Body and Brain: Who’s in Charge?
Until recently, both scientists and the general public believed that the brain and body were two relatively independent systems. The brain was thought to issue the main commands, and the body simply followed orders—unless it was seriously ill. Mental health issues and physical illnesses were considered unrelated. Depression, for example, was seen as just a bad mood, not comparable to a cold or fever. Some even imagined that, with enough technological progress, we could leave our damaged or sick bodies behind and keep our brains alive in nutrient tanks, allowing the mind to live on. However, recent research has shown that these ideas reflect a misunderstanding of the relationship between body and brain.
The Connection Between Brain and Immune System
Neurologist Jonathan Kipnis once said, “Perhaps we have two ancient forces: pathogens and the multicellular battlefield of the immune system. Part of our personality may actually be controlled by the immune system.”
Recent studies have found that psychiatric and neurological disorders—such as depression, schizophrenia, OCD, autism, and ALS—are linked to immune activity. How can the immune response be connected to mental illness? A groundbreaking 2018 study by Kipnis’s team showed that the body’s immune response can affect the brain through the lymphatic system, and that immune inflammatory factors can directly influence brain function.
We know that, in addition to arteries and veins, the circulatory system includes lymphatic vessels. Lymph nodes, which store immune cells that fight invading pathogens, are located throughout this network. For much of the 20th century, medicine believed that the brain and body were separated by the blood-brain barrier, and that the brain had no lymphatic system. This view was taught in medical textbooks for over a century. But in 2015, Professor Jonathan Kipnis and his team at the University of Virginia completely debunked this idea. Using neuroimaging, they discovered that the meninges (the membranes covering the brain and spinal cord) are lined with a network of lymphatic vessels that transport cerebrospinal fluid and lymphocytes to lymph nodes in the neck.
Kipnis’s team also found that the activity of the immune molecule interferon gamma in the brain affects social behavior. The more active it is, the more social the animal becomes. Interferon gamma is produced by the immune system in response to bacterial, viral, and parasitic infections. It’s active during socialization in fruit flies, mice, and rats; if it’s artificially blocked in mice, they become socially withdrawn. Scientists believe that when animals socialize, pathogens can easily spread. Since interferon gamma can fight pathogens, its activation during socialization may help protect against infections. In other words, our social behavior may be “approved” by the immune system.
Irritable Bowel Syndrome
A childhood friend of mine always ran to the bathroom when he was nervous. At age 25, a doctor friend told him he had classic irritable bowel syndrome (IBS)—an acute stress reaction that affects many people.
Indigestion and IBS often result from a combination of psychological symptoms in the brain and physical symptoms in the digestive tract. Studies show that people with high levels of anxiety and depression are much more likely to develop IBS or dyspepsia within a year. Conversely, people with IBS or dyspepsia who didn’t initially have anxiety or depression are also more likely to develop these mental health issues within a year. In other words, digestive disorders and emotional problems can influence each other. For about one-third of people, psychological issues appear before digestive problems; for the other two-thirds, it’s the opposite.
A survey of over 6,000 U.S. teenagers found that many mental and physical illnesses occur in close sequence. For example, arthritis or stomach problems are more likely to develop after depression; skin problems are more likely after anxiety disorders; and anxiety disorders are more common in teens with heart problems. Emotional and physical problems are not independent—they affect each other. Your stomach issues may be a result of depression, and unexplained skin rashes may be a physical manifestation of inner anxiety.
The Gut-Origin Hypothesis of Parkinson’s Disease
Parkinson’s disease is the second most common neurodegenerative disorder after Alzheimer’s, affecting 1% of people over 60 and 4% over 80. There is currently no cure. Years before motor symptoms appear, patients often experience early physical symptoms like loss of smell, insomnia, constipation, depression, and a one-sided thumb tremor. Later, motor symptoms such as hand tremors, slow movements, difficulty maintaining balance, stiffness, muscle weakness, and stooped posture may develop.
People with Parkinson’s have clear brain lesions, especially in a small area called the substantia nigra, which produces dopamine. Dopamine is crucial for voluntary movement, and in Parkinson’s, these neurons degenerate rapidly, impairing movement. By the time mild motor symptoms appear, up to 80% of these neurons are already lost. In mid-to-late stages, large deposits of alpha-synuclein protein are found in the brain.
In 2003, Frankfurt University pathologist Heiko Braak proposed a bold hypothesis: the earliest site of Parkinson’s is not the brain, but the digestive tract. This idea, once unusual, has gained increasing support. A 2015 study found that people who had the vagus nerve connection between the stomach and brain severed were less likely to develop Parkinson’s later in life. In 2018, researchers showed that appendectomy was linked to a 19.3% lower risk of Parkinson’s.
