Similarities and differences in the innervation of mystacial vibrissal follicle-sinus complexes in the rat and cat: a confocal microscopic study

ASICs mediate fast excitatory synaptic transmission for tactile discrimination

Tactile discrimination, the ability to differentiate objects' physical properties such as texture, shape, and edges, is essential for environmental exploration, social interaction, and early childhood development. This ability heavily relies on Merkel cell-neurite complexes (MNCs), the tactile end-organs enriched in the fingertips of humans and the whisker hair follicles of non-primate mammals. Although recent studies have advanced our knowledge on mechanical transduction in MNCs, it remains unknown how tactile signals are encoded at MNCs. Here, using rodent whisker hair follicles, we show that tactile signals are encoded at MNCs as fast excitatory synaptic transmission. This synaptic transmission is mediated by acid-sensing ion channels (ASICs) located on the neurites of MNCs, with protons as the principal transmitters. Pharmacological inhibition or genetic deletion of ASICs diminishes the tactile encoding at MNCs and impairs tactile discrimination in animals. Together, ASICs are required for tactile encoding at MNCs to enable tactile discrimination in mammals.

Feline papillomavirus-associated Merkel cell carcinoma: a comparative review with human Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare skin tumor that shares a similar immunophenotype with Merkel cells, although its origin is debatable. More than 80% of human MCC cases are associated with Merkel cell polyomavirus infections and viral gene integration. Recent studies have shown that the clinical and pathological characteristics of feline MCC are comparable to those of human MCC, including its occurrence in aged individuals, aggressive behavior, histopathological findings, and the expression of Merkel cell markers. More than 90% of feline MCC are positive for the Felis catus papillomavirus type 2 (FcaPV2) gene. Molecular changes involved in papillomavirus-associated tumorigenesis, such as increased p16 and decreased retinoblastoma (Rb) and p53 protein levels, were observed in FcaPV2-positive MCC, but not in FcaPV2-negative MCC cases. These features were also confirmed in FcaPV2-positive and -negative MCC cell lines. The expression of papillomavirus E6 and E7 genes, responsible for p53 degradation and Rb inhibition, respectively, was detected in tumor cells by in situ hybridization. Whole genome sequencing revealed the integration of FcaPV2 DNA into the host feline genome. MCC cases often develop concurrent skin lesions, such as viral plaque and squamous cell carcinoma, which are also associated with papillomavirus infection. These findings suggest that FcaPV2 infection and integration of viral genes are involved in the development of MCC in cats. This review provides an overview of the comparative pathology of feline and human MCC caused by different viruses and discusses their cell of origin.

Pre-neuronal processing of haptic sensory cues via dispersive high-frequency vibrational modes

Sense of touch is one of the major perception channels. Neural coding of object textures conveyed by rodents' whiskers has been a model to study early stages of haptic information uptake. While high-precision spike timing has been observed during whisker sweeping across textured surfaces, the exact nature of whisker micromotions that spikes encode remains elusive. Here, we discovered that a single micro-collision of a whisker with surface features generates vibrational eigenmodes spanning frequencies up to 10 kHz. While propagating along the whisker, these high-frequency modes can carry up to 80% of shockwave energy, exhibit 100× smaller damping ratio, and arrive at the follicle 10× faster than low frequency components. The mechano-transduction of these energy bursts into time-sequenced population spike trains may generate temporally unique "bar code" with ultra-high information capacity. This hypothesis of pre-neuronal processing of haptic signals based on dispersive temporal separation of the vibrational modal frequencies can shed light on neural coding of haptic signals in many whisker-like sensory organs across the animal world as well as in texture perception in primate's glabrous skin.

A nanonewton-scale biomimetic mechanosensor

Biomimetic mechanosensors have profound implications for various areas, including health care, prosthetics, human‒machine interfaces, and robotics. As one of the most important parameters, the sensitivity of mechanosensors is intrinsically determined by the detection resolution to mechanical force. In this manuscript, we expand the force detection resolution of current biomimetic mechanosensors from the micronewton to nanonewton scale. We develop a nanocrack-based electronic whisker-type mechanosensor that has a detection resolution of 72.2 nN. We achieve the perception of subtle mechanical stimuli, such as tiny objects and airflow, and the recognition of surface morphology down to a 30 nm height, which is the finest resolution ever reported in biomimetic mechanosensors. More importantly, we explore the use of this mechanosensor in wearable devices for sensing gravity field orientation with respect to the body, which has not been previously achieved by these types of sensors. We develop a wearable smart system for sensing the body's posture and movements, which can be used for remote monitoring of falls in elderly people. In summary, the proposed device offers great advantages for not only improving sensing ability but also expanding functions and thus can be used in many fields not currently served by mechanosensors.

DiI-CT-A bimodal neural tracer for X-ray and fluorescence imaging

Here, we present an X-ray-visible neural tracer, referred to as DiI-CT, which is based on the well-established lipophilic indocarbocyanine dye DiI, to which we conjugated two iodine atoms. The tracer is visible with microfocus computed tomography (microCT) imaging and shares the excellent fluorescent tracing properties of DiI. We document the discovery potential of DiI-CT by analyzing the vibrissa follicle-sinus complex, a structure where visual access is poor and 3D tissue structure matters and reveal innervation patterns of the intact follicle in unprecedented detail. In the brain, DiI-CT tracing holds promise for verification evaluation of indirect connectivity measures, such as diffusion tensor imaging. We conclude that the bimodal dye DiI-CT opens new avenues for neuroanatomy.

The functional anatomy of elephant trunk whiskers

Behavior and innervation suggest a high tactile sensitivity of elephant trunks. To clarify the tactile trunk periphery we studied whiskers with the following findings. Whisker density is high at the trunk tip and African savanna elephants have more trunk tip whiskers than Asian elephants. Adult elephants show striking lateralized whisker abrasion caused by lateralized trunk behavior. Elephant whiskers are thick and show little tapering. Whisker follicles are large, lack a ring sinus and their organization varies across the trunk. Follicles are innervated by ~90 axons from multiple nerves. Because elephants don't whisk, trunk movements determine whisker contacts. Whisker-arrays on the ventral trunk-ridge contact objects balanced on the ventral trunk. Trunk whiskers differ from the mobile, thin and tapered facial whiskers that sample peri-rostrum space symmetrically in many mammals. We suggest their distinctive features-being thick, non-tapered, lateralized and arranged in specific high-density arrays-evolved along with the manipulative capacities of the trunk.

On the intrinsic curvature of animal whiskers

Facial vibrissae (whiskers) are thin, tapered, flexible, hair-like structures that are an important source of tactile sensory information for many species of mammals. In contrast to insect antennae, whiskers have no sensors along their lengths. Instead, when a whisker touches an object, the resulting deformation is transmitted to mechanoreceptors in a follicle at the whisker base. Previous work has shown that the mechanical signals transmitted along the whisker will depend strongly on the whisker's geometric parameters, specifically on its taper (how diameter varies with arc length) and on the way in which the whisker curves, often called "intrinsic curvature." Although previous studies have largely agreed on how to define taper, multiple methods have been used to quantify intrinsic curvature. The present work compares and contrasts different mathematical approaches towards quantifying this important parameter. We begin by reviewing and clarifying the definition of "intrinsic curvature," and then show results of fitting whisker shapes with several different functions, including polynomial, fractional exponent, elliptical, and Cesàro. Comparisons are performed across ten species of whiskered animals, ranging from rodents to pinnipeds. We conclude with a discussion of the advantages and disadvantages of using the various models for different modeling situations. The fractional exponent model offers an approach towards developing a species-specific parameter to characterize whisker shapes within a species. Constructing models of how the whisker curves is important for the creation of mechanical models of tactile sensory acquisition behaviors, for studies of comparative evolution, morphology, and anatomy, and for designing artificial systems that can begin to emulate the whisker-based tactile sensing of animals.

