Final answer:
The proprioceptive cell responsible for the stretch reflex is the muscle spindle, which is a stretch receptor that monitors changes in muscle length.
Step-by-step explanation:
The proprioceptive cell responsible for the stretch reflex is the muscle spindle. Muscle spindles are specialized stretch receptors located within skeletal muscles, and they play a crucial role in detecting changes in muscle length. When a muscle stretches, muscle spindles send a signal to the spinal cord which then triggers an immediate contraction of the muscle, commonly referred to as the stretch reflex. This reflex helps to maintain muscle tone and posture, and protects muscles from overstretching. Along with muscle spindles, the body also contains Golgi tendon organs, which are tension receptors that monitor the force of muscle contractions, contributing to the body's overall proprioception.