The patellar tendon reflex corresponds to the monosynaptic patellar reflex and is triggered by pressure on the patellar tendon. The hamstring muscles contract as part of the involuntary intrinsic reflex movement, and the lower leg springs upward. An exaggerated patellar reflex is a pyramidal tract sign.
What is the patellar tendon reflex?
The patellar tendon reflex corresponds to the monosynaptic patellar reflex and is triggered by pressure on the patellar tendon. Reflexes are involuntary and automated movement responses to a specific stimulus. They usually have protective functions or support certain processes of human motor function. They are either present from birth or acquired through life experience. The patellar tendon reflex is understood to be an innate leg reflex. The reflex movement is one of the intrinsic reflexes. Thus, in this reflex, the stimulus reception and stimulus response take place in the same organ or muscle. The patellar tendon reflex is also called the hamstring reflex, knee phenomenon, or patellar reflex. The name quadriceps stretch reflex is equally common. The reflex is connected by only a single synapse and is therefore one of the monosynaptic reflexes. The involuntary reflex movement is triggered by a blow to the so-called patellar tendon in the area of the kneecap. This tendon is the attachment tendon of the thigh muscles. The blow therefore causes a contraction of the thigh extensor muscles (Musculus quadriceps femoris), which causes the knee joint to extend, thus causing the lower leg to shoot upwards. The femoral nerve acts as a mediator in the motor reflex response. In the central nervous system, the reflex is connected via the motoneurons in segment L3 and the neurons of the neighboring segments L2 and L4. The patellar reflex is one of the best known reflexes in the human body.
Function and task
The function and task of the patellar reflex is originally a functional and supportive one. For example, humans are able to walk upright on uneven ground because of the circuitry. If the patellar tendon is stimulated to stretch when jumping up, climbing stairs or tripping, then thanks to the reflex response, the correct muscles tense and thus prevent the person from falling over. Without the reflex, people would lose their balance and fall during numerous movements. To ensure that this does not occur, the speed of the automated stimulus response is crucial. Like all motor reflexes, the knee-jerk reflex is controlled by the spinal cord. This circuitry guarantees a fast response and ensures that the reflex can fulfill its purpose at all and is not triggered only after falling down. The muscle spindles in the quadriceps sense stretch and transmit it as receptor information to the spinal cord. The information from the stretch receptors is switched to motor efferent neurons in each of the lumbar segments via a synapse. The efferent neurons travel through the lumbar plexus and reach the femoral nerve back to the thigh muscle. A contraction is initiated. The leg flexor (biceps femoris muscle) is the antagonist of the thigh muscle. To prevent this antagonistic muscle of the thigh musculature from being activated at the same time, an inhibitory mechanism kicks in: the action potential of the leg extensors suppresses the potential of the leg flexor. This inhibitory mechanism is due to the branching of the axon that transmits the stimulus information to the spinal cord. This axon has what is called a divergence. One branch of it runs to the motor neurons that innervate the leg extensor. Via another synapse, the second branch runs to the inhibitory neurons of the leg extensor.
Diseases and disorders
The patellar reflex plays a role primarily in reflex examination. The physician preferably triggers the monosynaptic reflex with the patient seated. The patient loosely crosses one leg over the other for this purpose. In addition, the examiner often lifts the leg at the back of the knee. The physician delivers an accentuated blow with the reflex hammer on the patellar tendon below the kneecap. In the case of a lively reflex response, a little pressure on the applied fingers on the upper edge of the tendon is sufficient for triggering. The doctor repeats the triggering at intervals of two seconds. Subsequently, the second leg is also tested for the reflex. The results are finally interpreted.If the reflex is abolished, there is probably a lumbar disc herniation in segment L3. A peripheral nerve injury is also a possibility. If the reflex is merely attenuated, neuropathy is the most likely diagnosis. An increased reflex or even a widened reflex zone is considered a so-called pyramidal tract sign. Like all other pyramidal tract signs, this phenomenon refers to central nervous system damage to the motor neurons in the pyramidal system. Such damage usually manifests as either muscle weakness, gait unsteadiness and paralysis, or spasticity. The causes of the damage can be, for example, inflammatory manifestations of multiple sclerosis or degenerative manifestations in the context of ALS. ALS in particular attacks the motor nervous system. In addition to an increased patellar reflex, a number of pathological foot reflexes also belong to the pyramidal tract signs. This reflex group is also referred to as the Babinski group and encompasses reflexes such as the Babinski and Chaddock reflexes. Neurologic reflex testing and examination for pathologic reflexes is used primarily for differential diagnosis and localization of nerve lesions in the central and peripheral nervous systems. For example, the patellar reflex usually remains unchanged in stroke patients. This is generally true even when signs of paralysis are present.
