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stretch reflex (myotatic reflex) The reflex initiated when a muscle is stretched; an example is the knee-jerk reflex. Stretching of a muscle causes impulses to be generated in the muscle spindles. These impulses are transmitted by sensory neurons to the spinal cord, where the sensory neurons synapse with motor neurons; these initiate contraction of the same muscle so that it returns to its original length. Since the reflex action involves the transmission of impulses across only one set of synapses, the response is rapid and described as monosynaptic.