One giant neuron lets a fish flee in milliseconds
Every fish carries two huge Mauthner cells in its brainstem, one per side. A single impulse from one is enough to bend the whole body into a C and fling the fish away from danger within milliseconds. Humans, by contrast, are thought to have no neuron that unique at all.
A nerve is a sheathed cable of axons, the long fibres that extend from individual neurons, running between the brain or spinal cord and the rest of the body. Layers of connective tissue nest inside one another: the endoneurium around each fibre, the perineurium around bundles called fascicles, and the tough epineurium around the whole cable. Inside, a low-protein fluid forms a blood-nerve barrier, much as the brain has its own. When a nerve is irritated that fluid builds up, which is how MR neurography can spot injury.
Signals run three ways. Afferent fibres bring sensation in, efferent fibres carry commands out to muscles and glands, and mixed nerves, including every spinal nerve, do both. The fastest insulated fibres conduct at up to 120 m/s, and at each synapse the message switches from electrical to chemical and back. Cranial nerves are numbered 1 to 12, with a disputed cranial nerve zero sometimes added.
Cut axons can regrow if the cell body survives. The stretch beyond the injury is broken down, Schwann cells and surrounding tissue build a regeneration tube, and sprouts bud until one finds the tube and follows it home. Repair takes months and is never perfect. Damage comes from many directions: compression, as in carpal tunnel syndrome, diabetes, autoimmune attack in Guillain-Barré syndrome, and in multiple sclerosis immune cells stripping the myelin insulation. Pain may even surface far from the injured spot.
Identified neurons like the Mauthner cell, which is the same in every member of a species, are rare in vertebrates. The squid giant axon, part of another escape circuit, was big enough to power landmark experiments. Nerves were already studied around 300 BC by Herophilos, who told them apart from blood vessels and linked the optic nerve to sight. William Cullen's 1785 idea tying mental states to the nerves has not held up, though people still blame their nerves for worry.
Source: Nerve
