nerve vs tract

A nerve and a tract are basically the same thing — a bundle of axons, the long wire-like extensions that carry neurons' signals — but they get different names depending on where in the body the bundle runs. A nerve is a bundle of axons out in the peripheral nervous system, the wiring outside the brain and spinal cord. A tract is a bundle of axons inside the central nervous system, meaning within the brain and spinal cord itself.

Think of it like a cable of wires. Out in your arms, legs, and torso, those cables are wrapped in tough connective tissue and called nerves — the sciatic nerve in your leg or the optic nerve from your eye are familiar examples. Once that same kind of cable is bundled up inside the brain or spinal cord, anatomists stop calling it a nerve and start calling it a tract, such as the corticospinal tract that carries movement commands down from the brain. The name flips at the border between the central and peripheral nervous systems.

This naming rule is part of a wider pattern: many structures get one label inside the central nervous system and a different label outside it. A clump of neuron cell bodies, for instance, is called a nucleus inside the central nervous system but a ganglion outside it. Knowing that nerve means peripheral and tract means central saves a lot of confusion when you read anatomy, because the word itself tells you which side of the dividing line you are looking at.