Involvement of the reticulospinal system in the control of locomotion.

Human stepping movements are known to be co-ordinated by a neural coupling of upper and lower limbs. The aim of this study was to evaluate a potential involvement of the reticulospinal system in the interlimb co-ordination by neural coupling during locomotion. During stepping electrical stimuli (ES) were applied to the right tibial nerve to assess the mechanisms underlying neural coupling of lower limbs, and loud acoustic stimuli (LAS) were released, known to activate the reticulospinal system. EMG activity of the tibialis anterior (TA) and gastrocnemius medialis (GM) of both sides was recorded during the right swing and double support phases of the step cycle and during dorsi- and plantarflexion movements of the feet. In all motor tasks the responses to both ES and LAS were modulated in the same way. The largest responses to both stimuli appeared during the right swing in the left GM and during double support in the left TA. During the right swing the left GM contributes to maintain body equilibrium over the left leg. In the double support phase the TA has to secure foot clearance for swing initiation. There was an absolute time shift in latency between responses to both ES and LAS across all tasks, compatible with a common neural structure for their generation in the brainstem. These findings suggest that reticulospinal drive is dynamically modulated and is involved in interlimb co-ordination during locomotion. KEY POINTS: Unilateral electrical stimuli (ES), applied to the right tibial nerve, were used to explore the neural coupling mechanism and its contribution to co-ordination of the legs during stepping. Loud acoustic stimuli (LAS) were applied to assess the potential involvement of the reticulospinal system in locomotor co-ordination. LAS and ES evoked similar EMG responses in the left GM during the right swing and in the left tibialis anterior (TA) during double support. A 30 ms time shift between responses to LAS and ES across all tasks is compatible with a common origin of their generation in the brainstem. Indirect evidence suggests that neural leg coupling during stepping is mediated by brainstem structures, likely the reticulospinal system.