It keeps us up long hours, fearing sleep. Perhaps it’s the reason we toss and turn at night, or wake up in a panic, sometimes even struggling to breathe, or why you find yourself reluctant to look at the news alerts on your phone — or read email. As advanced as our species has become over the last two centuries, it seems we cannot elude the primal state of anxiety — it’s almost always right there waiting for us. Of course, some sense it more than others. Usually, you’ve got a good reason to be worried — will your yearly job review be a good one? Did you prepare enough for your final exam? Will the hospital test results come out OK?

It’s easy to think that we’re the only ones who agonize like this over events that are all too often inevitable, but what we’re experiencing is actually something our species has long evolved with. Our primate ancestors navigated a harsh and unforgiving terrain — having to retreat to trees or caverns when nightfall came — bringing with it an array of predators. Those who survived the prehistoric times — at least long enough to produce offspring, were those who could best recognize approaching danger — to spot leopards, snakes, even larger primates, hiding in the tall savannah grasses.

Crossing paths with a striking adder would have triggered the classic “fight-or-flight response” in their brains. The response starts off when the brain’s amygdala first processes the threat. It relays a message to the hypothalamus, activating the pituitary gland that begins to secrete the hormone epinephrine, rapidly increasing blood flow to the muscles and allowing our ancestors to either flee or intimidate their attacker. The rush of chemical signals from the brain produce cortisol increasing blood sugar and blood pressure to heighten that same energy and awareness we sense in a stressful situation.

To Flee Or Not To Flee?

From the time Walter Bradford Cannon first described this response back in 1929, we have come to equate the almond-shaped amygdala with the emotion of fear — thinking with our amygdala, the primal so-called reptilian brain, when we are easily startled. Recent research, however, suggests that basic emotions like fear and anger are hardly restricted to any particular region of the brain. As Aalto University doctoral candidate and researcher Heini Saarimäki describes it, “From the biological point of view, an emotion is a state of the entire brain at a given moment.”

Rather than specific regions of the brain being activated when we anxiously await our test results, the emotion we feel is actually the sum total of a number of factors. The doctor’s tone of voice or the way they enter the room, may be interpreted by the brain as ominous or reassuring. As that happens, the brain is also quickly pulling up a number of memories — where you were the last time you received bad news, for example.

You may wonder why painful memories remain so detailed in our minds for years after they happened — why it’s possible to replay them and revisit them so often, while forgetting mundane details from yesterday. We may now know why this has become part of our mental fabric. A recent study identified “anxiety neurons” residing in the hippocampuses of lab mice, using calcium imaging to highlight brain activity. The hippocampus is a part of the brain’s limbic system that plays a role in the formation of memories.

Shining A Light

To instill feelings of anxiety in the mice, the researchers at the University of California, San Francisco, placed them in an intricate maze. Some of the trails brought them to open spaces, others lifted them onto a different platform forcing them from the safety of the walls. Even though they have no natural enemies, the mice displayed the same feelings of vulnerability to predators, just as their cousins would in the wild. Miniature microscopes placed into their brains highlighted a burst of activity in the ventral CA1 region of the hippocampus, the more agitated the mice became.

The output from these neurons came from the hypothalamus, responsible for regulating the hormones behind emotions. The same regulation process occurs in people too, so the researchers suspect that the same anxiety neurons occur in human biology. The good news is that at least for the mice, there’s a way these neurons can be controlled.

The technique is known as “optogenetics” in which the cells are controlled by a beam of light, shone directly onto the ventral CA1 region. The cells that were activated during periods of high anxiety shut down, and the mice confidently explored their environment once again. Adjusting the light settings further allowed the researchers to reverse this effect. They were able to increase anxiety levels even when the rodents were safely enclosed in familiar surroundings. The team suspects that these neurons may exist in other parts of the brain as well.