Stretch reflex

The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length.

When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length.

Gamma motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle. There are several theories as to what may trigger gamma motoneurons to increase the reflex's sensitivity. For example, alpha-gamma co-activation might keep the spindles taut when a muscle is contracted, preserving stretch reflex sensitivity even as the muscle fibers become shorter. Otherwise the spindles would become slack and the reflex would cease to function.

This reflex has the shortest latency of all spinal reflexes including the Golgi tendon reflex and reflexes mediated by pain and cutaneous receptors.


A person standing upright begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the opposite side will stretch. The muscle spindles in those muscles will detect this stretching, and the stretched muscles will contract to correct posture.

Other examples (followed by involved spinal nerves) are responses to stretch created by a blow upon a muscle tendon:

Another example is the group of fibers in the calf muscle, which synapse with motor neurons supplying muscle fibers in the same muscle. A sudden stretch, such as tapping the Achilles' tendon, causes a reflex contraction in the muscle as the spindles sense the stretch and send an action potential to the motor neurons which then cause the muscle to contract; this particular reflex causes a contraction in the soleus-gastrocnemius group of muscles. Like the patellar reflex, this reflex can be enhanced by the Jendrassik maneuver.

Supraspinal control

The central nervous system can influence the stretch reflex via the gamma motoneurons, which as described above control the sensitivity of the reflex.

Inhibitory signals arrive at gamma neurons through the lateral reticulospinal tract from Brodmann area 6, the paleocerebellum and the red nucleus. Facilitatory signals arrive through the ventral reticulospinal tract from Brodmann area 4, the neocerebellum and the vestibular nucleus.

Spinal control


Grading of stretch reflexes upon tapping muscle tendon[1]
Grade Response Significance
0 no response always abnormal
1+ slight but definitely present response may or may not be normal
2+ brisk response normal
3+ very brisk response may or may not be normal
4+ clonus always abnormal

The clasp-knife response is a stretch reflex with a rapid decrease in resistance when attempting to flex a joint. It is one of the characteristic responses of an upper motor neuron lesion.

See also


  1. ^ Walker, H. K.; Walker, H. K.; Hall, W. D.; Hurst, J. W. (1990). "Deep Tendon Reflexes". PMID 21250237. [1]

External links

Ankle jerk reflex

The ankle jerk reflex, also known as the Achilles reflex, occurs when the Achilles tendon is tapped while the foot is dorsiflexed.

It is a type of stretch reflex that tests the function of the gastrocnemius muscle and the nerve that supplies it.

A positive result would be the jerking of the foot towards its plantar surface. Being a deep tendon reflex, it is monosynaptic. It is also a stretch reflex.

These are monosynaptic spinal segmental reflexes. When they are intact, integrity of the following is confirmed: cutaneous innervation, motor supply, and cortical input to the corresponding spinal segment.

Biceps reflex

Biceps reflex is a reflex test that examines the function of the C5 reflex arc and the C6 reflex arc. The test is performed by using a tendon hammer to quickly depress the biceps brachii tendon as it passes through the cubital fossa. Specifically, the test activates the stretch receptors inside the biceps brachii muscle which communicates mainly with the C5 spinal nerve and partially with the C6 spinal nerve to induce a reflex contraction of the biceps muscle and jerk of the forearm.

A strong contraction indicates a 'brisk' reflex, and a weak or absent reflex is known as 'diminished'. Brisk or absent reflexes are used as clues to the location of neurological disease. Typically brisk reflexes are found in lesions of upper motor neurons, and absent or reduced reflexes are found in lower motor neuron lesions.

A change to the biceps reflex indicates pathology at the level of musculocutaneous nerve, segment C5/6 or at some point above it in the spinal cord or brain.

Clasp-knife response

Clasp-knife response refers to a Golgi tendon reflex with a rapid decrease in resistance when attempting to flex a joint, usually during a neurological examination. It is one of the characteristic responses of an upper motor neuron lesion. It gets its name from the resemblance between the motion of the limb and the sudden closing of a claspknife after sufficient pressure is applied.


