Gregory J. Gerling

Merkel Cells are Essential for Light Touch Responses

Mysterious Merkel cells Anatomists have known about the existence of Merkel cells in our skin for over a century. However, the function of these cells has been unclear and controversial. To solve this mystery, Maricich et al. (p. 1580) created a genetic deletion of Merkel cells. When Atoh1, a transcription factor expressed by Merkel cells, was conditionally deleted in the skin, Merkel cells were completely absent from Atoh1CKO mice. Using ex vivo skin and nerve preparations from the animals showed that Merkel cells are needed to properly encode light touch sensation in the skin. In the skin, special cells with specific properties endow their afferent nerves with the ability to resolve fine spatial details. The peripheral nervous system detects different somatosensory stimuli, including pain, temperature, and touch. Merkel cell-neurite complexes are touch receptors composed of sensory afferents and Merkel cells. The role that Merkel cells play in light-touch responses has been the center of controversy for over 100 years. We used Cre-loxP technology to conditionally delete the transcription factor Atoh1 from the body skin and foot pads of mice. Merkel cells are absent from these areas in Atoh1CKO animals. Ex vivo skin/nerve preparations from Atoh1CKO animals demonstrate complete loss of the characteristic neurophysiologic responses normally mediated by Merkel cell-neurite complexes. Merkel cells are, therefore, required for the proper encoding of Merkel receptor responses, suggesting that these cells form an indispensible part of the somatosensory system.

The Regularity of Sustained Firing Reveals Two Populations of Slowly Adapting Touch Receptors in Mouse Hairy Skin

Touch is initiated by diverse somatosensory afferents that innervate the skin. The ability to manipulate and classify receptor subtypes is prerequisite for elucidating sensory mechanisms. Merkel cell-neurite complexes, which distinguish shapes and textures, are experimentally tractable mammalian touch receptors that mediate slowly adapting type I (SAI) responses. The assessment of SAI function in mutant mice has been hindered because previous studies did not distinguish SAI responses from slowly adapting type II (SAII) responses, which are thought to arise from different end organs, such as Ruffini endings. Thus we sought methods to discriminate these afferent types. We developed an epidermis-up ex vivo skin-nerve chamber to record action potentials from afferents while imaging Merkel cells in intact receptive fields. Using model-based cluster analysis, we found that two types of slowly adapting receptors were readily distinguished based on the regularity of touch-evoked firing patterns. We identified these clusters as SAI (coefficient of variation = 0.78 +/- 0.09) and SAII responses (0.21 +/- 0.09). The identity of SAI afferents was confirmed by recording from transgenic mice with green fluorescent protein-expressing Merkel cells. SAI receptive fields always contained fluorescent Merkel cells (n = 10), whereas SAII receptive fields lacked these cells (n = 5). Consistent with reports from other vertebrates, mouse SAI and SAII responses arise from afferents exhibiting similar conduction velocities, receptive field sizes, mechanical thresholds, and firing rates. These results demonstrate that mice, like other vertebrates, have two classes of slowly adapting light-touch receptors, identify a simple method to distinguish these populations, and extend the utility of skin-nerve recordings for genetic dissection of touch receptor mechanisms.

Predicting SA-I mechanoreceptor spike times with a skin-neuron model.

Slowly adapting type I (SA-I) mechanoreceptors encode the edges and curvature of touched objects by generating neural spikes in response to indentation of the skin. Beneath this general input-output relationship, models are of great utility for understanding the sub-processes, as SA-I transduction sites are inaccessible to whole-cell recording. This work develops and validates a SA-I skin-receptor model that combines a finite element model of skin mechanics, a sigmoidal function of transduction, and a leaky integrate-and-fire model of neural dynamics. The model produced a R(2)=0.80 goodness of fit between predicted and observed firing rates for 3 and 5mm grating stimuli. In addition, modulation indices of predicted firing rates for 3 and 5mm gratings are 0.46 and 0.59, respectively, compared to the 0.71 and 0.72 found in vivo. An analysis of predicted first spikes indicates their latency may also be enhanced by edges, as edge proximity shortened first spike latencies by 26.2 and 41.8 ms for the 3 and 5mm gratings, respectively. The model described here bridges the gap between those models that transform sustained indentation to firing rates and those that transform vibration to spike times.

