Neural Control of Movement Part IV: Reciprocal Inhibition

When we think about movement, we often focus on the central nervous system sending excitatory signals (turning things on) through the alpha motoneurons to the muscles so they will contract.  However, in order to control the motion and prevent unwanted movements, the nervous system also has to inhibit (or turn off) certain neurons to keep them from firing in order to prevented unwanted muscles from contracting.  This process is called inhibition.
There are two main types of inhibition: recurrent inhibition, and reciprocal inhibition.  Recurrent inhibition turns off alpha motoneurons that connect to the same muscle fibers that are contracting, and reciprocal inhibition turns off alpha motoneurons that control the opposing muscle group.  On the surface, reciprocal inhibition is the easiest to understand.  For example, if you are trying to actively flex your elbow (think about a dumbbell curl), you would use the elbow flexors (brachialis and biceps brachii).  You do not want your elbow extensors (triceps brachii) to turn on, because that would work against the desired movement.  So, through reciprocal inhibition, the nervous system will inhibit the alpha motoneurons that connect to the muscle fibers of the elbow extensors, in essence shutting them off. 

Another example of reciprocal inhibition is through the muscle spindles.  If a muscle is lengthening to much and too fast, the muscle spindles will send signals to the CNS.  In order to prevent the muscle from any further lengthening, the CNS will inhibit the alpha motoneurons of the opposing muscle group (that is causing the stretch to occur), and excite the alpha motoneurons of the muscle that is lengthening, so that the muscle will contract and shorten.  This inhibition of the opposing muscle group is know as reciprocal inhibition.  I will talk about recurrent inhibition during the next post.