Recruitment

Motor Units

We previously discussed how action potentials travel from your brain down your spinal cordThe central nervous system structure that relays signals between the brain and body.. They pass out a nerve to muscle fibers. Now, it’s worth examining this structure called a motor unitA motor neuron and all the muscle fibers it controls.. This is going to blow your mind but one neuron does not control just one muscle fiber. Iran can control many muscle fibers in one muscle. Therefore, you have one neuron coming down from your brain and out through your spinal cord. It then goes into your biceps brachiiFlexor / Supinator Front of upper arm; bends elbow and turns palm upward. to excite a collection of muscle fibers. Now, if your brain senses that you need to produce more tension, it will use another neuron. This neuron sends an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. The potential travels from your brain and out of your spinal cord. It then moves through a nerve to other muscle fibers in your biceps brachii. If your muscle senses this need again, it will use another neuron. This new neuron will excite certain muscle fibers in your biceps brachii. Each collection of muscle fibers controlled by one motor neuron is called a motor unit.

Exciting More Motor Units

When I went to adopt my cat Matilda she was in a cage all by herself at the ASPCA. When I saw the listing for her online I it said small so I was expecting a little kitty.  But, I failed to realize the picture had been taken 2 years prior. She’d been in this cage by herself because she was really afraid of all the other kitties.  Poor Matilda.  So, I open the cage. She comes right out. I go to pick her up. It was like picking up a 9 pound bowling ball.  I suspect the bag of treats on top of her cage was the culprit.  My brain assessed her unexpected weight and recruited more motor units to produce more tension.  This is called recruitment.  As I picked her off the floor, my brain sent action potentials to those motor units. It happened in cycles as I held her.   So motor unit 1 was stimulated, then 2 then 3, then 1 again, then 2 again.  This keeps the tension constant so I can hold my little bowling ball. 

Recruitment