Organization of the spinal trigeminal nucleus in star-nosed moles

Thermoconforming rays of the star-nosed mole

The star-nosed mole (Condylura cristata) is renowned for its densely innervated 22 appendage star-like rostrum ('star') specialized for tactile sensation. As a northerly distributed insectivorous mammal exploiting aquatic and terrestrial habitats, these vascularized nasal rays are regularly exposed to cold water and thermally conductive soil, leading us to ask whether the star surface temperature, a proxy for blood flow, conforms to the local ambient temperature to conserve body heat. Alternatively, given the exquisite sensory nature of the star, we posited that the uninsulated rays may be kept warm when foraging to maintain high mechanosensory function. To test these hypotheses, we remotely monitored surface temperatures in wild-caught star-nosed moles. Although the tail acted as a thermal window exhibiting clear vasoconstriction/vasodilation, the star varied passively in surface temperature, with little evidence for thermoregulatory vasomotion. This thermoconforming response may have evolved to minimize conductive heat loss to the water or wet soils when foraging.

Active Perception in Non-Visual Recognition Environments by Stretchable Tentacle Sensor Arrays

Smoke fog or other light-interference environments have intrinsic obstruction for visual recognition techniques to explore objects and surroundings. Alternatively, tactile perceptions, rather than visual observations, are commonly used by burrowing or deep-sea animals to communicate with environments. Bio-inspired by this natural wisdom, here, we demonstrate stretchable tentacle sensor arrays, which can recognize surrounding objects located in non-visual conditions such as smoke fog or dark environment. Each tentacle sensor is composed of two functional parts: a retractable tentacle with a magnetic top and an elastomer bottom containing copper coils. Different from traditionally passive tactile sensors, these tentacle sensors can actively stretch under the control of a syringe pump, yielding different electrical signals when in contact with the objects. Analyzing collected sensing signals of those tactile sensor arrays by the feature analysis model, complex morphological information of irregular objects in the smoke fog can be recognized. Our study reveals a fundamental connection between stretchable tactile sensors and feature analysis and demonstrates its practical potential for active perception in a non-visual recognition environment.

An inter-species computational analysis of vibrotactile sensitivity in Pacinian and Herbst corpuscles

Vibration sensing is ubiquitous among vertebrates, with the sensory end organ generally being a multilayered ellipsoidal structure. There is, however, a wide range of sizes and structural arrangements across species. In this work, we applied our earlier computational model of the Pacinian corpuscle to predict the sensory response of different species to various stimulus frequencies, and based on the results, we identified the optimal frequency for vibration sensing and the bandwidth over which frequencies should be most detectable. We found that although the size and layering of the corpuscles were very different, almost all of the 19 species studied showed very similar sensitivity ranges. The human and goose were the notable exceptions, with their corpuscle tuned to higher frequencies (130-170 versus 40-50 Hz). We observed no correlation between animal size and any measure of corpuscle geometry in our model. Based on the results generated by our computational model, we hypothesize that lamellar corpuscles across different species may use different sizes and structures to achieve similar frequency detection bands.

Visualizing the trigeminovagal complex in the human medulla by combining ex-vivo ultra-high resolution structural MRI and polarized light imaging microscopy

A trigeminovagal complex, as described in some animals, could help to explain the effect of vagus nerve stimulation as a treatment for headache disorders. However, the existence of a trigeminovagal complex in humans remains unclear. This study, therefore investigated the existence of the trigeminovagal complex in humans. One post-mortem human brainstem was scanned at 11.7T to obtain structural (T1-weighted) and diffusion magnetic resonance images ((d)MR images). Post-processing of dMRI data provided track density imaging (TDI) maps to investigate white matter at a smaller resolution than the imaging resolution. To evaluate the reconstructed tracts, the MR-scanned brainstem and three additional brainstems were sectioned for polarized light imaging (PLI) microscopy. T1-weighted images showed hyperintense vagus medullar striae, coursing towards the dorsomedial aspect of the medulla. dMRI-, TDI- and PLI-images showed these striae to intersect the trigeminal spinal tract (sp5) in the lateral medulla. In addition, PLI images showed that a minority of vagus fibers separated from the vagus trajectory and joined the trigeminal spinal nucleus (Sp5) and the sp5. The course of the vagus tract in the rostral medulla was demonstrated in this study. This study shows that the trigeminal- and vagus systems interconnect anatomically at the level of the rostral medulla where the vagus fibers intersect with the Sp5 and sp5. Physiological and clinical utility of this newly identified interconnection is a topic for further research.

Evolutionary Specialization of Tactile Perception in Vertebrates

Evolution has endowed vertebrates with the remarkable tactile ability to explore the world through the perception of physical force. Yet the sense of touch remains one of the least well understood senses at the cellular and molecular level. Vertebrates specializing in tactile perception can highlight general principles of mechanotransduction. Here, we review cellular and molecular adaptations that underlie the sense of touch in typical and acutely mechanosensitive vertebrates.

