Sweat Release Research Articles

Measurement of stress-induced sympathetic nervous activity using multi-wavelength PPG

The onset of stress triggers sympathetic arousal (SA), which causes detectable changes to physiological parameters such as heart rate, blood pressure, dilation of the pupils and sweat release. The objective quantification of SA has tremendous potential to prevent and manage psychological disorders. Photoplethysmography (PPG), a non-invasive method to measure skin blood flow changes, has been used to estimate SA indirectly. However, the impact of various wavelengths of the PPG signal has not been investigated for estimating SA. In this study, we explore the feasibility of using various statistical and nonlinear features derived from peak-to-peak (AC) values of PPG signals of different wavelengths (green, blue, infrared and red) to estimate stress-induced changes in SA and compare their performances. The impact of two physical stressors: and Hand Grip are studied on 32 healthy individuals. Linear (Mean, s.d.) and nonlinear (Katz, Petrosian, Higuchi, SampEn, TotalSampEn) features are extracted from the PPG signal’s AC amplitudes to identify the onset, continuation and recovery phases of those stressors. The results show that the nonlinear features are the most promising in detecting stress-induced sympathetic activity. TotalSampEn feature was capable of detecting stress-induced changes in SA for all wavelengths, whereas other features (Petrosian, AvgSampEn) are significant (AUC ≥ 0.8) only for IR and Red wavelengths. The outcomes of this study can be used to make device design decisions as well as develop stress detection algorithms.

An adjustable transpiration polyimide membrane for highly efficient personal drying and warming

Functional textiles with efficient sweat releasing and thermal insulating play a pivotal role in maintaining physiological comfort in cold winter. However, the breathability and warmth have conflicting structural demands on the conventional fabric. Here, a porous polyimide film (PIF) with tunable sweat release capability was prepared by growing a graphene layer through laser-ablation strategy. Jungle-like graphene layer offers the porous PI film with distinct advantages, including a desired super hydrophobic performance (water contact angle =153°), rapid air permeability (1572 g/m2·24 h), quick drying function based on high photo-thermal and electro-thermal conversion efficiency, and outstanding thermal insulating capability (0.033 W/m·K). Moreover, the graphene layer can also be custom printed into various patterns, which expanded the application scenarios for wearable flexible electronics. Overall, this Janus design offered insight into the proactive control of sweat release and heat transfer within the hierarchical and interconnected network, providing promising benefits to the development of multifunctional drying and warming textiles.

The role of the dorsolateral prefrontal cortex in control of skin sympathetic nerve activity in humans.

The dorsolateral prefrontal cortex (dlPFC) is primarily involved in higher order executive functions, with there being evidence of lateralization. Brain imaging studies have revealed its link to the generation of skin sympathetic nerve activity (SSNA), which is elevated in states of emotional arousal or anxiety. However, no studies have directly explored dlPFC influences on SSNA. Transcranial alternating current stimulation (-2 to 2mA, 0.08Hz, 100cycles) was applied between the left or right dlPFC and nasion via surface electrodes. Spontaneous bursts of SSNA were recorded from the common peroneal nerve via a tungsten microelectrode in 21 healthy participants. The modulation index was calculated for each stimulation paradigm by constructing cross-correlation histograms between SSNA and the sinusoidal stimulus. Stimulation of the dlPFC caused significant modulation of SSNA, but there was no significant difference in the median modulation index across sides. Stimulation also caused cyclic modulation of skin blood flow and sweat release. We have shown for the first time that stimulation of the dlPFC causes modulation of SSNA, also reflected in the effector-organ responses. This supports a role for the dlPFC in the control of SSNA, which likely contributes to the ability of emotions to bring about cutaneous vasoconstriction and sweat release.