Effects of potassium channel modulators on the responses of mammalian slowly adapting mechanoreceptors

Introduction

slowly adapting mechanoreceptors in the skin provide vital tactile information to animals. The ionic channels that underlie their functioning is the subject of intense research. Previous work suggests that potassium channels may play particular roles in the activation and firing of these mechanoreceptors.

Objective

We used a range of potassium channel blockers and openers to observe their effects on different phases of mechanoreceptor responses.

Methods

Extracellular recording of neural activity of slowly adapting mechanoreceptors was carried out in an in vitro preparation of the sinus hair follicles taken from rat whisker pads. A range of potassium (K⁺) channel modulators were tested on these mechanoreceptor responses. The channel blockers tested were: tetraethylammonium (TEA), barium chloride (BaCl₂), dequalinium, 4-aminopyridine (4-AP), paxilline, XE 991, apamin, and charybdotoxin.

Results

Except for charybdotoxin and apamin, these drugs increased the activity of both types of slowly adapting units, St I and St II. Generally, both spontaneous and evoked (dynamic and static) activities increased. The channel opener NS1619 was also tested. NS1619 clearly decreased evoked activity (both dynamic and static) while leaving spontaneous activity relatively unaffected, with no clear discrimination of effects on the two types of St receptor

Conclusion

These findings are consistent with the targets of the drugs suggesting that K⁺ channels play an important role in the maintenance of spontaneous firing and in the production of and persistence of mechanoreceptor activity.

Demonstration of three-dimensional contact point determination and contour reconstruction during active whisking behavior of an awake rat

The rodent vibrissal (whisker) system has been studied for decades as a model of active touch sensing. There are no sensors along the length of a whisker; all sensing occurs at the whisker base. Therefore, a large open question in many neuroscience studies is how an animal could estimate the three-dimensional (3D) location at which a whisker makes contact with an object. In the present work we simulated the shape of a real rat whisker to demonstrate the existence of several unique mappings from triplets of mechanical signals at the whisker base to the three-dimensional whisker-object contact point. We then used high speed video to record whisker deflections as an awake rat whisked against a peg, and used the mechanics resulting from those deflections to extract the contact points along the peg surface. These results demonstrate that measurement of specific mechanical triplets at the base of a biological whisker can enable 3D contact point determination during natural whisking behavior. The approach is viable even though the biological whisker has non-ideal, non-planar curvature, and even given the rat’s real-world choices of whisking parameters. Visual intuition for the quality of the approach is provided in a video that shows the contour of the peg gradually emerging during active whisking behavior.

Reducing Merkel cell activity in the whisker follicle disrupts cortical encoding of whisker movement amplitude and velocity

Merkel cells (MCs) and associated primary sensory afferents of the whisker follicle-sinus complex, accurately code whisker self-movement, angle, and whisk phase during whisking. However, little is known about their roles played in cortical encoding of whisker movement. To this end, the spiking activity of primary somatosensory barrel cortex (wS1) neurons was measured in response to varying the whisker deflection amplitude and velocity in transgenic mice with previously established reduced mechanoelectrical coupling at MC-associated afferents. Under reduced MC activity, wS1 neurons exhibited increased sensitivity to whisker deflection. This appeared to arise from a lack of variation in response magnitude to varying the whisker deflection amplitude and velocity. This latter effect was further indicated by weaker variation in the temporal profile of the evoked spiking activity when either whisker deflection amplitude or velocity was varied. Nevertheless, under reduced MC activity, wS1 neurons retained the ability to differentiate stimulus features based on the timing of their first post-stimulus spike. Collectively, results from this study suggest that MCs contribute to cortical encoding of both whisker amplitude and velocity, predominantly by tuning wS1 response magnitude, and by patterning the evoked spiking activity, rather than by tuning wS1 response latency.

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The role of Merkel cells (MCs) in cortical encoding of whisker deflection amplitude and velocity was investigated.

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Reducing MC synaptic activity increased barrel cortex neurons response sensitivity to whisker deflection.

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This effect occurred from a lack of variation in response magnitude to varying whisker deflection amplitude and velocity.

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However, stimuli differentiation through changes in cortical response latency was preserved.

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MCs are thus suggested to play a predominant role in tuning the cortical response magnitude over the response latency.

Anatomy of bristles on the nares and rictus of western barn owls (Tyto alba)

Many nocturnal avian species, such as Strigiformes, Caprimulgiformes and Apterygiformes, have sensitive vibrotactile bristles on their upper bill, especially on their rictus. The anatomy of these bristles can vary, especially in terms of sensitivity (Herbst corpuscle number), bristle length and bristle number. This variation is thought to be associated with foraging – such that diurnal, open foragers have smaller and less‐sensitive bristles. Here, we describe bristle morphology and follicle anatomy in the western barn owl (Tyto alba) for the first time, using both live and roadkill wild owls. We show that T. alba have both narial and rictal bristles that are likely to be vibrotactile, since they have Herbst corpuscles around their follicles. We observed more numerous (~8) and longer bristles (~16 mm) on the nares of T. alba, than on the rictal region (~4 and ~13 mm respectively). However, the narial bristle follicles contained fewer Herbst corpuscles in their surroundings (~5) than the rictal bristles (~7); indicating that bristle length is not indicative of sensitivity. As well as bristle length and number varying between different facial regions, they also varied between individuals, although the cause of this variation remains unclear. Despite this variation, the gross anatomy of facial bristle follicles appears to be conserved between nocturnal Strigiformes, Caprimulgiformes and Apterygiformes. Understanding more about how T. alba use their bristles would, therefore, give us greater insights into the function of avian bristles in general.

A novel stimulator to investigate the tuning of multi-whisker responsive neurons for speed and the direction of global motion: Contact-sensitive moving stimulator for multi-whisker stimulation

BACKGROUND

The rodent vibrissal (whisker) systcnsorimotor integration and active tactile sensing. Experiments on the vibrissal system often require highly repeatable stimulation of multiple whiskers and the ability to vary stimulation parameters across a wide range. The stimulator must also be easy to position and adjust. Developing a multi-whisker stimulation system that meets these criteria remains challenging.

NEW METHOD

We describe a novel multi-whisker stimulator to assess neural selectivity for the direction of global motion. The device can generate repeatable, linear sweeps of tactile stimulation across the whisker array in any direction and with a range of speeds. A fiber optic beam break detects the interval of whisker contact as the stimulator passes through the array.

RESULTS

We demonstrate the device's function and utility by recording from a small number of multi-whisker-responsive neurons in the trigeminal brainstem. Neurons had higher firing rates in response to faster stimulation speeds; some also exhibited strong direction-of-motion tuning.

COMPARISON WITH EXISTING METHODS

The stimulator complements more standard piezo-electric stimulators, which offer precise control but typically stimulate only single whiskers, require whisker trimming, and travel through small angles. It also complements non-contact methods of stimulation such as air-puffs and electromagnetic-induced stimulation. Tradeoffs include stimulation speed and frequency, and the inability to stimulate whiskers individually.

CONCLUSIONS

The stimulator could be used - in either anesthetized or awake, head-fixed preparations - as an approach to studying global motion selectivity of multi-whisker sensitive neurons at multiple levels of the vibrissal-trigeminal system.

