title: Enhanced D1 and D2 inhibitions induced by low-frequency trains of conditioning stimuli: differential effects on H- and T-reflexes and possible mechanisms.
abstract: Mechanically evoked reflexes have been postulated to be less sensitive to presynaptic inhibition (PSI) than the H-reflex. This has implications on investigations of spinal cord neurophysiology that are based on the T-reflex. Preceding studies have shown an enhanced effect of PSI on the H-reflex when a train of ~10 conditioning stimuli at 1 Hz was applied to the nerve of the antagonist muscle. The main questions to be addressed in the present study are if indeed T-reflexes are less sensitive to PSI and whether (and to what extent and by what possible mechanisms) the effect of low frequency conditioning, found previously for the H-reflex, can be reproduced on T-reflexes from the soleus muscle. We explored two different conditioning-to-test (C-T) intervals: 15 and 100 ms (corresponding to D1 and D2 inhibitions, respectively). Test stimuli consisted of either electrical pulses applied to the posterior tibial nerve to elicit H-reflexes or mechanical percussion to the Achilles tendon to elicit T-reflexes. The 1 Hz train of conditioning electrical stimuli delivered to the common peroneal nerve induced a stronger effect of PSI as compared to a single conditioning pulse, for both reflexes (T and H), regardless of C-T-intervals. Moreover, the conditioning train of pulses (with respect to a single conditioning pulse) was proportionally more effective for T-reflexes as compared to H-reflexes (irrespective of the C-T interval), which might be associated with the differential contingent of Ia afferents activated by mechanical and electrical test stimuli. A conceivable explanation for the enhanced PSI effect in response to a train of stimuli is the occurrence of homosynaptic depression at synapses on inhibitory interneurons interposed within the PSI pathway. The present results add to the discussion of the sensitivity of the stretch reflex pathway to PSI and its functional role.
authors: Laboratory of Signal Processing and Motor Control, College of Physical Education, Universidade de Brasília-UnB, Brasília, Brazil; Biomedical Engineering Laboratory, Escola Politécnica, PTC, Universidade de São Paulo, São Paulo, Brazil.
authors: Rinaldo André
authors: RA
authors: Mezzarane
authors: Biomedical Engineering Laboratory, Escola Politécnica, PTC, Universidade de São Paulo, São Paulo, Brazil; School of Arts, Sciences and Humanities-EACH, Universidade de São Paulo, São Paulo, Brazil.
authors: Fernando Henrique
authors: FH
authors: Magalhães
authors: Biomedical Engineering Laboratory, Escola Politécnica, PTC, Universidade de São Paulo, São Paulo, Brazil; Department of Electrical Engineering, Universidade Federal do Triângulo Mineiro-UFTM, Uberaba, Brazil.
authors: Vitor Martins
authors: VM
authors: Chaud
authors: Biomedical Engineering Laboratory, Escola Politécnica, PTC, Universidade de São Paulo, São Paulo, Brazil; Department of Biomedical Engineering, School of Electrical and Computer Engineering, University of Campinas-UNICAMP, Campinas, Brazil.
authors: Leonardo Abdala
authors: LA
authors: Elias
authors: Biomedical Engineering Laboratory, Escola Politécnica, PTC, Universidade de São Paulo, São Paulo, Brazil.
authors: André Fabio
authors: AF
authors: Kohn
mesh_terms: D000125:Achilles Tendon / Q000502:physiology; D000328:Adult; D003213:Conditioning, Psychological / Q000502:physiology*; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods; D005260:Female; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D007866:Leg / Q000502:physiology; D008297:Male; D009068:Movement / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D010543:Peroneal Nerve / Q000502:physiology; D017729:Presynaptic Terminals / Q000502:physiology; D012026:Reflex, Stretch / Q000502:physiology*; D013979:Tibial Nerve / Q000502:physiology