15.3 Passage of action potential

Question 1

Where does the action potential go once it has been made

Answer 1

It moves rapidly along the axon. The size of action potential remains the same from the beginning.

Question 2

How does it move down the axon

Answer 2

Nothing physically moves.

One region of the axon produces an action potential and becomes depolarised so acts as a stimulus for depolarisation of the next region of the axon.

Action potential is therefore a travelling wave of depolarisation, whilst the previous region undergoes repolarisation

Question 3

Describe passage of action potential along an unmyelinated axon

Step 1

Answer 3

• At resting potential the concentration of sodium ions outside axon membrane is higher than inside.
• Whereas concentration of potassium ions is higher inside than outside
• The overall concentration of positive ions greater on the outside making this positive compared with the inside
• This axon membrane is polarised.

Question 4

Describe passage of action potential along an unmyelinated axon

Step 2

Answer 4

• Stimulus causes a sudden influx of sodium ions and hence a reversal of charge on the axon membrane
• This is the action potential and the membrane is depolarised . So the inside of axon is more positive than the outside here.

Question 5

Describe passage of action potential along an unmyelinated axon

Step 3

Answer 5

• The localised electrical currents established by the influx of sodium ions causes the opening of sodium channels further along the axon.
• This resulting influx of sodium ions in this region causes depolarisation. Behind this region of depolarisation, the sodium channels close and the potassium ones open so they diffuse out.
• So once initiated, depolarisation moves along membrane

Question 6

Describe passage of action potential along an unmyelinated axon

Step 4

Answer 6

• The action potential (depolarisation) is propagated in the same way further along the axon.
• The outward movement of potassium ions has continued to the extent that the axon membrane behind the action potential has returned to its original charged state.
  This means it will be re-polarised.

Question 7

Describe passage of action potential along an unmyelinated axon

Step 5

Answer 7

• Repolarisation of the axon allows sodium ions to be actively transported out which returns the axon to its resting potential.
• It is now ready for a new stimulus to come.

Question 8

What would the action potential be along a myelinated axon

Saltatory conduction

Answer 8

• The fatty sheath of myelin acts as electrical insulator which prevents action potentials from forming
• action potentials can only occur at nodes of ranvier where there are breaks in the myelin insulation.
• This means localised circuits arise between adjacent nodes of Ranvier so the action potentials jump from node to node in process called saltatory conduction

Question 9

What is faster, action potential through myelinated sheath or unmyelinated sheath

Answer 9

• It is faster through Myelinated sheath
• Because in the unmyelinated ones the events of depolarisation take place all the way along the axon and this takes more time.