Sensory Adaptation in the Whisker-Mediated Tactile System: Physiology, Theory, and Function

In the natural environment, organisms are constantly exposed to a continuous stream of sensory input. The dynamics of sensory input changes with organism's behaviour and environmental context. The contextual variations may induce >100-fold change in the parameters of the stimulation that an animal experiences. Thus, it is vital for the organism to adapt to the new diet of stimulation. The response properties of neurons, in turn, dynamically adjust to the prevailing properties of sensory stimulation, a process known as "neuronal adaptation." Neuronal adaptation is a ubiquitous phenomenon across all sensory modalities and occurs at different stages of processing from periphery to cortex. In spite of the wealth of research on contextual modulation and neuronal adaptation in visual and auditory systems, the neuronal and computational basis of sensory adaptation in somatosensory system is less understood. Here, we summarise the recent finding and views about the neuronal adaptation in the rodent whisker-mediated tactile system and further summarise the functional effect of neuronal adaptation on the response dynamics and encoding efficiency of neurons at single cell and population levels along the whisker-mediated touch system in rodents. Based on direct and indirect pieces of evidence presented here, we suggest sensory adaptation provides context-dependent functional mechanisms for noise reduction in sensory processing, salience processing and deviant stimulus detection, shift between integration and coincidence detection, band-pass frequency filtering, adjusting neuronal receptive fields, enhancing neural coding and improving discriminability around adapting stimuli, energy conservation, and disambiguating encoding of principal features of tactile stimuli.

Continuous, multidimensional coding of 3D complex tactile stimuli by primary sensory neurons of the vibrissal system

Across all sensory modalities, first-stage sensory neurons are an information bottleneck: they must convey all information available for an animal to perceive and act in its environment. Our understanding of coding properties of primary sensory neurons in the auditory and visual systems has been aided by the use of increasingly complex, naturalistic stimulus sets. By comparison, encoding properties of primary somatosensory afferents are poorly understood. Here, we use the rodent whisker system to examine how tactile information is represented in primary sensory neurons of the trigeminal ganglion (Vg). Vg neurons have long been thought to segregate into functional classes associated with separate streams of information processing. However, this view is based on Vg responses to restricted stimulus sets which potentially underreport the coding capabilities of these neurons. In contrast, the current study records Vg responses to complex three-dimensional (3D) stimulation while quantifying the complete 3D whisker shape and mechanics, thereby beginning to reveal their full representational capabilities. The results show that individual Vg neurons simultaneously represent multiple mechanical features of a stimulus, do not preferentially encode principal components of the stimuli, and represent continuous and tiled variations of all available mechanical information. These results directly contrast with proposed codes in which subpopulations of Vg neurons encode select stimulus features. Instead, individual Vg neurons likely overcome the information bottleneck by encoding large regions of a complex sensory space. This proposed tiled and multidimensional representation at the Vg directly constrains the computations performed by more central neurons of the vibrissotrigeminal pathway.

Somatosensory innervation of healthy human oral tissues

The oral somatosensory system relays essential information about mechanical stimuli to enable oral functions such as feeding and speech. The neurochemical and anatomical diversity of sensory neurons across oral cavity sites have not been systematically compared. To address this gap, we analyzed healthy human tongue and hard-palate innervation. Biopsies were collected from 12 volunteers and underwent fluorescent immunohistochemistry (≥2 specimens per marker/structure). Afferents were analyzed for markers of neurons (βIII tubulin), myelinated afferents (neurofilament heavy, NFH), and Merkel cells and taste cells (keratin 20, K20). Hard-palate innervation included Meissner corpuscles, glomerular endings, Merkel cell-neurite complexes, and free nerve endings. The organization of these somatosensory endings is reminiscent of fingertips, suggesting that the hard palate is equipped with a rich repertoire of sensory neurons for pressure sensing and spatial localization of mechanical inputs, which are essential for speech production and feeding. Likewise, the tongue is innervated by afferents that impart it with exquisite acuity and detection of moving stimuli that support flavor construction and speech. Filiform papillae contained end bulbs of Krause, as well as endings that have not been previously reported, including subepithelial neuronal densities, and NFH+ neurons innervating basal epithelia. Fungiform papillae had Meissner corpuscles and densities of NFH+ intraepithelial neurons surrounding taste buds. The differing compositions of sensory endings within filiform and fungiform papillae suggest that these structures have distinct roles in mechanosensation. Collectively, this study has identified previously undescribed neuronal endings in human oral tissues and provides an anatomical framework for understanding oral mechanosensory functions.

Diversity of vibrissal follicle anatomy in cetaceans

Most cetaceans are born with vibrissae but they can be lost or reduced in adulthood, especially in odontocetes. Despite this, some species of odontocetes have been found to have functioning vibrissal follicles (including the follicle itself and any remaining vibrissal hair shaft) that play a role in mechanoreception, proprioception and electroreception. This reveals a greater diversity of vibrissal function in odontocetes than in any other mammalian group. However, we know very little about vibrissal follicle form and function across the Cetacea. Here, we qualitatively describe the gross vibrissal follicle anatomy of fetuses of three species of cetaceans, including two odontocetes: Atlantic white-sided dolphin (Lagenorhynchus acutus), harbour porpoise (Phocoena phocoena), and one mysticete: minke whale (Balaenoptera acutorostrata), and compared our findings to previous anatomical descriptions. All three species had few, short vibrissae contained within a relatively simple, single-part follicle, lacking in muscles. However, we observed differences in vibrissal number, follicle size and shape, and innervation distribution between the species. While all three species had nerve fibers around the follicles, the vibrissal follicles of Balaenoptera acutorostrata were innervated by a deep vibrissal nerve, and the nerve fibers of the odontocetes studied were looser and more branched. For example, in Lagenorhynchus acutus, branches of nerve fibers travelled parallel to the follicle, and innervated more superficial areas, rather than just the base. Our anatomical descriptions lend support to the observation that vibrissal morphology is diverse in cetaceans, and is worth further investigation to fully explore links between form and function.

Conveyance of texture signals along a rat whisker

Neuronal activities underlying a percept are constrained by the physics of sensory signals. In the tactile sense such constraints are frictional stick-slip events, occurring, amongst other vibrotactile features, when tactile sensors are in contact with objects. We reveal new biomechanical phenomena about the transmission of these microNewton forces at the tip of a rat's whisker, where they occur, to the base where they engage primary afferents. Using high resolution videography and accurate measurement of axial and normal forces at the follicle, we show that the conical and curved rat whisker acts as a sign-converting amplification filter for moment to robustly engage primary afferents. Furthermore, we present a model based on geometrically nonlinear Cosserat rod theory and a friction model that recreates the observed whole-beam whisker dynamics. The model quantifies the relation between kinematics (positions and velocities) and dynamic variables (forces and moments). Thus, only videographic assessment of acceleration is required to estimate forces and moments measured by the primary afferents. Our study highlights how sensory systems deal with complex physical constraints of perceptual targets and sensors.

Comparing vibrissal morphology and infraorbital foramen area in pinnipeds

Pinniped vibrissae are well-adapted to sensing in an aquatic environment, by being morphologically diverse and more sensitive than those of terrestrial species. However, it is both challenging and time-consuming to measure vibrissal sensitivity in many species. In terrestrial species, the infraorbital foramen (IOF) area is associated with vibrissal sensitivity and increases with vibrissal number. While pinnipeds are thought to have large IOF areas, this has not yet been systematically measured before. We investigated vibrissal morphology, IOF area, and skull size in 16 species of pinniped and 12 terrestrial Carnivora species. Pinnipeds had significantly larger skulls and IOF areas, longer vibrissae, and fewer vibrissae than the other Carnivora species. IOF area and vibrissal number were correlated in Pinnipeds, just as they are in terrestrial mammals. However, despite pinnipeds having significantly fewer vibrissae than other Carnivora species, their IOF area was not smaller, which might be due to pinnipeds having vibrissae that are innervated more. We propose that investigating normalized IOF area per vibrissa will offer an alternative way to approximate gross individual vibrissal sensitivity in pinnipeds and other mammalian species. Our data show that many species of pinniped, and some species of felids, are likely to have strongly innervated individual vibrissae, since they have high values of normalized IOF area per vibrissa. We suggest that species that hunt moving prey items in the dark will have more sensitive and specialized vibrissae, especially as they have to integrate between individual vibrissal signals to calculate the direction of moving prey during hunting.

