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

A novel path to chronic proprioceptive disability with oxaliplatin: Distortion of sensory encoding

Persistent neurotoxic side effects of oxaliplatin (OX) chemotherapy, including sensory ataxia, limit the efficacy of treatment and significantly diminish patient quality of life. The common explanation for neurotoxicity is neuropathy, however the degree of neuropathy varies greatly among patients and appears insufficient in some cases to fully account for disability. We recently identified an additional mechanism that might contribute to sensory ataxia following OX treatment. In the present study, we tested whether that mechanism, selective modification of sensory signaling by muscle proprioceptors might result in behavioral deficits in rats. OX was administered once per week for seven weeks (cumulative dose i.p. 70mg/kg) to adult female Wistar rats. Throughout and for three weeks following treatment, behavioral analysis was performed daily on OX and sham control rats. Compared to controls, OX rats demonstrated errors in placing their hind feet securely and/or correctly during a horizontal ladder rung task. These behavioral deficits occurred together with modification of proprioceptor signaling that eliminated sensory encoding of static muscle position while having little effect on encoding of dynamic changes in muscle length. Selective inability to sustain repetitive firing in response to static muscle stretch led us to hypothesize that OX treatment impairs specific ionic currents, possibly the persistent inward Na currents (NaPIC) that are known to support repetitive firing during static stimulation in several neuron types, including the class of large diameter dorsal root ganglion cells that includes muscle proprioceptors. We tested this hypothesis by determining whether the chronic effects of OX on the firing behavior of muscle proprioceptors in vivo were mimicked by acute injection of NaPIC antagonists. Both riluzole and phenytoin, each having multiple drug actions but having only antagonist action on NaPIC in common, reproduced selective modification of proprioceptor signaling observed in OX rats. Taken together, these findings lead us to propose that OX chemotherapy contributes to movement disability by modifying sensory encoding, possibly via a chronic neurotoxic effect on NaPIC in the sensory terminals of muscle proprioceptors.

Computational modeling indicates that surface pressure can be reliably conveyed to tactile receptors even amidst changes in skin mechanics

Distinct patterns in neuronal firing are observed between classes of cutaneous afferents. Such differences may be attributed to end-organ morphology, distinct ion-channel complements, and skin microstructure, among other factors. Even for just the slowly adapting type I afferent, the skin's mechanics for a particular specimen might impact the afferent's firing properties, especially given the thickness and elasticity of skin can change dramatically over just days. Here, we show computationally that the skin can reliably convey indentation magnitude, rate, and spatial geometry to the locations of tactile receptors even amid changes in skin's structure. Using finite element analysis and neural dynamics models, we considered the skin properties of six mice that span a representative cohort. Modeling the propagation of the surface stimulus to the interior of the skin demonstrated that there can be large variance in stresses and strains near the locations of tactile receptors, which can lead to large variance in static firing rate. However, variance is significantly reduced when the stimulus tip is controlled by surface pressure and compressive stress is measured near the end organs. This particular transformation affords the least variability in predicted firing rates compared with others derived from displacement, force, strain energy density, or compressive strain. Amid changing skin mechanics, stimulus control by surface pressure may be more naturalistic and optimal and underlie how animals actively explore the tactile environment.

Investigation of mechanosensation in C. elegans using light field calcium imaging

We describe a new experimental approach to investigate touch sensation in the model organism C. elegans using light field deconvolution microscopy. By combining fast volumetric image acquisition with controlled indentation of the organism using a high sensitivity force transducer, we are able to simultaneously measure activity in multiple touch receptor neurons expressing the calcium ion indicator GCaMP6s. By varying the applied mechanical stimulus we show how this method can be used to quantify touch sensitivity in C. elegans. We describe some of the challenges of performing light field calcium imaging in moving samples and demonstrate that they can be overcome by simple data processing.

