This is a monosynaptic reflex and also called a “stretch reflex”.  Intrafusal fibers in the muscle spindle apparatus located deep in the muscle belly (extrafusal fibers) of the extensor muscle group.  When the intrafusal fibers are stretched, type Ia afferent axons in the femoral nerve generate axon potentials.  These neurons synapse in the spinal cord to excite alpha motor neurons that innervate extrafusal muscle fibers.  Collaterals of Ia axons also stimulate Ia inhibitory interneurons.  These inhibitory neurons decrease the activity of antagonistic muscle groups (flexors) during opposing muscle contraction. 

*Gamma motor neurons directly excite intrafusal fibers.  UMN excitation or inhibition of gamma fibers lead to increased or decreased sensitivity of the intrafusal fibers to stretch. 

*The golgi tendon organ at the muscle-tendon junction helps to moderate strength of contraction (keeps it from being too exuberant).  The sensory nerve endings of Ib afferent axons are sensitive to muscle tension and increase the firing rate as the muscle contracts.  This stimulates a type Ib inhibitory interneuron which inhibits the alpha motor neuron that is causing the muscle contraction. 

**stretch – intrafusal fibers in the muscle belly – Ia afferents – causes muscle contraction and opposing muscle relaxation

**tension – golgi tendon organ at the muscle-tendon junction – Ib afferents – causes relaxation of the contracting muscle

For good diagrams:

http://neuroscience.uth.tmc.edu/s3/chapter02.html

A practical Guide to Canine and Feline Neurology, 2^nd Edition. 

Dewey.  Wiley-Blackwell 2008.