Simulation · Biology · Class 11

A wave along the axon

From the lesson Nerve impulse in Neural Control and Coordination. Change the values and watch what happens.

The idea behind it

NCERT §18.3.1

When a stimulus hits a site on the polarised membrane (site A), the membrane there becomes freely permeable to Na⁺.
Na⁺ rushes in, and the polarity at that site reverses: the outer surface turns negative and the inner surface positive. The site is depolarised.
The potential difference across the membrane at site A is the action potential; this is the nerve impulse.
Just ahead, at site B, the membrane is still positive outside and negative inside. So current flows along the inner surface from A to B, and along the outer surface from B to A, completing the circuit.
This reverses the polarity at B and an action potential forms there: the impulse has moved from A to B. Repeated along the axon, the impulse is conducted.
The rise in Na⁺ permeability lasts only a very short time. It is quickly followed by a rise in K⁺ permeability; within a fraction of a second K⁺ diffuses out, restoring the resting potential at the site, which can then respond to a new stimulus.
The impulse is thus a wave of depolarisation followed by repolarisation travelling along the axon membrane.

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