A lullaby, a speech by Jimmy Carter, a dozen imaginary sheep: people have long known things that help bring on sleep. But until recently they didn’t have much of a clue which brain cells were responsible. Last January neurologist Clifford Saper of Harvard Medical School and his colleagues announced that they had identified what looks like the somnolence command center. And though they did the research on rats, Saper is convinced there’s a similar mechanism in our brains, too.
The researchers studied a group of cells that send rousing signals to the cortex during wakefulness, but slow down or stop completely during sleep. We wanted to know what controls this cell group, says Saper. We looked for the inputs to these cells, and we found that they came from a specific little cluster of nerve cells in a part of the hypothalamus called the ventrolateral preoptic area, or vlpo. It’s on the undersurface of the brain, just behind your eyes.
When Saper’s group examined the connections between the vlpo and the wakefulness cells, they found the kind of synapses that carry inhibitory messages--the kind, in other words, that could turn off wakefulness and keep it off. And when they tested the vlpo’s daily activity pattern, says Saper, in the entire brain, this was the only cell group that consistently showed more activity during sleep.
More recently, the researchers found that the vlpo has inhibitory links to all the other major cell groups known to be involved in promoting wakefulness. This one little cluster, concludes Saper, has the properties of a master switch that can turn off all the arousal systems in the brain.