Interactions of Whisking and Touch Signals in the Rat Brainstem

Perception is an active process, requiring the integration of both proprioceptive and exteroceptive information. In the rat's vibrissal system, a classical model for active sensing, the relative contribution of the two information streams was previously studied at the peripheral, thalamic, and cortical levels. Contributions of brainstem neurons were only indirectly inferred for some trigeminal nuclei according to their thalamic projections. The current work addressed this knowledge gap by performing the first comparative study of the encoding of proprioceptive whisking and exteroceptive touch signals in the oralis (SpVo), interpolaris (SpVi), and paratrigeminal (Pa5) brainstem nuclei. We used artificial whisking in anesthetized male rats, which allows a systematic analysis of the relative contribution of the proprioceptive and exteroceptive information streams along the ascending pathways in the absence of motor or cognitive top-down modulations. We found that (1) neurons in the rostral and caudal parts of the SpVi convey whisking and touch information, respectively, as predicted by their thalamic projections; (2) neurons in the SpVo encode both whisking and (primarily) touch information; and (3) neurons of the Pa5 encode a complex combination of whisking and touch information. In particular, the Pa5 contains a relatively large fraction of neurons that are inhibited by active touch, a response observed so far only in the thalamus. Overall, our systematic characterization of afferent responses to active touch in the trigeminal brainstem approves the hypothesized functions of SpVi neurons and presents evidence that SpVo and Pa5 neurons are involved in the processing of active vibrissal touch.SIGNIFICANCE STATEMENT The present work constitutes the first comparative study of the encoding of proprioceptive (whisking) and exteroceptive (touch) information in the rat's brainstem trigeminal nuclei, the first stage of vibrissal processing in the CNS. It shows that (1) as expected, the rostral and caudal interpolaris neurons convey primarily whisking and touch information, respectively; (2) the oralis nucleus, whose function was previously unknown, encodes both whisking and (primarily) touch touch information; (3) a subtractive computation, reported at the thalamic level, already occurs at the brainstem level; and (4) a novel afferent pathway probably ascends via the paratrigeminal nucleus, encoding both proprioceptive and exteroceptive information.

Of mice and monkeys: Somatosensory processing in two prominent animal models

Our understanding of the neural basis of somatosensation is based largely on studies of the whisker system of mice and rats and the hands of macaque monkeys. Results across these animal models are often interpreted as providing direct insight into human somatosensation. Work on these systems has proceeded in parallel, capitalizing on the strengths of each model, but has rarely been considered as a whole. This lack of integration promotes a piecemeal understanding of somatosensation. Here, we examine the functions and morphologies of whiskers of mice and rats, the hands of macaque monkeys, and the somatosensory neuraxes of these three species. We then discuss how somatosensory information is encoded in their respective nervous systems, highlighting similarities and differences. We reflect on the limitations of these models of human somatosensation and consider key gaps in our understanding of the neural basis of somatosensation.

Constraints on the deformation of the vibrissa within the follicle

Nearly all mammals have a vibrissal system specialized for tactile sensation, composed of whiskers growing from sensor-rich follicles in the skin. When a whisker deflects against an object, it deforms within the follicle and exerts forces on the mechanoreceptors inside. In addition, during active whisking behavior, muscle contractions around the follicle and increases in blood pressure in the ring sinus will affect the whisker deformation profile. To date, however, it is not yet possible to experimentally measure how the whisker deforms in an intact follicle or its effects on different groups of mechanoreceptors. The present study develops a novel model to predict vibrissal deformation within the follicle sinus complex. The model is based on experimental results from a previous ex vivo study on whisker deformation within the follicle, and on a new histological analysis of follicle tissue. It is then used to simulate whisker deformation within the follicle during passive touch and active whisking. Results suggest that the most likely whisker deformation profile is “S-shaped,” crossing the midline of the follicle right below the ring sinus. Simulations of active whisking indicate that an increase in overall muscle stiffness, an increase in the ratio between deep and superficial intrinsic muscle stiffness, and an increase in sinus blood pressure will all enhance tactile sensitivity. Finally, we discuss how the deformation profiles might map to the responses of primary afferents of each mechanoreceptor type. The mechanical model presented in this study is an important first step in simulating mechanical interactions within whisker follicles.

Many mammals rely on whiskers as a mode of tactile sensation, especially when exploring in darkness. Active, rhythmic protraction and retraction of the whiskers, commonly referred to as “whisking,” is observed among many whisker specialist animals. During whisker-based sensing, forces and moments generated by external stimuli are transmitted to the base of the whisker shaft inside the follicle. Within the follicle, the interaction between the whisker’s deformation and the surrounding tissue determines how different groups of mechanoreceptors will deform, thereby transducing the mechanical signals into electrical signals. However, it is not yet possible to experimentally measure this interaction in vivo. We therefore created a mechanical model of the follicle sinus complex to simulate whisker deformation within the follicle resulting from external whisker deflection. Our results provide the first estimate of whisker shape as it deforms in the follicle, during both passive touch and active whisking. In turn, these shape estimates allow us to predict how the whisker will deform against different types of mechanoreceptors at different locations within the follicle. In addition, we find that both intrinsic muscle contraction and an increase in blood pressure will improve the tactile sensitivity of the whisker system.

The follicle-sinus complex of the bottlenose dolphin (Tursiops truncatus). Functional anatomy and possible evolutional significance of its somato-sensory innervation

Vibrissae are tactile hairs found mainly on the rostrum of most mammals. The follicle, which is surrounded by a large venous sinus, is called "follicle-sinus complex" (FSC). This complex is highly innervated by somatosensitive fibers and reached by visceromotor fibers that innervate the surrounding vessels. The surrounding striated muscles receive somatomotor fibers from the facial nerve. The bottlenose dolphin (Tursiops truncatus), a frequently described member of the delphinid family, possesses this organ only in the postnatal period. However, information on the function of the vibrissal complex in this latter species is scarce. Recently, psychophysical experiments on the river-living Guiana dolphin (Sotalia guianensis) revealed that the FSC could work as an electroreceptor in murky waters. In the present study, we analyzed the morphology and innervation of the FSC of newborn (n = 8) and adult (n = 3) bottlenose dolphins. We used Masson's trichrome stain and antibodies against neurofilament 200 kDa (NF 200), protein gene product (PGP 9.5), substance P (SP), calcitonin gene-related peptide, and tyrosine hydroxylase (TH) to characterize the FSC of the two age classes. Masson's trichrome staining revealed a structure almost identical to that of terrestrial mammals except for the fact that the FSC was occupied only by a venous sinus and that the vibrissal shaft lied within the follicle. Immunostaining for PGP 9.5 and NF 200 showed somatosensory fibers finishing high along the follicle with Merkel nerve endings and free nerve endings. We also found SP-positive fibers mostly in the surrounding blood vessels and TH both in the vessels and in the mesenchymal sheath. The FSC of the bottlenose dolphin, therefore, possesses a rich somatomotor innervation and a set of peptidergic visceromotor fibers. This anatomical disposition suggests a mechanoreceptor function in the newborns, possibly finalized to search for the opening of the mother's nipples. In the adult, however, this structure could change into a proprioceptive function in which the vibrissal shaft could provide information on the degree of rotation of the head. In the absence of psychophysical experiments in this species, the hypothesis of electroreception cannot be rejected.

Sensing and processing whisker deflections in rodents

The classical view of sensory information mainly flowing into barrel cortex at layer IV, moving up for complex feature processing and lateral interactions in layers II and III, then down to layers V and VI for output and corticothalamic feedback is becoming increasingly undermined by new evidence. We review the neurophysiology of sensing and processing whisker deflections, emphasizing the general processing and organisational principles present along the entire sensory pathway—from the site of physical deflection at the whiskers to the encoding of deflections in the barrel cortex. Many of these principles support the classical view. However, we also highlight the growing number of exceptions to these general principles, which complexify the system and which investigators should be mindful of when interpreting their results. We identify gaps in the literature for experimentalists and theorists to investigate, not just to better understand whisker sensation but also to better understand sensory and cortical processing.

