The Multimodal and Polysynaptic Nature of Deep Tendon Reflexes (DTRs) in Motor Control and Gait​

Deep tendon reflexes (DTRs) such as the patellar and Achilles reflex are often described as simple monosynaptic spinal loops confined to a single joint or myotome. Contemporary neuroscience, however, shows that they are multimodal, polysynaptic, and embedded in the global sensorimotor network of the central nervous system (CNS) (Côté et al., 2018; Jankowska, 2023). Through this extended architecture, a localized tendon tap can influence whole-body motor patterns, including interlimb coordination, postural stabilization, and gait-phase regulation, contributing to stance–swing transitions, limb loading, and the rhythmic inhibition–facilitation cycles of human locomotion (Di Russo et al., 2021; Nakajima et al., 2015; Ramadan et al., 2022). This explains why reflex-based assessment, as used in P-DTR, is often more informative about sensory integration than voluntary strength testing alone.

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1. Multisegmental Spinal Integration

Ia afferents from the patellar tendon arise from L2–L4 segments but do not terminate exclusively at their segment of origin (Jankowska, 2023). They send ascending and descending branches across multiple spinal levels, activating both local and distant interneuron networks. These Ia projections contribute to integration with trunk, hip, and contralateral limb musculature and help couple segmental reflex activity to global movement patterns (Côté et al., 2018). Propriospinal interneurons link lumbar, thoracic, and cervical regions, coordinating left–right alternation, trunk–limb coupling, and the timing of spinal central pattern generators (CPGs), so that sensory input from one limb can modulate whole-body movement a nd gait-phase timing (Takei and Seki, 2021; Nakajima et al., 2015; Nakajima et al., 2022).

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2. Supraspinal Modulation of Reflexes

Reflex responses are strongly shaped by descending pathways, especially the reticulospinal, vestibulospinal, and corticospinal tracts (Riddle et al., 2023; Côté et al., 2018). The reticulospinal tract integrates proprioceptive, vestibular, and cutaneous input to support postural stabilization, interlimb coordination, rapid corrective responses to perturbations, and phase-dependent modulation during gait (Riddle et al., 2023). Reflex activation can increase reticulospinal excitability, altering step timing, trunk stiffness, and global protective synergies. In addition, transcortical (long-latency) reflexes incorporate task relevance, environmental demands, and balance requirements, meaning that a single reflex response reflects both spinal and supraspinal sensorimotor integration rather than only local segment function (Dimitriou, 2022).

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3. Polysynaptic Components and Gait

Beyond the classic monosynaptic Ia loop, DTRs contain robust polysynaptic components that recruit group II spindle afferents, Ib Golgi afferents, cutaneous mechanoreceptors, propriospinal interneurons, and brainstem-mediated circuits (Prochazka and Ellaway, 2012; Côté et al., 2018). These polysynaptic pathways are essential for stance-phase stability, swing initiation, limb stiffness regulation, and interlimb locomotor coordination, and they help couple sensory input to CPG timing (Nakajima et al., 2015; Ramadan et al., 2022). As a result, reflexive sensory input can influence cadence, step length, and locomotor symmetry, linking a simple tendon tap to high-level gait characteristics (Di Russo et al., 2021).

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4. Locomotor Synergies and Heteronymous Effects

Reflex pathways not only activate homonymous muscles but also heteronymous motor pools across joints and limbs (Guerra et al., 2020; Dimitriou, 2022). This heteronymous facilitation allows stretch in one limb to affect contralateral limb activation, pelvic and trunk stabilization, global protective extension patterns, and phase-dependent gait synergies. The same circuits underlie the natural alternation of inhibition and facilitation observed in normal human gait, where muscles and joints across the body are rhythmically co-organized rather than acting in isolation (Côté et al., 2018; Nakajima et al., 2015).

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5. Reflex Testing vs Voluntary (Cortico-Somatic) Testing

Both reflex testing and cortico-somatic (voluntary strength) testing ultimately drive α-motor neurons, but they do so via different pathways and therefore measure different aspects of neuromuscular function. Voluntary testing primarily engages the motor cortex and corticospinal tract and is heavily influenced by strength, effort, attention, planning, and pain-avoidance strategies (Riddle et al., 2023). In contrast, reflex testing is sensory-driven: sensory afferents activate spinal, propriospinal, brainstem, and transcortical reflex circuits that reveal how the CNS weighs and integrates sensory input (Dimitriou, 2022; Côté et al., 2018). For this reason, reflex testing is particularly well suited to detecting dysfunctional sensory input, which is central to P-DTR methodology.

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6. γ-Motor Neurons: Setting Spindle Sensitivity

A frequent misconception is that γ-motor neurons “control the reflex.” In reality, γ-motor neurons adjust the sensitivity (gain) of muscle spindles rather than the reflex response itself (Prochazka and Ellaway, 2012). By regulating intrafusal fiber preload and the dynamic–static responsiveness of Ia and II fibers, γ-motor activity determines how strongly the spindle responds to stretch (Dimitriou, 2022). The final reflex output is then shaped by the interaction of sensory input, interneuronal processing, supraspinal modulation, and α-motor neuron activation.

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Conclusion

Modern evidence demonstrates that DTRs are not isolated monosynaptic events but complex multimodal and polysynaptic sensorimotor interactions that shape whole-body coordination, posture, and gait (Côté et al., 2018; Nakajima et al., 2015; Di Russo et al., 2021). Reflex-based assessment therefore offers a powerful window into how the CNS interprets and integrates sensory input, while voluntary testing mainly reflects conscious motor command and effort (Riddle et al., 2023). This framework underpins the use of DTRs in P-DTR, where treatment targets dysfunctional sensory signals to restore normal motor integration and efficient global movement (Ramadan et al., 2022; Prochazka and Ellaway, 2012).

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