Clonus (from the Greek for "violent, confused motion") is a series of involuntary, rhythmic, muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron lesions involving descending motor pathways, and in many cases is, accompanied by spasticity (another form of hyperexcitability). Unlike small, spontaneous twitches known as fasciculations (usually caused by lower motor neuron pathology), clonus causes large motions that are usually initiated by a reflex. Studies have shown clonus beat frequency to range from three to eight Hz on average, and may last a few seconds to several minutes depending on the patient’s condition.

Golgi tendon reflex

The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. In a Golgi tendon reflex, skeletal muscle contraction causes the antagonist muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex. Though muscle tension is increasing during the contraction, alpha motor neurons in the spinal cord supplying the muscle are inhibited. However, antagonistic muscles are activated.


The H-reflex (or Hoffmann's reflex) is a reflectory reaction of muscles after electrical stimulation of sensory fibers (Ia afferents stemming from muscle spindles) in their innervating nerves (for example, those located behind the knee). The H-reflex test is performed using an electric stimulator, which gives usually a square-wave current of short duration and small amplitude (higher stimulations might involve alpha fibers, causing an F-wave, compromising the results), and an EMG set, to record the muscle response. That response is usually a clear wave, called H-wave, 28-35 ms after the stimulus, not to be confused with an F-wave. An M-wave, an early response, occurs 3-6 ms after the onset of stimulation. The H and F-waves are later responses. As the stimulus increases, the amplitude of the F-wave increases only slightly, and the H-wave decreases, and at supramaximal stimulus, the H-wave will disappear. The M-wave does the opposite of the H-wave. As the stimulus increases the M-wave increases. There is a point of minimal stimulus where the M-wave is absent and the H-wave is maximal.

H-reflex is analogous to the mechanically induced spinal stretch reflex (for example, knee jerk reflex). "The primary difference between the H-reflex and the spinal stretch reflex is that the H-reflex bypasses the muscle spindle, and, therefore, is a valuable tool in assessing modulation of monosynaptic reflex activity in the spinal cord." Although stretch reflex gives just qualitative information about muscle spindles and reflex arc activity; if the purpose of the test to compare performances from different subjects, H-reflex should be used. In that case, in fact, latencies (ms) and amplitudes (mV) of H-wave can be compared.

H-reflex amplitudes measured by EMG are shown to decrease significantly with applied pressure such as massage and tapping to the cited muscle. The amount of decrease seems to be dependent on the force of the pressure, with higher pressures resulting in lower H-reflex amplitudes. H-reflex levels return to baseline immediately after pressure is released except in high pressure cases which had baseline levels returned within the first 10 seconds.After about 5 days in zero gravity, for instance in orbit around Earth, the h-reflex diminishes significantly. It is generally assumed that this is due to a marked reduction in the excitability of the spinal cord in zero gravity. Once back on Earth, a marked recovery occurs during the first day, but it can take up to 10 days to return to normal. The H-reflex was the first medical experiment completed on the International Space Station.


Hypertonia is a term sometimes used synonymously with spasticity and rigidity in the literature surrounding damage to the central nervous system, namely upper motor neuron lesions. Impaired ability of damaged motor neurons to regulate descending pathways gives rise to disordered spinal reflexes, increased excitability of muscle spindles, and decreased synaptic inhibition. These consequences result in abnormally increased muscle tone of symptomatic muscles. Some authors suggest that the current definition for spasticity, the velocity-dependent over-activity of the stretch reflex, is not sufficient as it fails to take into account patients exhibiting increased muscle tone in the absence of stretch reflex over-activity. They instead suggest that "reversible hypertonia" is more appropriate and represents a treatable condition that is responsive to various therapy modalities like drug and/or physical therapy.

Jaw jerk reflex

The jaw jerk reflex or the masseter reflex is a stretch reflex used to test the status of a patient's trigeminal nerve (cranial nerve V) and to help distinguish an upper cervical cord compression from lesions that are above the foramen magnum. The mandible—or lower jaw—is tapped at a downward angle just below the lips at the chin while the mouth is held slightly open. In response, the masseter muscles will jerk the mandible upwards. Normally this reflex is absent or very slight. However, in individuals with upper motor neuron lesions the jaw jerk reflex can be quite pronounced.