Doctors have long noticed that digestive symptoms (like constipation) often appear 10–20 years before Parkinson’s. Closer study of nerves in the gut of Parkinson’s patients revealed alpha-synuclein proteins, usually found in the brain at later stages. Animal models show that alpha-synuclein in the stomach can travel up the vagus nerve to the brain. In 2019, a Johns Hopkins team led by Professor Dawson provided strong evidence: they found that the misfolded protein associated with Parkinson’s can move from the gut to the brain via the vagus nerve, causing neuron death and classic Parkinson’s symptoms. Cutting the vagus nerve prevented the protein from reaching the brain and stopped symptoms. If mice lacked the gene for this protein, it couldn’t spread in their brains.
The gut-origin hypothesis of Parkinson’s gives us a deeper, more intuitive understanding of the mind-body continuum. We now know that brain diseases are often linked to physical illness.
Toxoplasmosis Can Change Human Behavior
Cat lovers, have you ever wondered why you adore your pet—even when it looks at you with disdain? Rats are usually afraid of cats, but when their brains are infected with the parasite Toxoplasma, they’re actually attracted to the smell of cats.
Toxoplasma is a parasite about five microns long. One-third of the world’s population is infected, with rates as high as 80% in France and Brazil. People usually get toxoplasmosis from eating raw meat, unwashed vegetables, or contact with cat feces.
When rats are infected, their sense of smell changes, and they’re drawn to cat urine, making them more likely to approach cats. Why does this tiny parasite so dramatically alter rat behavior? Many parasites need to pass through several hosts to complete their life cycle, and some manipulate their hosts’ behavior to reach their final host. Toxoplasma can only reproduce in cats, so its goal is to get its current host eaten by a cat.
In rats, Toxoplasma invades the brain, altering smell and behavior to make them easy prey for cats. Infected cats can then spread the parasite through their feces, infecting other animals and people. Toxoplasma is also found in human brains, where it can change behavior and personality, slow reaction times, and reduce concentration. Unlike in rats, Toxoplasma can’t manipulate human behavior to reach cats, since humans are no longer prey for large felines.
To study behavioral changes in humans, scientists looked at chimpanzees, our close relatives. Infected chimps are attracted only to their natural predator—the leopard—not to other big cats. This suggests that parasite manipulation is highly specific: hosts are drawn to the animal most likely to eat them. Scientists hypothesize that behavioral changes in humans after Toxoplasma infection may reflect the food chain of our ancestors, who were once prey for big cats.
When Toxoplasma enters the body, the immune system fights back, causing flu-like symptoms. Soon after, the parasite becomes dormant and resistant to antibiotics, maintaining a balance with the immune system so that most people have no symptoms. Toxoplasmosis is usually harmless for healthy people but can be deadly for those with weakened immune systems or for fetuses, so pregnant women should avoid cat litter and unwashed vegetables.
Although Toxoplasma infection isn’t fatal, it can cause mental and behavioral changes. Schizophrenia, depression, and anxiety disorders are more common in those infected. Studies show that infection can increase aggression and risk-taking: infected people are 2.65 times more likely to cause car accidents. In one study, 24% of drivers in accidents were infected, compared to 6.5% of the general population. The infection also affects men and women differently: infected men may become withdrawn and suspicious, while women may become more sociable, trusting, and submissive.
These changes may be due to alterations in brain chemistry caused by the parasite. Swedish researchers found that Toxoplasma affects the GABA neurotransmitter system in the human brain. Infected neurons release GABA to counter the invader, and GABA helps suppress fear and anxiety. Disruptions in GABA are seen in schizophrenia, anxiety, depression, and bipolar disorder.
Toxoplasma infection also increases glutamate levels between neurons. Glutamate is the main excitatory neurotransmitter, and high levels are found in brain injuries and neurodegenerative diseases like epilepsy and ALS. In experiments, infected mouse brains showed swollen astrocytes that couldn’t process glutamate efficiently, leaving neurons overexcited and firing abnormally, causing various brain symptoms.
Cat lovers shouldn’t panic: your pet isn’t necessarily infected, and if you wash your hands after cleaning the litter box, you’re unlikely to catch it. Cooking food thoroughly also greatly reduces the risk.
Toxoplasma has evolved alongside humans for millions of years. The infection is generally not fatal, but it does increase the risk of mental illness and suicide (due to depression and risky behavior). So—who says we’re always in control of our actions?