Task‐dependent changes in the responses to low‐threshold cutaneous afferent volleys in the human lower limb.

Abstract

SUMMARY1. In seven human subjects who were standing without support the sural nerves were stimulated electrically using trains of non-painful stimuli (five pulses at 300 Hz), designed to activate afferents from cutaneous mechanoreceptors. The reflex effects of the stimulus train on different muscles of the ipsilateral and contralateral legs were sought in post-stimulus averages of rectified EMG. Changes in the pattern of reflex influence were investigated when the subjects maintained different postures.2. Clear reflex responses were seen in ipsilateral tibialis anterior, soleus, biceps femoris and vastus lateralis, but only when the muscles were actively contracting. In each muscle, inhibition was the dominant reflex response within the first 100 ms. In four of the seven subjects, reflex changes were detectable in the contralateral tibialis anterior and soleus, the peak-to-peak modulation within the first 200 ms being 25-50% of that for the homologous ipsilateral muscle.3. When subjects attempted to stand on a tilted platform, an unstable platform or on one leg with the other flexed, different combinations of muscles were active, involving both flexors and extensors or predominantly flexors or predominantly extensors. In each posture the reflex effects were demonstrable only in the active muscles.4. With ipsilateral tibialis anterior, there were task-dependent changes in the short-latency components of the EMG response, approximately 60 ms and 80 ms after the stimulus. When seated performing voluntary contractions these components were difficult to define, and when standing on a platform tilted toe-up they were small. When the ipsilateral leg was flexed or when standing on an unstable base, these early components were more prominent in each subject. With contralateral tibialis anterior, the dominant reflex pattern was inhibition when seated and contracting voluntarily, and facilitation during bipedal stance tilted toe-up. These changes in reflex pattern could not be explained by different levels of background contraction.5. It is concluded that cutaneous mechanoreceptors of the foot have widespread reflex actions on muscles throughout both limbs, particularly the ipsilateral limb. and that the reflex pattern in different muscles and within a single muscle may MS 8459 D. BURKE, H. G. DICKSON AND N. F. SKUSE change dependent on the task that the subject is undertaking. These task-dependent changes indicate plasticity in the expression of cutaneous reflex activity, affecting both short-latency spinal as well as long-latency pathways.

Modulation of Cutaneous Reflexes From the Foot During Gait in Parkinson's Disease

“…5) the amplitude of the subtracted responses were often larger than fourfold the maximum background activation encountered during gait. In control subjects the contralateral medium-latency cutaneous reflex responses are normally smaller than the corresponding ipsilateral responses during gait (Van Wezel et al 1997) and during nonlocomotor tasks (Burke et al 1991). Furthermore, in our laboratory such high amplitudes of contralateral reflex responses were never observed in healthy subjects during normal gait (Duysens et al 1990(Duysens et al , 1991Van Wezel et al 1997).…”

“…For other reflexes some abnormalities were reported as well. For example, abnormalities in postural reflexes have been observed (Beckley et al 1991;Bloem et al 1995;Carpenter et al 2004;Chong et al 2000;Dietz et al 1988Dietz et al , 1995Dimitrova et al 2004;Schieppati and Nardone 1991). However, these abnormalities did not seem to involve the basic synergies (Dimitrova et al 2004).…”

Evidence for Strong Synaptic Coupling Between Single Tactile Afferents From the Sole of the Foot and Motoneurons Supplying Leg Muscles

“…Electrical stimulation of populations of cutaneous afferents innervating the sole and toes has been shown to exert both excitatory and inhibitory reflex effects on on-going voluntary contractions of muscles acting about the ankle (Aniss et al 1992;Gibbs et al 1995); reflex modulation has also been observed in remote muscles (Gibbs et al 1995;Zehr et al 2001). However, these reflex effects are not immutable-they can be modulated by the gait cycle (De Serres et al 1995;Duysens et al 1990;Haridas and Zehr 2003;Yang and Stein 1990) and postural task (Burke et al 1991). The pathways involved in the reflex coupling between cutaneous mechanoreceptors and muscles of the lower limb are proposed to include a range of pathways, from oligosynaptic spinal pathways (Aniss et al 1992) to transcortical pathways (Christensen et al 1999;Nielsen et al 1997).…”

“…Reflex modulation of lower limb muscles by cutaneous afferents had been proposed by Sherrington (1910), and more recent work has highlighted the important role of such reflexes in gait (Zehr et al 1997(Zehr et al , 1998 and posture (Day and Cole 2002) and emphasized that the reflexes are not simple "limb flexion"/withdrawal reflexes (Burke et al 1991). In fact, efforts are already underway to attempt to augment cutaneous feedback in an effort to increase postural control (Priplata et al 2003).…”

“…The distribution of reflex latencies found using whole nerve stimulation of the tibial and/or sural nerves at the level of the ankle is often divided into "early" (Ͻ70 ms), "mid" (70 -120 ms), and "late" (Ͼ120 ms) responses (Aniss et al 1992;Brooke et al 1997;Burke et al 1991;Gibbs et al 1995;Kukulka 1994), with the earliest reflex responses seen after whole nerve stimulation at the ankle being in the order of 30 ms (Kukulka 1994). As our recordings of afferent activity were made at the level of the popliteal fossa (on average 0.25 m above the ankle) and the conduction velocity of large myelinated afferents in the tibial nerve is in the order of 50 ms Ϫ1 (Macefield et al 1989), we would expect reflex latencies to be ϳ5 ms shorter than those reported in studies involving wholenerve stimulation at level of the ankle.…”