How Fast Does a Signal Travel Along a Nerve? Slower Than You Would Guess

A nerve impulse is not electricity flowing along a wire but a wave of electrical change moving along a membrane, regenerated at every point as it goes. A neuron maintains a voltage difference across its membrane by pumping ions, and when a region is disturbed past a threshold, channels open and ions rush across, reversing the voltage locally, which disturbs the neighbouring region past its threshold and repeats the process. The signal therefore propagates by being continuously recreated rather than by anything moving from one end to the other, which is why it does not weaken with distance and why it is far slower than a current in a wire. The mechanism also means the nerve needs a recovery period before it can fire again.

Two properties dominate and both are visible in the anatomy:

• •Diameter, with wider fibres conducting faster because current spreads along them more readily
• •Myelination, an insulating sheath wrapped around the fibre by supporting cells
• •Myelin forces the signal to jump between gaps in the sheath rather than regenerating continuously
• •That jumping raises speed enormously without requiring a wider fibre
• •The fastest myelinated fibres reach around one hundred and twenty metres per second
• •The slowest unmyelinated fibres run around half a metre per second

The body sorts information by urgency and allocates fibre types accordingly, which is directly experienced. Signals for position, touch and rapid movement travel on fast myelinated fibres, because acting on them late is useless. Sharp pain travels on moderately fast fibres and dull aching pain on slow unmyelinated ones, which is why an injury produces an immediate sharp sensation followed a second or so later by a duller one, and the gap is the difference in conduction speed over the same distance. Signals regulating digestion and other functions that need no urgent response travel slowly. Building every fibre fast would require enormous nerves, since speed costs diameter and myelin, so the arrangement is an economy as much as a design.

The finite speed of conduction has consequences that are directly observable and are worth noticing. A signal from a toe takes roughly twenty milliseconds longer to reach the brain than one from a hand, which is measurable and which the brain apparently compensates for, since touches to different parts of the body feel simultaneous when they are not. Reaction time to a stimulus runs around two hundred milliseconds, most of which is processing rather than conduction, which is why a sprinter's start is judged as a false start below a threshold reflecting what is physiologically possible. A reflex bypasses the brain entirely by looping through the spinal cord, which is why pulling a hand from something hot begins before the pain is felt. And a giraffe's longest nerves run several metres, with the delays that implies.

Conduction speed is measured clinically and abnormalities point to specific problems. Nerve conduction studies stimulate a nerve at one point and record the response at another, timing the interval, which gives a speed directly. Slowed conduction across a short segment indicates compression at that point, which is how a trapped nerve is localised. Generally reduced speed across a nerve suggests damage to the myelin sheath, which is the pattern in several conditions including some inherited neuropathies and the demyelinating diseases. Reduced signal size with preserved speed suggests loss of fibres instead, which distinguishes the two main categories of nerve damage. The test is uncomfortable and informative, and it is one of the few places where a physical property of the nervous system is measured directly in a patient.

The signal is a wave of electrical change regenerated at every point rather than a current flowing, which is why it does not weaken and why it is slow. Wider fibres and a myelin sheath that forces the signal to jump between gaps both raise speed, from half a metre per second to around one hundred and twenty. The delay between sharp and dull pain is that speed difference.