By: Grace Huff
A creak in the hallway or a shadow that moves when it shouldn’t, before you realize what’s happening, your heart speeds up, your breath quickens, and your hands begin to shake. It’s the season of jump scares and haunted houses, but the real fright is happening inside your brain, not from the ghosts and goblins outside.
That shaking response starts in a small structure deep in the brain called the amygdala. It’s your internal alarm system, built to detect threats and send instant warnings to other regions to help you react. Once sensory information from the environment is gathered, whether it’s a loud bang or a figure in the dark, the amygdala rapidly evaluates it for potential danger. If a threat is perceived, a network involving the hypothalamus, which triggers the sympathetic nervous system, initiates the body’s “fight-or-flight” response. At the same time, other structures, including the anterior cingulate cortex (ACC), contribute by processing the cognitive components associated with that threat.
When this network kicks into gear, it releases powerful hormones like adrenaline and norepinephrine. These chemicals prepare your body for action by increasing heart rate, tightening muscles, and redirecting blood flow from your skin to your larger muscle groups. That flood of energy can make your body tremble or shake, not because you’re weak, but because your body is getting ready to move fast, whether that means running from a zombie or jumping at a sudden “boo.” But fear not, the ACC has got your back! As the wise overseer, it communicates extensively with other areas in that network, and more extensively with the prefrontal cortex and the brainstem, to modulate activation of the fight-or-flight response and hit the brakes once the threat has been removed.
Scientists have even captured this spooky science on film. In a 2016 NeuroImage study, participants watched scary movie clips inside an MRI scanner while researchers tracked their brain activity and fingertip temperature. The more frightened they felt, the stronger the connection became between the amygdala and ACC, and the colder their fingertips grew. That chill wasn’t supernatural; it came from vasoconstriction, the narrowing of blood vessels caused by the sympathetic response. The result: icy fingers, a racing pulse, and the unmistakable shake of fear. “What’s ironic about all of this,” said Franchesca Arias, PhD, a clinical neuropsychologist at the Fixel Institute, “is that the more bothersome symptoms are a telltale sign your body is being strategic. It’s rerouting blood and oxygen to your core muscles and gearing up to sprint and swing.”
Luckily, most fear-induced trembling fades as quickly as it appears. Short-term shaking triggered by adrenaline is perfectly normal, a temporary effect of your body’s built-in defense system. But when tremors happen without an emotional or chemical trigger, or persist long after the scare has passed, they may indicate a neurological condition such as essential tremor or Parkinson’s disease. These disorders involve motor circuits that control movement, but they misfire in ways that cause ongoing, involuntary shaking.
Understanding how the brain and body communicate during fear helps researchers learn more about what happens when those same pathways malfunction. By studying how emotions influence movement, scientists are uncovering new clues that could improve treatments for tremors and other neurological conditions. But how do we know all this, you may ask? It’s thanks to the scientists who spend their days mapping brain pathways, and to people like you who support research, whether by participating in studies, volunteering time, or donating to advance discovery. Translational studies that integrate imaging, such as those conducted at the Norman Fixel Institute for Neurological Diseases, are key to unlocking these networks and transforming that knowledge into real-world therapies.
“As a researcher, my work focuses on how people engage in research and the barriers that can keep them from doing so,” said Arias, “Progress happens when the community joins us, through participation, partnership, and philanthropy, to move science forward together.”
So, this Halloween, when your pulse quickens and your hands start to shake during a haunted house or scary movie, remember that’s your brain doing exactly what it was designed to do. The trembling you feel is a centuries-old survival reflex, not a trick. And for researchers at the Norman Fixel Institute for Neurological Diseases, those very reactions offer valuable insights into how the brain controls movement, in moments of fright, and in everyday life.
References:
Yoshihara, K., Tanabe, H. C., Kawamichi, H., Koike, T., Yamazaki, M., Sudo, N., & Sadato, N. (2016). Neural correlates of fear-induced sympathetic response associated with the peripheral temperature change rate. NeuroImage, 134, 522–531. https://doi.org/10.1016/j.neuroimage.2016.04.040
Fell, A. (2024, October). The roots of fear: Understanding the amygdala. UC Davis Health. https://health.ucdavis.edu/news/headlines/the-roots-of-fear-understanding-the-amygdala/2024/10
Alshak, M. N., & Das, J. M. (2023, May 8). Neuroanatomy, sympathetic nervous system. In StatPearls [Internet]. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK542195/
Harvard Health Publishing. (2024, November 5). What is essential tremor? Harvard Health. https://www.health.harvard.edu/diseases-and-conditions/essential-tremor-a-to-z