Freestyle Neurons

There's not much to love about a lamprey, an eellike parasite that clamps onto other fish and sucks their blood until they die. But physiologists have latched onto the lamprey's primitive nervous system as a way to understand how the brain directs movement. Now, for the first time, researchers have observed the lamprey's brainstem converting a sensory input--a bump on the snout--into a command to swim away. This kind of wiring, described in the current issue of Science, may be present in higher vertebrates, including humans--and if so, it might provide insights for scientists studying how to treat paralysis from spinal cord injuries.

Scientists have taken advantage of the lamprey's simplicity--its neurons number in the thousands, while humans have billions--to produce the first complete blueprint of a vertebrate motor system. In 1987, they discovered that the repetitive motions of swimming are choreographed by a group of neurons in the lamprey's spinal cord called the central pattern generator (CPG). The brain, instead of controlling each movement individually, merely turns on the CPG's swimming program.

To find out how the brain knows when to issue the command, physiologist Réjean Dubuc of the University of Quebec in Montreal and colleagues at the University of Montreal applied pressure to the skin of a tied-down lamprey's head. Using tiny electrodes, they monitored the response of brain cells responsible for movement. When the pressure was mild, the electrodes registered a small increase in voltage. But at a certain threshold--like a tickle turning to a sneeze--the electrical activity suddenly spiked, and simultaneously the lamprey's tail began to swish back and forth, as if the creature were swimming, continuing for about 20 seconds.

The new findings prove that a group of cells in the brain can turn a short-lived sensation into complex, long-lasting muscle movements, says physiologist Simon Alford of Northwestern University. The work also bodes well for helping people with injured spinal cords to walk again, he says: If just one connection from the brain has to be repaired, "then hope becomes much stronger that spinal cord regeneration could be successful."