March 22, 2013 | 1
A little shuteye refreshes.
Right, but what does that really mean?
Not talking here about leaping out of bed ready for a five-mile run upon awakening, but rather about what’s happening at the level of individual brain cells deep inside your head.
A new study by R. Douglas Fields, a pioneer in researching out-of-the-mainstream brain areas and neural activity, holds one promising suggestion. Fields’s team at the National Institutes of Child Health and Development in Bethesda, Maryland, built on an earlier observation that during sleep (or even when just chilling out), neural signals travel the “wrong way” in cells of a critical region of the hippocampus, the brain structure involved with forming some types of new memories. The new study by Fields demonstrates, in a lab dish, that this reverse trafficking functions as a form of “editing,” a physical paring back of inessential parts of a brain cell to ensure that you don’t forget what you learned the previous day.
Specifically, electrical signals in the CA1 area of the hippocampus reverse direction like the opposite flow of cars during the evening rush hour. The spiking electrical pulses move up instead of down the long extensions of nerve cells known as axons. The train of spikes pass through the cell body where the nucleus resides before reaching the ends of thousands of tiny branching tendrils called dendrites.
Upon arrival, the signals act as dimmer switches that cause neurons to fire less strongly when they receive chemical signals from other neurons across the small gaps known as synapses—in neurospeak, the synaptic strength diminishes. “That allows you to learn the next day because you haven’t saturated your synapses,” Fields says. During this synaptic tuneup, some of the synapses disappear as part of a process that helps integrate the sights and sounds of the past day into memory, a process that involves blotting out irrelevant detail and “refreshing” synapses to better absorb the sensory onslaught of the coming day.
In the experiment, Olena Bukalo, the first author on the paper that appeared in Proceedings of the National Academy of Sciences, working with the rest of a team in Fields’s lab, provided reverse stimulation to a slice of hippocampal tissue. When the researchers then turned around and sent electrical signals in the opposite direction, from dendrites to axons, the tuned-up neurons produced stronger signals. The re-stimulation (similar to spacing out studying for a test) was essential for strengthening connections. Without reminder zaps, firing did not improve.
“What has been discovered is a remarkable new mechanism of plasticity at the global cell level,” says Giulio Tononi, of the University of Wisconsin. “While it has been characterized in vitro, it is quite possible that it represents a fundamental way of resetting synaptic strength also in vivo.” Tononi researches the weakening and “resetting” of synapses during sleep and an article on his work will appear in Scientific American during coming months.
Reverse transmission up the axon, known as antidromic firing, occurs as part of a larger set of events in the hippocampus in which experiences of the previous day replay like a sportscaster’s video tape. Ultimately, understanding these night moves—and the benefits of weakening synapses—may help address PTSD, OCD and other disorders in which a mind, unable to detach, replays an endless tape loop that is incapable of refreshing and wiping the slate clean.
Source: National Institutes of Health