Final answer:
Stretching activates the muscle spindle, a proprioceptive stretch receptor within the muscle, increasing the rate of action potentials (APs) and leading to muscle contraction and maintenance of muscle length.
Step-by-step explanation:
In this type of muscle, stretching activates the muscle spindle, which is a proprioceptive stretch receptor located within the muscle. The muscle spindle detects the amount of stretch in the muscle and triggers an increase in the rate of action potentials (APs). This leads to a direct contraction of the muscle and inhibition of the motor neurons of antagonist muscles, helping to maintain the muscle at a constant length.