nervous system

Up until recently, habituation -- a simple form of learning -- was deemed the exclusive domain of complex organisms with brains and nervous systems, such as worms, insects, birds, and mammals.

When pain signals are passed along the nervous system, proteins called calcium channels play a key role.

The buildup can lead to a variety of health issues, including increased risks of cancer, hormone disruption, fertility effects and damage to the brain and nervous system.

"They're marked by progressive loss of motor functions -- from sitting, standing, and swallowing to even breathing -- as neurons in the nervous system die. Watching someone go through this is heartbreaking."

“Deep breathing can reset the nervous system,” Harris said.