Biology subjects

Colard, J.

Publications and source records attributed to Colard, J..

4 recordsLinked to original sources

bioRxiv2026

Recurrent inhibition crosses the spinal cord midline in humans

Recurrent inhibition is known to modulate motoneuron output within an active motor pool, but it is unclear whether Renshaw cells receive projections from a contralateral pathway. Using intramuscular single motor unit recordings in humans, we demonstrated that electrical activation of the contralateral quadriceps motor axons elicits a robust decrease in soleus motor unit discharge rate, consistent with the characteristic features of recurrent inhibition. The duration of the inhibition scaled with motor unit firing rates and exhibited substantial interindividual variability. To uncover the underlying circuitry, we developed a biophysically grounded spiking network model constrained by individual experimental data. The model reproduced the observed contralateral inhibitory dynamics only when incorporating a polysynaptic commissural pathway mediated by V3-like interneurons. Model-based inference further revealed that intrinsic motoneuron properties critically shape the duration of inhibition. Together, these findings provide the first evidence for a commissural pathway influencing human spinal recurrent inhibitory networks, revealing a previously unrecognized mechanism that may contribute to bilateral motor coordination.

neuroscience↗

bioRxiv2026

Recurrent inhibition, not presynaptic inhibition, contributes to the velocity-dependent control of motoneuron output during eccentric contractions

It is well documented that both motoneuron output and the effectiveness of activated Ia afferents to discharge soleus -motoneurons decrease during eccentric (muscle lengthening) contractions. Evidence suggests that these modulations can be explained by recurrent inhibition and greater presynaptic inhibition of Ia afferents. However, the influence of angular velocity on the modulation of the effectiveness of activated Ia afferents to discharge -motoneurons observed during eccentric contractions remains unclear. We investigated the influence of angular velocity on spinal mechanisms involved in the effectiveness of activated Ia afferents to discharge -motoneurons during eccentric plantar flexor contractions using 16 healthy adults. We used both simple and conditioned Hoffmann reflex with different conditioning techniques to assess presynaptic inhibition, heteronymous Ia facilitation and heteronymous recurrent inhibition coupled with electromyography during eccentric contractions of the plantar flexors at three angular velocities. Our results showed that during eccentric contractions, the effectiveness of Ia afferents to discharge -motoneurons was lower at 90{degrees}{middle dot}s-{superscript 1} than 60{degrees}{middle dot}s-{superscript 1} and 20{degrees}{middle dot}s-{superscript 1} angular velocities. A similar velocity-dependent pattern was observed for heteronymous recurrent inhibition, decreasing at 90{degrees}{middle dot}s-{superscript 1} when compared with 60{degrees}{middle dot}s-{superscript 1} and 20{degrees}{middle dot}s-{superscript 1}. In contrast, presynaptic inhibition of Ia afferents was not different between the velocities. These demonstrate a differential influence of angular velocity on spinal recurrent inhibitory mechanisms during eccentric contractions and support distinct functional roles of recurrent and presynaptic inhibition in modulating -motoneurons discharge with increasing movement velocity. The findings provide new insights into the velocity-dependent and mechanism-specific modulation of spinal inhibitory circuits during eccentric contractions. KEY POINTSO_LIDuring eccentric contractions in soleus muscle, the effectiveness of activated Ia afferents to discharge -motoneurones decreases with increasing angular velocity, indicating a velocity-dependent modulation. C_LIO_LIPresynaptic inhibition of Ia afferents does not differ between angular velocities, suggesting that it does not contribute to the observed changes. C_LIO_LIHeteronymous recurrent inhibition from the quadriceps to the soleus increases with angular velocity, indicating that increasing movement velocity promotes a functional reorganization of intermuscular recurrent inhibition. C_LIO_LIThese findings suggest a differential functional role of the two spinal inhibitory mechanisms, indicating that increasing angular velocity primarily influences recurrent postsynaptic inhibition rather than presynaptic inhibition. C_LI

neuroscience↗

bioRxiv2026

Spinal rotational dynamics orchestrate locomotor recovery

Locomotion is one of the most essential functions of the nervous system, yet the principles that generate it and how it might be restored after spinal cord injury remain unresolved. Walking is possible even after complete spinal cord injury and a traditional assumption is that this activity emerges from modular flexor-extensor transitions driven by somatosensory feedback. To test this assumption, we recorded bilateral motor unit activity from sixteen flexor/extensor hindlimb muscles in awake cats walking on a treadmill. We found that spinal motor populations operate according to a fundamentally different principle. Rather than alternating between flexion and extension, population activity continuously rotated through all phases of the locomotor cycle within a low-dimensional neural manifold. This rotational organisation persisted after complete spinal cord injury, revealing the preservation of a cyclic attractor despite substantial alterations in motor outputs. Computational modeling further indicated that locomotor activity was more consistent with an autonomous regime than with somatosensory-driven control. These findings suggest that locomotor recovery after complete spinal cord injury reflects the preservation of an intrinsic spinal attractor and that somatosensory feedback contributes to, rather than generates, the locomotor rhythm. More broadly, they establish rotational population dynamics as a fundamental principle of spinal locomotor recovery.

neuroscience↗

bioRxiv2024

Muscle length modulates recurrent inhibition and post-activation depression differently according to contraction type

It is well documented that, in soleus, motoneuron output and the effectiveness of activated Ia afferents to discharge -motoneurons both decrease during eccentric contractions. Evidence suggests that these regulations can be explained by (1) recurrent inhibition and (2) greater post-activation depression by primary afferent depolarization. However, the influence of muscle length on the regulation of the effectiveness of Ia afferents to discharge -motoneurons observed during eccentric contractions remains unclear. We conducted a study on 16 healthy young individuals. We used simple and conditioned Hoffmann reflex with different conditioning techniques such as paired H reflex, D1 method and heteronymous Ia facilitation coupled with electromyography during eccentric, isometric and concentric contractions at long, intermediate and short soleus muscle lengths. Our results confirm that during eccentric contraction the effectiveness of Ia afferents to discharge U-motoneurons decreases only at intermediate and short muscle lengths but is similar between all contraction types at long muscle length. Findings are similar for recurrent inhibition. Post-activation depression is significantly more pronounced during eccentric contractions compared with isometric and concentric contractions at long muscle length. Our analysis also shows that recurrent inhibition and post-activation depression are greater at long muscle length compared with short muscle length, whatever the contraction type. These new findings demonstrate an important influence of muscle length on the activity of spinal regulatory mechanisms and the effectiveness of activated Ia afferents to discharge -motoneurons during eccentric contractions.

neuroscience↗