Patellar reflex

The patellar reflex or knee jerk is a monosynaptic reflex. Striking the patellar tendon with a tendon hammer just below the patella stretches the quadriceps tendon. This stimulates stretch sensory receptors (most importantly, muscle spindles) that trigger an afferent impulse in a sensory nerve fiber of the femoral nerve leading to the lumbar region of the spinal cord. There, the sensory neuron synapses directly with a motor neuron that conducts an efferent impulse to the quadriceps femoris muscle, triggering contraction. This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick. It has been thought that this type of reflex helps maintain the upright posture. However, Russian scientists Gurfinkel, Lipshits and Popov showed that different control mechanisms govern human vertical posture.

The patellar tendon reflex tests the function of the femoral nerve and spinal cord segements L2-L4.

The patellar reflex is a clinical and classic example of the monosynaptic reflex arc. There is no interneuron. Instead the bipolar sensory neuron synapses directly on a motor neuron in the spinal cord. (Ganong, 2005)

That was coined by Sir Michael Foster in his textbook of physiology in 1877: "Striking the tendon below the patella gives rise to a sudden extension of the leg, known as the knee-jerk."

Popular culture
The term began to be used figuratively from the early 20th century onwards. O. O. McIntyre, in his New York Day-By-Day column in The Coshocton Tribune, October 1921, wrote this: "Itinerant preacher stemming Broadway on a soap box. And gets only an occasional knee-jerk."