Bilateral Soleus H-Reflexes in Humans Elicited by Simultaneous Trains of Stimuli: Symmetry, Variability, and Covariance

Abstract

Experiments using electrical and mechanical activation of spinal reflexes have contributed important results toward the understanding of neuronal and synaptic dynamics involved in spinal neural circuits as well as their response to different inputs. In this work, data obtained from the simultaneous stimulation of both legs are analyzed to provide information on the degree of symmetry of the respective spinal reflex circuits and on the characteristics of reflex variability. H-reflexes recorded from relaxed muscles show a frequency-dependent amplitude depression when elicited by a train of stimuli. This effect has been attributed to homosynaptic depression. Soleus H-reflexes were recorded in response to trains of simultaneous stimuli applied to both legs in right-handed subjects that were sitting in a relaxed state. The first objective was to verify the existence of asymmetries in H-reflex parameters obtained from the two legs. We measured the mean, variance, and coefficient of variation of the depressed H-reflex amplitudes and the time constant of decay toward the depressed plateau. The second objective was the analysis of the time correlation of subsequent H-reflex amplitudes in a long train of responses recorded from a given leg. The statistical dependence of H-reflex amplitudes in the long trains recorded from both legs was also investigated. Data obtained from preliminary experiments showed that there was no effect of a given stimulus on the contralateral leg applied simultaneously or 1 s before, therefore validating the simultaneous stimulation paradigm. Paired t-tests indicated that several parameters measured bilaterally from soleus H-reflex trains of right-handed subjects were not statistically different in the overall, although individually there were statistically significant asymmetries, toward either the right or left leg. Sequences of H-reflex amplitudes, as measured by the auto-covariance, were either white or had a memory ranging from 2 up to 50 s. This indicates that the random fluctuations in presynaptic inhibition and/or postsynaptic inputs to motoneurons may have either fast or slow time courses. The average auto-covariance sequences of the right and left legs, computed from all subjects, were practically superposable. The cross-covariance between the bilateral H-reflex amplitudes showed a statistically significant peak at zero lag in some experiments, suggesting a correlation between the synaptic inputs to the Ia-motoneuron systems of the soleus muscles of both legs.

Changes in FDB and soleus muscle activity after a train of stimuli during upright stance

“…An electrolytic gel interface was placed between the skin and the electrode, which was then secured to the skin with micropore tape. Electrodes were attached at the following locations: the soleus (SOL) -distal to the end of the gastrocnemius muscles, the tibialis anterior (TA) -over the largest corresponding girth and at the FDB -over the plantar region of the foot, near the medial arch 16,19,20 . TA muscle activity was recorded to check for any occurrence of co-contraction or reciprocal inhibition.…”

Spinal Source for the Synchronous Fluctuations of Bilateral Monosynaptic Reflexes in Cats

“…The present work is a counterpart of experiments done in awake humans by Mezzarane and Kohn (2002). They studied the correlation between H-reflex amplitudes recorded bilaterally to simultaneous stimulation of the right and left tibial nerves.…”

“…This finding indicates that, in 50% of the cases, H-reflex-amplitude variations tend to fluctuate synchronously in both legs. Mezzarane and Kohn (2002) also reported the case in which a subject changed from a nonflat (synchronous fluctuations) to a flat (absence of synchronous fluctuations) cross-covariance between bilateral H-reflexes on the second day of recording. However, because experiments in humans offer certain limitations, no experimental evidence was provided about the sources modulating the synchronous fluctuations of such bilateral H-reflexes.…”

“…The purpose of the present research was to extend the study of Mezzarane and Kohn (2002) to examine the correlation between the amplitude of the bilaterally synchronous nSCDPs and the amplitude of the bilateral MSRs in cats. Analysis of this correlation, before and after spinal sections, could be important to identify the sources contributing to this synchrony.…”

Absence of effects of contralateral group I muscle afferents on presynaptic inhibition of Ia terminals in humans and cats

“…PSI of Ia terminals could be a potential target for such crossed effects, as it modulates reflex actions in different motor contexts (Rudomin and Schmidt 1999). For instance, to meet the requirements of a variety of motor tasks, the PSI onto Ia terminals can be altered by ipsi-or contralateral peripheral input and descending pathways, in both humans (Hultborn et al 1987a;Mezzarane and Kohn 2002;Roby-Brami and Bussel 1990) and cats (Gossard and Rossignol 1990;Quevedo et al 1995).…”