Perspiration-Wicking and Luminescent On-Skin Electronics Based on Ultrastretchable Janus E-Textiles

Stretchable electronics have attracted surging attention for next-generation smart wearables, yet traditional flexible devices fabricated on hermetical elastic substrates cannot satisfy lengthy wearing comfort and signal stability due to their poor moisture and air permeability. Herein, perspiration-wicking and luminescent on-skin electrodes are fabricated on superelastic nonwoven textiles with a Janus configuration. Through the electrospin-assisted face-to-face assembly of all-SEBS microfibers with differentiated diameters and composition, porosity and wettability asymmetry are constructed across the textile, endowing it with antigravity water transport capability for continuous sweat release. Also, the phosphor particles evenly encapsulated in the elastic fibers empower the Janus textile with stable light-emitting capability under extreme stretching in a dark environment. Additionally, the precise printing of highly conductive liquid metal (LM) circuits onto the matrix not only equips the electronic textile with broad detectability for various biophysical and electrophysiological signals but also enables successful implementation of human-machine interface (HMIs) to control a mechanical claw.

Review on the Development and Application of Directional Water Transport Textile Materials

Moisture (sweat) management in textile products is crucial to regulate human thermo-physiological comfort. Traditional hydrophilic textiles, such as cotton, can absorb sweat, but they retain it, leading to undesired wet adhesion sensation and even excessive cooling. To address such issues, the development of functional textiles with directional water transport (DWT) has garnered great deal of interest. DWT textile materials can realize directional water transport and prevent water penetration in the reverse direction, which is a great application for sweat release in daily life. In this review article, the mechanism of directional water transport is analyzed. Then, three key methods to achieve DWT performance are reviewed, including the design of the fabric structure, surface modification and electrospinning. In addition, the applications of DWT textile materials in functional clothing, electronic textiles, and wound dressing are introduced. Finally, the challenges and future development trends of DWT textile materials in the textile field are discussed.

Three‐Layer Composite Fabric with the Gradient of Wettability Inspired by Populus euphratica Root Pressure for Drying and Cooling

AbstractFabric with efficient sweat release and heat dissipation is highly desirable in a broad range of applications. However, creating such materials still remains a tough challenge. Here, inspired by the root pressure water of Populus euphratica and jade heat dissipation, a three‐layer composite fabric (T‐CF) for highly efficient personal drying and cooling is presented. The obtained T‐CF is formed by three individual layers with designed functions. The inner layer, middle layer, and surface layer are composed of cool polyester yarns, cool polyester/bamboo composite yarns, and bamboo yarns, respectively, to form a wetting gradient property. The biomimetic design offers the fabric with promising advantages, such as the unidirectional water conductivity of 800.34%, the water evaporation rate of 0.2611 g h−1, the instantaneous contact cool feeling of 0.186 ± 0.009 w cm−2, and the maximum cool temperature in wet state of 4.74 °C. Consequently, the T‐CF not only has good moisture absorption and moisture conduction, but also has heat dissipation and a certain sense of contact cooling. It is believed that the strategy reported in this work can provide a facile route for new‐generation functional fabric development regarding human health management.

A Finite Element Approach to Evaluate Thermoregulation in the Human Body due to the Effects of Sweat Evaporation during Cooking, Cleaning, and Walking

Sweat evaporation is the principal process of dissipating heat energy in a hot environment and during activities. Sweat loss is significantly affected by the level of energy expenditure, hormones, and the number of sweat glands. The thickness of the skin layer plays a vital role to maintain body temperature. The rate of sweat evaporation varies with ambient temperature and activity level. On increasing both metabolism and ambient temperature, sweat rate loss also increases and controls the body in the thermoregulatory system. The evaporative sweat release rate has a linear behavior. The appropriate physical and physiological parameters that affect thermoregulation have been incorporated into the model. The study presents the temperature distribution in three layers: epidermis, dermis, and subcutaneous tissue (SST) of the human dermal parts during cooking, cleaning, and walking. The solution is obtained by using the finite element method. The results demonstrate that the body mechanism keeps the body in thermoregulation by increasing the sweat evaporation rate exhibited by increasing the ambient temperature and metabolism during strenuous activities.

