Selective skin sensitivity changes and sensory reweighting following short-duration space flight

Abstract

Skin sensory input from the foot soles is coupled with vestibular input to facilitate body orientation in a gravitational environment. Anecdotal observations suggest that foot sole skin becomes hypersensitive following space flight. The veritable level of skin sensitivity and its impact on postural disequilibrium observed post space flight have not been documented. Skin sensitivity of astronauts (n = 11) was measured as vibration perception at the great toe, fifth metatarsal and heel. Frequencies targeted four classes of receptors: 3 and 25 Hz for slow-adapting (SA) receptors and 60 and 250 Hz for fast-adapting (FA) receptors. Data were collected pre- and post-space flight. We hypothesized that skin sensitivity would increase post-space flight and correlate to balance measures. Decreased skin sensitivity was found on landing day at 3 and 25 Hz on the great toe. Hypersensitivity was found for a subset of astronauts (n = 6) with significantly increased sensitivity to 250 Hz at the heel. This subset displayed a greater reduction in computerized dynamic posturography (CDP) equilibrium (EQ) scores (-54%) on landing vs. non-hypersensitive participants (-11%). Observed hyposensitivity of SA (pressure) receptors may indicate a strategy to reduce pressure input during periods of unloading. Hypersensitivity of FAs coupled with reduced EQ scores may reflect targeted sensory reweighting. Altered gravito-inertial environments reduce vestibular function in balance control which may trigger increased weighting of FAs (that signal foot contact, slips). Understanding modulations to skin sensitivity has translational implications for mitigating postural disequilibrium following space flight and for on-Earth preventative strategies for imbalance in older adults.

Skin Hardness and Epidermal Thickness Affect the Vibration Sensitivity of the Foot Sole

“…Sinusoidal vibrations were applied to the foot sole via a plastic probe (width: 2 mm; length: 5 mm) attached to a minishaker (model 201, Ling Dynamic Systems, Royston, UK). As recommended by Lowrey and coworkers [15], prior to the onset of each trial the probe of the minishaker was placed in contact with the foot sole and a preload force of 2N was applied, manipulated by a vertical adjustment of the shaker and confirmed with a force transducer (model K13-0.02 kN, Scaime, Annemasse, France). Our vibrator device allowed to deliver different amplitudes of vertical motions of the probe and seven levels were retained (1,2,5,10,15,20 and 25 arbitrary units).…”

“…Based on the vibration frequencies known to recruit mechanoreceptors in the foot sole [14,15], the frequencies were chosen to target the activation of either the SAI receptors (25 Hz: the Merkel disks and Ruffini corpuscles) or the FAII ones (150 Hz: the Meissner and Pacinian corpuscles). In a limited number of subjects who showed the highest skin hardness at the level of the 5 th metatarsal head, the podiatrists performed superficial skin abrasion to search for the benefits of a reduced epidermal layer of skin keratinization on the skin sensitivity.…”

Psychophysical estimate of plantar vibration sensitivity brings additional information to the detection threshold in young and elderly subjects

“…The vibration frequencies were chosen to target the activation of three different skin receptors in the glabrous skin of the foot (25 Hz for SAI, 50 Hz for FAI, and 150 Hz for FAII) based on the frequencies known to best recruit individual mechanoreceptors in the foot sole (Johansson et al, 1982, Lowrey et al, 2014). Sinusoidal vibrations were applied to the foot sole via a plastic probe (width: 2 mm; length: 5 mm) attached to a minishaker (model 201, Ling Dynamic Systems, Royston, UK).…”

“…Sinusoidal vibrations were applied to the foot sole via a plastic probe (width: 2 mm; length: 5 mm) attached to a minishaker (model 201, Ling Dynamic Systems, Royston, UK). As recommended by Lowrey et al (2014), prior to the onset of each trial the probe of the minishaker was placed in contact with the foot sole and a preload force of 2N was applied, manipulated by a vertical adjustment of the shaker and confirmed with a force transducer (model K13 – 0.02 kN, Scaime, Annemasse, France). Fig.…”

In-vivo function of human plantar intrinsic foot muscles

“…However the involuntary activation of these muscles during tasks such as the foot loading experiments performed in Chapter four, suggests these muscles are highly susceptible to afferent feedback and it is highly likely that a combination of various sensory afferents contribute to the regulation of intrinsic foot muscle activation during standing and gait, including the joint mechanoreceptors, muscles spindles and golgi tendon organs. One area of particular interest is the role that sensory cutaneous receptors of the foot sole play in the recruitment and activation of the plantar intrinsic foot muscles, with suggestions that these receptors play an important function in providing afferent feedback for postural control (Lowrey et al, 2010;Mouchnino & Blouin, 2013;Lowrey et al, 2014). The slowly adapting type one (SAI) and type two (SAII) receptors are particularly sensitive to pressure and skin stretch, respectively (Macefield, 2005;Lowrey et al, 2013;.…”