Final Answer:
A stretch reflex occurs when a muscle spindle is stretched, resulting in the contraction (shortening) of the muscle containing the muscle spindle.
Step-by-step explanation:
The stretch reflex is a type of monosynaptic reflex that involves the stretching of a muscle. The key component in this reflex arc is the muscle spindle, a sensory organ embedded within the muscle tissue. When the muscle is stretched, the muscle spindle is also stretched, leading to the activation of sensory neurons. These sensory neurons transmit signals to the spinal cord, where they synapse directly with motor neurons. The motor neurons, in turn, stimulate the contraction of the stretched muscle, causing it to shorten. This reflex mechanism serves as a rapid and automatic response to changes in muscle length, helping to maintain posture, stability, and prevent overstretching.
In summary, the stretch reflex results in the contraction of the muscle containing the stimulated muscle spindle.