The heart contains its own nervous system. That has been known for decades, and neuroscientists have long called it the "little brain on the heart," a phrase that is useful shorthand but slightly misleading. It does not think. It is a network of neurons embedded in the fatty tissue surrounding the organ, processing signals locally while remaining in constant contact with the brain.
What nobody could establish was what those neurons do, because there are almost none of them. Cardiac neurons make up less than 0.01 percent of the cells in heart tissue and are scattered sparsely across the organ. A study published in Cell has now sorted them into categories, and the results are stark: remove either of the two main types, and the heart stops working properly.
