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Brains ~in Space~
Brains have been evolving for 500 million years to exist on a planet with gravity. However, when astronauts enter outer space, their brains have to overcome some serious challenges contrary to the way they were designed. Astronauts report all types of side effects, both during their time in Space and upon their return to Earth, including several neurological problems. While understanding the causes behind these problems can help us understand how the brain works, there are many limitations to studying the brains of astronauts. In order to get a better picture of how long-term periods without gravity may affect humans, scientists have done all sorts of ingenious experiments to tackle this problem. However, the results of these studies suggest that long-term space colonization may be further away than we think.
One of the most notable symptoms of space-flight is dubbed “puffy face syndrome”- when astronauts’ faces appear more swollen. Normally, gravity helps regulate the flow of fluids throughout the body; however, in a gravity-free environment, fluids are not properly circulated throughout the body, and thus both blood and cerebrospinal fluid build up in astronauts’ heads. This causes the brain to swell and become compressed by the skull, in turn causing visual issues and leading to smaller structures in the brain becoming compacted [1]. Medical evaluations on Earth have revealed that astronauts’ optic nerves swell and their eyeballs flatten in shape. Some astronauts experience retinal hemorrhages. These effects are still seen one year after space flight.
In addition to parts of the brain becoming squished for long periods of time, space travel may also “rewire” our brains. Researchers imaged astronauts’ brains to see the structure of the brain before and after 6 months of space flight. Through these images, researchers learned that white matter, the material in the brain that helps relay information and signals quickly and efficiently, was significantly decreased in the part of the brain responsible for motor control and a region of the brain responsible for balance, the insular cortex [2]. These effects never went away. Astronauts may experience this loss of white matter due to decreased need for balance and movement while floating around in zero gravity, but the fact that the white matter never recovers means that astronauts might struggle with movement for the rest of their lives while back on earth
Another notable effect on astronauts is the disruption of their sleep-wake cycles. In the space shuttles, the sun rises and sets every 45 minutes and the brightness of light is variable. Astronauts need to be alert during their shifts, but also have to sleep soundly without gravity- meaning they can’t lay down on a soft mattress and rest their heads on a pillow. Instead, they must get used to sleeping weightlessly while strapped into a sleep chamber. Disruptions in natural biological rhythms are known to cause several cognitive and behavioral issues like diminished focus, vigilance, attention, motor skills, and memory. According to NASA research, and first-hand reports from astronauts, microgravity has an effect on the human ability to function, both physically and mentally. Astronauts have a harder time controlling their movements and completing cognitive tasks [3].
Before humans can achieve longer duration space exploration, more work is needed to understand the long-term effects of these changes on the brain and other potential ways the brain may change or be harmed by spaceflight and life without gravity. Because there are a limited number of astronauts who can be studied, and studying the human brain poses both practical and ethical challenges, scientists have come up with ways to use other types of brains to perform necessary experiments in space.
How can we study the brain in space?
In order to study how lack of gravity plays into both nervous system development and our ability to navigate the world, scientists in the International Space Station performed several experiments with rats. Previous spaceflight experiments showed a dramatic increase in the synapses between cells in the ear responsible for hearing and balance. Synapses are the points of contact at which nerve cells communicate, either by sending and receiving chemical messengers or through electrical impulses. This team wanted to confirm the increase in synapses and to determine how early this change takes place and how long it lasts. Tissue samples taken from the parts of the brain responsible for balance in rats on board showed that, in the inner ear, the number of synapses increased dramatically as early as day 2 of the flight. Although the number declined by day 14, the space rats still had more synapses overall than control rats on the ground. This may mean that in order to overcome the challenges of weightlessness, the brain was able to adapt to spaceflight relatively rapidly, illustrating how adaptable the brain is and how our brains may be able to adjust to life without gravity.
