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Stretch reflex

The stretch reflex, also known as the myotatic reflex, is a fundamental monosynaptic reflex arc in the spinal cord that automatically contracts a stretched skeletal muscle to resist lengthening and maintain posture or limb position.^[1] This reflex is initiated when muscle spindles—specialized sensory receptors within the muscle—detect sudden or sustained stretch, sending signals via fast-conducting Ia afferent nerve fibers directly to alpha motor neurons in the spinal cord, which then trigger muscle contraction without higher brain involvement.^[1] The pathway involves a single synapse between the sensory and motor neurons, making it one of the simplest and fastest reflexes in the human body, with response times as short as 20-50 milliseconds.^[2] Physiologically, the stretch reflex exists in two main forms: the dynamic stretch reflex, which responds to rapid changes in muscle length via nuclear bag fibers and produces a phasic contraction (as seen in deep tendon reflexes like the knee-jerk), and the static stretch reflex, which maintains tonic muscle activity during prolonged stretch through nuclear chain fibers.^[3] Muscle spindles are innervated by both sensory afferents and gamma motor neurons, which adjust spindle sensitivity to ensure the reflex operates effectively across different muscle lengths—a process known as alpha-gamma co-activation.^[1] Reciprocal inhibition occurs simultaneously, where the antagonist muscle is relaxed via interneurons, promoting smooth coordinated movement.^[4] The stretch reflex plays a critical role in everyday motor control, such as stabilizing joints during walking or standing, and preventing muscle overstretching that could lead to injury.^[3] Clinically, it is assessed through deep tendon reflexes (e.g., patellar at L2-L4 spinal levels or Achilles at S1-S2), graded on a 0-4 scale, to evaluate the integrity of the nervous system; hyperreflexia may indicate upper motor neuron lesions like stroke, while hyporeflexia suggests lower motor neuron damage such as peripheral neuropathy.^[4] Abnormalities in stretch reflex modulation are also implicated in conditions like spasticity or dystonia, highlighting its importance in neuromuscular health.^[3]

Overview

Definition and Types

The stretch reflex, also known as the myotatic reflex, is a fundamental monosynaptic reflex arc that detects muscle stretch through sensory receptors and triggers a rapid contraction to resist further lengthening of the muscle.^[5] This involuntary response helps maintain muscle length and tone, forming a basic feedback mechanism in the neuromuscular system.^[6] The reflex operates via a simple circuit involving sensory detection, a direct spinal synapse, and motor neuron activation to produce the contractile output.^[7] First described by Liddell and Sherrington in 1924, the stretch reflex was characterized as a myotatic response elicited by passive muscle extension, distinguishing it from other spinal reflexes through its brevity and specificity to stretch stimuli.^[8] Their work on decerebrate cats demonstrated that the reflex could be isolated and quantified, laying the groundwork for understanding spinal reflex integration.^[9] The basic components include specialized sensory receptors within the muscle that transduce stretch into afferent signals, a monosynaptic connection in the spinal cord to alpha motor neurons, and efferent output that innervates the same or synergistic muscles to generate contraction.^[10] The primary types of stretch reflexes are classified by latency and synaptic complexity: the short-latency monosynaptic reflex, mediated primarily by Ia afferent fibers from muscle spindles, which produces a rapid, direct excitatory response to sudden stretch; and the long-latency polysynaptic reflex, involving additional interneurons and possibly supraspinal inputs, which contributes to more sustained or modulated responses.^[11]^[12] The short-latency type exemplifies the core monosynaptic pathway, with Ia afferents synapsing directly onto motor neurons to ensure minimal delay in counteracting perturbations.31151-9) In contrast, long-latency reflexes allow for integration of contextual information, though they remain spinal in origin for many muscles.^[7]

