Subthreshold Electrical Noise Applied to the Plantar Foot Enhances Lower-Limb Cutaneous Reflex Generation.

Tushar Sharma,Ryan M Peters,Leah R Bent

doi:10.3389/fnhum.2020.00351

Tushar Sharma, Ryan M Peters + Show 1 more

Open Access

https://doi.org/10.3389/fnhum.2020.00351

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Abstract

Reflex responses generated by cutaneous mechanoreceptors of the plantar foot are important for the maintenance of balance during postural tasks and gait. With aging, reflex generation, particularly from fast adapting type I receptors, is reduced, which likely contributes to impaired postural stability in this population. Therefore, improving reflex generation from these receptors may serve as a tool to improve balance performance. A mechanism to enhance reflexes may lie in the phenomenon of stochastic resonance, whereby the addition of certain intensities and frequencies of noise stimuli improves the performance of a system. This study was conducted to determine whether tactile noise stimuli could improve cutaneous reflex generation. In 12 healthy young adults, we evoked cutaneous reflex responses using a 0–50 Hz Gaussian noise vibration applied to the plantar heel. Concurrently, we applied one of six subthreshold intensities of electrical tactile noise to the plantar heel [0%, 20%, 40%, 60%, 80% or 100% (threshold)] and were able to analyze data from 0%, 20% and 40% trials. Across participants, it was found that the addition of a 20% perceptual threshold (PT) noise resulted in enhanced reflex responses when analyzed in both the time and frequency domains. These data provide evidence that cutaneous reflex generation can be enhanced via a stochastic resonance effect and that 20% PT is the optimal intensity of noise to do so. Therefore, the addition of noise stimuli may be a valuable clinical intervention to improve reflex responses associated with postural balance in populations with impairments.

Highlights

Sensory information from the cutaneous mechanoreceptors of the foot sole is important for the successful maintenance of standing balance (Magnusson et al, 1990; Day and Cole, 2002) and gait (Yang and Stein, 1990; Zehr et al, 2014)
While all four types of cutaneous mechanoreceptors found in the glabrous skin of the foot can generate cutaneous reflex responses (Fallon et al, 2005; Bent and Lowrey, 2013), fast adapting type-I (FAI) receptors are believed to be important for the maintenance of balance
We saw that the addition of electrical tactile noise at an intensity of 20% perceptual threshold (PT) increased the magnitude of pooled cutaneous reflex responses and significantly increased pooled coherence at ∼30 Hz in SOL

Summary

Introduction

Sensory information from the cutaneous mechanoreceptors of the foot sole is important for the successful maintenance of standing balance (Magnusson et al, 1990; Day and Cole, 2002) and gait (Yang and Stein, 1990; Zehr et al, 2014). When the stimulation is moved anteriorly to the metatarsal region, a reversal of these reflexes occurs such that the soleus response becomes inhibitory and an excitatory reflex occurs in the tibialis anterior (Sonnenborg et al, 2000; Nakajima et al, 2006; Sayenko et al, 2009) This pattern of activation may function to remove the foot from obstacles that may contact the plantar foot during postural balance or gait (Nakajima et al, 2006). Enhancing the ability of these cutaneous mechanoreceptors to generate reflex responses may be a viable method of reducing the risk of falls in older adults

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