Astronauts are no strangers to the harsh and wild conditions of outer space. From ionizing radiation to microgravity, these space explorers are exposed to a myriad of stressors that play a toll on their psychological and physiological systems. While it may take a trained professional to diagnose radiation sickness, almost anyone could recognize some of the other, more obvious stressors like microgravity. Videos from the International Space Station (ISS), for example, show astronauts floating around with puffy faces and “chicken legs” – a result of the upwards fluid shift that happens in the absence of Earth’s gravity. While the effects of this weightlessness are most obvious in the musculoskeletal systems of astronauts (who often struggle to stand up right after they land back on Earth), more research in space physiology has revealed an entire network of changes throughout the body, as well. Some of these changes are better documented and more widely reported than others, like the effect of microgravity on astronauts’ immune systems or hormone regulation. Curiously enough, however, weightlessness also seems to be impacting the actual structure of the human brain.
In 2015, a group of neuroscience researchers performed an analog spaceflight study using a technique called head-down tilt bed rest (HDBR). Instead of sending people into orbit, researchers confined participants to a bed with the pillow end tilted downward a few degrees. This allows fluid to shift up towards the head, which is essentially what we see in astronauts in orbit. Participants are confined to these beds for about a month or so, sometimes with limited lower limb exercise, in order to maintain the fluid shift and simulate long-term spaceflight. By comparing the MRI scans of these participants before and after they were confined to HDBR, researchers were able to observe any changes in their brain density and volume.
It may suggest that these areas of the brain are undergoing some element of neural plasticity or maybe even compensation, all in response to simulated weightlessness.
Interestingly, they found that in response to extended simulated weightlessness, participants’ brains showed significant changes in the volume of neuronal cell bodies (or gray matter volume). These changes especially cropped up in areas throughout the brain associated with performance, learning, memory, movement, and coordination. For example, researchers observed decreases in gray matter volume in the bilateral frontal lobes, which are associated with voluntary movement and higher cognitive processing. Decreases in brain volume and/or density are typically linked with some kind of neurodegeneration, and this particular decrease could be linked to suppressed synapse formation and decreased neuronal activity. On the other hand, significant increases were found in a specific region of the cerebellum linked with limb movement and coordination. And as one might guess, increases in brain volume are often associated with the formation of new neurons and connections. The possibility of new neuron formation in an area linked to movement and coordination is interesting, especially in participants whose movement was limited. It may suggest that these areas of the brain are undergoing some element of neural plasticity or maybe even compensation, all in response to simulated weightlessness.
But what about actual spaceflight? Is there some mysterious variable in our Earth-based analog studies that is linking simulated weightlessness with these brain changes, or do we see similar things happening in real astronauts? Another study in 2016 looked into this very question by retrospectively analyzing pre-flight and post-flight MRI scans of Space Shuttle and ISS astronauts, in order to pinpoint any structural changes that happened to their brains. Indeed, the brains of these astronauts were not quite the same after they had landed back on Earth. For instance, gray matter volume decreased in areas of the brain linked to higher cognitive processing, semantic knowledge (knowledge about objects, definitions, and facts), and timing accuracy. Other non-related studies also found that astronauts exhibit impaired performance on certain types of attentional tasks, although it is unclear which stressor (or combination of stressors) may be contributing. While it is still not certain whether these findings are definitive signs of neurodegeneration, it certainly seems that the spaceflight environment is somehow impacting astronauts’ complex thinking skills, memory, and overall operational capabilities.
It may be that the brain is actually compensating for the lack of weight on the lower limbs by changing the structural neural pathways connected to them.
Other areas of the brain, like the somatosensory and motor cortex, experienced increased gray matter volume instead. These sections of the brain are responsible for limb movement and coordination – interestingly enough, lower limbs especially, are known to experience muscle atrophy and calcium loss due to weightlessness. It may be that the brain is actually compensating for the lack of weight on the lower limbs by changing the structural neural pathways connected to them.
These researchers took their study one step further and decided to compare their actual spaceflight data to their own HDBR analog studies to compare the two environments. What they found was an interesting overlap of changes in the brain, which seems to support the robustness and validity of both the analog studies and the spaceflight studies. Many of the same regions of the brain increase and decrease in gray matter volume in both analog and spaceflight participants, but the changes appeared to be much more dramatic and widespread in the analog participants and much more localized in actual astronauts. The reason for this disparity may be due to the differences between the physical environment of the two studies and between the activities of the two groups (astronauts are likely more active than the HDBR participants, for example). However, this is merely a hypothesis – the real difference remains a mystery.
One particularly interesting finding that came out of the 2016 study, however, was a slight difference in the magnitude of changes between the Space Shuttle astronauts and the ISS astronauts. Prior to their pre-flight scans, the Shuttle crew members had spent about twice as many days in space as their ISS counterparts; they also experienced less dramatic decreases in gray matter volume compared to the ISS crew. This could suggest that being exposed to spaceflight for an extended period of time may offer some kind of protective preconditioning response, or at the very least, result in smaller changes in gray matter volume the second or third time one goes into space. While the differences between the two groups were statistically significant, the association between prior spaceflight experience and the changes in volume were not significant. Still, the fact that the difference existed in the first place and appeared to be a significant change is an interesting observation. After all, preconditioning responses would not be completely unheard of in astronauts. For instance, many astronauts who visit space a second time experience less difficulties with space sickness and have an easier time adjusting to microgravity. However, the only way to ascertain if there really is a connection between prior spaceflight experience and neural plasticity would be to look for similar things in more astronaut cohorts.
Before drawing definitive conclusions, however, it is important to note that these research studies have their limitations, which may impact the data that was gathered and the analysis and interpretation that followed. For instance, the first HDBR study from 2015 was performed on 18 young, healthy male participants, which – as you might imagine – are not the most representative sample of individuals. Also, the 2016 study with astronauts was done retrospectively, meaning that not all MRI images were gathered using the same protocol. Researchers actually went in afterwards and did their own analysis on pre-existing scans of astronauts pre- and post-flight. Yet, the overlap of changes that researchers saw among the analog and spaceflight studies seems to suggest that something is indeed changing in the brains of individuals exposed to weightlessness (both simulated and real).
How permanent are some of these changes, and how will they impact Mars explorers as they transition from outer space to planet surfaces?
Overall, many of these findings suggest interesting and sometimes concerning changes in the brains of individuals exposed to microgravity – which raises the question of how future astronauts headed to Mars may fare. Would more experienced astronauts with prior spaceflight experience be better candidates for a Mars mission, if they really do experience less dramatic changes to their brain volume? How permanent are some of these changes, and how will they impact Mars explorers as they transition from outer space to planet surfaces? And perhaps, most importantly, what can we do to lessen any detrimental impacts that spaceflight may have on our astronauts’ neurological systems? Advances in technology, like the development of portable and lightweight MRI machines, could help us answer many of these questions by giving us real data from the actual spaceflight environment. As space neuroscience takes its steps into the unknown, perhaps future studies will help us understand just how much our brains are changing – not just in response to microgravity, but to outer space as a whole.
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