Thresholds of cutaneous afferents related to perceptual threshold across the human foot sole

NewsBot · Aug 22, 2015

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Thresholds of cutaneous afferents related to perceptual threshold across the human foot sole.
Strzalkowski ND, Mildren RL, Bent LR.
J Neurophysiol. 2015 Aug 19

Perceptual thresholds are known to vary across the foot sole, despite a reported even distribution in cutaneous afferents. Skin mechanical properties have been proposed to account for these differences, however, a direct relationship between foot sole afferent firing, perceptual threshold and skin mechanical properties has not been previously investigated. Using the technique of microneurography, we recorded the monofilament firing thresholds of cutaneous afferents and associated perceptual thresholds across the foot sole. In addition, receptive field hardness measurements were taken to investigate the influence of skin hardness on these threshold measures. Afferents were identified as Fast Adapting; FAI (n=48), FAII (n=13), or Slowly Adapting; SAI (n=21) or SAII (n=20), and were grouped based on receptive field location (Heel, Arch, Metatarsals, Toes). Overall, perceptual thresholds were found to most closely align with firing thresholds of FA afferents. In contrast, SAI and SAII afferent firing thresholds were found to be significantly higher than perceptual thresholds and are not thought to mediate monofilament perceptual threshold across the foot sole. Perceptual thresholds and FAI afferent firing thresholds were significantly lower in the Arch compared to other regions, and skin hardness was found to positively correlate with both FAI and FAII afferent firing and perceptual thresholds. These data support a perceptual influence of skin hardness, which is likely the result of elevated FA afferent firing threshold at harder foot sole sites. The close coupling between FA afferent firing and perceptual threshold across foot sole indicates that small changes in FA afferent firing can influence perceptual thresholds.
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NewsBot · Nov 7, 2015

Foot sole skin vibration perceptual thresholds are elevated in a standing posture compared to sitting
Robyn L. Mildren et al
Gait and Posture; article in press

Highlights
•We measured skin vibration thresholds at the foot sole while standing and sitting.
•Multiple frequencies were used to activate different cutaneous afferent populations.
•Thresholds across all frequencies were increased during standing.
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Foot sole sensitivity is commonly assessed while individuals are seated or prone; however the primary role of foot sole cutaneous feedback is for the control of upright stance and gait. The aim of this study was to compare vibration perceptual thresholds across the foot sole between sitting and standing postures. Vibration perceptual thresholds were measured in sitting and standing postures in 18 healthy participants (8 male) using a custom vibration device. Two foot sole locations (heels and metatarsals) were tested at four vibration frequencies (3, 15, 40, and 250 Hz) selected to target different cutaneous afferent populations. At each frequency, perceptual thresholds across the foot sole were significantly higher in the standing posture compared to the sitting posture; this is indicative of lower sensitivity while standing. In addition, threshold differences between the heels and metatarsals for lower frequency vibratory stimuli were more pronounced while standing, with higher thresholds observed at the heels. Our results demonstrate that standing significantly alters sensitivity across the foot sole. Therefore, conducting perceptual tests at the foot sole during stance could potentially provide more direct information about the ability of cutaneous afferents to signal tactile information in a state where this feedback can contribute to postural control.
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NewsBot · Nov 23, 2015

Differences in foot sensitivity and plantar pressure between young adults and elderly.
Machado ?S et al
Arch Gerontol Geriatr. 2015 Nov 10

OBJECTIVE:
The understanding of foot sensitivity and plantar pressure contributes to the design of insoles, shoes, as well as to guide therapeutic interventions. Here we investigate differences in plantar pressure and foot sensitivity between young adults and community-dwelling elderly.
METHODS:
Thirty-eight participants (19 young adults and 19 elderly) underwent clinical assessment of foot sensitivity and upright standing with eyes open and closed for measurement of plantar pressure in each foot. Data were compared between feet, groups, and visual conditions.
RESULTS:
Foot sensitivity was lower in the elderly and, in contrast to young adults, differed between the foot regions (loss of sensitivity was primarily seen at the heel). Elderly shift plantar pressure to more distal foot zones, namely towards midfoot and forefoot. Asymmetries in foot sensitivity and plantar pressure were not observed. Visual condition did not influence plantar pressure distribution.
CONCLUSIONS:
The forward shift in plantar pressure (away from the insensitive heel) constitutes a strategy of elderly to maintain balance.
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NewsBot · Jan 30, 2016

Vibrotactile stimulation of fast adapting cutaneous afferents from the foot modulates proprioception at the ankle joint.
Mildren RL, Bent LR.
J Appl Physiol (1985). 2016 Jan 28:jap.00810.2015.

It has previously been shown that cutaneous sensory input from across a broad region of skin can influence proprioception at joints of the hand. The current experiment tested whether cutaneous input from different skin regions across the foot can influence proprioception at the ankle joint. The ability to passively match ankle joint position (17? and 7? plantarflexion and 7? dorsiflexion) was measured while cutaneous vibration was applied to the sole (heel, distal metatarsals) or dorsum of the target foot. Vibration was applied at two different frequencies to preferentially activate Meissner's corpuscles (45Hz, 80μm) or Pacinian corpuscles (255Hz, 10μm) at amplitudes ~3dB above mean perceptual thresholds. Results indicated that cutaneous input from all skin regions across the foot could influence joint matching error and variability, although the strongest effects were observed with heel vibration. Furthermore, the influence of cutaneous input from each region was modulated by joint angle; in general, vibration had a limited affect on matching in dorsiflexion compared to matching in plantarflexion. Unlike previous results in the upper limb, we found no evidence that Pacinian input exerted a stronger influence on proprioception compared to Meissner input. Findings from this study suggest that fast adapting cutaneous input from the foot modulates proprioception at the ankle joint in a passive joint matching task. These results indicate that there is interplay between tactile and proprioceptive signals originating from the foot and ankle.
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NewsBot · May 26, 2017

Plantar Sole Unweighting Alters the Sensory Transmission to the Cortical Areas.
Mouchnino L et al
Front Hum Neurosci. 2017 May 10;11:220. doi: 10.3389/fnhum.2017.00220. eCollection 2017.

It is well established that somatosensory inputs to the cortex undergo an early and a later stage of processing. The later has been shown to be enhanced when the earlier transmission decreased. In this framework, mechanical factors such as the mechanical stress to which sensors are subjected when wearing a loaded vest are associated with a decrease in sensory transmission. This decrease is in turn associated with an increase in the late sensory processes originating from cortical areas. We hypothesized that unweighting the plantar sole should lead to a facilitation of the sensory transmission. To test this hypothesis, we recorded cortical somatosensory evoked potentials (SEPs) of individuals following cutaneous stimulation (by mean of an electrical stimulation of the foot sole) in different conditions of unweighting when standing still with eyes closed. To this end, the effective bodyweight (BW) was reduced from 100% BW to 40% BW. Contrary to what was expected, we found an attenuation of sensory information when the BW was unweighted to 41% which was not compensated by an increase of the late SEP component. Overall these results suggested that the attenuation of sensory transmission observed in 40 BW condition was not solely due to the absence of forces acting on the sole of the feet but rather to the current relevance of the afferent signals related to the balance constraints of the task.
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