Definition: a motor unit is a single motor neuron plus all the muscle fibers to which it connects.

Some motor neurons only connect to a few muscle fibers. Other motor neurons connect to hundreds of muscle fibers. Which of these two options do you think allows for greater control of your movement?

Have you guessed yet?

How about now?

OK. If a motor neuron connects to only a few muscle fibers you will have greater control of movement, because you can very gradually recruit more and more motor neurons.

I think you'll need more information.

Here's an image that I stole from 2motorunit.jpg (5633 bytes) that other professor's powerpoint presentation, slide #22. It shows two motor neurons. Each one connects to certain muscle fibers. There is no overlap between the two neurons. Motor neuron #1 happens to be connecting to 2 muscle fibers while motor neuron #2 is connecting to 4 muscle fibers. All of the muscle fibers would be inside of one muscle. When motor neuron #1 fires its AP, 2 muscle fibers will contract. This will only produce half the force of the activity from motor neuron #2.

Let's define motor unit #1 as the motor unit that includes motor neuron #1. And, likewise, motor unit #2 is the motor unit that includes motor neuron #2. We can say that the motor unit #1 produces half the force of motor unit #2 (assuming that every muscle fiber can produce the same force). Right?

Motor unit #1 is considered a smaller motor unit than motor unit #2. A small motor unit is when fewer muscle fibers are connected to a motor neuron. Using the example above, the smaller the motor unit, then, the smaller the amount of force produced.

If this muscle only had these 6 muscle fibers (no muscles are really that small), whenever motor unit #1 was activated, 1/3 of the muscle's muscle fibers would contract, while when motor unit #2 was activated, 2/3 of the muscle's muscle fibers would contract. If you wanted to be able to have more graded force production, rather than an all-or-none type of force production, you would rather have smaller motor units than larger motor units.

Summary to this point: the smaller the motor unit, the less force it produces, and the more graded the force production of the entire muscle.

The only thing left to consider is how big is your entire muscle and how much force does it need to produce. Your quadriceps muscles are huge, containing many thousands of muscle fibers. They also need to be able to produce a lot of force, since they help support your body to stand up. So quadriceps muscles should have pretty large motor units... since there are so many muscle fibers overall in these muscles, it won't cause our motion to be too jerky if we have large motor units. And we will get a lot of force out of them.

Another image from that PowerPoint presentation is shown 4motorunit.jpg (7025 bytes) here. The only reason I added it is so that you can picture a muscle with more motor units. In this figure we are up to having 4 motor units. And just that little leap from 2 to 4 makes it look so much more complicated. Imagine now having hundreds... with each motor unit containing hundreds of muscle fibers! That's the situation in your quadriceps muscles!

Now think about your eye muscles. They are small. They have many fewer muscle fibers. They have to be able to contract in tiny amounts so that your eyes can make tiny movements rather than jumping all over as they look around. I hope you can guess that these muscles need to have small motor units.