What is the longest nerve in the human body?

The sciatic, running from the lower spine through the buttock and down the back of the leg, about as thick as a finger where it starts - the largest single nerve there is. It is not one nerve so much as a bundle of five nerve roots that travel together before dividing behind the knee.

Sciatica is not a diagnosis but a symptom, and the pain is usually not coming from where it hurts. Something compresses the nerve roots near the spine - most often a herniated disc bulging into the space they pass through - and the brain interprets the signal as coming from the whole length of the nerve's territory, so a problem in the back is felt as a line of pain down the calf.

The longest single cell, though, is elsewhere and stranger. A motor neuron running from the base of the spine to a muscle in the foot is one cell about a metre long, with a cell body microscopic at one end and an axon extending the length of a leg. In a blue whale the equivalent cells are tens of metres, which raises the question of how a single cell manages logistics over that distance - the answer is a continuous internal transport system moving materials along tracks, at rates that mean a protein made in the cell body can take days to reach the far end.