1. Stretch reflex

2. The stretch (myotatic) reflex is exemplified by the knee jerk reflex
used routinely in clinical neurophysiology as a tool for diagnosis of certain neurological condition
The stretch reflex has only one synapse betweenThe sensory afferent nerves (group Ia afferents)and the motor efferent nerves (α moroneurons).

3. When the muscle is stretched, group Ia afferent fibers in the muscle spindle are activated and their firing rate increase
The group Ia afferents enter the spinal cord and synapse directly on and activate α motoneurons.
When these α motoneurons are activated, they cause contraction of the muscle that was originally stretched

4. When the muscle contracts, it shortens, thereby decreasing stretch on the muscle spindle.
The muscle spindle returns to its original length, and the firing rate of the group Ia afferents returns to baseline
Simultaneously, information is sent from the spinal cord to cause contraction of synergistic muscles and relaxation of antagonistic muscles.