title: Convergence in reflex pathways from multiple cutaneous nerves innervating the foot depends upon the number of rhythmically active limbs during locomotion.
abstract: Neural output from the locomotor system for each arm and leg influences the spinal motoneuronal pools directly and indirectly through interneuronal (IN) reflex networks. While well documented in other species, less is known about the functions and features of convergence in common IN reflex system from cutaneous afferents innervating different foot regions during remote arm and leg movement in humans. The purpose of the present study was to use spatial facilitation to examine possible convergence in common reflex pathways during rhythmic locomotor limb movements. Cutaneous reflexes were evoked in ipsilateral tibialis anterior muscle by stimulating (in random order) the sural nerve (SUR), the distal tibial nerve (TIB), and combined simultaneous stimulation of both nerves (TIB&SUR). Reflexes were evoked while participants performed rhythmic stepping and arm swinging movement with both arms and the leg contralateral to stimulation (ARM&LEG), with just arm movement (ARM) and with just contralateral leg movement (LEG). Stimulation intensities were just below threshold for evoking early latency (<80 ms to peak) reflexes. For each stimulus condition, rectified EMG signals were averaged while participants held static contractions in the stationary (stimulated) leg. During ARM&LEG movement, amplitudes of cutaneous reflexes evoked by combined TIB&SUR stimulation were significantly larger than simple mathematical summation of the amplitudes evoked by SUR or TIB alone. Interestingly, this extra facilitation seen during combined nerve stimulation was significantly reduced when performing ARM or LEG compared to ARM&LEG. We conclude that locomotor rhythmic limb movement induces excitation of common IN reflex pathways from cutaneous afferents innervating different foot regions. Importantly, activity in this pathway is most facilitated during ARM&LEG movement. These results suggest that transmission in IN reflex pathways is weighted according to the number of limbs directly engaged in human locomotor activity and underscores the importance of arm swing to support neuronal excitability in leg muscles.
authors.affiliation: Department of Integrative Physiology, Kyorin University School of Medicine, Tokyo, Japan; Rehabilitation Neuroscience Laboratory, University of Victoria, Victoria, BC, Canada.
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Nakajima
authors.affiliation: Rehabilitation Neuroscience Laboratory, University of Victoria, Victoria, BC, Canada; Laboratory of Signal Processing and Motor Control, College of Physical Education, University of Brasília, Brasília, Brazil.
authors.forename: Rinaldo A
authors.initials: RA
authors.lastname: Mezzarane
authors.affiliation: Motion and Mobility Rehabilitation Laboratory, University of Victoria, Victoria, BC, Canada; Human Discovery Science, International Collaboration on Repair Discoveries (ICORD), Vancouver, BC, Canada; Centre for Biomedical Research, University of Victoria, Victoria, BC, Canada.
authors.forename: Sandra R
authors.initials: SR
authors.lastname: Hundza
authors.affiliation: Division of Sports and Health Science, Chiba University, Chiba, Japan.
authors.forename: Tomoyoshi
authors.initials: T
authors.lastname: Komiyama
authors.affiliation: Rehabilitation Neuroscience Laboratory, University of Victoria, Victoria, BC, Canada; Human Discovery Science, International Collaboration on Repair Discoveries (ICORD), Vancouver, BC, Canada; Centre for Biomedical Research, University of Victoria, Victoria, BC, Canada; Division of Medical Sciences, University of Victoria, Victoria, BC, Canada.
authors.forename: E Paul
authors.initials: EP
authors.lastname: Zehr
mesh_terms: D000328:Adult; D001132:Arm / Q000502:physiology*; D004576:Electromyography; D005260:Female; D005528:Foot / Q000294:innervation* / Q000502:physiology*; D006801:Humans; D007866:Leg / Q000294:innervation / Q000502:physiology; D008124:Locomotion; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D009424:Nervous System Physiological Phenomena; D010507:Periodicity; D012018:Reflex; D013497:Sural Nerve / Q000502:physiology; D013979:Tibial Nerve / Q000502:physiology; D055815:Young Adult