The patellar reflex, also known as the knee-jerk reflex, is a simple, involuntary response that occurs when the patellar tendon is tapped. This action stretches the tendon and the connected muscles, triggering a signal that travels through the spinal cord. The result is a quick contraction of the quadriceps muscle, causing the leg to kick forward. This reflex is a type of monosynaptic reflex, meaning it involves only one synapse between the sensory neuron and the motor neuron. It is commonly tested in medical examinations to assess the function of the nervous system, particularly the L2-L4 spinal segments.