On top of adapting to experiencing weightlessness, the brain also has to learn how to move around differently and navigate the world in new ways. Researchers in space created a track that would give the rats’ visual and balance systems contradictory signals, allowing them to study the role of each in creating links between neurons and places. Researchers recorded signals from a region of the brain known to elicit cues about where we are in regards to spatial navigation, the hippocampus. At first, the brain signals from the rats were abnormal as they tried to navigate the special track, and their brains struggled to establish clear links with places. However, by day 9, the brains resumed normal signaling. Being able to gradually adapt to the environment shows the mechanism astronauts themselves may use and also shows how flexible the brain can be even when experiencing an environment it was not evolved for.
We can also use rats to study how life in space might affect human development. If humans one day want to achieve long-term space travel, we will need to understand how babies born in space may be affected. In humans, shortly after birth, if you stretch a newborn’s arms over its head, it will make stepping movements. About a year later, the child’s first real steps are an important milestone, brought about by many months of practice. Infant rats also show a reflex related to gravity, known as the righting reflex. If you hold an infant rat on its back and then let it go, it will turn over, or right itself, in whatever way it can. As it matures, it will learn to right itself efficiently and smoothly. Researchers in space brought rats that later gave birth on the International Space Station. When the rats reached the age that they would have ordinarily been learning to walk and right themselves, instead these young rats floated around without ever righting themselves. Since they had no input from the gravity sensors that told them they were upside down, they never felt a need to flip over. Interestingly, after returning to Earth, the rats could right themselves, but they were never able to do so smoothly as rats who were born on Earth.
Back on Earth, scientists looked at what may have been different in their brains. They found that there were fewer dendrites (protrusions that receive incoming signals) in the motor neurons that were involved in posture and motor control. This may mean that the rats needed gravity to develop a normal righting reflex, and by experiencing weightlessness, that part of their brain remained underdeveloped. Another interesting finding is that there were more synapses in parts of the brain related to hind limb movement, perhaps because in space, rats can walk around in a cage in three dimensions instead of just two. This is another example of how brains can rapidly adapt to new environments. Although the rats were deficient in the righting reflex, they were enriched in other ways as a result of a new environment they were raised in.
Another way to study brain development in space is by growing small clusters of human brain cells and measuring their differences compared to those grown on Earth. These clusters of brain cells are called organoids. While the human brain is made up of about 86 trillion neurons, organoids are often only thousands of cells and about the size of a pin. In 2017, the Muotri lab at UCSD packaged these cells to be sent to space and then grown on plates that can measure the electrical activity of the cells within the organoids. These cells developed electrical patterns similar to those of developing fetuses, a positive outcome. While there is much more to be learned about brain development in space, this study opens the door for how future experiments can happen.
Studying the development and adaptability of the brain in space brings up an interesting question: even though we evolved to live on Earth, could we live as easily elsewhere? Most of the work done in the lab on the Space Station has taught us that the brain has astonishing adaptability, but adaptability does not last forever. The developing nervous system adapts to the influences it receives at particular times in development. After a critical period, however, the opportunity for wide-ranging adaptation fades; changes made during the adaptive period appear to be permanent. While results are not conclusive, humans may not be ready for long-term space colonization just yet- at least maybe until we can create artificial gravity.
References:
[1] Robitzski, D. (2019, August 29). Scientists grew tiny human brains and hooked them up to Robots. Futurism. Retrieved May 10, 2022, from https://futurism.com/scientists-grew-human-brains-robots
[2] Penttila, N. (2019, September 8). Astronauts study the brain in space. Dana Foundation. Retrieved May 10, 2022, from https://www.dana.org/article/astronauts-study-the-brain-in-space/
[3] Magazine, S. (2022, February 24). Long-term space travel may ‘rewire’ astronauts’ brains. Smithsonian.com. Retrieved May 10, 2022, from https://www.smithsonianmag.com/smart-news/spacefaring-astronauts-return-home-with-rewired-brains-180979626/
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