The patellar reflex, also called the knee reflex or knee-jerk, is a

stretch reflex The stretch reflex (myotatic reflex), or more accurately ''muscle stretch reflex'', is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length but ...

which tests the L2, L3, and L4 segments of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

. Many animals, most significantly humans, have been seen to have the patellar reflex, including dogs, cats, horses, and other mammalian species.

Mechanism

Striking of the

patellar tendon The patellar tendon is the distal portion of the common tendon of the quadriceps femoris, which is continued from the patella to the tibial tuberosity. It is also sometimes called the patellar ligament as it forms a bone to bone connection whe ...

with a

reflex hammer A reflex hammer is a medical instrument used by practitioners to test deep tendon reflexes, the best known possibly being the patellar reflex. Testing for reflexes is an important part of the neurological examination, neurological physical examin ...

just below the

patella The patella (: patellae or patellas), also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in m ...

stretches the

muscle spindle Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...

in the

quadriceps muscle The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...

. This produces a signal which travels back to the spinal cord and synapses (without interneurons) at the level of L3 or L4 in the spinal cord, completely independent of higher centres. From there, an

alpha motor neuron Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar neuron, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for i ...

conducts an efferent impulse back to the

quadriceps femoris muscle The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...

, triggering contraction. This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick. There is a latency of around 18 ms between stretch of the patellar tendon and the beginning of contraction of the quadriceps femoris muscle. This is a reflex of

proprioception Proprioception ( ) is the sense of self-movement, force, and body position. Proprioception is mediated by proprioceptors, a type of sensory receptor, located within muscles, tendons, and joints. Most animals possess multiple subtypes of propri ...

which helps maintain

posture Posture or posturing may refer to: Medicine * List of human positions ** Abnormal posturing, in neurotrauma **Spinal posture * Posturography, in neurology Other uses * Posture (psychology) * Political posturing Political posturing, also known a ...

and

balance Balance may refer to: Common meanings * Balance (ability) in biomechanics * Balance (accounting) * Balance or weighing scale * Balance, as in equality (mathematics) or equilibrium Arts and entertainment Film * Balance (1983 film), ''Balance'' ( ...

, allowing to keep one's balance with little effort or conscious thought. The patellar reflex is a clinical and classic example of the monosynaptic

reflex arc A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons synapse in the spinal cord and the signal then travels through it into the brain. This allows for faster reflex actions to occur by activating spinal mo ...

. There is no

interneuron Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enab ...

in the pathway leading to contraction of the quadriceps muscle. Instead, the sensory neuron synapses directly on a motor neuron in the spinal cord. However, there ''is'' an inhibitory interneuron used to relax the antagonistic hamstring muscle (

reciprocal innervation René Descartes (1596–1650) was one of the first to conceive a model of reciprocal innervation (in 1626) as the principle that provides for the control of agonist and antagonist muscles. Reciprocal innervation describes skeletal muscles as ...

). This test of a basic automatic reflex may be influenced by the patient consciously inhibiting or exaggerating the response; the doctor may use the

Jendrassik maneuver The Jendrassik maneuver is a medical maneuver wherein the patient clenches the teeth, flexes both sets of fingers into a hook-like form, and interlocks those sets of fingers together. The tendon below the patient's knee is then hit with a refl ...

in order to ensure a more valid reflex test.

Clinical significance

After the tap of a hammer, the leg is normally extended once and comes to rest. The absence or decrease of this reflex is problematic, and known as Westphal's sign. This reflex may be diminished or absent in

lower motor neuron lesion A lower motor neuron lesion is a lesion which affects nerve fibers traveling from the lower motor neuron(s) in the anterior horn/anterior grey column of the spinal cord, or in the motor nuclei of the cranial nerves, to the relevant muscle(s). O ...

s and during sleep. On the other hand, multiple oscillation of the leg (pendular reflex) following the tap may be a sign of cerebellar diseases. Exaggerated (brisk) deep tendon reflexes such as this can be found in

upper motor neuron lesion An upper motor neuron lesion (also known as pyramidal insufficiency) Is an injury or abnormality that occurs in the neural pathway above the anterior horn cell of the spinal cord or motor nuclei of the cranial nerves. Conversely, a lower motor ...

hyperthyroidism Hyperthyroidism is a endocrine disease in which the thyroid gland produces excessive amounts of thyroid hormones. Thyrotoxicosis is a condition that occurs due to elevated levels of thyroid hormones of any cause and therefore includes hyperth ...

, anxiety or nervousness. The test itself assesses the nervous tissue between and including the L2 and L4 segments of the spinal cord. The patellar reflex is often tested in

infant In common terminology, a baby is the very young offspring of adult human beings, while infant (from the Latin word ''infans'', meaning 'baby' or 'child') is a formal or specialised synonym. The terms may also be used to refer to juveniles of ...

s to test the nervous system.

History

Wilhelm Heinrich Erb Wilhelm Heinrich Erb (30 November 1840 – 29 October 1921) was a German neurologist. He was born in Winnweiler, and died in Heidelberg. Academic career In 1864 he received his medical degree from the University of Heidelberg, where for se ...

(1840–1921) and

Karl Friedrich Otto Westphal Karl Friedrich Otto Westphal (23 March 1833 – 27 January 1890) was a German psychiatrist from Berlin. He was the son of Otto Carl Friedrich Westphal (1800–1879) and Karoline Friederike Heine and the father of Alexander Karl Otto Westphal (18 ...

(1833–1890) simultaneously reported the patellar tendon or knee reflex in 1875. The term ''knee-jerk'' was recorded by Sir Michael Foster in his ''Textbook of

physiology Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...

'' in 1877: "Striking the tendon below the patella gives rise to a sudden extension of the leg, known as the knee-jerk."

In popular culture

The term began to be used figuratively from the early 20th century onwards. O. O. McIntyre, in his ''New York Day-By-Day'' column in ''

The Coshocton Tribune ''Coshocton Tribune'' is a daily newspaper that serves the community of Coshocton, Ohio, United States, and the surrounding Coshocton County. History The ''Coshocton Tribune'' was founded in 1909 by William J. Bahmer, a former teacher. The p ...

'', October 1921, wrote: "Itinerant preacher stemming Broadway on a soap box. And gets only an occasional knee-jerk."

References

Sources