Biology · Neural Control and Coordination · NEET
A fresh one is made at each point. The action potential does not physically slide along like a marble. When a site depolarises, the small local current it creates depolarises the next site, which then generates its OWN action potential. So conduction is a chain of new action potentials, each one regenerated at the next spot. This is why the signal does not fade with distance.
The region just behind the impulse is in the refractory phase. After a point fires, its Na+ channels are briefly inactivated and it cannot be re-stimulated for a short time. So the local current can only excite the region ahead (which is still at resting potential), not the region behind. This gives the impulse a one-way direction along the axon. This exact idea was tested in NEET (refractory phase stops backward movement).
A local (circuit) current. At the depolarised point the inside becomes positive while the neighbouring point is still negative inside. Because there is now a charge difference between the two neighbouring points, a tiny current flows between them. This current depolarises the next point up to threshold, so it fires. Then that point does the same to the point after it, and so on down the axon.
No. Because a brand-new full-strength action potential is regenerated at every point, the impulse arrives at the axon terminal just as strong as it started. This is the all-or-none property: the size of the impulse does not shrink over distance. Only its speed differs between myelinated and non-myelinated fibres.
In a non-myelinated axon the impulse moves smoothly, point after point, which is slower. In a myelinated axon the myelin insulates the membrane, so the impulse jumps from one Node of Ranvier to the next (this is saltatory conduction). Fewer points need to fire, so the impulse travels much faster. The step-by-step mechanism (Na+ in, local current, next point fires) is the same, just at the nodes.
Which of the following statements is not correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
As a moving wave of depolarisation: Na+ enters at one point, the local current it makes depolarises the next point, and a new action potential fires there, repeating down the axon.
Closely linked but not identical. An action potential is the electrical event at one point (depolarisation then repolarisation). Conduction is that action potential being regenerated at point after point so the signal travels along the axon.
Because a full-strength action potential is freshly generated at every point (all-or-none). Energy is added at each step, so the impulse arrives at the end as strong as it began.
Sodium (Na+) influx. Na+ rushing in reverses the membrane to positive inside (depolarisation), which creates the local current that triggers the next point.
It makes conduction one-directional. The point that just fired cannot fire again for a moment, so the impulse can only move to the resting region ahead, never backward.
