Researchers have used a common molecular model, the fruit fly, to learn more about why we suddenly freeze when we're startled. This phenomenon has been observed in almost every animal where researchers have looked for it. The startle response was found to be triggered by a chemical messenger called serotonin, which stops our bodies when we perceive a sudden threat. When flies unexpectedly experience a rapid change in their environment, a burst of serotonin is released, stopping their movement. The findings have been reported in Current Biology.
"Imagine sitting in your living room with your family and all of a sudden the lights go out, or the ground begins to shake; your response and that of your family will be the same: You will stop, freeze and then move to safety. With this study, we show in flies that a rapid release of the chemical serotonin in their nervous system drives that initial freeze," said the senior author of the study Richard Mann, Ph.D., a principal investigator at Columbia's Mortimer B. Zuckerman Mind Brain Behavior Institute. "And because serotonin also exists in people, these findings shed light on what may be going on when we get startled as well."
Serotonin has been linked to the regulation of emotion and mood in the brain. Other studies have connected it to the speed of movement in animals. In this work, the researchers sought to learn more about how serotonin is related to motion. They began by tracking fruit flies as they moved, and manipulated the levels of serotonin in a structure called the ventral nerve cord (VNC), which is similar to the spinal cord in vertebrates.
The scientists found that by triggering serotonin-producing neurons in the VNC, they could slow fly movement, and deactivating those neurons sped them up. They also determined that altering serotonin levels could affect the speed of fly walking.
"We witnessed serotonin's biggest effects when the flies experienced rapid environmental changes," noted the first author of the study Clare Howard, Ph.D. "In other words, when they were startled."
The researchers went on to stimulate the startle response in flies by exposing them to a total blackout and an earthquake simulation. They also changed serotonin levels during both conditions.
"We found that when a fly is startled in these scenarios, serotonin acts like an emergency brake; its release is needed for them to freeze, and that part of this response may be a result of stiffening both sides of the animal's leg joints," said Dr. Mann. "This co-contraction could cause the brief pause in walking, after which the insect begins to move."
"We think this pause is important," added Dr. Howard, "It could allow the fly's nervous system to gather the information about this sudden change and decide how it should respond."
While the flies immediately paused after being exposed to a blackout or earthquake simulation, their walking speeds afterward were different. "After being startled in the blackout scenario, the fly's gait was slow and deliberate," Dr. Howard revealed. "But the earthquake caused the flies to walk faster after the initial pause."
Although a fruit fly model (discussed in the video below) was used in these experiments, the researchers suggested that the startle response and serotonin are found in so many animals, the biochemical processes may also apply to more complex animals.
"Our results indicate that serotonin has the potential to interact with many different types of nerve cells in the fly nervous system, such as those that guide movement and process sensory information," said Dr. Mann. "As we and others continue to investigate, we hope to develop a detailed, molecular blueprint for locomotion that can be applied broadly to other animals, perhaps even people."