Nerve Impulse Conduction: From Intuition to Precision
Imagine a long fuse that burns from one end to the other. The flame doesn't jump — it ignites the next bit of gunpowder, which ignites the next, and so on. A nerve impulse works much the same way, except the "flame" is a wave of electrical reversal that travels down the axon, and the "gunpowder" is the set of voltage-gated ion channels embedded in the axon membrane.
The key intuition is this: the signal is not a single electron racing from your toe to your brain. It is a chain reaction of local electrical events, each one triggering the next. The impulse is regenerated at every point along the axon, which is why it never fades out, no matter how long the distance.
The Resting State: A Charged Battery
Before anything happens, the axon is at resting potential — about −70 mV inside relative to outside. This is maintained by the sodium-potassium pump (which pushes 3 Na⁺ out for every 2 K⁺ in) and by leaky potassium channels. The inside is negative, the outside positive. Think of it as a charged battery waiting to be used.
The resting membrane potential is negative inside because:
- The membrane is far more permeable to K⁺ than to Na⁺ at rest.
- The Na⁺/K⁺ pump actively maintains the concentration gradient.
The Action Potential: A Rapid Reversal
When a stimulus (chemical, mechanical, or electrical) depolarises the membrane to a threshold of about −55 mV, voltage-gated sodium channels snap open. Na⁺ rushes into the cell, driven by both its concentration gradient and the electrical attraction of the negative interior. This influx reverses the membrane potential to about +30 mV — the depolarisation phase.
Almost immediately, the sodium channels inactivate (they close and lock), and voltage-gated potassium channels open. K⁺ flows out of the cell, repolarising the membrane back toward −70 mV. In fact, the K⁺ efflux briefly overshoots, causing a hyperpolarisation (about −90 mV) before the resting potential is restored by the pump.
Action potential sequence:
- Depolarisation to threshold (−55 mV)
- Rapid Na⁺ influx → spike to +30 mV
- Na⁺ channels inactivate; K⁺ channels open
- K⁺ efflux → repolarisation (and brief hyperpolarisation)
- Resting potential restored by Na⁺/K⁺ pump
Propagation: How the Impulse Moves
Here is the crucial part. The depolarisation at one point of the axon does not stay local. The positive charge that rushed into the cell spreads laterally along the inside of the membrane, depolarising the adjacent region to threshold. That adjacent region then fires its own action potential. Meanwhile, the region that just fired is in its refractory period — its sodium channels are inactivated and cannot reopen for a few milliseconds. This ensures the impulse travels only forward, never backward.
So the impulse is not a flow of charge along the whole axon; it is a self-regenerating wave of channel openings and closings. Each patch of membrane acts as a tiny amplifier, boosting the signal anew.
The absolute refractory period (when Na⁺ channels are inactivated) guarantees one-way propagation and sets an upper limit on firing frequency.
Saltatory Conduction: The Myelin Shortcut
In many vertebrate axons, the membrane is wrapped in myelin — a fatty insulating sheath produced by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system). Myelin is an electrical insulator; it prevents ion flow across the membrane where it covers the axon.
But the myelin is not continuous. It is interrupted at regular intervals by nodes of Ranvier — bare patches of axon membrane packed with voltage-gated sodium channels. …