Abstract
Reciprocal Ia inhibition constitutes a key segmental neuronal pathway for coordination of antagonist muscles. In this study, we investigated the soleus H-reflex and reciprocal inhibition exerted from flexor group Ia afferents on soleus motoneurons during standing and walking in 15 healthy subjects following transcranial magnetic stimulation (TMS). The effects of separate TMS or deep peroneal nerve (DPN) stimulation and the effects of combined (TMS + DPN) stimuli on the soleus H-reflex were assessed during standing and at mid- and late stance phases of walking. Subthreshold TMS induced short-latency facilitation on the soleus H-reflex that was present during standing and at midstance but not at late stance of walking. Reciprocal inhibition was increased during standing and at late stance but not at the midstance phase of walking. The effects of combined TMS and DPN stimuli on the soleus H-reflex significantly changed between tasks, resulting in an extra facilitation of the soleus H-reflex during standing and not during walking. Our findings indicate that corticospinal inputs and Ia inhibitory interneurons interact at the spinal level in a task-dependent manner, and that corticospinal modulation of reciprocal Ia inhibition is stronger during standing than during walking.
Highlights
Reciprocal Ia inhibition, the postsynaptic pathway mediating inhibition to antagonist motoneurons through Ia inhibitory interneurons, is a key spinal pathway for coordination of antagonist muscles activation, and is the most thoroughly studied spinal circuit in human subjects (Pierrot-Deseilligny and Burke 2012)
Reciprocal inhibition from flexor group Ia afferents on soleus motoneurons decreases during ankle plantarflexion and may increase or remain unaltered during or at the onset of ankle dorsiflexion (Shindo et al 1984; Crone et al 1987; Petersen et al 1998a; Morita et al 2001), decreases upon imposed hip angle movements (Knikou 2005), decreases during ankle co-contraction (Nielsen and Kagamihara 1992), is present when sensory afferent feedback is absent and before the onset of antagonist muscle activity (Crone and Nielsen 1989, 1994), and is modulated in a phase-dependent manner during human walking (Capaday et al 1990, 1995; Lavoie et al 1997; Petersen et al 1999; Kido et al 2004; Mummidisetty et al 2013)
The transcranial magnetic stimulation (TMS)-mediated soleus H-reflex facilitation, reciprocal Ia inhibition, and TMS-mediated effects on reciprocal Ia inhibition were modulated in a phase- and task-dependent manner
Summary
Reciprocal Ia inhibition, the postsynaptic pathway mediating inhibition to antagonist motoneurons through Ia inhibitory interneurons, is a key spinal pathway for coordination of antagonist muscles activation, and is the most thoroughly studied spinal circuit in human subjects (Pierrot-Deseilligny and Burke 2012). Reciprocal inhibition from flexor group Ia afferents on soleus motoneurons decreases during ankle plantarflexion and may increase or remain unaltered during or at the onset of ankle dorsiflexion (Shindo et al 1984; Crone et al 1987; Petersen et al 1998a; Morita et al 2001), decreases upon imposed hip angle movements (Knikou 2005), decreases during ankle co-contraction (Nielsen and Kagamihara 1992), is present when sensory afferent feedback is absent and before the onset of antagonist muscle activity (Crone and Nielsen 1989, 1994), and is modulated in a phase-dependent manner during human walking (Capaday et al 1990, 1995; Lavoie et al 1997; Petersen et al 1999; Kido et al 2004; Mummidisetty et al 2013). Ia inhibitory interneurons are influenced by segmental interneuronal circuits, afferents, and supraspinal inputs (Eccles et al 1956; Hongo et al 1969; Hultborn et al 1971, 1976), with corticospinal descending volleys to facilitate transmission in Ia inhibitory interneurons (Lundberg and Voorhoeve 1962)
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