Physiological Role

The stretch reflex plays a fundamental role in maintaining muscle tone and posture, particularly during static positions such as standing. By providing continuous feedback to counteract gravitational forces and minor displacements, it ensures that antigravity muscles remain appropriately activated to support body weight without constant voluntary input. For instance, when an individual sways slightly while upright, the reflex induces contraction in the stretched muscles of the legs and trunk, thereby restoring balance and preventing falls.^[6] This automatic stabilization allows higher motor centers to focus on other tasks, enhancing overall postural control.^[7] In dynamic activities like locomotion, the stretch reflex contributes to smooth movement by rapidly countering external perturbations, such as uneven terrain or unexpected pushes, that could disrupt gait. During walking or running, it stabilizes limb trajectory and assists in force production during the stance phase, minimizing deviations and promoting efficient progression. Studies indicate that stretch reflexes integrate with central pattern generators to adjust muscle activity in response to load changes, thereby reducing energy expenditure and improving stability across varied speeds.^[13] This perturbation resistance is evident in how the reflex augments muscle stiffness to oppose length changes, fostering coordinated stepping.^[14] The reflex exhibits adaptive scaling, where its gain modulates based on factors like muscle length and stretch velocity, allowing context-appropriate responses. For example, faster stretches elicit proportionally stronger reflexes to handle rapid disturbances, while slower changes produce milder adjustments suitable for gradual movements. This velocity-dependent sensitivity ensures the reflex supports both precise positioning and robust resistance without overreacting.^[15] Such modulation optimizes motor performance across a range of conditions.^[16] From an evolutionary standpoint, the stretch reflex is highly conserved across vertebrates, serving to preserve limb position against external forces and enabling reliable motor behaviors in diverse environments. Present even in basal species like the lamprey, it provides an ancestral mechanism for proprioceptive feedback that underpins postural and locomotor stability, adapting through variability to environmental demands.^[17] This preservation highlights its essential role in the foundational neural circuitry for movement control.^[18]

Anatomy

Muscle Spindles

Muscle spindles are specialized sensory receptors embedded within skeletal muscles, consisting of a bundle of 3 to 12 intrafusal muscle fibers enclosed in a fusiform connective tissue capsule that runs parallel to the extrafusal muscle fibers.^[19] The intrafusal fibers are categorized into nuclear bag fibers and nuclear chain fibers based on the arrangement of their nuclei in the central, equatorial region. Nuclear bag fibers include dynamic (bag₁) fibers, which are larger and more fusiform, and static (bag₂) fibers, which are smaller; nuclear chain fibers are thinner and shorter, with nuclei aligned in a chain-like fashion.^[20] Sensory innervation of the muscle spindle occurs primarily through two types of afferent nerve endings located in the central region of the intrafusal fibers. Group Ia afferents form primary annulospiral endings that encircle the equatorial zones of all intrafusal fiber types (bag₁, bag₂, and chain), providing rapid conduction and sensitivity to both the velocity and amplitude of muscle stretch. Group II afferents form secondary flower-spray endings mainly on the bag₂ and nuclear chain fibers, conveying information primarily about static muscle length with slower conduction velocities.^[19]^[20] The functional properties of muscle spindles distinguish dynamic and static responses to mechanical stimuli. Dynamic sensitivity, mediated by Ia afferents from bag₁ fibers, detects the speed of muscle lengthening during rapid stretches, generating high-frequency discharge rates proportional to velocity. Static sensitivity, involving Ia and II afferents from bag₂ and chain fibers, maintains firing rates that reflect sustained muscle length, ensuring ongoing feedback during isometric conditions or slow movements.^[20] These properties allow muscle spindles to serve as length transducers, unloading during muscle contraction unless adjusted.^[19] Motor innervation of muscle spindles is provided by gamma (γ) motor neurons, which target the contractile polar regions of the intrafusal fibers to regulate spindle sensitivity independently of extrafusal fiber activity. Dynamic γ neurons preferentially activate bag₁ fibers to enhance velocity sensitivity, while static γ neurons innervate bag₂ and chain fibers to maintain length sensitivity; some beta (β) innervation from alpha motor neurons also contributes to this adjustment. This forms the gamma loop, a feedback mechanism where γ activation contracts intrafusal fibers, keeping the sensory region taut and preventing spindle unloading during alpha-driven muscle shortening, thus preserving reflex responsiveness across varying lengths.^[20]^[19] Muscle spindles are distributed throughout most skeletal muscles, with an average density of 1 to 100 spindles per gram of muscle tissue and approximately 30,000 total in the adult human body. Density varies by muscle function and size, with higher concentrations observed in smaller, slow-twitch dominant antigravity muscles such as the soleus, which support posture and contain large proportions of slow fibers correlated with elevated spindle numbers. These spindles initiate stretch reflex responses by relaying length and velocity signals to the spinal cord.^[20]^[21]

