The Patellar Tendon Reflex, commonly known as the knee jerk reflex, is an involuntary somatic reflex often used by medical professionals. This rapid, automatic response involves a sudden extension of the lower leg when the tendon below the kneecap is lightly struck. The reflex is a stretch reflex that helps the body maintain muscle tone and posture. Testing this reflex provides a simple, non-invasive window into the health and function of the nervous system.
The Anatomy of the Reflex Arc
The mechanism begins when a tap on the patellar tendon causes a stretch of the quadriceps femoris muscle. This stretch is immediately detected by specialized sensory receptors within the muscle, called muscle spindles. The muscle spindle generates a signal that travels swiftly along a sensory neuron (a type of Ia afferent fiber) toward the spinal cord.
The sensory neuron enters the spinal cord and travels to the lumbar region, primarily involving the L2, L3, and L4 segments. Within the gray matter, the sensory neuron makes a direct connection, or synapse, with a motor neuron. This direct sensory-to-motor connection defines the knee jerk as a monosynaptic reflex, making it one of the fastest reflexes in the human body.
The motor neuron transmits an impulse back out to the quadriceps muscle. This signal instructs the quadriceps to contract, which causes the lower leg to kick forward in a jerking motion. A secondary pathway involves the sensory neuron branching off to activate an inhibitory interneuron. This interneuron signals the motor neuron controlling the antagonistic hamstring muscle to relax, ensuring the quadriceps can contract without resistance.
Clinical Testing and Diagnostic Value
When a healthcare provider performs this examination, they use a reflex hammer to deliver a tap to the patellar tendon, located just below the kneecap. The patient is often seated with legs dangling freely or with one leg crossed over the other to ensure the muscles are relaxed. The response is graded on a standardized scale based on the speed and force of the leg movement.
This simple test assesses the functional integrity of the nerve circuit responsible for the reflex. Since the nerve pathway runs through the L2, L3, and L4 segments of the lumbar spinal cord, the reflex confirms that the nerves and corresponding spinal segments are transmitting signals correctly. If the reflex response is difficult to elicit, the provider may use the Jendrassik maneuver, which involves the patient interlocking their fingers and pulling them apart, to distract them and enhance the reflex. This assessment is a valuable screening tool for evaluating the peripheral nerves and the central nervous system.
Interpreting Abnormal Responses
A response that is outside the normal range provides clues about the presence and location of neurological damage. When the reflex is diminished or entirely absent, a condition known as hypo-reflexia, it often indicates an issue within the reflex arc itself. This reduced response is commonly associated with damage to the peripheral nerves or the lower motor neurons. Conditions such as peripheral neuropathy (which might occur with diabetes) or muscle diseases can cause this diminished reaction.
Conversely, an exaggerated or overactive response is termed hyper-reflexia, resulting in a kick that is brisker than expected. This heightened response points to damage in the central nervous system, specifically an issue with the upper motor neurons. These upper motor neurons normally exert an inhibitory, or dampening, effect on the reflex arc. When this control is disrupted by a condition like a stroke or a spinal cord injury located above the lumbar segments, the reflex arc becomes overly sensitive and reacts strongly. Observing whether the reflex is too weak or too strong helps the physician narrow down the potential origin of a neurological problem, differentiating between damage to the peripheral nerves versus the brain and spinal cord.