A Biomimetic Transpiration Textile for Highly Efficient Personal Drying and Cooling

AbstractEfficient sweat release and heat dissipation are required for functional textiles that improve comfort and productivity while being worn in daily life. However, the porous structure of textiles exhibits an opposite effect on water transport and heat transfer capacities, leading to a longstanding bottleneck for the design of multifunctional drying and cooling textiles. Here, a biomimetic transpiration textile based on the hierarchical and interconnected network of vascular plants is demonstrated for highly efficient personal drying and cooling. The transpiration‐inspired design offers a textile with distinct advantages, including a desired one‐way water transport index (1072%), rapid water evaporation rate (0.36 g h−1), and outstanding through‐plane (0.182 W m−1 K−1) and in‐plane (1.137 W m−1 K−1) thermal conductivities. Moreover, based on the optimized performance, plausible mechanisms are proposed and calculated to provide insight into the water transport and heat transfer within the hierarchical and interconnected network, which provide promising benefits to the development of multifunctional drying and cooling textiles. Overall, the successful synthesis of this biomimetic transpiration textile provides a comfortable microclimate to the human body, thus satisfying the growing demand for better health, productivity, and sustainability.

One-step fabrication of multi-scaled, inter-connected hierarchical fibrous membranes for directional moisture transport

HypothesisFunctional textiles engineered for maintaining body comfort by fast sweat release using the directional moisture transport concept are in high demand. However, designing these functional textiles remains a critical job and generally requires multi-step complex fabrication routes. In this regard, developing one-step strategy to fabricate multi-scaled, inter-connected nonwoven-nanofiber/nets hierarchical fibrous composite membranes with asymmetric wettability for enhanced directional moisture transport would be a very fruitful approach. ExperimentComposite membranes were fabricated by the rational combination of commercial polyethylene terephthalate nonwoven (CNW) as hydrophobic layer, and polyamide and silver nanoparticles (PA-Ag) nanofiber/nets as hydrophilic layer via one-step electrospinning process. FindingsThe resultant CNW/PA-Ag nanofiber/net composite membranes were carefully investigated for water vapor transport, moisture management performance, and antibacterial activity. The subsequent membranes not only exhibit exceptionally high one-way moisture transport index (1253%), considerably high water vapor transport rate (11.45 Kg m-2d-1), and overall moisture management capacity (0.91), but also offer high resistance of 16.9 cm H2O to prohibit the moisture drive in the opposite way, and considerable antibacterial activity against Escherichia coli and Staphylococcus aureus. The effective one-step fabrication of such fascinating directional moisture transport membranes with decent antibacterial activity opens a new intuition into the designing of novel functional materials for rapid sweat release and personal drying applications.

Multi-scaled interconnected inter- and intra-fiber porous janus membranes for enhanced directional moisture transport

HypothesisGrowing use of comfortable functional textiles has resulted in increased demand of excellent directional moisture (sweat) transport feature in textiles. However, designing such anisotropic functional textiles that allow fast penetration of sweat through one direction but prevent its movement in the reverse direction is still a challenging task. In this regard, fabrication of a novel Janus membrane with multi-scaled interconnected inter- and intra-fiber pores for enhanced directional moisture transport designed by a rational combination of superhydrophilic hydrolyzed porous polyacrylonitrile (HPPAN) nanofibers and hydrophobic polyurethane (PU) fibers via electrospinning may be a very useful approach. ExperimentPAN/PVP composite nanofibers were electrospun using PAN/PVP composite solution dissolved in DMF. After electrospinning, electrospun fibers were subjected extensive washing process to selectively remove PVP from the fiber matrix to develop highly rough and porous PAN (PPAN) nanofibers. The resultant PPAN nanofibers were then hydrolyzed to further improve their wettability. Finally, a layer of PU fibers was directly deposited on the HPPAN nanofibers via electrospinning to fabricate the subsequent Janus membrane. FindingsThe resultant PU/HPPAN Janus membranes display instant moisture transport in the positive direction with exceptional directional moisture transport index (R = 1311.3%), whereas, offer superior resistance (i.e. breakthrough pressure ≥17.1 cm H2O) to the moisture movement in the reverse direction. Moreover, a plausible mechanism articulating the role of inter- and intra-porosity for the enhanced directional moisture transport has been proposed. Successful fabrication of such fascinating Janus membranes based on the proposed coherent mechanism opens a new insight into the engineering of novel functional textiles for fast sweat release and personal drying applications.

