Stretch reflex mechanism. Computer illustration showing the muscle and associated structures involved in the stretch reflex (myotatic reflex). The stretch reflex is a monosynaptic reflex that causes a muscle to contract in response to stretching. It provides automatic regulation of skeletal muscle length in order to maintain the muscle at a constant length. When the muscle lengthens, the muscle spindle (centre) is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibres to contract and thus resist the stretching. At the same time, a secondary set of neurons causes the opposing muscle to relax.