Muscle spindles are complex sensory receptors found in muscle in parallel to regular muscle fibers. They contain nuclear bag and nuclear chain intrafusal muscle fibers. Group la afferent fibers form primary endings on nuclear bag and chain fibers, and group 11 fibers form secondary endings on nuclear chain fibers. Golgi tendon organs are located in the tendons of muscles and are arranged in series. Ib afferent fibers and are excited both by stretch and by contraction of the muscle.
Reflexes are simple, stereotyped motor responses to a stimulus.
A reflex arc includes the afferent fibers, interneurons, and motoneurons responsible for the reflex. The reflex arcs of many reflexes are located in the spinal cord or brainstem. Phasic stretch reflexes are triggered by the dynamic responses of group la fibers and tonic stretch reflexes by the static responses of group la and II afferents. The inverse myotatic reflex is evoked by Golgi tendon organs. However, because Golgi tendon organs monitor force, their reflex action actually complements that of the stretch reflex.