Development of a sweating thermal skin simulant for heat transfer evaluation of clothed human body under radiant heat hazard

Human perspiration highly affects body heat dissipation as well as heat and mass transfer in clothing systems. Nevertheless, current skin simulant systems for assessing heat transfer under high heat exposures simply simulate the dry skin state. This fact limits the validity of such systems as perspiration and mass transfer exist during high heat exposure conditions. In this study, an active sweating skin simulant was developed using the polydimethylsiloxane material and with controllable sweat rates for heat transfer assessment under radiant heat hazards, considering mass transfer as well. In the dry state, the temperature and its variation rate of the skin simulant with water-filled sweating channels showed good agreement with the ISO standard numerical model. For sweating conditions, the results showed that the wet conduction facilitated heat transfer towards the skin by 5–8 % at the beginning of sweating. With continuous sweat release, the surface temperature rise was attenuated due to the additional heat capacity induced by additional moisture in the clothing-skin system. The low standard deviations, below 2 °C, and coefficients of variation, below 10%, for the temperature measurements under various sweat rates, indicated good reproducibility of the skin simulant. This research provides the first approach to consider active sweating in skin simulant systems under high heat exposures. This further provides new insights into heat transfer in protective clothing-human skin systems under extreme heat exposures, contributing to realistically evaluating the thermal protective performance of clothing materials under sweating conditions.

Continuous, Spontaneous, and Directional Water Transport in the Trilayered Fibrous Membranes for Functional Moisture Wicking Textiles.

Directional water transport is a predominant part of functional textiles used for continuous sweat release in daily life. However, it has remained a great challenge to design such textiles which ensure continuous directional water transport and superior prevention of water penetration in the reverse direction. Here, a scalable strategy is reported to create trilayered fibrous membranes with progressive wettability by introducing a transfer layer, which can guide the directional water transport continuously and spontaneously, thus preventing the skin from being rewetted. The resulting trilayered fibrous membranes exhibit a high one-way transport index R (1021%) and a desired breakthrough pressure (16.1 cm H2 O) in the reverse direction, indicating an ultrahigh directional water transport capacity. Moreover, on the basis of water transport behavior, a plausible mechanism is proposed to provide insight into the integrative and cooperative driving forces at the interfaces of trilayered hydrophobic/transfer/superhydrophilic fibrous membranes. The successful synthesis of such fascinating materials would be valuable for the design of functional textiles with directional water transport properties for personal drying applications.

Neck movement but not neck position modulates skin sympathetic nerve activity supplying the lower limbs of humans.

