The Mapping of Our Internal Clock
The SCN (suprachiasmatic nucleus) makes the body run properly and on time. It acts as the control center for our genetic clock and the circadian rhythm, which regulates multiple functions including insulin sensitivity, hunger, sleep, body temperature, hormonal levels, cell cycles, etc. The suprachiasmatic nucleus has been extensively studied, but its neural network has remained a mystery to scientists.
In a new study from Harvard John A. Paulson School of Engineering and Applied Sciences, Washington University in St. Louis, and the University of California Santa Barbara, researchers have found that SCN neurons are linked to each other, which sheds immeasurable light on this area of the brain. By understanding this brain structure and how it responds to disruption, scientists will be able to tackle more illnesses such as posttraumatic stress disorder and diabetes. Additionally, researchers of this study report that disruption to the rhythms that come from the SCN, i.e., shiftwork and nighttime blue light exposure, negatively interferes with health.
The findings were published in the Proceedings of the National Academies of Science (PNAS).
The first author of the paper, John Abel from SEAS, stated that due to the noisiness of the cells within the SCN, it has been incredibly difficult to understand. There are in excess of 20,000 neurons that reside within the SCN, and each of them have their own task of regulating the circadian rhythm; however, they also communicate with their fellow neurons in order to maintain relationships. Now, scientists are able to cut through that noise of communication and determine which cells are sharing what with the others.
There are two hemispheres in the SCN, so it looks like a small brain. It is located inside the hypothalamus. It takes cues of light from the retina, which is what allows it to keep track of time and reset itself when needed. When the SCN is functioning normally, all neurons in both hemispheres demonstrate a synchronized pattern in their oscillation.
Abel and the research team disrupted this synchronized pattern in order to better understand the network structure. They did this by using a potent neurotoxin found in pufferfish, which turned the usually steady, rhythmic oscillating pulse into disconnected beats. Then, they removed the toxin and used information theory to determine how the network re-established communication between cells.
John Abel likened this process to trying to determine if a group of people were friends without being able to look at their text messages or phone calls. In large groups, you probably would not be able to tell who was a friend of whom, but if a group of people showed up to a party together, it is safe to assume that they communicate on a friendly basis.
At a single-cell resolution, the research team was able to identify groups of friendly neurons in the center of each hemisphere. They shared a lot of information with each other during the resynchronization process. Additionally, researchers found that there were dense connections between the hubs of each hemisphere. The area called the shell outside the hub showed neurons that behaved like acquaintances rather than friends and shared very little information.
Researchers were surprised to find that the shell did not contain functionally connected collections of neurons. Previously, it was assumed that the neurons in the shell played a bigger role in communication; however, this study shows that it is the core neurons that mediated the clustering.
It was also assumed in past research that the core SCN dominated the process because of its role in receiving light cues from the retina. The use of the neurotoxin to disrupt circadian rhythm, however, allowed researchers to demonstrate the core to be the key player in resynchronization without light cues.
Co-author of the paper, Professor Frank Doyle, stated that in the 15 years of studying the complex control mechanisms involved in the circadian rhythms, this is the work that brings them closer to understanding the communication between neurons. This would demonstrate the importance of seeing the link between genes, the cells, and the SCN tissue.
Author: Rachael Herman is a professional writer with an extensive background in medical writing, research, and language development. Her hobbies include hiking in the Rockies, cooking, and reading.
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