By Mariam Melkumyan

If you have ever been to a primary care physician, they have almost certainly used a little hammer to tap your knee to test for your knee-jerk reflex. In response to this tap, your leg likely jerked a little, hence the name of the reflex. This simple test is used to assure that everything is working correctly in your spinal cord. A diminished reflex response can be associated with peripheral nervous system disorders such as sensory polyneuropathies, neuromuscular junction disorders, and others, while hyper reflexive response is related to central nervous system disorders. It is important to note that an absence of the reflex in response to a tap does not necessarily mean that an individual has a disease, as the test is not 100% accurate¹. The knee-jerk reflex is not just used to test your neurological state when you are at the doctor’s office. It is used in everyday life, even if you don’t notice it.

Before we talk about when the reflex is used, it is important to understand what a reflex is in general, and how the knee-jerk reflex specifically works. A reflex is a fast, automatic response to a stimulus, requiring no input from the brain. There are two types of healthy reflexes: muscle stretch and cutaneous reflexes. There are also pathological reflexes, that is, reflexes that are not expected to be found in healthy individuals². Such reflexes include the Babinski reflex, a reflex elicited by drawing a dull object down the sole of the foot. If extension of the big toe and abduction of the other toes is seen, the patient is thought to have problems with the corticospinal tract. Both healthy and pathological reflexes help us learn about the neurological health of an individual, particularly at birth. When an infant is born, multiple tests, including the Babinski test, are conducted to assure the infant is healthy. Reflexes are crucial in that they allow us to react fast to a potentially dangerous stimulus, such as a hot surface.  Not only do reflexes keep us safe from harm, but they also are involved in daily activities, such as balancing when we walk. The reflex that helps us not fall down on our faces when we trip as we are walking is the knee-jerk reflex. The knee-jerk reflex falls under the category of muscle stretch reflexes, where tapping a tendon leads to the activation of a muscle spindle, leading to nerve fiber activation in the spinal cord, which further leads to a response through a motor neuron².

The knee-jerk reflex is also known as the patellar reflex. The name of the reflex comes from the patellar tendon, a tendon that attaches the bottom of the kneecap to the top of the shinbone, and it is the tendon that the doctor taps when they test for the knee-jerk reflex. The knee-jerk reflex is a deep tendon reflex that is mediated by the nerves in the L2, L3, and L4 of the spinal cord³. This reflex is a monosynaptic reflex, meaning that one neuron synapses onto a second neuron, leading to a response in the muscle. This monosynaptic connection is part of why the knee-jerk reflex is so fast. Additionally, the reflex only goes to the lumbar region of the spinal cord, instead of getting feedback from the brain first, increasing its speed. So what exactly happens when your patellar tendon is hit with the hammer (Fig. 1)⁴?

1. The stretch of the tendon is detected through stretch receptors in the muscle spindle in the quadriceps muscle.
2. The muscle spindle then stimulates the sensory neurons which travel to the lumbar region of the spinal cord.
3. In the gray matter of the spinal cord, the sensory neurons synapse on a motor neuron (this is the monosynaptic reflex).
4. The motor neuron projects to the muscle spindle in the quadriceps contracting the muscle.

These are the basic steps of how the knee jerk reflex works. However, when one muscle contracts, the opposing muscle needs to relax to allow for movement. Therefore, the knee jerk reflex, like any other reflex, has additional, non-monosynaptic, steps that lead to the relaxation of the hamstring (Fig. 1)⁴.

1. When the sensory neuron reaches the spinal cord, in addition to synapsing on a motor neuron, it also synapses on an inhibitory interneuron.
2. This inhibitory interneuron then synapses on a different motor neuron.
3. This second motor neuron travels to the hamstring muscle, relaxing it.

The combination of these two sets of steps leads to the movement of the leg in response to a tap on the tendon. It takes about 30-50ms for the leg to move after the patellar tendon is tapped^5,6 which is incredibly fast, considering the number of steps involved in the reflex pathway.

The efficiency and speed of the knee-jerk reflex is very important, as the reflex is used in our everyday life. The stimulation of the patellar tendon helps us contract and relax the quadriceps and hamstrings, making the reflex important for balance and movement, for example when you walk, accidentally trip, or rock back and forth, since the without the knee-jerk reflex the pull of gravity could make the knee bend, leading us to fall down. So, the next time you decide to go on a walk, pay attention to the stretch and contraction of your quadriceps and hamstrings and admire the rhythmic and accurate movement. Consider how incredibly fast and efficient the nervous system is, especially when you trip and look around to make sure that no one saw you.

TL:DR

• Reflexes are fast, automatic responses to a stimulus
• The knee-jerk reflex involves a very simple yet efficient neurocircuitry
• The knee-jerk reflex is used in daily life

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References

1.         Falkson SR, Hinson JW. Westphal Sign. In: StatPearls. StatPearls Publishing; 2021. Accessed October 20, 2021. http://www.ncbi.nlm.nih.gov/books/NBK553214/

2.         Sanders RD, Gillig PM. Reflexes in Psychiatry. Innov Clin Neurosci. 2011;8(4):24-29.

3.         Salazar-Muñoz Y, López-Pérez GA, García-Caballero BE, Muñoz-Rios R, Ruano-Calderón LA, Trujillo L. Classification and Assessment of the Patelar Reflex Response through Biomechanical Measures. J Healthc Eng. 2019;2019:1614963. doi:10.1155/2019/1614963

4.         Experiment: The Patellar Reflex and Reaction. Accessed October 15, 2021. https://backyardbrains.com/experiments/Musclekneejerk

5.         Mamizuka N, Sakane M, Kaneoka K, Hori N, Ochiai N. Kinematic quantitation of the patellar tendon reflex using a tri-axial accelerometer. Journal of Biomechanics. 2007;40(9):2107-2111. doi:10.1016/j.jbiomech.2006.10.003

6.         Neuroscience for Kids – Reflexes. Accessed October 20, 2021. https://faculty.washington.edu/chudler/chreflex.html