What structure is activated during the stretch reflex in a stretched muscle?

During the stretch reflex, the structure that is activated in a stretched muscle is the muscle spindle. Muscle spindles are specialized sensory receptors located within the belly of skeletal muscles. They are sensitive to changes in muscle length and the rate of that change. When a muscle is stretched, the muscle spindles detect this elongation and send signals to the spinal cord indicating that the muscle has been stretched.

The activation of muscle spindles leads to a reflexive response that causes the muscle to contract, helping to resist the stretching and maintain posture or protect the muscle from excessive stretching. This reflexive action happens very quickly and is an essential mechanism for maintaining muscle tone and preventing injury.

In contrast, other structures like the sarcomere are the fundamental contractile units of muscle fibers and do not play a direct role in the detection of stretch. The musculotendinous band refers to the region where muscle and tendon meet, and while it contributes to overall muscle function, it is not directly responsible for the stretch reflex. The Golgi tendon organs, which monitor muscle tension rather than stretch, are involved in a different reflex pathway focusing on muscle relaxation when excessive tension is detected.