High-Resolution Images From Sleep Research Reveals How The Brain Resets During Sleep

Astonishing photos of electron microscope pictures inside the brains of mice reveals that the synapses grow strong and large during the day, and then shrink by nearly 20 percent when sleeping, thus, creating room for more learning and growth the next day. The four-year research project by Dr. Chiara Cirelli and Dr. Giulio Tononi of Wisconsin Center for Sleep and Consciousness, provides a visual evidence of the "synaptic homeostasis hypothesis" (SHY).

The researchers found that sleep is the price people pay for elastic brains that are able to keep learning new things. The activation of the synapse when a person wakes up makes it to grow stronger which is believed to be crucial for the process of learning and memory retention. However, the growth needs to be balanced in order to prevent the saturation of synapses and the obliteration of neural signaling and memories.

The study authors believe that this process of renormalization is best achieved through sleep as people pay less attention to the external world and are free from the "here and now." When synapses get stronger and more effective they also become bigger, and conversely they shrink when they weaken, according to Science Daily.

The researchers reasoned that a direct test of SHY was to determine whether the size of synapses changes between sleep and wake and to do so, they used a technique with an extremely high spatial resolution known as serial scanning 3-D electron microscopy. The study was a massive undertaking, which engaged many research specialists working for four years to reconstruct, analyze, and photograph two areas of the cerebral cortex in mice brain.

They were able to measure the size of and reconstruct 6,920 synapses. It was not clear whether the researchers were analyzing the brain cells of sleeping mice or one that is awake. But when they cracked the code and compared the measurements with the amount of sleep the mice achieved during the eight hours before the image was taken, they discovered that a few hours sleep led to an 18 percent decrease in the size of the synapses.

The study authors noticed that the changes occurred in both areas of the cerebral cortex and corresponded to the size of the synapses. The scaling was reported to have occurred in up to 80 percent of the synapses but not in the largest ones, which is said to be related with the most stable memory traces. This reveals that the balance of synaptic strength and size is altered when mice wakes up and is automatically restored when sleeping.

The researchers explained that it is interesting that the majority of synapses in the cortex face such a huge change in size and strength with the few hours period of wake and sleep. Given this result in mice, the findings invariably mean that trillions of synapses in human cortex could get slimmer by up to 20 percent every night, according to Medical Express.

The findings of the research confirms SHY's prediction that synapses undergo a process of scaling down during sleep using biochemical and molecular methods, and also identifies genes necessary for the process. The researchers published the findings of the study in Science with the findings of a study from Dr. Richard Huganir's laboratory at Johns Hopkins University, Baltimore.