The jaw jerk reflex can be classified as a dynamic stretch reflex. As with most other reflexes, the response to the stimulus is monosynaptic, with sensory neurons of the trigeminal mesencephalic nucleus sending axons to the trigeminal motor nucleus, which in turn innervates the masseter. This reflex is used to judge the integrity of the upper motor neurons projecting to the trigeminal motor nucleus. Both the sensory and motor aspects of this reflex are through CN V.

It is not part of a standard neurological examination. It is performed when there are other signs of damage to the trigeminal nerve.

The clinical presentation of cervical spondylotic myelopathy can be similar to multiple sclerosis (MS) or amyotrophic lateral sclerosis (ALS), however, a hyperactive jaw reflex suggests the pathology is above the foramen magnum. In other words, a normal jaw jerk reflex points the diagnosis toward cervical spondylotic myelopathy and away from MS or ALS.

List of reflexes

A list of reflexes in humans.

Abdominal reflex

Accommodation reflex — coordinated changes in the vergence, lens near object.

Acoustic reflex or attenuation reflex — contraction of the stapedius and tensor tympani muscles in the middle ear in response to high sound intensities.

Anal wink - contraction of the external anal sphincter upon stroking of the skin around the anus.

Ankle jerk reflex — jerking of the ankle when the Achilles tendon is hit with a tendon hammer while the foot is relaxed, stimulating the S1 reflex arc.

Arthrokinetic reflex — muscular activation or inhibition in response to joint mobilization

Asymmetric tonic neck reflex (ATNR) or tonic neck reflex a primitive reflex— in infants up to four months of age, when the head is turned to the side, the arm on that side will straighten and the contralateral arm will bend.

Babinski reflex — in infants up to one year of age, and also in older individuals with neurological damage, a spreading of the toes and extension of the big toe in response to stroking the side of the foot.

Bainbridge reflex - increasing heart rate in response to increased central venous pressure.

Baroreflex or baroreceptor reflex — homeostatic countereffect to a sudden elevation or reduction in blood pressure detected by the baroreceptors in the aortic arch, carotid sinuses, etc.

Bezold-Jarisch reflex — involves a variety of cardiovascular and neurological processes which cause hypopnea and bradycardia.

Biceps reflex — a jerking of the forearm when the biceps brachii tendon is struck with a tendon hammer, stimulating the C5 and C6 reflex arcs.

Blushing — a reddening of the face caused by embarrassment, shame, or modesty.

Brachioradialis reflex — a jerking of the forearm when the brachioradialis tendon is hit with a tendon hammer while the arm is resting, stimulating the C5 and C6 reflex arcs.

Brain's reflex

Bulbocavernosus reflex

Cervico-collic reflex

Cervico-Spinal reflex

Churchill cope reflex

Corneal reflex — blinking of both eyes when the cornea of either eye is touched.

Coronary reflex

Cough reflex — a rapid expulsion of air from the lungs after sudden opening of the glottis, and usually following irritation of the trachea.

Cremasteric reflex — elevation of the scrotum and testis elicited by stroking of the superior and medial part of the thigh.

Crossed extensor reflex — a contraction of a limb in response to ipsilateral pain, and extension of the contralateral limb.

Cushing reflex - triad of hypertension, bradycardia, and irregular breathing in response to elevated ICP.

Diving reflex

Galant reflex — a primitive reflex in infants up to four months of age, a rotation of the upper body towards one or other side of the back when that side is stroked.

Glabellar reflex

Golgi tendon reflex

Hering–Breuer reflex — is a reflex triggered to prevent over-inflation of the lung

Hoffmann's reflex — also known as the finger flexor reflex; middle finger and thumb response. Test can indicate both neurological damage and nerve regeneration; often combined with the Babinski reflex test.

Jaw jerk reflex

Knee jerk or patellar reflex — a kick caused by striking the patellar tendon with a tendon hammer just below the patella, stimulating the L4 and L3 reflex arcs.

Moro reflex, a primitive reflex— only in all infants up to 4 or 5 months of age: a sudden symmetric spreading of the arms, then unspreading and crying, caused by an unexpected loud noise or the sensation of being dropped. It is the only unlearned fear in humans.

Palmar grasp reflex — in infants up to six months of age, a closing of the hand in response to an object being placed in it.

Photic sneeze reflex — a sneeze caused by sudden exposure to bright light.

