Fight-Or-Flight, Behind The Scenes: What Happens In The Brain When Danger Is Clear And Present

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Astrocytes, a type of brain cell found in great concentrations, may be responsible for your feelings of focus in the face of fear. Finizio, CC BY-ND 2.0

A clanging set of harmless pots and pans or a real-life threat of assault — they both produce a sense of fear. And according to a new study, the mechanism in our brains that causes us to choose one way or another in our fight-or-flight response may be helping us to focus, too.

Researchers from Johns Hopkins University analyzed the brains of mice, and found that one of the most abundant cells in the animals’ brains, which also lives in great numbers in human brains, lies mostly dormant until something terrifying happens. These star-shaped cells, known as astrocytes, surround the individual neurons in the brain. Recent research suggests they play a key role in helping you cope with fear.

For instance, “we know that astrocytes can regulate local blood flow, provide energy to neurons, and release signaling molecules that alter neuronal activity,” said Dr. Dwight Bergles, study leader and professor of neuroscience, in a statement. Bergles and his colleagues now believe the cells are responsible for regulating the flow of norepinephrine in the brain, causing a cascade of cortisol (a stress hormone) and adrenaline, both of which heighten your sense of alertness.

A “fight-or-flight” dichotomy is actually the wrong way to think about how we respond to fear, science is increasingly learning. We can run and hide, or we can stay and confront the problem. Or we can do nothing — we can freeze. (Among PTSD sufferers, even, the distinction comes with a lesser-known fourth response: fawn.) Before each response takes place, a stimulus has to cause a chain of events.

This stimulus, of course, is the source of our fear: a horror movie, an angry dog, flushing the toilet on an airplane. When the stimulus hits your eyes and ears it travels straight to the amygdala, where it sends a signal to your pituitary gland, which secretes a mix of hormones responsible for either high-tailing it out of there or putting up your dukes — or, just quivering in a lifeless heap.

Not all fear produces such extreme responses, of course. Your state of arousal may simply heighten. You’ll feel alert and focused, and it’s all thanks to norepinephrine. And more distally, thanks to astrocytes. When the mice in Bergles’ study were moving, the astrocytes responded only sometimes. They also remained dormant and activated when the mice were stationary. Likewise, a flash of light had little effect; astrocytes were just as happy to activate in darkness and stay dormant after a flash.

It wasn’t until the team realized the astrocytes needed to be “woken up” before they would respond to nearby neurons that the investigation picked up speed. Norepinephrine, they found, alerts the astrocytes to the neighboring neuronal signals, causing the cells to monitor and trigger sensations of alertness. "Since memory formation and other important functions of the brain require a state of attention,” Bergles said, “we're interested in learning more about how astrocytes help create that state."

He and his team hope the findings can further the research on brain function. If astrocytes indeed act as a megaphone to “broadcast local norepinephrine signals to every neuron in the brain,” then scientists would have a better understanding of fear’s main pathway. And ultimately, it would help uncover more of the great mystery that is the human brain.

Source: Paukert M, Agarwal A, Cha J, et al. Norepinephrine Controls Astroglial Responsiveness to Local Circuit Activity. Neuron. 2014.

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