What detects skeletal muscle lengthening and sends corrective signals to the spinal cord?

Muscle spindles are specialized sensory receptors located within the belly of muscles. They are primarily responsible for detecting changes in muscle length and the rate of that change. When a muscle lengthens, the muscle spindles are stretched, activating the sensory nerve endings within them. This activation results in the transmission of signals to the spinal cord, where they can trigger reflexes to adjust muscle tone and maintain posture. This process is crucial for coordinating movements and ensuring that muscles can respond appropriately to changes in demand or position.

In contrast, Golgi tendon organs predominantly sense changes in muscle tension rather than length. Free nerve endings serve various functions, including the detection of pain and temperature, but are not specifically involved in sensing muscle length. Interneurons are involved in processing signals within the spinal cord, including reflex actions, but they do not directly detect muscle lengthening. Therefore, muscle spindles are the correct answer as they fulfill the role of sensing skeletal muscle lengthening and relaying that information to the spinal cord for corrective actions.