Patellar Tendon Reflex

What is the patellar tendon reflex?

The patellar tendon reflex (PSR) or “knee-cap reflex” is a reflex of its own that is frequently used in everyday clinical practice. This reflex is triggered by a light blow with the reflex hammer on the ligamentum patellae, a broad and strong ligament just below the patella, which represents the end section of the quadriceps tendon between the patella and the rough bony process at the end of the anterior tibia (tuberosity tibiae). The impact causes excitation of the muscle spindles (spindle-shaped connective tissue capsules enclosing 3 to 10 thin, 1 to 3 mm long, specialized muscle fibers) in the quadriceps femoris muscle, resulting in a brief stretching of this muscle.

Procedure

During the procedure, the patient sits down on a chair or examination couch and lets his legs hang freely and loosely. The leg to be examined can be additionally lifted by the examiner in the hollow of the knee. The blow to the kneecap should not be too strong and should always be performed with a reflex hammer.

Alternatively, the reflex can also be triggered by a light blow of the reflex hammer on two placed fingers. The reflex should be triggered at intervals of 2-5 seconds. For comparison, the other leg should always be examined as well. If the reflex is weak, it is possible to let the patient perform the Jendrassik handle to strengthen 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

A reflex arc is the distance covered by the stimulus. The stimulus begins at the point of release and ends at the reacting organ or muscle. Reflex arcs can be relatively simple.

A good example is the patellar tendon reflex. It consists of only two nerve cells connected by a synapse. A reflex bow 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 between sensory and motor fibers in the form of a synapse, this is called a monosynaptic reflex; if there are several synapses, this is called a polysynaptic reflex. If the receptors and effectors of a reflex are located in the same organ, one speaks of a self-reflex.