Rooted along the human spine, emerging through spaces between vertebrae, are 31 pairs of spinal nerves, part of the body’s expansive peripheral nervous system that transmits signals from throughout the body to the brain and back again.
In this scanning electron micrograph, created by Thomas Deerinck, a cross-sectioned spinal nerve reveals hundreds of smaller, varied nerve fibers bundled within, each wrapped in an insulating sheath of fatty myelin. Some of these fibers carry sensory information to the brain, like the touched sharp edge of a shard of glass or the cold wetness of a dog’s nose. Other fibers transmit signals back from the responding brain, neural missives to muscles in the arm and hand to drop the glass shard or scratch the dog’s head.
The speed of these impulses varies, from a sluggish 2 miles per hour to more than 200 mph, depending upon the type of nerve fiber and its job. The senses of touch and body position are notably speedy, rushing information to the brain at up to 350 feet per second. Pain signals travel a bit more slowly, which is why you “feel” a stubbed toe before it actually starts to hurt.
At the center of the spinal nerve is a blood vessel, packed with blood cells. Despite its overall complexity, the whole nerve assembly is remarkably small, in this case about the diameter of a pencil lead. The spinal nerve in this image has been magnified 500 times. (from UCSD health sciences tumblr)