How the Barrel Cortex Became a Working Model for Developmental Plasticity: A Historical Perspective

For half a century now, the barrel cortex of common laboratory rodents has been an exceptionally useful model for studying the formation of topographically organized maps, neural patterning, and plasticity, both in development and in maturity. We present a historical perspective on how barrels were discovered, and how thereafter, they became a workhorse for developmental neuroscientists and for studies on brain plasticity and activity-dependent modeling of brain circuits. What is particularly remarkable about this sensory system is a cellular patterning that is induced by signals derived from the sensory receptors surrounding the snout whiskers and transmitted centrally to the brainstem (barrelettes), the thalamus (barreloids), and the neocortex (barrels). Injury to the sensory receptors shortly after birth leads to predictable pattern alterations at all levels of the system. Mouse genetics have increased our understanding of how barrels are constructed and revealed the interplay of the molecular programs that direct axon growth and cell specification, with activity-dependent mechanisms. There is an ever-rising interest in this sensory system as a neurobiological model to study development of somatotopy, patterning, and plasticity at both the morphologic and physiological levels. This article is part of a group of articles commemorating the 50th anniversary of the Society for Neuroscience.

Histomorphological investigations on the lips of Rahmani sheep (Ovis aries): A scanning electron and light microscopic study

This study was conducted to provide comprehensive information on the anatomical and histological features of the upper (UL) and lower (LL) lips of Rahmani sheep by gross examination, morphometric analysis in addition to Light and scanning electron microscope (SEM). Samples from normal healthy adult male sheep heads were collected directly after slaughtering. UL and LL were dissected, and specimens were collected for both light and SEM. The UL was longer approximately by one-fold and thicker by one-fold at the median and the oral angle areas, and by one- and half-folds at the paramedian area than the LL. The free border of both lips was characterized rostrally by the presence of labial projections. By SEM the edges of the inner aspect of the lips and of the philtrum were distinguished by labial projections. These projections which surrounding the philtrum subdivided into polygonal areas with numerous keratinized scales especially at the apical parts which increased dorsally toward the nostril. Most of the openings of the upper labial salivary glands were volcanic crater-shaped while that of the lower labial salivary glands were various shapes; round folded margin, rosette and whirlpool shaped. Histologically, the shape of the projection or papillae differs at the median and paramedian areas of the UL than the LL. However, there was no differences near the oral angle area. In conclusion, the shape, size and amount of keratinization of the papillae may offer efficiency to the lips during feeding process.

Neuronal Circuits in Barrel Cortex for Whisker Sensory Perception

The array of whiskers on the snout provides rodents with tactile sensory information relating to the size, shape and texture of objects in their immediate environment. Rodents can use their whiskers to detect stimuli, distinguish textures, locate objects and navigate. Important aspects of whisker sensation are thought to result from neuronal computations in the whisker somatosensory cortex (wS1). Each whisker is individually represented in the somatotopic map of wS1 by an anatomical unit named a 'barrel' (hence also called barrel cortex). This allows precise investigation of sensory processing in the context of a well-defined map. Here, we first review the signaling pathways from the whiskers to wS1, and then discuss current understanding of the various types of excitatory and inhibitory neurons present within wS1. Different classes of cells can be defined according to anatomical, electrophysiological and molecular features. The synaptic connectivity of neurons within local wS1 microcircuits, as well as their long-range interactions and the impact of neuromodulators, are beginning to be understood. Recent technological progress has allowed cell-type-specific connectivity to be related to cell-type-specific activity during whisker-related behaviors. An important goal for future research is to obtain a causal and mechanistic understanding of how selected aspects of tactile sensory information are processed by specific types of neurons in the synaptically connected neuronal networks of wS1 and signaled to downstream brain areas, thus contributing to sensory-guided decision-making.

Variability in the number of infraorbital foramina in rhesus macaques (Macaca mulatta) and cynomolgus macaques (Macaca fascicularis)

This study aimed to determine the number of infraorbital foramina in monkeys of the Papionini tribe. The authors performed a μCT analysis of the morphology of the infraorbital foramina. A total number of 52 simian skulls belonged to two macaque species: Macaca mulatta and Macaca fascicularis were used in the study. The number of infraorbital foramina was counted macroscopically and with the use of a magnifying glass. Next, the skull representing the most common morphological type was selected and scanned by micro-computed tomography (μCT). The Shapiro-Wilk normality test was used in the study. To compare the differences in the number of infraorbital foramen between species, sex and sides, the Mann-Whitney U test was applied. Three infraorbital foramina were present in most individuals from the test group. The Mann-Whitney test revealed no statistically significant difference between the number of foramina on the right- and left-hand side. Likewise, no statistically significant differences between the numbers of infraorbital foramina across sexes were observed. Volumetric reconstructions revealed the presence of separate infraorbital canals for each infraorbital foramen. Craniofacial innervation in macaques is formed by complex branching patterns of cranial nerves. Variability in the number of infraorbital foramina suggests a variable maxillary innervation pattern in these animals. Based on the analysis of volumetric projections, the presence of two labial branches and a single nasal branch of the infraorbital nerve is suggested. Detailed descriptions are supported by quantitative data and μCT evidence.

Evaluation of whisker stress in cats

OBJECTIVES

The aim of this study was to determine if cats fed from a commercially advertised whisker-friendly dish vs their normal food dish would spend more time at the food dish, eat more and drop less food.

METHODS

Forty indoor cats were enrolled in the study. Owners fasted their cats for 12 h and fed them their normal measured amount of dry food in their normal dish. Owners filmed their cats eating for up to 5 mins, and measured how much food was eaten and dropped from the dish. Owners then switched to feeding their cats from a whisker-friendly dish for a 7-day transition period. Following this transition, owners were instructed to fast their cats for 12 h and then feed them their normal food from the new dish and film them eating, as previously described. The following day the owners offered food in both dishes to determine their cat's preference.

RESULTS

No evidence was found that eating from the whisker-friendly dish increased the amount of time spent eating (P = 0.8), decreased the amount of food dropped (P = 0.9) or increased the amount of food eaten (P = 0.7). The estimated probability for the cats to prefer the whisker-friendly dish was 0.74 with a 95% confidence interval.

CONCLUSIONS AND RELEVANCE

Cats fed from a whisker-friendly dish did not spend more time eating, drop less food or eat more food in a 5-min period. Some cats appeared to prefer the new whisker-friendly dish over their normal food dish. Overall, food dish-associated whisker stress did not affect the eating habits of the study cats.

Gap Junctions Interconnect Different Subtypes of Parvalbumin-Positive Interneurons in Barrels and Septa with Connectivity Unique to Each Subtype

Parvalbumin (PV)-positive interneurons form dendritic gap junctions with one another, but the connectivity among gap junction-coupled dendrites remains uninvestigated in most neocortical areas. We visualized gap junctions in layer 4 of the mouse barrel cortex and examined their structural details. PV neurons were divided into 4 types based on the location of soma and dendrites within or outside barrels. Type 1 neurons that had soma and all dendrites inside a barrel, considered most specific to single vibrissa-derived signals, unexpectedly formed gap junctions only with other types but never with each other. Type 2 neurons inside a barrel elongated dendrites outward, forming gap junctions within a column that contained the home barrel. Type 3 neurons located outside barrels established connections with all types including Type 4 neurons that were confined inside the inter-barrel septa. The majority (33/38, 86.8%) of dendritic gap junctions were within 75 μm from at least 1 of 2 paired somata. All types received vesicular glutamate transporter 2-positive axon terminals preferentially on somata and proximal dendrites, indicating the involvement of all types in thalamocortical feedforward regulation in which proximal gap junctions may also participate. These structural organizations provide a new morphological basis for regulatory mechanisms in barrel cortex.