Plantar reflex — in infants up to 1 year of age, a curling of the toes when something rubs the ball of the foot.

Pupillary accommodation reflex — a reduction of pupil size in response to an object coming close to the eye.

Pupillary light reflex — a reduction of pupil size in response to light.

Rectoanal inhibitory reflex - a transient relaxation of the internal anal sphincter in response to rectal distention.

Rooting reflex — turning of an infant's head toward anything that strokes the cheek or mouth.

Righting reflex - a proprioceptive reflex

Shivering — shaking of the body in response to early hypothermia in warm-blooded animals.

Sneeze or sternutation — a convulsive expulsion of air from the lungs normally triggered by irritation of the nasal mucosa in the nose.

Startle evoked movement — involuntary initiation of a planned movement in response to a startling stimuli.

Startle reflex

Sucking reflex — a primitive reflex, sucking at anything that touches the roof of an infant's mouth.

Stretch reflex

Triceps reflex — jerking of the forearm when the triceps tendon is hit with a tendon hammer, stimulating the C7 and C6 reflex arcs.

Vagovagal reflex — contraction of muscles in the gastrointestinal tract in response to distension of the tract following consumption of food and drink.

Vestibulocollic reflex

Vestibulo-spinal reflex

Vestibulo-ocular reflex — movement of the eyes to the right when the head is rotated to the left, and vice versa.

Withdrawal reflex



A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. Normally there are four main types in glabrous, or hairless, mammalian skin: lamellar corpuscles, tactile corpuscles, Merkel nerve endings, and bulbous corpuscles. There are also mechanoreceptors in hairy skin, and the hair cells in thoreceptors of primates like rhesus monkeys and other mammals are similar to those of humans and also studied even in early 20th century anatomically and neurophysiologically.Invertebrate mechanoreceptors include campaniform sensilla and slit sensilla, among others.

Muscle spindle

Muscle spindles are stretch receptors within the body of a 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 processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch.

The muscle spindle has both sensory and motor components.

Sensory information conveyed by primary type Ia sensory fibers and secondary type II sensory fibers, which spiral around muscle fibres within the spindle

Motor action by up to a dozen gamma motor neurons and to a lesser extent by one or two beta motor neurons that activate muscle fibres within the spindle.

Myofascial release

Myofascial release (MFR, self-myofascial release) is an alternative medicine therapy that claims to treat skeletal muscle immobility and pain by relaxing contracted muscles, improving blood and lymphatic circulation, and stimulating the stretch reflex in muscles.Fascia is a thin, tough, elastic type of connective tissue that wraps most structures within the human body, including muscle. Fascia supports and protects these structures. Osteopathic theory proposes that this soft tissue can become restricted due to psychogenic disease, overuse, trauma, infectious agents, or inactivity, often resulting in pain, muscle tension, and corresponding diminished blood flow.The use of myofascial release as a treatment is not supported by good evidence; as a replacement for conventional treatment, it risks causing harm.

Patellar reflex

The patellar reflex or knee-jerk (myotatic) (monosynaptic) (American spelling knee reflex) is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord.

Reciprocal inhibition

Reciprocal inhibition describes the process of muscles on one side of a joint relaxing to accommodate contraction on the other side of that joint. In some allied health disciplines this is known as reflexive antagonism. Joints are controlled by two opposing sets of muscles, extensors and flexors, which must work in synchrony for smooth movement. When a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent it from working against the resulting contraction of the homonymous muscle. This inhibition is accomplished by the actions of an inhibitory interneuron in the spinal cord.

The afferent of the muscle spindle bifurcates in the spinal cord. One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the reflex. The other branch innervates the inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle. Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.

If opposing muscles were to contract at the same time, a muscle tear can occur. This may occur during physical activities, such as running, during which muscles that oppose each other are engaged and disengaged sequentially to produce coordinated movement. Reciprocal inhibition facilitates ease of movement and is a safeguard against injury. However, if a "misfiring" of motor neurons occurs, causing simultaneous contraction of opposing muscles, a tear can occur. For example, if the quadriceps femoris and hamstring contract simultaneously at a high intensity, the stronger muscle (traditionally the quadriceps) overpowers the weaker muscle group (hamstrings). This can result in a common muscular injury known as a pulled hamstring, more accurately called a muscle strain.