We previously showed that dynamic, but not static, neck displacement modulates muscle sympathetic nerve activity (MSNA) to lower limbs of humans. However, it is not known whether dynamic neck displacement modulates skin sympathetic nerve activity (SSNA). Tungsten microelectrodes inserted into the common peroneal nerve were used to record SSNA in 5 female and 4 male subjects lying supine on a table that fixed their head in space but allowed trapezoidal ramp (8.1 ± 1.2°/s) and hold (17.5° for 53 s) or sinusoidal (35° peak to peak at 0.33-0.46 Hz) horizontal displacement of the body about the head. SSNA recordings were made before, during, and after trapezoidal and sinusoidal displacements of the body. Spike frequency analysis of trapezoidal displacements and cross-correlation analysis during sinusoidal displacements revealed that SSNA was not changed by trapezoid body-only displacement but was cyclically modulated during sinusoidal angular displacements (median, 95% CI: 27.9%, 19.6-48.0%). The magnitude of this modulation was not statistically ( P > 0.05) different from that of cardiac and respiratory modulation at rest (47.1%, 18.7-56.3% and 48.6%, 28.4-59.3%, respectively) or during sinusoidal displacement (10.3%, 6.2-32.1% and 26.9%, 13.6-43.3%, respectively). Respiratory frequency was entrained above its resting rate (0.26 Hz, 0.2-0.29 Hz) during sinusoidal neck displacement; there was no significant difference ( P > 0.05) between respiratory frequency (0.38 Hz, 0.25-0.49 Hz) and sinusoidal displacement frequency (0.39 Hz, 0.35-0.42 Hz). This study provides evidence that SSNA is modulated during neck movement, raising the possibility that neck mechanoreceptors may contribute to the cutaneous vasoconstriction and sweat release associated with motion sickness. NEW & NOTEWORTHY This study demonstrates that dynamic, but not static, stretching of the neck modulates skin sympathetic nerve activity in the lower limbs.

Moisture Transport: Tailoring Differential Moisture Transfer Performance of Nonwoven/Polyacrylonitrile‐SiO2 Nanofiber Composite Membranes (Adv. Mater. Interfaces 15/2017)

Xianfeng Wang, Bin Ding, and co-authors design a nonwoven/polyacrylonitrile-SiO2 nanofiber dual-layer membrane with high wettability gradient in article number 1700062. The developed fibrous membrane offers an excellent moisture management performance with an outstanding one-way transport capacity (1413%), excellent overall moisture management capacity (0.99), and a superior breathability (water vapor transfer rate of 12.73 Kg m−2 d−1), indicating a potential candidate for faster sweat release applications (e.g. sportswear, footwear and workwear).

Tailoring Differential Moisture Transfer Performance of Nonwoven/Polyacrylonitrile‐SiO2 Nanofiber Composite Membranes

Liquid moisture (sweat) transport properties of textile materials play critical role in maintaining the comfort of body which are mainly controlled by surface chemistry, structure, and morphology of substrate. Herein, a dual layer surface treated nonwoven (NW)/electrospun nanofiber membrane with excellent differential liquid moisture transport characteristics is reported. NW is used as inner layer for its high moisture releasing and low moisture absorbing features, and polyacrylonitrile (PAN) as outer layer because of its decent hydrophilic nature. Wettability of NW is tailored by surface treatment with polydopamine, and the wicking characteristics of PAN nanofibers are enhanced by inducing hydrophilic SiO2 nanoparticles (PAN‐SiO2). Performance of resultant surface treated NW (TNW)/PAN‐SiO2 nanofiber composite membranes is thoroughly characterized by moisture management tester (MMT). Subsequent composite membranes offer high wettability gradient which leads to their marvelous MMT performance with an outstanding one‐way transport capacity of 1413%, excellent overall moisture management capacity of 0.99, and a decent water vapor transfer rate of 12.73 kg m−2 d−1, indicating a potential candidate for faster sweat release applications.

Skin sympathetic nerve activity in humans during exposure to emotionally-charged images: sex differences

While it is known that anxiety or emotional arousal affects skin sympathetic nerve activity (SSNA), the galvanic skin response (GSR) is the most widely used parameter to infer increases in SSNA during stress or emotional studies. We recently showed that SSNA provides a more sensitive measure of emotional state than effector-organ responses. The aim of the present study was to assess whether there are gender differences in the responses of SSNA and other physiological parameters such as blood pressure, heart rate, skin blood flow and sweat release, while subjects viewed neutral or emotionally-charged images from the International Affective Picture System (IAPS). Changes in SSNA were assessed using microneurography in 20 subjects (10 male and 10 female). Blocks of positively-charged (erotica) or negatively-charge images (mutilation) were presented in a quasi-random fashion, following a block of neutral images, with each block containing 15 images and lasting 2 min. Images of both erotica and mutilation caused significant increases in SSNA, with increases being greater for males viewing erotica and greater for females viewing mutilation. The increases in SSNA were often coupled with sweat release and cutaneous vasoconstriction; however, these markers were not significantly different than those produced by viewing neutral images and were not always consistent with the SSNA increases. We conclude that SSNA increases with both positively-charged and negatively-charged emotional images, yet sex differences are present.

