Serotonin Triggers the Body’s Startle Response: Study
Mon, November 29, 2021

Serotonin Triggers the Body’s Startle Response: Study

Imagine being in your living room with your family members when, suddenly, the ground starts to shake or the room goes dark. You and your family’s immediate response would be to stop whatever it is that you’re doing and then move to safety / Photo by: sirtravelalot via Shutterstock

 

Imagine being in your living room with your family members when, suddenly, the ground starts to shake or the room goes dark. You and your family’s immediate response would be to stop whatever it is that you’re doing and then move to safety. We call this the startle response and it is triggered by the chemical nerve cells called serotonin, which sends the signals between our nerve cells.

A team of researchers from The Zuckerman Institute at Columbia University conducted a study using fruit flies and they found that the serotonin chemical triggers the startle response in the body. 

Understanding Startle Response

Startle response, sometimes referred to as the startle reaction or startle pattern, is an extremely rapid psychophysiological response of an organism after an unexpected or sudden stimulus, like a flash of light or a loud sound. The human startle response is characterized by the involuntary bending of the limbs as well as the spasmodic avoidance movement of the head. In less than one second, the musculature will again return to normal after the startle pattern.

Startle response, sometimes referred to as the startle reaction or startle pattern, is an extremely rapid psychophysiological response of an organism after an unexpected or sudden stimulus, like a flash of light or a loud sound / Photo by: fizkes via Shutterstock

 

The Use of FlyWalker

To come up with their theory that there is a link between startle response and serotonin, the researchers used an apparatus called FlyWalker to analyze the steps of the fruit fly. The FlyWalker was developed by Columbia physicist Szabolcs Marka, Ph.D. and senior author Richard Mann, Ph.D. to track the steps of the insects while inside a special type of glass. The researchers then monitored how the fruit flies moved inside the apparatus and manipulated the serotonin as well as dopamine levels through the insect’s ventral nerve cord (VNC). The VNC is like the vertebrate spinal cord.

Their initial findings were that by activating the neurons that produce serotonin chemicals slows down the fruit flies. If the neurons are, however, silenced, it speeds up the flies. They also conducted additional experiments, which revealed that the level of serotonin in the flies impacts their walking speed while exposed to various conditions. These include different temperatures, walking upside down, or when the flies were hungry. They also used other situations that would normally affect the normal walking speed of the insect.

First author Clare Howard, Ph.D. said that they witnessed the biggest effects of the serotonin when the flies experienced rapid changes in the environment. This means that they were startled.

Evoking the Insect’s Startle Response

To elicit the startle response of the fruit flies, Howard and colleagues conducted further experiments. They used two scenarios. First, they turned off the lights or initiated total blackouts for the flies and second, they simulated an earthquake in a fly-sized arena with specialized vibrating motors. The arena was made possible with the help of Zuckerman Institute’s Director of Advanced Instrumentation Tanya Tabachnik and her group of engineers and machinists. 

Serotonin as an Emergency Brake

Dr. Mann stated that when the fly was startled because of the two scenarios, their serotonin served as their “emergency brake.” Both of their leg joints momentarily stiffened, after which, the insects would start to move again.

This kind of pause is important not only in insects but also in humans because it enables the nervous system to collect the needed information about the sudden change. It also helps one decide what is the proper way to respond. After the immediate pause, the flies’ subsequent walking speed changed. For instance, their gait was more deliberate and slow after they were startled in the off lights scenario. The team said they want to know more about the role of serotonin in movement and not just in startle response.

Dr. Mann stated that when the fly was startled because of the two scenarios, their serotonin served as their “emergency brake” / Photo by: Danijela Maksimovic via Shutterstock

 

Excessive Startle Reaction

However, excessive startle reaction (body spasms or eye blinking) to sudden movement, touch, or noise is not good. Instead of it serving as a defensive response to a threatening or sudden stimuli, excessive startle reaction (hyperekplexia) prevents voluntary movement and may cause the affected person to fall like a log but remain conscious. In medical terms, exaggeration of reflexes is referred to as hyperreflexia. 

Hyperekplexia, on the other hand, is a rare neurological disorder characterized by an excessive startle reaction and it is hereditary.  It is frequently misdiagnosed as a form of epilepsy so it may lead to prolonged treatment because getting an accurate diagnosis was delayed. In the major form of hyperekplexia, symptoms include unusually extreme startle reaction, spastic jerking movements, arching of the head, and falling stiffly tot he ground while being conscious.

Hyperekplexia, sometimes called the exaggerated surprise, affects one in 40,000 people in the United States, according to patient advocacy organization the National Organization for Rare Disorders. Most often, the condition is present at birth and affects both males and females. In some cases, the disorder is delayed until adulthood or adolescence. Testing for hyperekplexia includes electromyography or checking the electrical impulses produced by the muscles and the EEG, which records the electrical activity of the brain.