AV node

The AV node also contains many heart muscle cells that can transmit the “contract now” signal.

“If the light is switched on, the AV node functions better,” Nahrendorf says.

For scientists studying AV node biology, “this paper really puts macrophages on the map,” says molecular and developmental biologist Vincent Christoffels of the University of Amsterdam in the Netherlands.

But he notes that previous studies have implicated other non–heart muscle cells, such as fibroblasts, in the AV node’s operation.

Nahrendorf says the rodents appeared to have an AV block, in which the AV node doesn’t forward the atrial signal to the ventricles.