Lecture 28: Action potentials: Generation and transmission Part II

Question 1

What are two types of axons?

Answer 1

Myelinated and unmyelinated

Question 2

What is the diameter of an unmyelinated axon?

Answer 2

smaller than myelinated (-1um)

Question 3

What is the diameter of a myelinated axon?

Answer 3

larger than unmyelinated (5-10um)

Question 4

What is the difference in transmission speed between a myelinated and unmyelinated axon?

Answer 4

Transmission speeds through myelinated axons are much faster due to saltatory conduction (20-100m/sec) compared to -1m/sec in unmyelinated axons

Question 5

What are the two stages of action potential transmission?

Answer 5

1) passive

2) generation of action potentials

Question 6

How does passive spread occur?

Answer 6

1. Passive spread occurs when depolarisation (subthreshold) occurs at one region of the membrane
2. Current flows inside and outside of the axon from + to - charge
3. Depolarisation of adjacent parts of membrane

Question 7

How far can passive spread occur?

Answer 7

Not very far as current quickly dissipates due to leaky channels in the membrane

Question 8

How are action potentials transmitted in unmyelinated axons?

Answer 8

1. Action potential occurs
2. Initially current flows passively
3. Depolarisation of adjacent parts of membrane to threshold
4. Action potentials generated - depolarisation occurs with opening of voltage-gated Na ion channels on adjacent parts of membrane

Question 9

How is the myelin sheath formed?

Answer 9

By the action of glia cells

1) oligodendrocytes in the CNS
2) schwann cells in PNS

Question 10

What is the nodes of Ranvier?

Answer 10

Gaps in the myelin sheath of along axons

Question 11

What is the role of myelin sheath?

Answer 11

1) Increase passive spread of current (less dissipation due to insulating properties)
2) Increase rate of action potential transmission (by saltatory conduction)

Question 12

What is saltatory conduction?

Answer 12

Increase of passive spread where APs are only generated at the nodes of Ranvier - this means APs do not need to be generated at every point along the cell membrane

Question 13

How are action potentials only transmitted in one direction?

Answer 13

Absolute refractory period -> once depolarisation and repolarisation occur the voltage-gated Na+ become inactive for a period of 1-2ms therefore action potentials cannot travel backwards

Question 14

How are action potentials generated in sensory neurons?

Answer 14

1) A stimuli is detected by receptors in sensory neurons
2) A receptor potential is created - this is a graded depolarisation (depends on strength of stimuli)
3) The receptor potential spreads passively to the trigger zone where action potentials are generated
4) Action potentials then spread along the axon towards the CNS

Question 15

How do action potentials carry information?

Answer 15

The frequency of APs give information on intensity of stimuli based on amplitude of the receptor potential