Afferent and Efferent Pathways

The afferent pathways of the stretch reflex originate from muscle spindles, where sensory neurons detect muscle stretch. Primary afferents, classified as group Ia fibers, arise from annulospiral endings that encircle the central regions of all types of intrafusal fibers (dynamic and static nuclear bag and nuclear chain); these fibers are large-diameter, heavily myelinated, and conduct action potentials rapidly at velocities of 70-110 m/s.^[22] Secondary afferents, known as group II fibers, originate from flower-spray endings that primarily innervate the juxta-equatorial regions of nuclear chain and static nuclear bag intrafusal fibers; these are thinner and conduct more slowly, typically at 30-70 m/s, providing input sensitive to sustained stretch.^[5]^[20]^[19] The efferent pathway involves alpha motor neurons located in the ventral horn of the spinal cord, which innervate extrafusal muscle fibers to elicit contraction; these neurons have large axons with conduction velocities ranging from 60-120 m/s.^[23] The key synaptic connection in the stretch reflex is the direct monosynaptic linkage between group Ia afferent terminals and alpha motor neurons within the same spinal segment, enabling rapid transmission without intervening synapses.^[1] This wiring ensures that stretch detected by Ia fibers promptly excites homonymous alpha motor neurons, contributing to the reflex's speed and specificity.^[6]

Mechanism

Monosynaptic Arc

The monosynaptic arc represents the simplest and fastest pathway in the stretch reflex, involving a direct connection between sensory afferents and motor neurons within the spinal cord. This arc was first characterized in decerebrate cats, where passive stretching of a muscle elicits a reflexive contraction mediated by a single central synapse.^[8] Subsequent electrophysiological studies confirmed its monosynaptic nature, demonstrating that group Ia afferent fibers from muscle spindles synapse directly onto alpha motor neurons innervating the same muscle.^[24] The process unfolds in a precise sequence: sudden muscle stretch deforms the intrafusal fibers within muscle spindles, increasing the firing rate of Ia afferents; these afferents convey the signal via their central processes to the spinal cord, where they form excitatory synapses onto alpha motor neurons; the activated motor neurons then propagate action potentials along efferent axons to extrafusal muscle fibers, resulting in rapid contraction that resists the stretch.^[8]^[24] This pathway relies on the Ia afferents originating from primary endings of muscle spindles and the alpha motor neurons projecting to homonymous muscle fibers, as detailed in spinal cord anatomy.^[14] Due to the single synaptic delay, the latency of this response is notably short, typically 20-50 ms in human upper limb muscles such as the biceps brachii.^[14] The magnitude of the reflex response, often quantified as electromyographic (EMG) activity, exhibits basic proportionality to the velocity of the stretch, reflecting the dynamic sensitivity of Ia afferents where response amplitude ∝ stretch velocity.^[25] In parallel, the monosynaptic excitation is accompanied by reciprocal inhibition of antagonist muscles, mediated by collaterals from Ia afferents synapsing onto inhibitory interneurons that suppress alpha motor neurons of the opposing muscle group.^[26]

