Striking of the
patellar tendon with a
reflex hammer just below the
patella stretches the
muscle spindle in the
quadriceps muscle. This produces a signal which travels back to the spinal cord and synapses (without interneurons) at the level of L3 or L4 in the spinal cord, completely independent of higher centres. From there, an
alpha motor neuron conducts an efferent impulse back to the
quadriceps femoris muscle, triggering contraction. This is a reflex of
proprioception which helps maintain
posture and
balance, allowing to keep one's balance with little effort or conscious thought. The patellar reflex is a clinical and classic example of the monosynaptic
reflex arc. There is no
interneuron in the pathway leading to contraction of the quadriceps muscle. Instead, the sensory neuron synapses directly on a motor neuron in the spinal cord. However, there
is an inhibitory interneuron used to relax the antagonistic hamstring muscle (
reciprocal innervation). This test of a basic automatic reflex may be influenced by the patient consciously inhibiting or exaggerating the response; the clinician may use the
Jendrassik maneuver in order to ensure a more valid reflex test. ==Clinical significance==