Neural Control of Movement Part V: Recurrent Inhibition

Last week we talked about reciprocal inhibition, in which the CNS would inhibit the muscles of the one muscle group because their shortening would cause the opposite muscle group to lengthen.  This week, we are going to talk about recurrent inhibition, in which the CNS actually inhibits muscle fibers of the same muscle that is contracting.  This is known as recurrent inhibition, and the inhibitory neuron found in the spinal cord is called a Renshaw cell.

So, how does recurrent inhibition work?  The Renshaw cells are located near the cell bodies of the alpha motoneurons in the spinal cord.  When the alpha motoneurons are excited and send signals to the muscle fibers to contract, the Renshaw cells are also excited.  These Renshaw cells make inhibitory connections on these same alpha motoneurons, thus preventing them from sending more signals down to the muscle fibers.

Now, why would the CNS want to inhibit the same neurons that are causing a muscle to contract?  Well, it gives the CNS more control over the movement.  One way to increase force/decrease force/maintain force is to control the number of muscle fibers that are activated.  Renshaw cells can serve this function by inhibiting alpha motoneurons and limiting the number of muscle fibers involved in the movement.  However, this may not always be the most effective strategy.  The Renshaw cells can be inhibited (turned off) by descending inputs in the spinal cord, so that they cannot inhibit the alpha motoneurons, which would allow for the activation of more motor units and muscle fibers.