After a long and tiring day, we all love to go to our beds and get lost in the sweet world of sleep. Sleep provides us a break from the outside world and rejuvenates our bodies and minds. It is essential for our physical and mental well-being. Research has shown that sleep serves many important purposes including energy conservation, replenishment of cellular supplies, waste clearance, memory processing, and learning. However, sleep is still a mysterious biological phenomenon as we do not completely understand its mechanisms and functions.
Researchers have specifically linked sleep to the normal functioning of the brain through the synaptic homeostasis hypothesis (SHY). According to this hypothesis, a core function of sleep is to restore the strength of the synapses, the structures that allow the neurons in our brains to communicate with each other. SHY states that the learning occurs during the wake, when we are under the influence of signals from the environment, through the process of synaptic potentiation which leads to an increase in the strength of synapses. On the other hand, while we are asleep, synaptic depression takes place in our brains, resulting in the decrease in the strength of the synapses. Hence, sleep helps in the renormalization of the synaptic strength. The strengthening and weakening of synapses, termed as synaptic scaling, occur regularly across the wake/sleep cycle and is crucial for the integration of new information in our brains.
In a recent study published in Science, scientists (Vivo et al.) at the University of Wisconsin-Madison provided the morphological evidence of synaptic scaling occurring in the mouse brain across the wake/sleep cycle. To study this phenomenon, the researchers isolated the brains from three groups of mice. The first group of mice got proper sleep, whereas the mice in the other two groups were forcefully kept awake or stayed awake on their own. The researchers then used a technique called block-face scanning electron microscopy to image the synapses in two different regions of the isolated brains. Based on these images, they calculated the axon-spine interface (ASI) i.e., the surface area of direct contact between the axonal bouton and dendritic spine head, which serve as the transmitting and receiving ends of the neuron, respectively. The ASI was used as the parameter to assess the strength of the synaptic connections.
Researchers made the interesting observation that the ASI decreased by about 18% in the first group of mice that got sufficient sleep as compared to the other two groups that were awake, indicating a downscaling in the synaptic connections during sleep. However, downscaling was not observed uniformly across all the synapses but was limited to small and medium synapses, which represented about 80% of the total synapses. The remaining 20% of the synapses, which were larger, did not undergo a decrease in ASI. In addition, while downscaling was observed in the spines which were structurally unstable and contained endosomes which facilitated the structural changes by recycling of cellular material, it did not occur in the spines that lacked the endosomes. Also, the decrease in ASI during sleep was observed to be minimal in the dendritic spines with high synaptic density. The researchers suggest that the synapses that are large or do not contain endosomes or have high synaptic density might be associated with committed and stable memory circuits and hence, they escape the process of downscaling during sleep.
Although it might not be possible to replicate this study in humans, the researchers suggest that synaptic scaling during wake/sleep cycle also occurs in our brains. Taken together, this study provides a definite evidence for the SHY and establishes that while wake results in an increase in synaptic strength, an important function of sleep is to selectively reduce the synaptic strength to bring it back to the normal levels. In simpler terms, sleep provides a useful mechanism of “smart forgetting”. When we are asleep, our brains can comprehensively analyze all the memories made during wake and keep the ones that are important, while discarding the ones that are irrelevant. Another group of researchers has confirmed the results of this study by identifying the gene involved in synaptic downscaling during sleep.
This study highlights the importance of sleep for the proper functioning of our brains. A good night’s sleep enhances our reasoning and problem-solving skills and helps us to concentrate and memorize. So, next time whenever you are feeling confused, unable to make a decision, just try to sleep on it. And hopefully, when you wake up, your brain will be able to think much more clearly!
Journal reference:
de Vivo L, Bellesi M, Marshall W, Bushong EA, Ellisman MH, Tononi G, Cirelli C. Ultrastructural evidence for synaptic scaling across the wake/sleep cycle. Science. 2017 Feb 3;355(6324):507-510. doi: 10.1126/science.aah5982.
Other references:
http://science.sciencemag.org/content/355/6324/511.long
http://www.cell.com/neuron/abstract/S0896-6273(13)01186-0
https://www.sciencedaily.com/releases/2017/02/170202141913.htm
http://www.learningscientists.org/blog/2017/2/23-1
https://www.psychologytoday.com/blog/memory-medic/201702/sleep-perhaps-learn
Featured image source: Pixabay
About the author:
Isha Verma is currently pursuing her PhD in Stem cell research from the Indian Institute of Science, Bangalore. She loves reading and traveling.
