title: Temporal depression of the soleus H-reflex during passive stretch.
abstract: 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.
authors.affiliation: Department of Sport and Exercise Science, Jacksonville University, Jacksonville, FL 32211, USA.
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Robertson
authors.forename: Koichi
authors.initials: K
authors.lastname: Kitano
authors.forename: David M
authors.initials: DM
authors.lastname: Koceja
authors.forename: Zachary A
authors.initials: ZA
authors.lastname: Riley
mesh_terms: D005260:Female; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D036881:Long-Term Synaptic Depression / Q000502:physiology*; D008297:Male; D052580:Muscle Stretching Exercises / Q000379:methods; D018482:Muscle, Skeletal / Q000502:physiology*; D012026:Reflex, Stretch / Q000502:physiology*; D013997:Time Factors; D055815:Young Adult