Polysynaptic and Long-Latency Responses

Polysynaptic pathways in the stretch reflex involve interneurons within the spinal cord, creating longer reflex loops that integrate sensory input from muscle spindles with other proprioceptive signals. These pathways contrast with the direct monosynaptic connection by incorporating one or more interneurons, allowing for more coordinated responses such as reciprocal inhibition of antagonist muscles during stretch. For instance, activation of Ia afferents from muscle spindles can polysynaptically excite antagonist inhibition via Ia inhibitory interneurons, contributing to the overall stabilization of joint position.^[4] Long-latency stretch reflexes, often denoted as M2 and M3 components, emerge 50-100 ms after the initial short-latency (M1) response, reflecting multi-synaptic processing beyond the spinal level. The M2 response, typically occurring at 50-80 ms, is primarily mediated by transcortical pathways involving the primary motor cortex, where afferent signals from muscle spindles ascend via the dorsal columns to the sensorimotor cortex and descend rapidly back to spinal motoneurons. In contrast, the M3 component, with latencies exceeding 100 ms, incorporates additional brainstem or cerebellar influences, enabling more context-dependent modulation.^[27]^[28]^[29] These long-latency reflexes play a critical role in fine-tuning motor output during voluntary movements, particularly by interrupting or adjusting ongoing actions in response to perturbations. They facilitate adaptive feedback control, integrating biomechanical constraints of the limb to enhance stability and accuracy, such as compensating for unexpected loads in upper limb tasks. Unlike the automatic spinal M1, M2 and M3 responses can be voluntarily modulated based on task demands, supporting rapid corrections that bridge reflex and intentional control.^[30]^[31]^[32] Recent research has highlighted adaptive modulation of these reflexes in shoulder muscles, where gain scaling varies with the aggregate activity of synergistic muscles crossing the glenohumeral joint. For example, studies using torque perturbations during multi-joint reaching tasks demonstrate that stretch reflex amplitudes in shoulder abductors like the deltoid are enhanced when synergistic muscles are co-activated, promoting joint-level stability. This modulation is goal-directed and reduced in the non-dominant limb, underscoring hemispheric asymmetries in reflex tuning for bimanual coordination.^[33]^[34]

Control and Modulation

Spinal Integration

Spinal integration of the stretch reflex occurs through local circuits in the spinal cord that refine sensory inputs and motor outputs, ensuring precise and adaptive responses to muscle stretch without reliance on supraspinal processing. A key component involves inhibitory interneurons that provide feedback to modulate reflex excitability. Renshaw cells, glycinergic and GABAergic interneurons located in the ventral horn, receive excitatory input from recurrent axon collaterals of alpha-motoneurons and in turn inhibit homonymous and synergistic motoneurons, thereby limiting overexcitation and promoting smooth motor control during reflexive activation.^[35] This recurrent inhibition helps stabilize the monosynaptic stretch reflex arc by reducing the duration and intensity of motoneuron bursts. Complementing this, Ib interneurons, activated by high-threshold afferents from Golgi tendon organs, mediate autogenic inhibition of homonymous motoneurons, which counteracts excessive force generation in the stretched muscle and protects against tendon overload during intense contractions.^[36] Presynaptic inhibition further refines spinal integration by directly targeting Ia afferent terminals from muscle spindles. This process, driven by GABAergic interneurons in a trisynaptic pathway, induces primary afferent depolarization that diminishes glutamate release at Ia-motoneuron synapses, effectively gating the strength of stretch reflex transmission.^[37] Such modulation allows the spinal cord to adjust reflex sensitivity based on ongoing activity, with inhibition strengthening postnatally to balance sensory feedback as motor circuits mature. In the context of the basic Ia afferent pathway to alpha-motoneurons, this presynaptic control provides a segmental mechanism to fine-tune gain without altering postsynaptic excitability.^[37] Central pattern generators (CPGs) in the spinal cord integrate stretch reflexes into rhythmic locomotion by embedding sensory feedback within locomotor circuits. These CPGs, comprising interconnected interneurons and motoneurons, use group Ia inputs from extensor muscle spindles to reinforce stance-phase activation, contributing up to 70% of extensor burst amplitude and resetting the step cycle toward extension via disynaptic excitatory pathways.^[38] Flexor group II afferents similarly interact with CPG elements to enhance swing-phase flexion or inhibit extensors, enabling adaptive gait adjustments through local suppression of non-locomotor inhibitory paths and emergence of locomotion-specific reflex reversals.^[38] Gamma-alpha coactivation represents a critical spinal coordination mechanism that maintains muscle spindle function during active contraction. In this process, gamma motoneurons innervating intrafusal fibers are activated concurrently with alpha motoneurons driving extrafusal muscle shortening, which adjusts spindle sensitivity to prevent unloading and sustain Ia afferent discharge proportional to muscle length changes.^[39] This coactivation, regulated at propriospinal and segmental levels, ensures reliable proprioceptive signaling for both postural stability and dynamic movements, with static gamma drive supporting length feedback and dynamic components timing reciprocal activations.^[39]

