane resistance
2. Low membrane capacitance
3. Large axon diameter - as it leads to cytoplasmic resistance

Question 6

What is capacitance and of which structure is it a property of?

Answer 6

Capacitance, C, is the ability to store charge and is a property of the lipid bilayer

Question 7

How is high capacitance related to local current spread?

Answer 7

High capacitance takes more current to charge and can decrease in spread of local current especially with brief current pulses

Question 8

What does membrane resistance depend on?

Answer 8

Then number of ion channels open. The lower the resistance, the more ion channels are open which means that there’s more loss of local current across the membrane. This limits the spread of the local current.

Question 9

What cells produce the myelin sheath?

Answer 9

1. Oligodendrocytes - myelinate axons in the CNS

2. Schwann cells - myelinate axons in the PNS

Question 10

How does a myelin sheath affect the conduction velocity of an axon?

Answer 10

Increases it by lowering membrane capacitance by 100x and increasing membrane resistance by 100x - this increases the length constant and decreases time (as c.v. is distance over time, this increases c.v.)

Question 11

What types of axons are myelinated and which type?

Answer 11

Axons with large diameters are myelinated e.g. Motor neurones
Axons with small diameters are un-myelinated e.g. Sensory neurones

Question 12

How does myelination of an axon relate to conduction velocity?

Answer 12

In a myelinated axon, the conduction velocity is proportional to the diameter
In an un myelinated axon, the conduction velocity is proportional to the square root of the diameter

Question 13

What is the conduction velocity of a myelinated mammalian motor-neurone?

Answer 13

120m per second

Question 14

What is the conduction velocity of an unmyelinated axon?

Answer 14

20m per second

Question 15

What is Multiple Sclerosis?

Answer 15

AI destruction of proteins that hold myelin sheath together so therefore destroying the myelin sheath in parts of the CNS. Has a dramatic effect on the ability of a previously myelinated axon to conduct AP effectively. Leads to decreased conduction velocity or complete block or to cases where only some AP are transmitted.

Question 16

Why is the density of the action current reduced in demyelination?

Answer 16

Due to resistive and capactive shunting

Question 17

How do the spread of sodium channels in myelinated and unmyelinated axons differ?

Answer 17

In myelinated axons, the sodium channels are concentrated at the nodes of Ranvier.
In unmyelinated axons, the sodium channels are evenly distributed throughout the axon.

Question 18

How does a myelin sheath allow for saltatory conduction?

Answer 18

It acts as a good insulator causing local circuit currents to depolarise the next Node of Ranvier to above threshold value and cause an AP. The AP jumper from node to node, allowing for a much faster conduction. APs only occur at the nodes of R where the sodium channels are concentrated