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

Abstract

Evidence for strong synaptic coupling between single tactile afferents from the sole of the foot and motoneurons supplying leg muscles. J Neurophysiol 94: 3795-3804, 2005. First published August 3, 2005 doi:10.1152/jn.00359.2005. It has been known for some time that populations of cutaneous and muscle afferents can provide short-latency facilitation of motoneuron pools. Recently, it has been shown that the input from individual low-threshold mechanoreceptors in the glabrous skin of the hand can modulate ongoing activity in muscles acting on the fingers via spinally mediated pathways. We have extended this work to examine whether such strong synaptic coupling exists between tactile afferents in the sole of the foot and motoneurons supplying muscles that act about the ankle. We recorded from 53 low-threshold mechanoreceptors in the glabrous skin of the foot via microelectrodes inserted percutaneously into the tibial nerve of awake human subjects. Reflex modulation of ongoing whole muscle electromyography (EMG) was observed for each of the four classes of low-threshold cutaneous mechanoreceptors (17 of 21 rapidly adapting type I; 2 of 4 rapidly adapting type II; 7 of 18 slowly adapting type I; and 4 of 10 slowly adapting type II). Reflex modulation of the firing probability in single motor units (5 of 11) was also observed. These results indicate that strong synaptic coupling between tactile afferents and spinal motoneurons is not a specialization of the hand and emphasizes the potential importance of cutaneous inputs from the sole of the foot in the control of gait and posture.

Cooling reduces the cutaneous afferent firing response to vibratory stimuli in glabrous skin of the human foot sole

“…2 -Analyses revealed that the number of units that fell within each classification did not differ from previous reports of receptors on the foot sole (P Ͼ 0.05; Fallon et al 2005;Kennedy and Inglis 2002). Similar to previous findings, there did not appear to be a clear pattern of receptor distribution across the foot sole (Kennedy and Inglis 2002).…”

“…2 -Tests were used to determine differences in the number of receptors from each class between the present work and previous studies that have recorded from populations of skin afferents in the foot sole (Fallon et al 2005;Kennedy and Inglis 2002). Student's t-tests were used to determine the difference between the percentage of type I versus type II and the percentage of slowly adapting versus fast-adapting receptors that were reduced to 0% BFR.…”

“…IN RECENT YEARS cutaneous receptors have been ascribed an important role in proprioception (Collins et al 2005;Lowrey et al 2010), modulation of muscle activation (Fallon et al 2005;McNulty and Macefield 2001), and control of movement (Do et al 1990;Eils et al 2002;Perry et al 2000). Skin on the foot sole has a particularly notable role in the control of upright stance in humans.…”

Changes in Motoneuron Properties and Synaptic Inputs Related to Step Training after Spinal Cord Transection in Rats

“…It is known that even brief noxious input can significantly interfere with the effects of training at the spinal level for long periods (Grau et al, 1998(Grau et al, , 2004Crown et al, 2002a,b;Ferguson et al, 2006). This putative effect of noxious cutaneous input is perhaps not surprising in light of the significant role that cutaneous input plays in locomotion (Bouyer and Rossignol, 2003;Fallon et al, 2005).…”

Parallel facilitatory reflex pathways from the foot and hip to flexors and extensors in the injured human spinal cord

“…For example, a short latency inhibitory pathway from plantar cutaneous afferents to ankle extensors following low-threshold stimulation to the posterior tibial nerve is enabled in spinal intact subjects when the foot sole contacts a surface (Abbruzzese et al, 1996). Excitation of cutaneous afferents arising from the sural nerve or foot sole induces long-lasting facilitation of the ipsilateral extensor motoneurons in the cat (Schieppati and Crenna, 1984) and human (Fallon et al, 2005) and induces crossed inhibitory postsynaptic potentials in the knee and ankle extensors (Edgley and Aggelopoulos, 2006). These effects may be related to the postulated action of plantar cutaneous afferents on the presynaptic inhibition acting on Ia afferent terminals (Iles, 1996), to their interaction with the machinery of reciprocal innervation (Rossi and Mazzocchio, 1988), and to their actions on Ib inhibitory pathways during which soleus exerts facilitation (instead of inhibition) to quadriceps when the triceps surae muscle is contracting (Pierrot-Deseilligny et al, 1981,1982.…”