Supraspinal Influences

Supraspinal influences on the stretch reflex arise primarily from descending pathways originating in the cerebral cortex, brainstem, and cerebellum, which modulate spinal reflex circuits to support voluntary movement, posture, and adaptive responses to perturbations. The corticospinal tract, arising from the primary motor cortex, provides direct and indirect control over alpha and gamma motor neurons, enabling voluntary override of reflex responses and fine-tuning of reflex gain during goal-directed actions.^[6] This tract facilitates precise adjustments in limb position by altering the threshold and amplitude of stretch reflex activation, particularly in distal muscles.^[40] Brainstem descending tracts, including the vestibulospinal and reticulospinal pathways, play crucial roles in maintaining postural stability by modulating stretch reflexes in antigravity muscles. The lateral vestibulospinal tract, originating from the lateral vestibular nucleus, excites extensor motor neurons to counteract gravitational forces and enhance reflex-mediated balance during standing or locomotion.^[6] Similarly, the pontine and medullary reticulospinal tracts integrate sensory inputs to facilitate or inhibit reflex excitability, promoting coordinated postural adjustments in response to body sway.^[41] During voluntary movements, supraspinal modulation often reduces stretch reflex gain to prevent unwanted oscillations and ensure smooth execution, as seen in reaching tasks where motor cortical inputs suppress long-latency components.^[42] Conversely, techniques like the Jendrassik maneuver— involving remote muscle contraction, such as interlocking fingers and pulling apart—enhance stretch reflex amplitude (e.g., increasing patellar reflex response by approximately 95%) through supraspinal facilitation, likely via reduced presynaptic inhibition of Ia afferents.^[43] This maneuver demonstrates how descending pathways can amplify reflex sensitivity for diagnostic or adaptive purposes. The cerebellum contributes to stretch reflex modulation by refining timing and coordination, particularly through adjustments in agonist-antagonist interactions during movement initiation. Cerebellar outputs, relayed via the thalamus to the motor cortex, advance the phase of reflex responses in dynamic conditions, such as sinusoidal stretches at 0.1–10 Hz, thereby stabilizing joint torque and preventing instability.^[44] Ablation studies in animal models reveal diminished reflex gain and prolonged recovery of motor stability post-lesion, underscoring the cerebellum's role in adaptive reflex tuning.^[45] Research from the 2020s highlights the involvement of supraspinal loops in long-latency stretch reflexes (LLRs) for postural stability, where motor cortical and brainstem pathways selectively engage LLRs to coordinate multi-joint responses during balance challenges like wobble board training.^[46] These LLRs, occurring 50–100 ms post-perturbation, contribute a significant portion of corrective responses in compliant environments, enhancing equilibrium through context-dependent gain modulation.^[47] Such findings emphasize supraspinal contributions to reflexive adaptability beyond spinal circuits alone.

