We have talked about the muscles being “turned off” when there is joint effusion or injury. But what happens to the motor system that drives the muscles (ie the cortex)?
It seems the brain actually becomes MORE excited and it contributes little, if any to the “muscle inhibition” that is occurring in the injured or swollen joint (ie; it is a spinal cord segmental reflex).
Take home message?
When a joint is injured, the muscles crossing the joint become “turned off” (or defacilitated/weak) when the joint is swollen
The “turing off” that occurs is a local or spinal segmental (read spinal cord) phenomenon. This is great because we all work with these reflexes on a daily basis
The lack of muscle activity appears due to decreased inhibition (which causes increased excitation) of the cortex. So the brain is working hard to figure out a way around the problem!
“The results of this study provide no evidence for a supraspinal contribution to quadriceps Arthrogenic Muscle Inhibition. Paradoxically, but consistent with previous observations in patients with chronic knee joint pathology, quadriceps corticomotor excitability increased after experimental knee joint effusion. The increase in quadriceps corticomotor excitability may be at least partly mediated by a decrease in gamma-aminobutyric acid (GABA)-ergic inhibition within the motor cortex.”