Day 19: Mouse neurons vary like (pretty) trees

Motor neurons associated with the mouse interscutularis muscle, which connects the base of the ear to the middle of the skull. Credit: Ju Lu, Harvard University

Mostly, I just wanted to share this image because I think it’s beautiful, especially once you know what it represents. This is the branched network of nerves that communicate with a small muscle in the head of a mouse, connecting the base of the ear to the top of the skull.

Researchers want to diagram neural networks throughout animals’ bodies, but the task is daunting. So far progress has been made in roundworms and earthworms, but the complexity and variability of the nervous system is proving formidable when it comes to making maps in mammalian subjects.

Ju Lu, a post-doctoral researcher at Harvard, is lead author a new study that started small, by diagramming a mouse muscle innervated by an average of just 14 nerve fibers. Even that was a lot of work.

Richard Robinson, who wrote about the study for the open access journal PLoS (Public Library of Science), noted that the research team ”took thousands of individual photographs and reconstructed them to trace individual axons as they stretched across the skull, entered the muscle, branched repeatedly, and ultimately formed synapses with individual muscle fibers.” Each axon, at 7 millimeters (not quite 3/10 of an inch), took 14 hours to trace, he wrote.

The researchers found that, even in this tiny muscle, neural networks show an incredible amount of variation between individual animals and between two sides of the same animal. The number of axons innervating a single muscle ranged from 13 to 16. The length of each nerve varied much more, ranging between 1.5 millimeters (less than 1/10 an inch) to 13.3 millimeters (just over half an inch). The number of synapses — or connections to the muscle — at the end of each nerve cell ranged from one to 37.

Gets pretty mind-blowing to think of the variation between larger muscles, larger organisms and — whoa — the human brain. No wonder so few of us think alike.

Source: In Mammalian Muscle, Axonal Wiring Takes Surprising Paths, by Richard Robinson PLoS Biology Vol. 7, No. 2. The original research paper is here.