Final Answer:
A mono-synaptic reflex that contracts a muscle to counter passive stretch is known as the stretch reflex.
Step-by-step explanation:
The stretch reflex is a mono-synaptic reflex arc involving sensory and motor neurons. When a muscle undergoes passive stretch, specialized sensory receptors called muscle spindles detect the change in muscle length. The sensory neurons associated with these spindles transmit signals directly to the spinal cord, where they synapse with motor neurons. Subsequently, motor neurons stimulate the contracting of the stretched muscle, leading to a quick and automatic response that helps maintain muscle tone and prevent overstretching. This reflex is vital for maintaining posture and coordinating smooth movements.
In summary, the stretch reflex is a mono-synaptic mechanism that contracts muscles in response to passive stretch.