Genetic Identification of an Expansive Mechanoreceptor Sensitive to Skin Stroking

Touch perception begins with activation of low-threshold mechanoreceptors (LTMRs) in the periphery. LTMR terminals exhibit tremendous morphological heterogeneity that specifies their mechanical receptivity. In a survey of mammalian skin, we found a preponderance of neurofilament-heavy-chain(+) circumferential endings associated with hair follicles, prompting us to develop a genetic strategy to interrogate these neurons. Targeted in vivo recordings revealed them to be Aβ field-LTMRs, identified 50 years ago but largely elusive thereafter. Remarkably, while Aβ field-LTMRs are highly sensitive to gentle stroking of the skin, they are unresponsive to hair deflection, and they encode skin indentation in the noxious range across large, spotty receptive fields. Individual Aβ field-LTMRs form up to 180 circumferential endings, making them the most anatomically expansive LTMR identified to date. Thus, Aβ field-LTMRs are a major mammalian LTMR subtype that forms circumferential endings in hairy skin, and their sensitivity to gentle skin stroking arises through integration across many low-sensitivity circumferential endings.

Emergence and coherence of oscillations in star networks of stochastic excitable elements

We study the emergence and coherence of stochastic oscillations in star networks of excitable elements in which peripheral nodes receive independent random inputs. A biophysical model of a distal branch of sensory neuron in which peripheral nodes of Ranvier are coupled to a central node by myelinated cable segments is used along with a generic model of networked stochastic active rotators. We show that coherent oscillations can emerge due to stochastic synchronization of peripheral nodes and that the degree of coherence can be maximized by tuning the coupling strength and the size of the network. Analytical results are obtained for the strong-coupling regime of the active rotator network. In particular, we show that in the strong-coupling regime, the network dynamics can be described by an effective single active rotator with rescaled parameters and noise.

Somatosensory substrates of flight control in bats

Flight maneuvers require rapid sensory integration to generate adaptive motor output. Bats achieve remarkable agility with modified forelimbs that serve as airfoils while retaining capacity for object manipulation. Wing sensory inputs provide behaviorally relevant information to guide flight; however, components of wing sensory-motor circuits have not been analyzed. Here, we elucidate the organization of wing innervation in an insectivore, the big brown bat, Eptesicus fuscus. We demonstrate that wing sensory innervation differs from other vertebrate forelimbs, revealing a peripheral basis for the atypical topographic organization reported for bat somatosensory nuclei. Furthermore, the wing is innervated by an unusual complement of sensory neurons poised to report airflow and touch. Finally, we report that cortical neurons encode tactile and airflow inputs with sparse activity patterns. Together, our findings identify neural substrates of somatosensation in the bat wing and imply that evolutionary pressures giving rise to mammalian flight led to unusual sensorimotor projections.

Compressive viscoelasticity of freshly excised mouse skin is dependent on specimen thickness, strain level and rate

Although the skin’s mechanical properties are well characterized in tension, little work has been done in compression. Here, the viscoelastic properties of a population of mouse skin specimens (139 samples from 36 mice, aged 5 to 34 weeks) were characterized upon varying specimen thickness, as well as strain level and rate. Over the population, we observed the skin’s viscoelasticity to be quite variable, yet found systematic correlation of residual stress ratio with skin thickness and strain, and of relaxation time constants with strain rates. In particular, as specimen thickness ranged from 211 to 671 μm, we observed significant variation in both quasi-linear viscoelasticity (QLV) parameters, the relaxation time constant (τ₁ = 0.19 ± 0.10 s) and steady-state residual stress ratio (G_∞ = 0.28 ± 0.13). Moreover, when τ₁ was decoupled and fixed, we observed that G_∞ positively correlated with skin thickness. Second, as steady-state stretch was increased (λ_∞ from 0.22 to 0.81), we observed significant variation in both QLV parameters (τ₁ = 0.26 ± 0.14 s, G_∞ = 0.47 ± 0.17), and when τ₁ was fixed, G_∞ positively correlated with stretch level. Third, as strain rate was increased from 0.06 to 22.88 s⁻¹, the median time constant τ₁ varied from 1.90 to 0.31 s, and thereby negatively correlated with strain rate. These findings indicate that the natural range of specimen thickness, as well as experimental controls of compression level and rate, significantly influence measurements of skin viscoelasticity.

