Passage of an Action Potential

Question 1

STEP-BY-STEP GUIDE TO ACTION
POTENTIAL PROPAGATION in UNMYELINATED
neurones

Answer 1

1. The Resting Potential:
   At resting potential the concentration of sodium ions outside the axon
   the membrane is very high.
   The concentration of potassium ions inside the axon is lower, which
   means that the outside of the axon is POSITIVE in relation to the inside.
   The axon membrane is POLARISED
   ————————————————-
2. Initiation of the 1st Action Potential:
   A stimulus causes a sudden influx of sodium ions and hence a reversal of
   charge on the axon membrane.
   This is the action potential.
   The axon membrane is DEPOLARISED
   —————————————————
3. Stimulation of the NEXT Action Potential:

The first action potential caused by the influx of sodium ions, causes the
opening of sodium voltage-gated channels further along the axon.
The resulting influx of sodium ions, causes a new action potential
(depolarisation) here.
Behind this region, the sodium voltage-gated channels close, and the
potassium voltage-gated channels open, causing potassium to leave
——————————————————

4. Repolarisation of the Axon:

The action potential is propagated once again (3rd time in our example).
Notice how the area where the 1
st action potential occurred, is returning
to its resting potential.
That area has been REPOLARISED
Now notice how the area of the second action potential, is removing
potassium ions in just the same way…
——————————————————
5. Getting Back to Normal:
Once again, everything has just shifted to the right by ‘one section’.
Notice how the original ‘section’ is now pumping out sodium ions,
ensuring that the resting potential of around -65mV is established.
That part of the axon is now ready to receive a new stimulus and start the
whole process off again.

Question 2

Describe the process of ACTION
POTENTIAL PROPAGATION in MYELINATED
neurons

Answer 2

The myelin sheath is an electrical insulator, which prevents action
potentials from forming in area of ‘myelination’.

• Action potentials can only occur at Nodes of Ranvier.
• Because of this, action potentials effectively ‘jump’ from node to
  node.
• As a result of this node-hopping, the propagation of action
  potentials through myelinated neurones is much faster than in
  unmyelinated ones.

Question 3

Whats another name for ACTION
POTENTIAL PROPAGATION in MYELINATED
neurons

Answer 3

SALTATORY
CONDUCTION