Examples

Knee-Jerk Reflex

The knee-jerk reflex, also known as the patellar tendon reflex, is elicited by a sharp tap on the patellar tendon just below the kneecap while the leg is relaxed and hanging freely, such as when seated with the knee flexed at about 90 degrees.^[4] This mechanical stimulus rapidly stretches the quadriceps femoris muscle, activating muscle spindles within it and producing a brisk extension of the lower leg at the knee joint due to quadriceps contraction.^[4] The procedure is straightforward and non-invasive, making it a standard clinical test for assessing spinal cord integrity.^[4] The neural pathway involves Ia afferent fibers from the quadriceps muscle spindles, which enter the spinal cord at the L2-L4 segments via the dorsal root ganglion.^[4] These afferents form a monosynaptic connection directly onto alpha motor neurons in the anterior horn of the spinal cord at the same levels, leading to rapid excitation and contraction of the quadriceps.^[4] Simultaneously, the reflex arc includes reciprocal inhibition of the antagonistic hamstring muscles through polysynaptic interneurons, promoting relaxation of the hamstrings to facilitate the knee extension.^[4] The femoral nerve provides the primary innervation for this pathway.^[4] In healthy adults, the normal response exhibits a latency of approximately 20 milliseconds from tendon tap to the onset of quadriceps electromyographic activity, reflecting the speed of the monosynaptic transmission.^[48] The amplitude of the reflex kick varies with factors such as age, where it tends to decrease in magnitude among older individuals due to neuromuscular changes, and overall health status, which can influence muscle tone and spindle sensitivity.^[49] This reflex serves as a classic example for demonstrating the monosynaptic nature of stretch reflexes in humans, owing to its simplicity and reliability in experimental and clinical settings.^[50]

Other Stretch Reflexes

The ankle jerk reflex, elicited by a brisk tap on the Achilles tendon, primarily involves the gastrocnemius and soleus muscles of the lower leg, resulting in plantar flexion of the foot. This monosynaptic stretch reflex arc is mediated through the tibial nerve and corresponds to the S1-S2 spinal segments.^[4] The biceps reflex, tested by striking the biceps tendon above the elbow, assesses the integrity of the elbow flexor muscles, particularly the biceps brachii. This reflex induces flexion at the elbow and is associated with the C5-C6 spinal segments via the musculocutaneous nerve.^[4] The jaw jerk reflex, provoked by tapping the chin with the jaw slightly open, engages the masseter and other jaw-closing muscles, leading to a brief upward jerk of the jaw. It involves afferent and efferent pathways within the trigeminal nerve (cranial nerve V) and the mesencephalic and motor nuclei, functioning as a monosynaptic stretch reflex.^[51] Stretch reflexes in the upper limbs, such as the biceps reflex, enable finer motor control compared to those in the lower limbs, owing to enhanced supraspinal modulation through long-latency responses that incorporate cortical and subcortical influences for adaptive adjustments during voluntary movements.^[14]

Clinical Significance

Testing and Disorders

Clinical testing of the stretch reflex primarily involves eliciting deep tendon reflexes (DTRs) through percussion of tendons, such as the patellar or Achilles, to assess the integrity of the reflex arc and motor neuron pathways.^[4] These reflexes are graded on a standardized 0-4 scale, where 0 indicates no response (areflexia), 1+ a diminished response, 2+ a normal response, 3+ a brisk or exaggerated response (hyperreflexia), and 4+ a response accompanied by transient clonus.^[4] This grading helps differentiate upper motor neuron (UMN) from lower motor neuron (LMN) involvement, with hyperreflexia (grades 3+ or 4+) signaling UMN lesions and hyporeflexia (grades 0 or 1+) indicating LMN damage.^[52] Hyperreflexia arises from UMN lesions, which disrupt descending inhibitory pathways, leading to unchecked spinal reflex excitability and spasticity.^[52] Common causes include stroke, where cortical or subcortical damage results in brisk reflexes often with increased muscle tone, and multiple sclerosis, in which demyelination of central pathways produces similar hyperreflexic responses alongside other pyramidal signs.^[52] In contrast, hyporeflexia or areflexia occurs due to LMN damage, impairing the reflex arc at the spinal cord, nerve roots, or peripheral nerves, as seen in peripheral neuropathy from conditions like diabetes or Guillain-Barré syndrome, where axonal degeneration reduces sensory input and motor output.^[4] Clonus represents a pathological exaggeration of the stretch reflex, manifesting as a series of involuntary, rhythmic contractions and relaxations in the stretched muscle, considered pathological if more than 3 beats occur when elicited by rapid stretch, with sustained clonus (greater than 10 beats) indicating severe involvement.^[53] It results from reflex hyperactivity due to UMN lesions, often co-occurring with hyperreflexia and spasticity, and is commonly tested at the ankle in conditions like cerebral palsy or spinal cord injury.^[53] Studies, including a 2021 systematic review, have shown that musculoskeletal pain, including low back pain, consistently modulates supraspinal projections to motoneurons, leading to delayed or attenuated long-latency reflexes without affecting short-latency components.^[54]