MORPHOLOGICAL DIVERSITY OF FACIAL VIBRISSAE IN Chaetophractus vellerosus (MAMMALIA, XENARTHRA, DASYPODIDAE) AND DIFFERENTIAL MECHANOPERCEPTION

Vibrissae are specialized and complex mechanoreceptor organs present in the skin of most mammals that respond to a diverse mechanical stimuli (e.g. tension, pressure, movement, vibrations) and provide information on distance to the object, its location/orientation, and general characteristics of its surface; also, it may play diverse roles during food acquisition and attacking potential prey. There are scarce papers on the vibrissae of armadillos, only considering their presence/absence and distribution, but no histological analyses have been made. The goal of our contribution is to perform a histological study of the head vibrissae of Chaetophractus vellerosus, identify their morphological features, the tissues that form them, interpret their possible functions, and attempt to link the characteristics with ecological aspects of this species like its digging habits. Our results suggest that Chaetophractus vellerosus possesses two types of vibrissae: macro- and micro-vibrissae. Both types are similar in gross morphology, characterized mainly by an absence of annular sinus and ringwulst, but having a trabecular sinus that extends along the entire length of the follicle; these features might be linked to a reduction of its sensory capacity. Unlike other mammals, the macro-vibrissae are in the genal, anterobital and intermandibular regions, while micro-vibrissae are distributed in the superior labial and mental regions. In addition to size differences, the macro-vibrissae possess intrinsic muscles composed of smooth muscular fibers. The genal macro-vibrissae are very close to each other, with smooth muscle fibers connecting the capsules of adjacent ones (intrinsic muscles). Those from the superior labial and mental (micro-vibrissae), show bundles of striated muscle inserted on their capsules. These muscle fibers would be part of the facial musculature and could be considered as extrinsic muscles. The mobility of these two types of vibrissae must certainly be different, given that the respective muscles (intrinsic and extrinsic) have different origins and innervation. The presence of two types of vibrissae might indicate that these mechanoreceptors have differential perception capacities that would probably be complementary, thus providing more precise information about the environment. The presence of macro-vibrissae in the genal, anteorbital and intermandibular zone would be directly related to the life habits of Chaetophractus vellerosus.

Hydrodynamic reception in the Australian water rat, Hydromys chrysogaster

The Australian water rat, Hydromys chrysogaster, preys on a wide variety of aquatic and semiaquatic arthropods and vertebrates, including fish. A frequently observed predatory strategy of Hydromys is sitting in wait at the water's edge with parts of its vibrissae submersed. Here we show that Hydromys can detect water motions with its whiskers. Behavioural thresholds range from 1.0 to 9.4 mm s⁻¹ water velocity, based on maximal horizontal water velocity in the area covered by the whiskers. This high sensitivity to water motions would enable Hydromys to detect fishes passing by. No responses to surface waves generated by a vibrating rod and resembling the surface waves caused by struggling insects were found.

Anatomy of avian rictal bristles in Caprimulgiformes reveals reduced tactile function in open-habitat, partially diurnal foraging species

Avian rictal bristles are present in many species of birds, especially in nocturnal species. Rictal bristles occur along the upper beak and are morphologically similar to mammalian whiskers. Mammalian whiskers are important tactile sensors, guiding locomotion, foraging and social interactions, and have a well‐characterised anatomy. However, it is not yet known whether avian rictal bristles have a sensory function, and their morphology, anatomy and function have also not been described in many species. Our study compares bristle morphology, follicle anatomy and their association with foraging traits, across 12 Caprimulgiform species. Rictal bristle morphology and follicle anatomy were diverse across the 12 species. Nine of the 12 species had mechanoreceptors around their bristle follicles; however, there was large variation in their musculature, mechanoreceptor numbers and bristle morphology. Overall, species with short, thin, branching bristles that lacked mechanoreceptors tended to forage pre‐dusk in open habitats, whereas species with mechanoreceptors around their bristle follicle tended to forage at night and in more closed habitats. We suggest that rictal bristles are likely to be tactile in many species and may aid in navigation, foraging and collision avoidance; however, identifying rictal bristle function is challenging and demands further investigation in many species.

The Cellular and Mechanical Basis for Response Characteristics of Identified Primary Afferents in the Rat Vibrissal System

Compared to our understanding of the response properties of receptors in the auditory and visual systems, we have only a limited understanding of the mechanoreceptor responses that underlie tactile sensation. Here, we exploit the stereotyped morphology of the rat vibrissal (whisker) array to investigate coding and transduction properties of identified primary tactile afferents. We performed in vivo intra-axonal recording and labeling experiments to quantify response characteristics of four different types of identified mechanoreceptors in the vibrissal follicle: ring-sinus Merkel; lanceolate; clublike; and rete-ridge collar Merkel. Of these types, only ring-sinus Merkel endings exhibited slowly adapting properties. A weak inverse relationship between response magnitude and onset response latency was found across all types. All afferents exhibited strong "angular tuning," i.e., their response magnitude and latency depended on the whisker's deflection angle. Although previous studies suggested that this tuning should be aligned with the angular location of the mechanoreceptor in the follicle, such alignment was observed only for Merkel afferents; angular tuning of the other afferent types showed no clear alignment with mechanoreceptor location. Biomechanical modeling suggested that this tuning difference might be explained by mechanoreceptors' differential sensitivity to the force directed along the whisker length. Electron microscopic investigations of Merkel endings and lanceolate endings at the level of the ring sinus revealed unique anatomical features that may promote these differential sensitivities. The present study systematically integrates biomechanical principles with the anatomical and morphological characterization of primary afferent endings to describe the physical and cellular processing that shapes the neural representation of touch.

Somatosensation: The Cellular and Physical Basis of Tactile Experience

A fundamental question in sensory neuroscience is how perceptual experience arises from the cellular properties of sensory neurons. A new, tour de force study has dissected out the functional properties of identified mechanosensory nerve endings that innervate whisker follicles.

Sensorimotor integration in the whisker somatosensory brain stem trigeminal loop

The rodent's vibrissal system is a useful model system for studying sensorimotor integration in perception. This integration determines the way in which sensory information is acquired by sensory organs and the motor commands that control them. The initial instance of sensorimotor integration in the whisker somatosensory system is implemented in the brain stem loop and may be essential to the way rodents explore and sense their environment. To examine the nature of these sensorimotor interactions, we recorded from lightly anesthetized rats in vivo and brain stem slices in vitro and isolated specific parts of this loop. We found that motor feedback to the vibrissal pad serves as a dynamic gain controller that controls the response of first-order sensory neurons by increasing and decreasing sensitivity to lower and higher tactile stimulus magnitudes, respectively. This delicate mechanism is mediated through tactile stimulus magnitude-dependent motor feedback. Conversely, tactile inputs affect the motor whisking output through influences on the rhythmic whisking circuitry, thus changing whisking kinetics. Similarly, tactile influences also modify the whisking amplitude through synaptic and intrinsic neuronal interaction in the facial nucleus, resulting in facilitation or suppression of whisking amplitude. These results point to the vast range of mechanisms underlying sensorimotor integration in the brain stem loop.NEW & NOTEWORTHY Sensorimotor integration is a process in which sensory and motor information is combined to control the flow of sensory information, as well as to adjust the motor system output. We found in the rodent's whisker somatosensory system mutual influences between tactile inputs and motor output, in which motor neurons control the flow of sensory information depending on their magnitude. Conversely, sensory information can control the magnitude and kinetics of whisker movement.