Mimicking the End Organ Architecture of Slowly Adapting Type I Afferents May Increase the Durability of Artificial Touch Sensors

In effort to mimic the sensitivity and efficient information transfer of natural tactile afferents, recent work has combined force transducers and computational models of mechanosensitive afferents. Sensor durability, another feature important to sensor design, might similarly capitalize upon biological rules. In particular, gains in sensor durability might leverage insight from the compound end organ of the slowly adapting type I afferent, especially its multiple sites of spike initiation that reset each other. This work develops models of compound spiking sensors using a computational network of transduction functions and leaky integrate and fire models (together a spike encoder, the software element of a compound spiking sensor), informed by the output of an existing force transducer (hardware sensing elements of a compound spiking sensor). Individual force transducer failures are simulated with and without resetting between spike encoders to test the importance of both resetting and configuration on system durability. The results indicate that the resetting of adjacent spike encoders, upon the firing of a spike by any one, is an essential mechanism to maintain a stable overall response in the midst of transducer failure. Furthermore, results suggest that when resetting is enabled, the durability of a compound sensor is maximized when individual transducers are paired with spike encoders and multiple, paired units are employed. To explore these ideas more fully, use cases examine the design of a compound sensor to either reach a target lifetime with a set probability or determine how often to schedule maintenance to control the probability of failure.

Novel method to assess axonal excitability using channelrhodopsin-based photoactivation

Measuring the excitability of individual axons is complicated by the prohibitive difficulty in obtaining intracellular recordings. Here, we present an innovative methodology that enables local excitability to be measured anywhere in a channelrhodopsin (ChR2)-expressing neuron. The approach hinges on activating ChR2 in a spatially and temporally precise manner while recording the resulting spike train from a remote site. We validated this approach in primary afferent neurons (PANs). Initial encoding of somatosensory stimuli relies on transduction of the physical stimulus into a receptor potential and transformation of the receptor potential into a spike train; the transformation process depends on the excitability of the most distal PAN endings but, as explained above, is extraordinarily difficult to study in situ using traditional methods. Using ChR2-based photoactivation, we show 1) that excitability differs between the distal endings and more proximal portions of PAN axons, 2) that the transformation process differs between PANs, and 3) that the transformation process is directly affected by inflammation. Beyond presenting an innovative method by which to study axonal excitability, this study has validated its utility in helping to decipher the earliest stages of somatosensory encoding.

Merkel cells and neurons keep in touch

The Merkel cell-neurite complex is a unique vertebrate touch receptor comprising two distinct cell types in the skin. Its presence in touch-sensitive skin areas was recognized more than a century ago, but the functions of each cell type in sensory transduction have been unclear. Three recent studies demonstrate that Merkel cells are mechanosensitive cells that function in touch transduction via Piezo2. One study concludes that Merkel cells, rather than sensory neurons, are principal sites of mechanotransduction, whereas two other studies report that both Merkel cells and neurons encode mechanical inputs. Together, these studies settle a long-standing debate on whether or not Merkel cells are mechanosensory cells, and enable future investigations of how these skin cells communicate with neurons.

Segregation of tactile input features in neurons of the cuneate nucleus

Our tactile perception of external objects depends on skin-object interactions. The mechanics of contact dictates the existence of fundamental spatiotemporal input features—contact initiation and cessation, slip, and rolling contact—that originate from the fact that solid objects do not interpenetrate. However, it is unknown whether these features are represented within the brain. We used a novel haptic interface to deliver such inputs to the glabrous skin of finger/digit pads and recorded from neurons of the cuneate nucleus (the brain’s first level of tactile processing) in the cat. Surprisingly, despite having similar receptive fields and response properties, each cuneate neuron responded to a unique combination of these inputs. Hence, distinct haptic input features are encoded already at subcortical processing stages. This organization maps skin-object interactions into rich representations provided to higher cortical levels and may call for a re-evaluation of our current understanding of the brain’s somatosensory systems.

•

Specific haptic input features were selectively delivered to glabrous skin

•

The input features were segregated in the neurons of the cuneate nucleus

•

These observations may call for a shift in current views of tactile processing

Mechanotransduction in epidermal Merkel cells

The cellular and molecular basis of vertebrate touch reception remains least understood among the traditional five senses. Somatosensory afferents that innervate the skin encode distinct tactile qualities, such as flutter, slip, and pressure. Gentle touch is thought to be transduced by somatosensory afferents whose tactile end organs selectively filter mechanical stimuli. These tactile end organs comprise afferent terminals in association with non-neuronal cell types such as Merkel cells, keratinocytes, and Schwann cells. An open question is whether these non-neuronal cells serve primarily as passive mechanical filters or whether they actively participate in mechanosensory transduction. This question has been most extensively studied in Merkel cells, which are epidermal cells that complex with sensory afferents in regions of high tactile acuity such as fingertips, whisker follicles, and touch domes. Merkel cell-neurite complexes mediate slowly adapting type I (SAI) responses, which encode sustained pressure and represent object features with high fidelity. How Merkel cells contribute to unique SAI firing patterns has been debated for decades; however, three recent studies in rodent models provide some direct answers. First, whole-cell recordings demonstrate that Merkel cells are touch-sensitive cells with fast, mechanically activated currents that require Piezo2. Second, optogenetics and intact recordings show that Merkel cells mediate sustained SAI firing. Finally, loss-of-function studies in transgenic mouse models reveal that SAI afferents are also touch sensitive. Together, these studies identify molecular mechanisms of mechanotransduction in Merkel cells, reveal unexpected functions for these cells in touch, and support a revised, two-receptor site model of mechanosensory transduction.

