A new study has revealed that the human brain's tiniest blood vessels can activate genes known to trigger spinal motor neurons, prompting the neurons to grow during early development. The findings could provide insights into how amyotrophic lateral sclerosis (ALS) and other neurodegenerative disorders may develop. To make the discovery, investigators successfully re-created living tissues of the blood vessels and the spinal motor neurons -- which control muscles -- outside the body to show how they interact.