Propagation of action potentials in non-myelinated neurones

• A nerve impulse is an action potential that starts at one end of the neurone and is propagated along the axon to the other end of the neurone.
• The initial stimulus causes a change in the sensory receptor which triggers an action potential in the sensory receptor, so the first region of the axon membrane is depolarised. This acts as a stimulus for the depolarisation of the next region of the membrane. The process continues along the length of the axon forming a wave of depolarisation. Once sodium ions are inside the axon, they are attracted by the negative charge ahead and the concentration gradient to diffuse further along inside the axon, triggering the depolarisation of the next section.
• 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. The 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 the action potential and the membrane is depolarised.
• 3. The localised electrical circuits established by the influx of sodium ions cause the opening of sodium voltage-gated channels a little further along the axon. The resulting influx of sodium ions in this region causes depolarisation. Behind this new 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.
• 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. Following repolarisation the axon membrane returns to its resting potential in readiness for a new stimulus if it comes.
• movement of action potential along a neurone

  A) Na+

  B) K+

  C) refractory period

  D) stimulus

  E) wave of depolarisation