What gives birth to fear?

The sense that something is about to go wrong, what we call fear, is key to an animal's survival.

For example, smelling smoke and realising the presence of fire nearby or seeing a falcon's shadow above you before it swoops in front of you.

The question is: Where does fear come from?

While we are still not clear on where exactly fear comes from and how it is developed, a recent study has shown the existence of two circuits that work together to develop fear in our brains.

Researchers were suspicious that neurons were using a molecule called calcitonin gene-related peptide (CGRP) which plays a major role in the process with the brain's "fear centre", the amygdala.

The scientists tested their hypothesis on mice where they found two bunches of  CGRP neurons in the brainstem and thalamus connecting to the amygdala.

Researchers think it is possible that a similar circuit causes PTSD, migraines, and autism spectrum disorder in humans.

The team observed that CGRP neurons increased their activity when the mice were surrounded by a threatening situation like a sound, smell, or visual clues.

"Brain pathway that we discovered works like a central alarm system," said Sung Han, a neurobiologist at the Salk Institute for Biological Studies in California.

They then conducted another experiment on the same mice but this time they shut down the CGRP neurons. They wanted to see if these neurons were required for multi-sensory threat perception.

Mice without activated CGRP neurons were found less likely to respond to fearful stimuli like loud sounds.

"Results indicate that CGRPSPFp and CGRPPBel neurons are required for mediating behavioural responses to different sets of multi-sensory threats," the scientists wrote in their paper published in Cell Reports.

The team also said that these neurons were needed to form memories.