Therapeutic Applications

Physical therapy plays a central role in managing spasticity by employing stretching exercises to normalize muscle tone and reduce hyperactive stretch reflexes. Passive and active stretching techniques elongate spastic muscles, thereby decreasing the velocity-dependent increase in tonic stretch reflexes and improving range of motion, particularly in conditions such as post-stroke hemiparesis and cerebral palsy.^[55] These interventions are widely adopted as first-line conservative treatments, with evidence from randomized trials showing sustained reductions in spasticity scores after regular sessions over several weeks.^[56] Pharmacological approaches target the neural components of the stretch reflex to diminish its excitability. Baclofen, a GABA-B receptor agonist, is commonly administered orally or intrathecally to alleviate spasticity by enhancing presynaptic inhibition of Ia afferent terminals, thereby suppressing monosynaptic reflex transmission in the spinal cord.^[57] Clinical studies demonstrate that baclofen effectively lowers muscle tone and spasm frequency in patients with spinal cord injury or multiple sclerosis, with intrathecal delivery providing more precise control for severe cases.^[58] Neuromodulation techniques offer non-invasive or minimally invasive options to adjust stretch reflex activity by influencing muscle spindle feedback and reflex arcs. Functional electrical stimulation (FES) applied to antagonist muscles promotes reciprocal inhibition, reducing the gain of spastic stretch reflexes and enhancing motor control in lower limbs after stroke or spinal cord injury.^[59] Similarly, botulinum toxin type A injections into hypertonic muscles weaken excessive contractions, indirectly modulating spindle sensitivity and decreasing hyperactive reflexes, as evidenced by improved gait and reduced spasticity in cerebral palsy patients.^[60] Emerging post-2020 interventions include biofeedback training protocols that enable patients to voluntarily modulate stretch reflex gain for better management of movement disorders. Operant conditioning-based biofeedback, using real-time electromyographic or joint torque feedback, allows individuals with spasticity to downregulate reflexive contributions to joint stiffness, leading to improved voluntary control and reduced hyperreflexia in upper and lower extremities.^[61] These patient-driven methods show promise in neurorehabilitation, particularly when integrated with robotic assistance, for long-term reflex adaptation in chronic conditions like post-stroke spasticity.^[62] As of 2025, extracorporeal shock wave therapy (ESWT) has emerged as an effective non-invasive option for reducing spasticity in post-stroke and cerebral palsy patients by modulating muscle tone and reflex excitability.^[63] Ongoing clinical trials are also evaluating novel oral agents that enhance endogenous GABAergic mechanisms to treat multiple sclerosis-related spasticity.^[64]

