The Wiring Behind Your Reflexes

Isabella Gavazzi

Has a doctor ever tested your reflexes with a light whack of your knee? When that little red hammer taps your knee, your leg pops up almost as if it has a mind of its own. That's because — in a sense — it does. Your knee has access to its own bit of grey matter far from your head — your spinal cord.

The neurons that run from your skin to your spinal cord independently handle a range of reflexive reactions, protecting us from immediate threats like unseen thumbtacks to still-hot electric burners. To get the pain signal from your skin to your spine for on-the-spot processing, you need this orange blob of nerves — the dorsal root ganglion.

In this image, the nerve bundle stays strictly in its lane. In a normal petri dish it would sprawl in all directions, but here the green stripes corral it with a repulsive protein — EphrinA5. This protein family serves as a traffic light in the body that helps direct neurons as they grow.

About the Author

Charlie Wood

Charlie Wood is a science writer with a bachelor’s degree in physics from Brown University and a master’s degree in science journalism from New York University. In previous lives he taught physics in Mozambique and English in Japan, but these days he freelances from his home in New York.