Epidermal Merkel cells are mechanosensory cells that tune mammalian touch receptors

Touch submodalities, such as flutter and pressure, are mediated by somatosensory afferents whose terminal specializations extract tactile features and encode them as action potential trains with unique activity patterns. Whether non-neuronal cells tune touch receptors through active or passive mechanisms is debated. Terminal specializations are thought to function as passive mechanical filters analogous to the cochlea's basilar membrane, which deconstructs complex sounds into tones that are transduced by mechanosensory hair cells. The model that cutaneous specializations are merely passive has been recently challenged because epidermal cells express sensory ion channels and neurotransmitters; however, direct evidence that epidermal cells excite tactile afferents is lacking. Epidermal Merkel cells display features of sensory receptor cells and make 'synapse-like' contacts with slowly adapting type I (SAI) afferents. These complexes, which encode spatial features such as edges and texture, localize to skin regions with high tactile acuity, including whisker follicles, fingertips and touch domes. Here we show that Merkel cells actively participate in touch reception in mice. Merkel cells display fast, touch-evoked mechanotransduction currents. Optogenetic approaches in intact skin show that Merkel cells are both necessary and sufficient for sustained action-potential firing in tactile afferents. Recordings from touch-dome afferents lacking Merkel cells demonstrate that Merkel cells confer high-frequency responses to dynamic stimuli and enable sustained firing. These data are the first, to our knowledge, to directly demonstrate a functional, excitatory connection between epidermal cells and sensory neurons. Together, these findings indicate that Merkel cells actively tune mechanosensory responses to facilitate high spatio-temporal acuity. Moreover, our results indicate a division of labour in the Merkel cell-neurite complex: Merkel cells signal static stimuli, such as pressure, whereas sensory afferents transduce dynamic stimuli, such as moving gratings. Thus, the Merkel cell-neurite complex is an unique sensory structure composed of two different receptor cell types specialized for distinct elements of discriminative touch.

Piezo2 is required for Merkel-cell mechanotransduction

How we sense touch remains fundamentally unknown^1,2. The Merkel cell-neurite complex is a gentle touch receptor in the skin that mediates slowly-adapting (SA) responses of Aβ sensory fibers to encode fine details of objects^3-6. This mechanoreceptor complex was recognized to play an essential role in sensing gentle touch nearly 50 years ago^3,4. However, whether Merkel cells or afferent fibers themselves sense mechanical force is still debated, and the molecular mechanism of mechanotransduction is unknown^1,2,7-12. Interestingly, synapse-like junctions are observed between Merkel cells and associated afferents^6,13-15, and yet it is unclear if Merkel cells are inherently mechanosensitive or whether they can rapidly transmit such information to the neighboring nerve^1,2,16,17. Here we show for the first time that Merkel cells produce touch-sensitive currents in vitro. Piezo2, a mechanically-activated (MA) cation channel, is expressed in Merkel cells. We engineered mice deficient in Piezo2 in the skin, but not in sensory neurons, and show that Merkel cell mechanosensitivity completely depends on Piezo2. In these mice, Merkel cell-neurite complex-mediated SA responses in vivo show reduced static firing rates, and moreover, they display moderately decreased behavioral responses to gentle touch. Our results indicate that Piezo2 is the Merkel cell mechanotransduction channel and provide the first line of evidence that Piezos play a physiological role in mechanosensation in mammals. Furthermore, our data present evidence for a two-receptor site model, where both Merkel cells and innervating afferents act in concert as mechanosensors. The two-receptor system could provide this mechanoreceptor complex with a tuning mechanism to achieve highly sophisticated responses to a given mechanical stimulus^15,18,19.