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Muscles and spindles are activated by α and γ motor signals respectively after the regulation of PN and reflex network within spinal cord. The muscular dynamics ...
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https://pmc.ncbi.nlm.nih.gov/articles/PMC4598585/
[40]
https://doi.org/10.1152/jn.01094.2007
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https://doi.org/10.1152/jn.91013.2008
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https://doi.org/10.1186/2045-709X-20-29
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The cerebellum and initiation of movement: the stretch reflex - PMC
Studies of the stretch reflex in decerebrate cats indicate a phase advance of peak sinusoidal tension in steady-state cycles between 0.1 and 10 Hz.
[45]
https://pmc.ncbi.nlm.nih.gov/articles/PMC2590329/
[46]
Surface electromyography reveals males have a slower patellar ...
We found that the PR latency in females (17 ± 0.19 ms), was significantly (p < 0.001) faster than in males (21 ± 0.37 ms). This 4 ms difference in latency ...
[47]
Influence of Age on Patellar Tendon Reflex Response - PMC - NIH
Nov 18, 2013 · The major finding of this study confirms a decline in the magnitude of reflex response with increasing age.
[48]
Monosynaptic Reflex - an overview | ScienceDirect Topics
In the case of a monosynaptic reflex, two neurons, one sensory and one motor, are connected by a single synapse. The knee jerk is an involuntary neural response ...
[49]
Intracranial stimulation of the trigeminal nerve in man. II. Reflex ... - NIH
The response was sometimes followed by a later activity and showed the features of a polysynaptic reflex. No response was obtained in the jaw elevators when ...
[50]
Neuroanatomy, Upper Motor Neuron Lesion - StatPearls - NCBI - NIH
These symptoms can include weakness, spasticity, clonus, and hyperreflexia. UMN's lesions have a wide differential diagnosis which ranges from cerebrovascular ...Introduction · Structure and Function · Surgical Considerations · Clinical Significance
[51]
Clonus - StatPearls - NCBI Bookshelf - NIH
Clonus is a rhythmic oscillating stretch reflex that is related to upper motor neuron lesions. Therefore, clonus is generally accompanied by hyperreflexia.Missing: contractions | Show results with:contractions
[52]
Nociceptive withdrawal reflexes of the trunk muscles in chronic low ...
Jun 14, 2023 · Cutaneous low back pain did not influence the short-latency response [17,20] but reduced the long-latency response of the stretch reflex [17].
[53]
The effect of experimental and clinical musculoskeletal pain on ...
May 8, 2021 · This systematic review sought to understand the effect of pain on spinal and supraspinal projections to motoneurons and motor unit properties<|control11|><|separator|>
[54]
Effectiveness of Stretching in Post-Stroke Spasticity and Range of ...
Oct 24, 2021 · Stretching (muscle elongation) is currently the most widely used technique in the physical management of spasticity [25].
[55]
Stretching exercises in managing spasticity: effectiveness, risks, and ...
Jun 13, 2024 · The results of this study propose that several prospective and case studies have demonstrated the benefits of stretching to control spasticity.
[56]
Role of Baclofen in Modulating Spasticity and Neuroprotection in ...
The principal role of baclofen through GABAB receptor activation is to reduce the spinal reflexes. Baclofen acts at the spinal cord level blocking mono- and ...
[57]
Treatment of spasticity with intrathecal Baclofen administration
Aug 10, 2004 · The main effect of baclofen is to enhance presynaptic inhibition and thus inhibit monosynaptic and polysynaptic reflexes. Baclofen, 4 ...
[58]
The role of electrical stimulation for rehabilitation and regeneration ...
Jan 6, 2022 · NMES is thought to improve spasticity via disynaptic reciprocal inhibition in which the activation of one muscle produces an inhibition of the ...
[59]
Botulinum Toxin Intervention in Cerebral Palsy-Induced Spasticity ...
Nov 8, 2022 · The main goal of BTX-A treatment is to reduce the exaggerated muscle tone and hyperactive stretch reflexes seen in patients with CP, thus ...
[60]
Neurophysiological validation of simultaneous intrinsic and reflexive ...
Feb 17, 2021 · Training people using this joint impedance biofeedback was shown to achieve voluntary within-session modulation of both intrinsic and reflexive contributions ...
[61]
Effects of Acupuncture Combined with Biofeedback Therapy on ...
Jul 5, 2022 · Biofeedback therapy can improve lower limb muscle tension, spasticity, EMG integral value, and passive range of motion of ankle joint in children with cerebral ...