Somatosensory organ topography across the star of the star-nosed mole (Condylura cristata)

Quantifying somatosensory receptor distribution in glabrous skin is usually difficult because of the diversity of skin receptor subtypes and their location within the dermis and epidermis. However, the glabrous noses of moles are an exception. In most species of moles, the skin on the nose is covered with domed mechanosensory units known as an Eimer's organs. Eimer's organs contain a stereotyped array of different mechanosensory neurons, meaning that the distribution of mechanosensitive nerve endings can be inferred by visual inspection of the skin surface. Here we detail the distribution of Eimer's organs on the highly derived somatosensory star on the rostrum of the star-nosed mole (Condylura cristata). The star consists of 22 fleshy appendages, or rays, that are covered in Eimer's organs. We find that the density of Eimer's organs increases from proximal to distal locations along the length of the star's rays with a ratio of 1:2.3:3.1 from the surface nearest to the nostril, to the middle part of ray, to the ray tip, respectively. This ratio is comparable to the increase in receptor unit density reported for the human hand, from the palm, to the middle of the digits, to the distal fingertips. We also note that the tactile fovea of the star-nosed mole, located on the medial ventral ray, does not have increased sensory organ density, and we describe these findings in comparison with other sensory fovea.

Subcortical barrelette-like and barreloid-like structures in the prosimian galago (Otolemur garnetti)

Galagos are prosimian primates that resemble ancestral primates more than most other extant primates. As in many other mammals, the facial vibrissae of galagos are distributed across the upper and lower jaws and above the eye. In rats and mice, the mystacial macrovibrissae are represented throughout the ascending trigeminal pathways as arrays of cytoarchitecturally distinct modules, with each module having a nearly one-to-one relationship with a specific facial whisker. The macrovibrissal representations are termed barrelettes in the trigeminal somatosensory brainstem, barreloids in the ventroposterior medial subnucleus of the thalamus, and barrels in primary somatosensory cortex. Despite the presence of facial whiskers in all nonhuman primates, barrel-like structures have not been reported in primates. By staining for cytochrome oxidase, Nissl, and vesicular glutamate transporter proteins, we show a distinct array of barrelette-like and barreloid-like modules in the principal sensory nucleus, the spinal trigeminal nucleus, and the ventroposterior medial subnucleus of the galago, Otolemur garnetti. Labeled terminals of primary sensory neurons in the brainstem and cell bodies of thalamocortically projecting neurons demonstrate that barrelette-like and barreloid-like modules are located in areas of these somatosensory nuclei that are topographically consistent with their role in facial touch. Serendipitously, the plane of section that best displays the barreloid-like modules reveals a remarkably distinct homunculus-like patterning which, we believe, is one of the clearest somatotopic maps of an entire body surface yet found.

Organotopic organization of the primary Infrared Sensitive Nucleus (LTTD) in the western diamondback rattlesnake (Crotalus atrox)

Pit vipers (Crotalinae) have a specific sensory system that detects infrared radiation with bilateral pit organs in the upper jaw. Each pit organ consists of a thin membrane, innervated by three trigeminal nerve branches that project to a specific nucleus in the dorsal hindbrain. The known topographic organization of infrared signals in the optic tectum prompted us to test the implementation of spatiotopically aligned sensory maps through hierarchical neuronal levels from the peripheral epithelium to the first central site in the hindbrain, the nucleus of the lateral descending trigeminal tract (LTTD). The spatial organization of the anatomical connections was revealed in a novel in vitro whole-brain preparation of the western diamondback rattlesnake (Crotalus atrox) that allowed specific application of multiple neuronal tracers to identified pit-organ-supplying trigeminal nerve branches. After adequate survival times, the entire peripheral and central projections of fibers within the pit membrane and the LTTD became visible. This approach revealed a morphological partition of the pit membrane into three well-defined sensory areas with largely separated innervations by the three main branches. The peripheral segregation of infrared afferents in the sensory epithelium was matched by a differential termination of the afferents within different areas of the LTTD, with little overlap. This result demonstrates a topographic organizational principle of the snake infrared system that is implemented by maintaining spatially aligned representations of environmental infrared cues on the sensory epithelium through specific neuronal projections at the level of the first central processing stage, comparable to the visual system.

Neuronal mechanism for acute mechanosensitivity in tactile-foraging waterfowl

Relying almost exclusively on their acute sense of touch, tactile-foraging birds can feed in murky water, but the cellular mechanism is unknown. Mechanical stimuli activate specialized cutaneous end organs in the bill, innervated by trigeminal afferents. We report that trigeminal ganglia (TG) of domestic and wild tactile-foraging ducks exhibit numerical expansion of large-diameter mechanoreceptive neurons expressing the mechano-gated ion channel Piezo2. These features are not found in visually foraging birds. Moreover, in the duck, the expansion of mechanoreceptors occurs at the expense of thermosensors. Direct mechanical stimulation of duck TG neurons evokes high-amplitude depolarizing current with a low threshold of activation, high signal amplification gain, and slow kinetics of inactivation. Together, these factors contribute to efficient conversion of light mechanical stimuli into neuronal excitation. Our results reveal an evolutionary strategy to hone tactile perception in vertebrates at the level of primary afferents.

Brain mass and cranial nerve size in shrews and moles

We investigated the relationship between body size, brain size, and fibers in selected cranial nerves in shrews and moles. Species include tiny masked shrews (S. cinereus) weighing only a few grams and much larger mole species weighing up to 90 grams. It also includes closely related species with very different sensory specializations - such as the star-nosed mole and the common, eastern mole. We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size. Auditory nerves were similarly small but increased in fiber number with increasing brain and body size. Trigeminal nerve number was by far the largest and also increased with increasing brain and body size. The star-nosed mole was an outlier, with more than twice the number of trigeminal nerve fibers than any other species. Despite this hypertrophied cranial nerve, star-nosed mole brains were not larger than predicted from body size, suggesting that magnification of their somatosensory systems does not result in greater overall CNS size.