Computation identifies structural features that govern neuronal firing properties in slowly adapting touch receptors

Touch is encoded by cutaneous sensory neurons with diverse morphologies and physiological outputs. How neuronal architecture influences response properties is unknown. To elucidate the origin of firing patterns in branched mechanoreceptors, we combined neuroanatomy, electrophysiology and computation to analyze mouse slowly adapting type I (SAI) afferents. These vertebrate touch receptors, which innervate Merkel cells, encode shape and texture. SAI afferents displayed a high degree of variability in touch-evoked firing and peripheral anatomy. The functional consequence of differences in anatomical architecture was tested by constructing network models representing sequential steps of mechanosensory encoding: skin displacement at touch receptors, mechanotransduction and action-potential initiation. A systematic survey of arbor configurations predicted that the arrangement of mechanotransduction sites at heminodes is a key structural feature that accounts in part for an afferent’s firing properties. These findings identify an anatomical correlate and plausible mechanism to explain the driver effect first described by Adrian and Zotterman. DOI: http://dx.doi.org/10.7554/eLife.01488.001

Hyperelastic Material Properties of Mouse Skin under Compression

The skin is a dynamic organ whose complex material properties are capable of withstanding continuous mechanical stress while accommodating insults and organism growth. Moreover, synchronized hair cycles, comprising waves of hair growth, regression and rest, are accompanied by dramatic fluctuations in skin thickness in mice. Whether such structural changes alter skin mechanics is unknown. Mouse models are extensively used to study skin biology and pathophysiology, including aging, UV-induced skin damage and somatosensory signaling. As the skin serves a pivotal role in the transfer function from sensory stimuli to neuronal signaling, we sought to define the mechanical properties of mouse skin over a range of normal physiological states. Skin thickness, stiffness and modulus were quantitatively surveyed in adult, female mice (Mus musculus). These measures were analyzed under uniaxial compression, which is relevant for touch reception and compression injuries, rather than tension, which is typically used to analyze skin mechanics. Compression tests were performed with 105 full-thickness, freshly isolated specimens from the hairy skin of the hind limb. Physiological variables included body weight, hair-cycle stage, maturity level, skin site and individual animal differences. Skin thickness and stiffness were dominated by hair-cycle stage at young (6–10 weeks) and intermediate (13–19 weeks) adult ages but by body weight in mature mice (26–34 weeks). Interestingly, stiffness varied inversely with thickness so that hyperelastic modulus was consistent across hair-cycle stages and body weights. By contrast, the mechanics of hairy skin differs markedly with anatomical location. In particular, skin containing fascial structures such as nerves and blood vessels showed significantly greater modulus than adjacent sites. Collectively, this systematic survey indicates that, although its structure changes dramatically throughout adult life, mouse skin at a given location maintains a constant elastic modulus to compression throughout normal physiological stages.

Fingerprint lines may not directly affect SA-I mechanoreceptor response

Understanding how skin microstructure affects slowly adapting type I (SA-I) mechanoreceptors in encoding edge discontinuities is fundamental to understanding our sense of touch. Skin microstructure, in particular papillary ridges, has been thought to contribute to edge and gap sensation. Caunas 1954 model of touch sensibility describes a functional relationship between papillary ridges and edge sensation. His lever arm model proposes that the papillary ridge (exterior fingerprint line) and underlying intermediate ridge operate as a single unit, with the intermediate ridge acting as a lever which magnifies indentation imposed at the papillary ridge. This paper contests the validity of the lever arm model. While correctly representing the anatomy, this mechanism inaccurately characterizes the function of the papillary ridges. Finite element analysis and assessment of the critical anatomy indicate that papillary ridges have little direct effect on how SA-I receptors respond to the indentation of static edges. Our analysis supports a revised (stiff shell–elastic bending support) interpretation where the epidermis is split into two major layers with a stiff, deformable shell over an elastic bending support. Recent physiological, electrophysiological, and psychophysical findings support our conclusion that the function of the intermediate ridge is distinct from the function of the papillary ridge.

