Patellar tendon reflex

What is the patellar tendon reflex?

The patellar tendon reflex (PSR) or "kneecap reflex" is a self-reflex that is often used in everyday clinical practice.

This reflex is triggered by a light blow with the reflex hammer on the Patellar ligament, a wide and strong ligament just below the kneecap, which is the final stretch of the quadriceps tendon between the kneecap and the rough bony process at the end of the anterior tibia (Tibial tuberosity) represents.

The impact causes the muscle spindles (spindle-shaped connective tissue capsules that enclose 3 to 10 thin, 1 to 3 mm long, specialized muscle fibers) in the muscle spindles to be excited Quadriceps femoris musclecausing a brief stretch of this muscle.

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During the implementation, the patient takes a seat on a chair or an examination couch and leaves the Hang your legs freely and loosely. The examiner can also raise the affected leg in the hollow of the knee.

The Blow on the kneecap should not too strong and always with one Reflex hammer be performed. Alternatively, the reflex can also be triggered by tapping the reflex hammer gently on two fingers.

The triggering of the reflex should take place at an interval of 2-5 seconds. For comparison, the other leg should always be examined as well.

If the reflex is weak, it is possible for the patient Jendrassik handle Allow it to be performed to reinforce the reflex. To do this, the patient bends his arms in front of his body, crosses his hands and pulls his arms outwards.

The reflex arc

As Reflex arc is called the Distance traveled by the stimulus. The stimulus begins at the point of triggering and ends at the reacting organ or muscle.

Reflex arcs can be built relatively simply. A good example of this is the patellar tendon reflex. It just consists of two nerve cellsthat are connected to each other via a synapse.

A reflex arc basically consists of the following elements: a receptor (target molecule for certain substances), sensory (afferent) nerve fiber, Central nervous system (CNS, brain / spinal cord), motor (efferent fiber) and effector (muscle or gland).

If there is only one connection in the form of a synapse between the sensory and motor fibers, one speaks of a monosynaptic reflex, in the case of several synapses one speaks of a so-called polysynaptic reflex.

If the receptors and effectors of a reflex are in the same organ, one speaks of a self-reflex.

Segments of the spine

In humans, the sensitive neurons (Afferents) to the lumbar segments (lumbar vertebrae) L2-L4, in smaller animals to L3-L6. There the excitation is switched to the motor neurons (efferents) via a synapse. These neurons go through the Lumbar plexus and in Femoral nerve back to the muscle where there is a contraction of the Quadriceps femoris is triggered.

Failure of the patellar tendon reflex

By examining one's own reflexes, as well Sensors and Motor skills can cause injuries in the area of Spine can be localized more precisely.

This is also the case with the patellar reflex. If there is a weakening or even failure of the patellar tendon reflex, it can be assumed that a Nerve injury in the L3 / L4 area or at the level of LWK 2/3 and LWK 3/4. A very common example of this is the Herniated disc of the lumbar spine.

Rarer causes, however, are malignant tumors or a narrowing in the spinal canal (Spinal stenosis of the lumbar spine), as well as a cystic mass.