2024-03-15 12:42:38
Recent college graduate Elma Jashim is looking forward to starting medical school in the fall. The only downside: She dreads the emotional roller coaster she endures each month due to her period and the damage it might pose to her busy academic calendar.
“About two or three days before my period starts, I don’t feel much emotion. I’m not particularly sad, but not particularly happy either,” says Jashim. This flat mood makes her even more sensitive to emotional stimuli, even minimal ones, when her period begins. “It only takes one tiny mistake at work for tears to come to my eyes. »
The brain mechanisms that trigger these emotions are still poorly understood. However, progress has been made in visualizing how sex hormones can change certain areas of the brain. Previous studies carried out on rats as well as other mammals had already shown that the volume of certain regions of the brain might change, influenced by estrogen, a hormone necessary for the proper development of sexuality and reproduction in women. . However, it was not known whether this powerful hormone might change the structure of the human female brain.
Today, recent MRI scans of women’s brains show that the rise and fall of sex hormones during the menstrual cycle, the twenty-nine-day period of hormonal ebb and flow that prepares the reproductive organs for possible pregnancy , dramatically reshapes the regions of the brain that govern emotions, memory, behavior and the efficiency of information transfer.
“It’s amazing how quickly the adult brain can change,” says Julia Sacher, a psychiatrist and neuroscientist at the Max Planck Institute for Neurology and Cognitive Sciences in Leipzig, Germany, who led one of these studies.
The fact that the brain changes during the menstrual cycle is particularly notable because over the course of thirty to forty years, most women experience regarding 450 menstrual cycles, says Catherine Woolley, a neurobiologist at Northwestern University in Evantson, Illinois.
The brain imaging and hormone analyzes on which these studies are based were carried out on the same individuals, during specific phases of their menstrual cycles, which makes the results all the more reliable, Woolley explains.
“These studies allowed us to better understand the extent to which these hormones influence not only the morphology of the brain but also its functional architecture,” says Emily Jacobs, a neuroscientist at the University of California, Santa Barbara.
A menstrual cycle repeats every twenty-five to thirty days and begins with “menstruation”, during which the superficial layer of the endometrium, the inner lining of the uterus, sheds. At this time, the levels of female sex hormones in the blood are lowest, but they increase sharply over the following weeks. First, estrogen levels rise, signaling endometrial growth. Then they drop to release an egg from the ovary, which marks the middle of the cycle. Then, progesterone and estrogen levels increase once more for regarding seven days to prepare the uterine lining for the eventual fertilization of the egg. If pregnancy does not occur, these levels decrease, triggering menstrual bleeding.
The brain is a dense mass of miniature tree-like cells called neurons. These neurons, as well as their extensions, called dendrites, are contained in gray matter, the outer layer of brain tissue. On the dendrites are leaf-shaped protrusions, called dendritic spines. The roots, or axons, of neurons cluster in the white matter of the brain.
Gray matter is responsible for emotions, learning and memory while white matter, more anchored in brain tissue, is responsible for exchanging information and connecting the different regions of gray matter.
“This result was particularly surprising and aroused great skepticism in the community,” recalls Woolley. “At the time, estrogens were considered to be reproductive hormones only and did not affect cognitive regions of the brain such as the hippocampus. »
The hippocampus, the cognitive center of the brain that contains both gray and white matter, is a small curved structure buried in the brain, behind the ears, in a region filled with sex hormone receptors. It is also the region of the adult human brain most influenced by volume changes. Acquiring new skills, such as learning to juggle in old age or studying maps to take the taxi driving test, causes the hippocampus to enlarge. Conversely, a shrinking hippocampus is sometimes an early sign of dementia, particularly in Alzheimer’s disease.
Since Woolley’s groundbreaking discovery, scientists have understood that menopause causes the volume of gray matter in certain parts of the brain to shrink. The research, however, was limited to obtaining snapshot images of volunteer brains at a single point in time. The scientists wanted to know if the adult human brain changed during the monthly rise and fall of sex hormones.
“She was, in a way, the Marie Curie of neuroscience,” says Jacobs. After thirty scans of this woman’s brain, Jacobs’ team discovered that sex hormones were reshaping the hippocampus and reorganizing brain connections. However, it remained unclear how quickly this effect of waves of hormones occurred during the menstrual cycle.
To answer the question, scientists from Leipzig and Santa Barbara, each separately and for two separate studies, carried out scans of the brains of fifty women at different phases of their menstrual cycle.
In a study published in the journal Nature Mental Health, Sacher and his team performed ultrasound scans to identify the precise moment of ovulation in twenty-seven female volunteers. This allowed them to take blood samples from the volunteers at six specific times during their menstrual cycle, chosen based on ovulation and hormone levels in the blood. They then scanned the brains of these twenty-seven women at six specific time points using ultra-high-field MRI.
Using this more powerful MRI than is commonly used in clinics, Sacher’s team was able to image the living brain with such high resolution that was previously only possible by directly cutting into the brain when of an autopsy.
Despite the small size of the structure analyzed, Sacher’s team was able to observe a choreographed series of changes in different regions of the hippocampus as it remodeled throughout the menstrual cycle. The outer layer of the hippocampus thickened and gray matter expanded with increasing estrogen levels and decreasing progesterone. However, when progesterone levels increased, the memory-related layer expanded.
Another study, which has not yet been peer-reviewed, involved scanning the brains of thirty volunteers during ovulation, menstruation, and the period in between. This study found that not only the thickness of gray matter, but also the structural properties of white matter fluctuated, influenced by hormones.
“We used a kind of rule [pour mesurer la matière grise] and we observed it change alongside fluctuations in hormones,” explains Elizabeth Rizor, who co-led this study with Viktoriya Babenko. They are both neuroscientists at the University of California, Santa Barbara.
The study suggests that changes in white matter linked to hormonal fluctuations preceding ovulation might make the transfer of information between different parts of the brain more efficient.
“These changes are very widespread, not only in gray matter, but also in areas of the brain that are responsible for coordination between regions and between white matter pathways,” explains Babenko.
However, the changes observed in the volume or thickness of brain regions in these studies have not yet been associated with specific brain functions. While these studies show that certain areas of the brain can be remodeled based on oscillations of hormones during the menstrual cycle, scientists point out that this does not mean that memory or cognition are affected.
“For particular brain functions or processes, we can’t say bigger is better,” Woolley says.
Studies also don’t reveal whether volume changes are linked to the myriad emotional and cognitive symptoms women experience during their periods. In reality, these studies only considered healthy women who did not report such symptoms.
On the other hand, they highlight the urgency of research into the specific neuroscientific needs of women, says Jacobs.
“There are real structural changes happening in our brains that might be linked to these emotional elevators, mood shifts, etc.,” says Jashim.
“It’s high time to make the brain a major research focus in women’s health,” says Sacher.
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