"When the central nervous system sends a message to the agonist muscle (muscle causing movement) to contract, the tension in the antagonist muscle (muscle opposing movement) is inhibited by impulses from motor neurons, and thus must simultaneously relax. This neural phenomenon is called reciprocal inhibition."

Taken from Massage Therapy Principles & Practices by Susan Salvo 1999, pg 161


A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus. A reflex is made possible by neural pathways called reflex arcs which can act on an impulse before that impulse reaches the brain. The reflex is then an automatic response to a stimulus that does not receive or need conscious thought.

Tendon reflex

Tendon reflex (or T-reflex) may refer to:

A stretch reflex, when the stretch is created by a blow upon a muscle tendon. This is the usual definition of the term. A common example is the standard patellar reflex or knee-jerk response. Deep tendon reflex also usually refers to this sense. A deep tendon reflex is often associated with muscle stretching. Tendon reflex tests are used to determine the integrity of the spinal cord and peripheral nervous system, and they can be used to the presence of a neuromuscular disease.

The Golgi tendon reflex, motivated by that the sensory receptors for this reflex are anatomically located in the tendon, while the sensory receptors for the stretch reflex are actually inside the proper muscle.

Triceps reflex

The triceps reflex, a deep tendon reflex, is a reflex as it elicits involuntary contraction of the triceps brachii muscle. It is initiated by the Cervical (of the neck region) spinal nerve 7 nerve root (the small segment of the nerve that emerges from the spinal cord). The reflex is tested as part of the neurological examination to assess the sensory and motor pathways within the C7 and C8 spinal nerves.

Upper motor neuron syndrome

Upper motor neuron syndrome (UMNS) is the motor control changes that can occur in skeletal muscle after an upper motor neuron lesion.

Following upper motor neuron lesions, affected muscles potentially have many features of altered performance including:

weakness (decreased ability for the muscle to generate force)

decreased motor control including decreased speed, accuracy and dexterity

altered muscle tone (hypotonia or hypertonia) – a decrease or increase in the baseline level of muscle activity

decreased endurance

exaggerated deep tendon reflexes including spasticity, and clonus (a series of involuntary rapid muscle contractions)Such signs are collectively termed the "upper motor neuron syndrome". Affected muscles typically show multiple signs, with severity depending on the degree of damage and other factors that influence motor control. In neuroanatomical circles, it is often joked, for example, that hemisection of the cervical spinal cord leads to an "upper lower motor neuron syndrome and a lower upper motor neuron syndrome". The saying refers to lower motor neuron symptoms in the upper extremity (arm) and upper motor neurons symptoms in the lower extremity (leg).

Health professionals' understanding of impairments in muscles after an upper motor neuron lesion has progressed considerably in recent decades. However, a diagnosis of "spasticity" is still often used interchangeably with upper motor neuron syndrome, and it is not unusual to see patients labeled as spastic who demonstrate an array of UMN findings.Spasticity is an exaggerated stretch reflex, which means that a muscle has a reflex contraction when stretched, and that this contraction is stronger when the stretch is applied more quickly. The commonly quoted definition by Lance (1980) describes "a motor disorder, characterised by a velocity-dependent increase in tonic stretch reflexes with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neurone (UMN) syndrome".

Spasticity is a common feature of muscle performance after upper motor neuron lesions, but is generally of much less clinical significance than other features such as decreased strength, decreased control and decreased endurance. The confusion in the use of the terminology complicates assessment and treatment planning by health professionals, as many confuse the other findings of upper motor neuron syndrome and describe them as spasticity. This confusion potentially leaves health professionals attempting to inhibit an exaggerated stretch reflex to improve muscle performance, potentially leaving more significant UMNS changes such as weakness unaddressed. Improved understanding of the multiple features of the upper motor neuron syndrome supports more rigorous assessment, and improved treatment planning.

Westphal's sign

Westphal's sign is the clinical correlate of the absence or decrease of patellar reflex or knee jerk. Patellar reflex or knee jerk is a kind of deep or stretch reflex where an application of a stimulus to the patellar tendon such as strike by a solid object or hammer caused the leg to extend due to such stimulus causes the quadriceps femoris muscle to contract.

It is named for Carl Friedrich Otto Westphal (1833-1890).

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