Which structures send inhibitory signals to reduce motor unit activity when excessive tension is applied to a muscle?

The correct answer highlights the role of Golgi tendon organs, which are specialized proprioceptive sensory receptors located at the junctions between muscles and tendons. Their primary function is to monitor tension within the muscle and tendons. When excessive tension is detected—such as during heavy lifting or an intense contraction—the Golgi tendon organs send inhibitory signals to the spinal cord. This reduces the activity of the motor neurons that control the muscle, leading to a relaxation response in the muscle to prevent injury or stress.

This mechanism is known as autogenic inhibition. It acts as a protective reflex to prevent muscle damage that could result from excessive force production. By reducing motor unit activity in response to high tension, Golgi tendon organs help maintain muscle integrity and enhance safety during physical activity.

Other structures mentioned in the choices have different roles. Muscle spindles are responsible for detecting changes in the length of the muscle and aiding in the stretch reflex but do not primarily inhibit motor activity in response to high tension. Motor neurons transmit signals to initiate muscle contraction rather than sending inhibitory signals. Interneurons play a role in processing and relaying information but are not specifically responsible for reducing motor unit activity due to tension.