M&R 4.2 - Conduction of the Nerve Impulse

Question 1

Describe the process of propagation of the nerve impulse

Answer 1

• Depolarisation causes transmembrane currents (positive areas in membrane promotes a positive charge in adjacent regions of the membrane)
• Further current spreads = faster velocity current

Question 2

What are the factors that affect membrane conduction? (3)

Answer 2

• High membrane resistance
• Large axon diameter
• Low membrane capacitance

Question 3

What is the effect of a high membrane resistance on conduction? Why?

Answer 3

• Increased resistance = Increased potential difference across membrane
• Increased voltage = Increase Na+ channels open
• Easier to reach threshold = Easier to generate an AP
• Increases conduction velocity

Question 4

What is the effect of a large axon diameter on membrane conduction? Why?

Answer 4

• Results in a lower cytoplasmic resistance = Increased current
• Action potential travels further = Increased conduction velocity

Question 5

What is membrane capacitance?

Answer 5

The ability of a membrane to store charge

Question 6

What is the effect of a low membrane capacitance on membrane conduction? Why?

Answer 6

• Low capacitance = less time taken to charge

- Increases the conduction velocity

Question 7

What is the effect of myelination on conduction velocity? Why?

Answer 7

• Increases conduction velocity
• Reduces capacitance
• Increases membrane resistance

Question 8

What is saltatory conduction? Why does it happen?

Answer 8

• Action potential ‘jumps’ between Nodes of Ranvier
• Myelin sheath is a good insulator = depolarises local circuit currents at next node
• Reach threshold = action potential

Question 9

How do myelinated axons differ to unmyelinated axons?

Answer 9

• Nodes of Ranvier are dense in Na+ Vg Channels

- Unmyelinated have an equal distribution

Question 10

What forms myelin? How?

Answer 10

• Schwann Cells in PNS
• Oligodendrocytes in CNS
• Wrap around axons to in plasmalemma until it becomes myelin

Question 11

What is Multiple Sclerosis caused by?

Answer 11

• Autoimmune

- Myelin is destroyed in certain areas of the CNS

Question 12

What are the effects of Multiple Sclerosis?

Answer 12

• Can’t conduct action potentials properly

- Decreases conduction velocity (completely or only some can be transmitted)

Question 13

Describe the action of Procaine (a local anaesthetic)

Answer 13

• Binds and blocks Na+ channels when open
• Have a higher affinity to inactivated Na+ channels
• Prevents generation of action potentials

Question 14

What is the order of nerves in which local anaesthetics block conduction?

Answer 14

• Small myelinated axons
• Non-myelinated axons
• Large myelinated axons

(affect sensory before motor)