Have you ever wondered why a patient's muscles tend to spasm in the presence of injury? The answer lies in the alpha-gamma loop. For example, the other day I was performing PROM on a post-surgical rotator cuff repair patient. During the treatment, noticeable muscle spasms were occurring in several muscles surrounding the glenohumeral joint. In the alpha-gamma loop, sensory neurons, which provide information about length and tension of muscle fibers, send signals to the alpha motor neuron (AMN) located in the brainstem and spinal cord. The AMN sends the signal to the motor cortex. Finally, the signal is sent back to the muscle resulting in a muscle contraction. In normal healthy movement, the loop is uninterrupted.
When the body is injured, the gamma loop can become imbalanced. Sensory cells within the muscle spindles do not know how to interpret the input. The signal sends an improper impulse toward the brain and spinal cord. Since the impulse is abnormal, the alpha motor neuron also misinterprets the information. The efferent response, which would be a muscle contraction, is now a muscle spasm.
Check out the quick YouTube video below for more information!