Material characterization of ex vivo prostate tissue via spherical indentation in the clinic

BACKGROUND The mechanical characterization of prostate tissue has not received much attention and is often disconnected from the clinic, where samples are readily attained. METHODS We developed a spherical indenter for the clinic to generate force-displacement data from ex vivo prostate tissue. Indentation velocity, depth, and sphere diameter, and four means of estimating elastic modulus (EM) were validated. EM was then estimated for 26 prostate specimens obtained via prostatectomy and 6 samples obtained from autopsy. Prostatectomy prostates were evaluated clinically upon digital rectal exam and pathologically post-extirpation. FINDINGS Whole-mount measurements yielded median EM of 43.2 kPa (SD=59.8 kPa). Once sliced into cross-sections, median EM for stage T2 and T3 glands were 30.9 and 71.0 kPa, respectively, but not significantly different. Furthermore, we compared within-organ EM difference for prostates with (median=46.5 kPa, SD=22.2 kPa) and without (median=31.0 kPa, SD=63.1 kPa) palpable abnormalities. INTERPRETATION This work finds that diseased prostate tissue is stiffer than normal tissue, stiffness increases with disease severity, and large variability exists between samples, even though disease differences within a prostate are detectable. A further study of late-stage cancers would help to strengthen the findings presented in this work.

Mechanical modeling of a wrinkled fingertip immersed in water

Fingertips often wrinkle after extended exposure to water. The underlying mechanics issues, in particular the critical parameters governing the wrinkled morphology, are studied by using both finite element simulation and analytical modeling. The wrinkling behaviors, characterized by the wrinkle-to-wrinkle distance (wavelength), wrinkle depth (amplitude) and critical wrinkling stress/strain, are investigated as the geometry and material parameters of the fingertip are varied. A simple reduced model is employed to understand the effect of finger curvature and skin thickness, whereas a more refined full anatomical model provides the basis for analyzing the effect of a multilayered skin structure. The simulation results demonstrate that the stiffness of the stratum corneum and the dermal layer in the skin has a large effect on the wrinkling behavior, which agrees well with the analytical predictions. From the uncovered mechanical principles, potential ways for effectively slowing down and suppressing skin wrinkles are proposed; among them, increasing the modulus of the dermal layer in the skin appears to be the most effective.

Augmented, Pulsating Tactile Feedback Facilitates Simulator Training of Clinical Breast Examinations

Haptic training devices can facilitate tactile skill development by providing repeatable exposures to rare stimuli. Extant haptic training simulator research primarily emphasizes realistic stimuli representation; however, the experiments reported herein suggest that providing augmented feedback can improve training effectiveness, even when the feedback is not natural. A novel clinical breast examination training device uses inflated balloons embedded in silicone to simulate breast lumps. Oscillating the balloon water pressure makes the lumps pulsate. The pulsating lumps are easier to detect than the static lumps used in current simulators, and this manipulation seems to effectively introduce trainees to small, deep lumps that are initially difficult to perceive. A study of 48 medical students indicates that training with the dynamic breast model increased the number of lumps detected, F(1, 47) = 9.34, p = .004, decreased the number of false positives, F(1, 47) = 5.78, p = .020, and improved intersimulator skill transfer, F(1, 47) = 26.56, p < .001. The results suggest that at least in this case, augmented, tactile feedback increases training effectiveness, despite the fact that the feedback does not attempt to mimic any physical phenomenon present in the natural stimulus. Applications of this research include training techniques and tools for improved detection of palpable cancers.

Providing a sense of touch to prosthetic hands.

Summary: Each year, approximately 185,000 Americans suffer the devastating loss of a limb. The effects of upper limb amputations are profound because a person’s hands are tools for everyday functioning, expressive communication, and other uniquely human attributes. Despite the advancements in prosthetic technology, current upper limb prostheses are still limited in terms of complex motor control and sensory feedback. Sensory feedback is critical to restoring full functionality to amputated patients because it would relieve the cognitive burden of relying solely on visual input to monitor motor commands and provide tremendous psychological benefits. This article reviews the latest innovations in sensory feedback and argues in favor of peripheral nerve interfaces. First, the authors examine the structure of the peripheral nerve and its importance in the development of a sensory interface. Second, the authors discuss advancements in targeted muscle reinnervation and direct neural stimulation by means of intraneural electrodes. The authors then explore the future of prosthetic sensory feedback using innovative technologies for neural signaling, specifically, the sensory regenerative peripheral nerve interface and optogenetics. These breakthroughs pave the way for the development of a prosthetic limb with the ability to feel.