dendritic spine

A dendritic spine is a tiny knob-like bump that sticks out from the surface of a neuron's receiving branches. To picture it, imagine a neuron as a leafless tree in winter: the trunk is the cell body, and the bushy crown of fine twigs spreading out to catch incoming signals are the dendrites. A dendritic spine is like a small bud or thorn growing out along one of those twigs, each one usually less than a thousandth of a millimeter across — so small that for a long time scientists could only see them as faint dots after a special black stain soaked into the cell. A single neuron can carry tens of thousands of these spines, giving its branches a fuzzy, frosted look under the microscope.

Spines matter because they are the main spots where one neuron listens to another. Most of the excitatory contacts a neuron receives — the messages that nudge it toward firing its own signal — land on the head of a spine rather than on the bare branch. Each spine has a rounded head where the incoming message arrives and a thin neck that connects it back to the dendrite, and that narrow neck acts a bit like a private booth: it lets each contact be tuned up or quieted down on its own, without disturbing its neighbors. This is a big part of how the brain learns. When a connection is used a lot, its spine can swell and grow stronger; when it falls out of use, the spine can shrink or vanish. So the changing forest of spines on a neuron is, in a real sense, a physical record of experience and memory.