What is the muscle stretch reflex?
Muscle stretch reflex is a reflex arc that responds to stretching of muscle fibers to keep the muscle in an appropriate state of tension and tone, ready to contract or relax as needed.
The sensory input (afferent) of the reflex is from two structures in the muscle called spindles and Golgi tendon organs .
The output (efferent) is the alpha motor neuron that contracts (tightens) the muscle. (The muscle fibers are sometimes referred to as extrafusal fibers .)