Passage Of An Action Potential

Question 1

Describe the passage of an impulse along the axon of an unmyelinated neurone

Answer 1

1. At resting potential the concentration of sodium ions outside the axon membrane is high relative to the inside, whereas that of the potassium ions is high inside the membrane relative to the outside. The overall concentration of positive ions, is however greater on the outside, making this positive compared with the inside (axon membrane is polarised )
2. A stimulus causes a sudden influx of sodium ions and hence a reversal of charge on the axon membrane. This is an action potential and the membrane is depolarised
3. The localised electrical currents established by the influx of sodium ions cause the opening of sodium voltage gated channels a little further along the axon. The resting influx of sodium ions in this region causes depolarisation. Behind this bed region of depolarisation, the sodium voltage gated channels close and the potassium ones open. Potassium ions begin to leave the axon along their electrochemical gradient. So once initiated, the depolarisation moves along the membrane
4. The action potential (depolarisation) is propagated in the same way further along the axon. The outward movement of the potassium ions has continued to the extent that the axon membrane behind the action potential has returned to its original charged state ( positive outside, negative inside) that is, it has been repolarised
5. Repolarisation of the axon allows sodium ions to be actively transported out, once again returning the axon to its resting potential in readiness for a new stimulus if it comes back

Question 2

Describe the passage of an action potential along a myelinated axon

Answer 2

The fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials form forming.

There are breaks in the myelin sheath called nodes of ranvier, action potentials can occur at these points

Localised circuits therefore arise between adjacent nodes of ranvier and the action potentials in effect jump from node to node ( saltatory conduction )

Question 3

Why does a an action potential pass along a myelinated neurone faster than along an axon of a unmyelinated one of the same diameter

Answer 3

As In an unmyelinated neuron, the events of depolarisation have to take place all the way along an axon and this takes more time.

Question 4

Describe what happens to the size of an action potential as it moves along an axon

Answer 4

It remains the same