Cardiorespiratory coupling of sympathetic outflow in humans: a comparison of respiratory and cardiac modulation of sympathetic nerve activity to skin and muscle

What is the central question of this study?Muscle sympathetic nerve activity (MSNA) is well known to be modulated by the arterial baroreceptors and respiration, but what are the magnitudes of cardiac and respiratory modulation of skin sympathetic nerve activity (SSNA), which primarily subserves thermoregulation?What is the main finding and what is its importance?Using direct microelectrode recordings of MSNA and SSNA in awake humans, we show that the magnitude of respiratory modulation of SSNA is identical to that of MSNA, the primary difference between the two sources of sympathetic outflow being the greater cardiac modulation of MSNA. This emphasises the role of the baroreceptors in entraining sympathetic outflow to muscle. It is well known that microelectrode recordings of skin sympathetic nerve activity (SSNA) in awake human subjects reveal spontaneous bursts of activity with no overt modulation by changes in blood pressure or respiration, in contrast to the clear cardiac and respiratory modulation of muscle sympathetic nerve activity (MSNA). However, cross-correlation analysis has revealed that, like individual muscle vasoconstrictor neurones, the firing of individual cutaneous vasoconstrictor neurones is temporally coupled to both the cardiac and respiratory rhythms during cold-induced cutaneous vasoconstriction, and the same is true of single sudomotor neurones during heat-induced sweating. Here, we used cross-correlation analysis to determine whether SSNA exhibits cardiac and respiratory modulation in thermoneutral conditions and to compare respiratory and cardiac modulation of SSNA with that of MSNA. Oligounitary recordings of spontaneous SSNA (n = 20) and MSNA (n = 18) were obtained during quiet, unrestrained breathing. Respiration was recorded by a strain-gauge transducer around the chest and ECG recorded by surface electrodes. Respiratory and cardiac modulation of SSNA and MSNA were quantified by fitting polynomial equations to the cross-correlation histograms constructed between the sympathetic spikes and respiration or ECG. The amplitude of the respiratory modulation (52.5 ± 3.4%) of SSNA was not significantly different from the amplitude of the cardiac modulation (46.6 ± 3.2%). Both were comparable to the respiratory modulation of MSNA (47.7 ± 4.2%), while cardiac modulation of MSNA was significantly higher (89.8 ± 1.5%). We conclude that SSNA and MSNA share similar levels of respiratory modulation, the primary difference between the two sources of sympathetic outflow being the marked cardiac modulation of MSNA provided by the baroreceptors.

Viscerosympathetic reflexes in human spinal cord injury: relationships between detrusor pressure, blood pressure and skin blood flow during bladder distension