Mechanical coupling through the skin affects whisker movements and tactile information encoding

Rats use their whiskers to extract sensory information from their environment. While exploring, they analyze peripheral stimuli distributed over several whiskers. Previous studies have reported cross-whisker integration of information at several levels of the neuronal pathways from whisker follicles to the somatosensory cortex. In the present study, we investigated the possible coupling between whiskers at a preneuronal level, transmitted by the skin and muscles between follicles. First, we quantified the movement induced on one whisker by deflecting another whisker. Our results show significant mechanical coupling, predominantly when a given whisker's caudal neighbor in the same row is deflected. The magnitude of the effect was correlated with the diameter of the deflected whisker. In addition to changes in whisker angle, we observed curvature changes when the whisker shaft was constrained distally from the base. Second, we found that trigeminal ganglion neurons innervating a given whisker follicle fire action potentials in response to high-magnitude deflections of an adjacent whisker. This functional coupling also shows a bias toward the caudal neighbor located in the same row. Finally, we designed a two-whisker biomechanical model to investigate transmission of forces across follicles. Analysis of the whisker-follicle contact forces suggests that activation of mechanoreceptors in the ring sinus region could account for our electrophysiological results. The model can fully explain the observed caudal bias by the gradient in whisker diameter, with possible contribution of the intrinsic muscles connecting follicles. Overall, our study demonstrates the functional relevance of mechanical coupling on early information processing in the whisker system.NEW & NOTEWORTHY Rodents explore their environment actively by touching objects with their whiskers. A major challenge is to understand how sensory inputs from different whiskers are merged together to form a coherent tactile percept. We demonstrate that external sensory events on one whisker can influence the position of another whisker and, importantly, that they can trigger the activity of mechanoreceptors at its base. This cross-whisker interaction occurs pre-neuronally, through mechanical transmission of forces in the skin.

Whisker-Mediated Touch System in Rodents: From Neuron to Behavior

A key question in systems neuroscience is to identify how sensory stimuli are represented in neuronal activity, and how the activity of sensory neurons in turn is “read out” by downstream neurons and give rise to behavior. The choice of a proper model system to address these questions, is therefore a crucial step. Over the past decade, the increasingly powerful array of experimental approaches that has become available in non-primate models (e.g., optogenetics and two-photon imaging) has spurred a renewed interest for the use of rodent models in systems neuroscience research. Here, I introduce the rodent whisker-mediated touch system as a structurally well-established and well-organized model system which, despite its simplicity, gives rise to complex behaviors. This system serves as a behaviorally efficient model system; known as nocturnal animals, along with their olfaction, rodents rely on their whisker-mediated touch system to collect information about their surrounding environment. Moreover, this system represents a well-studied circuitry with a somatotopic organization. At every stage of processing, one can identify anatomical and functional topographic maps of whiskers; “barrelettes” in the brainstem nuclei, “barreloids” in the sensory thalamus, and “barrels” in the cortex. This article provides a brief review on the basic anatomy and function of the whisker system in rodents.

Molecular Mechanisms of the Sense of Touch: An Overview of Mechanical Transduction and Transmission in Merkel Discs of Whisker Hair Follicles and Some Clinical Perspectives

The Merkel disc is a main type of tactile end organs for sensing gentle touch and is essential for sophisticated sensory tasks including social interaction, environmental exploration, and tactile discrimination. Recent studies have shown that Merkel cells are primary sites of mechanotransduction using Piezo2 channels as a molecular transducer in Merkel discs. Furthermore, tactile stimuli trigger serotonin release from Merkel cells to excite their associated whisker Aβ-afferent endings and transmit tactile signals. The tactile transduction and transmission at Merkel discs may have important clinical implications in sensory dysfunctions such as the loss of tactile sensitivity and tactile allodynia seen in patients who have diabetes and inflammatory diseases and undergo chemotherapy.

Mechanics to pre-process information for the fine tuning of mechanoreceptors

Non-nervous auxiliary structures play a significant role in sensory biology. They filter the stimulus and transform it in a way that fits the animal’s needs, thereby contributing to the avoidance of the central nervous system’s overload with meaningless stimuli and a corresponding processing task. The present review deals with mechanoreceptors mainly of invertebrates and some remarkable recent findings stressing the role of mechanics as an important source of sensor adaptedness, outstanding performance, and diversity. Instead of organizing the review along the types of stimulus energy (force) taken up by the sensors, processes associated with a few basic and seemingly simple mechanical principles like lever systems, viscoelasticity, resonance, traveling waves, and impedance matching are taken as the guideline. As will be seen, nature makes surprisingly competent use of such “simple mechanics”.

Functional properties of mechanoreceptors in mouse whisker hair follicles determined by the pressure-clamped single-fiber recording technique

Several types of mechanoreceptors have been identified anatomically in rodent whisker hair follicles, but their functional properties have not been fully studied. Here we developed a pressure-clamped single-fiber recording technique to record impulses on mouse whisker hair follicle afferent nerves following displacements of whisker hair follicles. On the basis of the patterns of impulses evoked by the mechanical stimulation, three functional types of mechanoreceptors were identified, including rapidly adapting (RA), slowly adapting type 1 (SA1), and slowly adapting type 2 (SA2) mechanoreceptors. Impulses of all these mechanoreceptors were almost completely abolished by 30 nM TTX, and were largely suppressed by cooling temperatures at 15°C. Tested at different displacement distances as different stimulation intensity, RA mechanoreceptors showed a limited capacity for stimulation intensity encoding, but both SA1 and SA2 mechanoreceptors displayed linear increases of impulse numbers with increased stimulation intensity. Tested with different ramp speed of displacements, RA impulses were only evoked by rapid ramp stimulation but SA1 and SA2 impulses could be evoked by both rapid and slow ramp stimulation. Tested with different stimulation frequency, all three types of mechanoreceptors well followed the stimulation at 10-100 Hz. Taken together, this study revealed some important functional properties of RA, SA1 and SA2 mechanoreceptors, which helps better understand the encoding of tactile information by different types of low-threshold mechanoreceptors.

Dynamic changes in CGRP, PACAP, and PACAP receptors in the trigeminovascular system of a novel repetitive electrical stimulation rat model: Relevant to migraine

Migraine is the seventh most disabling disorder globally, with prevalence of 11.7% worldwide. One of the prevailing mechanisms is the activation of the trigeminovascular system, and calcitonin gene-related peptide (CGRP) is an important therapeutic target for migraine in this system. Recent studies suggested an emerging role of pituitary adenylate cyclase-activating peptide (PACAP) in migraine. However, the relation between CGRP and PACAP and the role of PACAP in migraine remain undefined. In this study, we established a novel repetitive (one, three, and seven days) electrical stimulation model by stimulating dura mater in conscious rats. Then, we determined expression patterns in the trigeminal ganglion and the trigeminal nucleus caudalis of the trigeminovascular system. Electrical stimulation decreased facial mechanical thresholds, and the order of sensitivity was as follows: vibrissal pad >inner canthus >outer canthus (P < 0.001). The electrical stimulation group exhibited head-turning and head-flicks (P < 0.05) nociceptive behaviors. Importantly, electrical stimulation increased the expressions of CGRP, PACAP, and the PACAP-preferring type 1 (PAC1) receptor in both trigeminal ganglion and trigeminal nucleus caudalis (P < 0.05). The expressions of two vasoactive intestinal peptide (VIP)-shared type 2 (VPAC1 and VPAC2) receptors were increased in the trigeminal ganglion, whereas in the trigeminal nucleus caudalis, their increases were peaked on Day 3 and then decreased by Day 7. PACAP was colocalized with NEUronal Nuclei (NeuN), PAC1, and CGRP in both trigeminal ganglion and the trigeminal nucleus caudalis. Our results demonstrate that the repetitive electrical stimulation model can simulate the allodynia during the migraine chronification, and PACAP plays a role in the pathogenesis of migraine potentially via PAC1 receptor.

