|MadSci Network: Anatomy|
Your question only looks simple. What could I add that would not be in biology texts?
I decided to add a historical perspective.
The American Physiological Society has a collection of classic papers, made freely online, with essays describing why they are classic. http://jn.physiology.org/cgi/ content/full/93/6/3024 is where you will find discussion of Elwood Henneman and the size principle, and links to Henneman's articles in PDF format.
The motor unit is the smallest functional unit of muscle. It is made up of a single spinal motoneuron and the muscle fibers the motoneuron activates. Henneman's work looked at the differences between motor units, and what those differences meant for muscles.
The text is a little stilted but very readable. You might want to share the articles with your science teacher.
Try the links in the MadSci Library for more information on Anatomy.