Stretch reflex is contraction of a muscle in response to stretching of that same muscle, initiated by the muscle spindle, its intrafusal stretch receptor lying parallel to the extrafusal fibers.
The muscle spindle and its reflex connection with motor neurons form a feedback device that controls muscle length.
Muscle Spindle: The Stretch Receptor
Muscle spindles lie parallel to the extrafusal fibers and are attached distally to the tendon or to extrafusal fibers, an arrangement that lets them detect muscle stretch.
Afferent signals arise from type Ia fibers (12 to 20 µm) and type II fibers (6 to 12 µm); Ia fibers have the faster conduction velocity.
Afferent discharge frequency is proportional to the degree to which the spindle is stretched.