Temporal depression of the soleus H-reflex during passive stretch

• Published in: Experimental brain research 2012-06, Vol.219 (2), p.217-225
• Main Authors: Robertson, Christopher T., Kitano, Koichi, Koceja, David M., Riley, Zachary A.
• Format: Article
• Language: English
• Subjects: Ankle; Biological and medical sciences; Biomedical and Life Sciences; Biomedicine; Complications and side effects; Depression, Mental; Electromyography; Feedback; Female; Firing rate; Fundamental and applied biological sciences. Psychology; H-Reflex - physiology; Humans; Inhibition; Kinesiology; Long-Term Synaptic Depression - physiology; Male; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Muscle spindles; Muscle Stretching Exercises - methods; Muscle, Skeletal - physiology; Neural transmission; Neurology; Neurophysiology; Neurosciences; Physiological aspects; Presynapse; Recruitment; Reflex, Stretch - physiology; Research Article; Sensory neurons; soleus muscle; Somesthesis and somesthetic pathways (proprioception, exteroception, nociception); interoception; electrolocation. Sensory receptors; synaptic depression; Synaptic strength; Time Factors; Vertebrates: nervous system and sense organs; Young Adult
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-012-3080-1

Synaptic efficacy associated with muscle spindle feedback is regulated via depression at the Ia-motoneurone synapse. The inhibitory effects of repetitive Ia afferent discharge on target motoneurones of different sizes were investigated during a passive stretch of ankle extensors in humans. H-reflex recruitment curves were collected from the soleus muscle for two conditions in ten subjects. H-reflexes were elicited during passive stretch at latencies of 50, 100, 300, and 500 ms after a slow (20°/s) dorsiflexion about the right ankle (from 100 to 90°). Control H-reflexes were recorded at corresponding static (without movement) ankle angles of 99, 98, 94, and 90° of flexion. The slope of the H-reflex recruitment curves (Hslp) was then calculated for both conditions. H-reflex values were similar for the static and passive stretch conditions prior to 50–100 ms, not showing the early facilitation typical of increased muscle spindle discharge rates. However, the H-reflex was significantly depressed by 300 ms and persisted through 500 ms. Furthermore, less than 300 ms into the stretch, there was significantly greater H-reflex depression with a lower stimulus intensity (20 % Mmax) versus a higher stimulus intensity (Hmax), though the effects begin to converge at later latencies (>300 ms). This suggests there is a distinct two-stage temporal process in the depression observed in the Ia afferent pathway for all motoneurones during a passive stretch. Additionally, there is not a single mechanism responsible for the depression, but rather both heterosynaptic presynaptic inhibition and homosynaptic post-activation depression are independently influencing the Ia-motoneurone pathway temporally during movement.