Autonomic dysreflexia, a dangerous and sustained increase in blood pressure brought about by widespread, reflexly generated vasoconstriction, can be induced by visceral or somatic sensory inputs originating below the lesion following spinal cord injury (SCI). We assessed whether cutaneous vasoconstriction below the lesion could serve as a proxy marker of incipient autonomic dysreflexia during bladder distension. Skin blood flow (pulse plethysmography), sweat release, blood pressure, heart rate, bladder and rectal pressures were recorded during routine cystometry (urodynamics) in 16 patients with SCI. Eight urological patients without SCI served as control subjects. In all SCI patients, who had sustained injuries 2 months to 44 years previously at levels C3-T3, bladder filling (mean ± SD, 339 ± 132 ml) induced increases in detrusor (bladder-rectal) pressure (52 ± 25 cmH(2)O) and cutaneous vasoconstriction in the fingers, but no consistent increases in sweat release. This occurred irrespective of whether the spinal lesions were complete [American Spinal Injury Association (ASIA) grade A, n = 6] or incomplete (ASIA B-D; n = 10). Group mean blood pressure for the SCI patients increased by 17 ± 15 mmHg, but in four patients the pressure decreased or did not change. Despite similar bladder volumes (423 ± 126 ml) in the control patients, the increases in detrusor pressure (14 ± 8 cmH(2)O) and blood pressure (9 ± 12 mmHg) were significantly smaller than in the SCI patients; moreover, there were no consistent changes in skin blood flow in the control subjects. In all SCI patients, changes in finger pulse amplitudes were inversely correlated to changes in detrusor pressure (mean r = -0.62 ± 0.17). Changes in finger pulse amplitudes correlated inversely to changes in blood pressure in nine of 15 patients. It is concluded that cystometry in SCI patients is associated with detrusor and cardiovascular reflex effects that are exaggerated compared with those in intact subjects and that measurement of skin blood flow from the fingers in patients with a high spinal lesion provides a supplementary, clinically useful, non-invasive and continuous marker of spinally mediated viscerosympathetic reflex activity below the lesion in such patients.

Increases in muscle sympathetic nerve activity, heart rate, respiration, and skin blood flow during passive viewing of exercise

The cardiovascular and respiratory effects of exercise have been widely studied, as have the autonomic effects of imagined and observed exercise. However, the effects of observed exercise in the first person have not been documented, nor have direct recordings of muscle sympathetic nerve activity (MSNA) been obtained during observed or imagined exercise. The aim of the current study was to measure blood pressure, heart rate, respiration, skin blood flow, sweat release, and MSNA (via microelectrodes inserted into the common peroneal nerve), during observation of exercise from the first person point of view. It was hypothesized that the moving stimuli would produce robust compensatory increases in the above-mentioned parameters as effectively as those generated by mental imagery and—to a lesser extent—actual exercise. Nine subjects watched a first-person running video, allowing them to view the action from the perspective of the runner rather than viewing someone else perform the exercise. On average, statistically significant increases from baseline during the running phase were seen in heart rate, respiratory rate, skin blood flow, and burst amplitude of MSNA. These results suggest that observation of exercise in the first person is a strong enough stimulus to evoke “physiologically appropriate” autonomic responses that have a purely psychogenic origin.

Low-frequency galvanic vestibular stimulation evokes two peaks of modulation in skin sympathetic nerve activity

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), delivered bilaterally at 0.2-2.0Hz, evokes a potent entrainment of sympathetic outflow to muscle and skin. Most recently, we showed that stimulation at 0.08-0.18Hz generates two bursts of modulation of muscle sympathetic nerve activity (MSNA), more pronounced at 0.08Hz, which we interpreted as reflecting bilateral projections from the vestibular nuclei to the medullary nuclei responsible for the generation of MSNA. Here, we test the hypothesis that these very low frequencies of sGVS modulate skin sympathetic nerve activity (SSNA) in a similar fashion. SSNA was recorded via tungsten microelectrodes inserted into the left common peroneal nerve in 11 awake-seated subjects. Bipolar binaural sGVS (±2mA, 100cycles) was applied to the mastoid processes at 0.08, 0.13 and 0.18Hz. As with MSNA, cross-correlation analysis revealed two bursts of modulation of SSNA for each cycle of stimulation but, unlike MSNA, this modulation was equally pronounced at all frequencies. These results further support our conclusion that bilateral sGVS causes cyclical modulation of the left and right vestibular nerves and a resultant modulation of sympathetic outflow that reflects the summed activity of bilateral projections from the vestibular nuclei onto, in this case, the primary output nuclei responsible for SSNA-the medullary raphé. Furthermore, these findings emphasise the role of the vestibular system in the control of skin sympathetic outflow, and the cutaneous expression of motion sickness: pallor and sweat release. Indeed, vestibular modulation of SSNA was higher in those subjects reporting nausea than in those who did not report nausea during this low-frequency sGVS.