neuronal cytoskeleton

The neuronal cytoskeleton is the internal scaffolding of a nerve cell — a web of microscopic fibers, far thinner than a hair, that gives the neuron its shape and holds it together from the inside. Picture the steel beams, tent poles, and guy-wires that keep a tall, oddly-shaped building or a circus tent standing: without that framework the structure would sag into a shapeless blob. A neuron has an unusually demanding shape — a tiny cell body sprouting a bushy crown of branches and a single wire-like fiber that can stretch a very long way — and the cytoskeleton is what lets it grow into that form and stay there.

It is built mainly from three kinds of fiber, each with its own job. Microtubules are stiff, hollow tubes that run the length of the long output fiber like internal rails; they not only brace the cell but also serve as tracks along which little molecular 'trucks' haul supplies — fresh proteins, packets of chemical messengers, worn-out parts heading back for recycling — between the cell body and its far-flung tips. Neurofilaments are tough, rope-like strands that pack the long fiber and set its thickness, which in turn helps decide how fast its electrical signals travel. Actin is a finer, more restless mesh, concentrated at the cell's edges and in the tiny knobs where neurons touch; it constantly assembles and dissolves, letting the neuron feel its way as it grows, change shape, and remodel its connections as you learn.

Because the cytoskeleton both shapes the neuron and runs its internal delivery system, damage to it shows up in disease. When microtubules or their helper proteins go wrong — as with the tangled tau protein in Alzheimer's, or in some forms of nerve degeneration — cargo stops reaching the ends of long fibers, the cell's distant parts starve, and the neuron sickens and can die.