Whisker Vibrations and the Activity of Trigeminal Primary Afferents in Response to Airflow

Rodents are the most commonly studied model system in neuroscience, but surprisingly few studies investigate the natural sensory stimuli that rodent nervous systems evolved to interpret. Even fewer studies examine neural responses to these natural stimuli. Decades of research have investigated the rat vibrissal (whisker) system in the context of direct touch and tactile stimulation, but recent work has shown that rats also use their whiskers to help detect and localize airflow. The present study investigates the neural basis for this ability as dictated by the mechanical response of whiskers to airflow. Mechanical experiments show that a whisker's vibration magnitude depends on airspeed and the intrinsic shape of the whisker. Surprisingly, the direction of the whisker's vibration changes as a function of airflow speed: vibrations transition from parallel to perpendicular with respect to the airflow as airspeed increases. Recordings from primary sensory trigeminal ganglion neurons show that these neurons exhibit responses consistent with those that would be predicted from direct touch. Trigeminal neuron firing rate increases with airspeed, is modulated by the orientation of the whisker relative to the airflow, and is influenced by the whisker's resonant frequencies. We develop a simple model to describe how a population of neurons could leverage mechanical relationships to decode both airspeed and direction. These results open new avenues for studying vibrissotactile regions of the brain in the context of evolutionarily important airflow-sensing behaviors and olfactory search. Although this study used only female rats, all results are expected to generalize to male rats.SIGNIFICANCE STATEMENT The rodent vibrissal (whisker) system has been studied for decades in the context of direct tactile sensation, but recent work has indicated that rats also use whiskers to help localize airflow. Neural circuits in somatosensory regions of the rodent brain thus likely evolved in part to process airflow information. This study investigates the whiskers' mechanical response to airflow and the associated neural response. Airspeed affects the magnitude of whisker vibration and the response magnitude of whisker-sensitive primary sensory neurons in the trigeminal ganglion. Surprisingly, the direction of vibration and the associated directionally dependent neural response changes with airspeed. These findings suggest a population code for airflow speed and direction and open new avenues for studying vibrissotactile regions of the brain.

Lessons from Worm Dendritic Patterning

The structural and functional properties of neurons have intrigued scientists since the pioneering work of Santiago Ramón y Cajal. Since then, emerging cutting-edge technologies, including light and electron microscopy, electrophysiology, biochemistry, optogenetics, and molecular biology, have dramatically increased our understanding of dendritic properties. This advancement was also facilitated by the establishment of different animal model organisms, from flies to mammals. Here we describe the emerging model system of a Caenorhabditis elegans polymodal neuron named PVD, whose dendritic tree follows a stereotypical structure characterized by repeating candelabra-like structural units. In the past decade, progress has been made in understanding PVD's functions, morphogenesis, regeneration, and aging, yet many questions still remain.

Optogenetic study of the response interaction among multi-afferent inputs in the barrel cortex of rats

We investigated the relationship between whisker mechanoreceptive inputs and the neural responses to optical stimulation in layer 2/upper 3 (L2/U3) of the barrel cortex using optogenetics since, ideally, we should investigate interactions among inputs with spatiotemporal acuity. Sixteen whisker points of a transgenic rat (W-TChR2V4), that expresses channelrhodopsin 2 (ChR2)-Venus conjugate (ChR2V) in the peripheral nerve endings surrounding the whisker follicles, were respectively connected one-by-one with 16 LED-coupled optical fibres, which illuminated the targets according to a certain pattern in order to evaluate interactions among the inputs in L2/U3. We found that the individual L2/U3 neurons frequently received excitatory inputs from multiple whiskers that were arrayed in a row. Although the interactions among major afferent inputs (MAIs) were negligible, negative interactions with the surrounding inputs suggest that the afferent inputs were integrated in the cortical networks to enhance the contrast of an array to its surroundings. With its simplicity, reproducibility and spatiotemporal acuity, the optogenetic approach would provide an alternative way to understand the principles of afferent integration in the cortex and should complement knowledge obtained by experiments using more natural stimulations.

Follicle sinus complexes (FSCs) in muzzle skin as postmortem diagnostic material of rabid dogs

Recently, we reported that follicle-sinus complexes (FSCs) in the muzzle skin are useful for postmortem diagnosis of rabid dogs. Here, we compared the sensitivity and specificity of detecting the viral antigen in the brain and FSCs of 226 suspected rabid dogs, and assessed whether the FSC harbored the virus genome and particles. The viral antigen was detected in 211 of 226 samples with 100% sensitivity and specificity. Viral RNA and particles were observed in the cytoplasm of Merkel cells (MCs). These results suggest that MCs are targets of virus infection and FSCs are useful material for diagnosing rabies.

The mesencephalic-hypoglossal nuclei loop as a possible central pattern generator for rhythmical whisking in rats

It has been previously demonstrated that the Me5 nucleus is involved in the genesis of reflex activities at whisker pad level. Specific Me5 neurons, which provide sensory innervation of the macrovibrissae, are monosynaptically connected with small hypoglossal neurons innervating the extrinsic muscles that control macrovibrissal movements. Artificial whisking, induced by the electrical stimulation of the peripheral stump of the facial nerve and the electrical stimulation of the XII nucleus or the infraorbital nerve, induced evoked responses in the whisker pad extrinsic motor units, along with a significant increase in the electromyographic activity of the extrinsic pad muscles (Mameli et al. in Acta Oto-Laryngol 126:1334-1338, 2006; in Pfűgers Arch Eur J Physiol 456:1189-1198, 2008; in Brain Res 1283:34-40, 2009; in Exp Brain Res 234:753-761, 2016). In anaesthetized rats, we evaluated the possible involvement of this Me5-XII loop in the genesis of rhythmical whisking. The anatomical findings showed that in addition to the ipsilateral, even the contralateral Me5 nucleus could be retrogradely labeled by the Dil tracer injected into the whisker pad of one side, they, furthermore, showed labeled axons extending across the midline between the two nuclei. The electrophysiological findings agreed with the neuroanatomical results, since the mechanical or artificially induced deflection of the whiskers of one side, evoked in the Me5 contralateral nucleus different patterns of responses. The hypothesis that the Me5-XII loops, along with their cross-linked relationship, could act as a "central generator" responsible for the stereotyped symmetrical pattern of macrovibrissal movements such as rhythmical whisking has been discussed.

Layer 4 barrel cortex neurons retain their response properties during whisker replacement

Bodies change continuously, but we do not know if and how these changes affect somatosensory cortex. We address this issue in the whisker-barrel-cortex-pathway. We ask how outgrowing whiskers are mapped onto layer 4 barrel neuron responses. Half of whisker follicles contained dual whiskers, a shorter presumably outgrowing whisker (referred to as young whisker) and a longer one (referred to as old whisker). Young whiskers were much thinner than old ones but were inserted more deeply into the whisker follicle. Both whiskers were embedded in one outer root sheath surrounded by a common set of afferent nerve fibers. We juxtacellularly identified layer 4 barrel neurons representing dual whiskers with variable whisker length differences in anesthetized rats. Strength and latency of neuronal responses were strongly correlated for deflections of young and old whiskers but were not correlated with whisker length. The direction preferences of young and old whiskers were more similar than expected by chance. Old whiskers evoked marginally stronger and slightly shorter latency spike and local field potential responses than young whiskers. Our data suggest a conservative rewiring mechanism, which connects young whiskers to existing peripheral sensors. The fact that layer 4 barrel neurons retain their response properties is remarkable given the different length, thickness, and insertion depth of young and old whiskers. Retention of cortical response properties might be related to the placement of young and old whisker in one common outer root sheath and may contribute to perceptual stability across whisker replacement. NEW & NOTEWORTHY A particularly dramatic bodily change is whisker regrowth, which involves the formation of dual whisker follicles. Our results suggest that both whiskers are part of the same mechanoreceptive unit. Despite their distinct whisker length and thickness, responses of single cortical neurons to young and old whisker deflection were similar in strength, latency, and directional tuning. We suggest the congruence of young and old whisker cortical responses contributes to perceptual stability over whisker regrowth.