Cutaneous sensory afferents recorded from the nervus intramandibularis ofGallus gallus vardomesticus

Summary The responses of single sensory afferent nerve fibres were recorded from small nerve bundles of the intramandibular nerve of the chicken following thermal and mechanical stimulation of the beak. Thermoreceptors, nociceptors and mechanoreceptors were identified and their responses characterized.Of the thermoreceptors identified 11 units were classified as cold receptors, which responded to cooling the receptive field by increasing the discharge rate and had conduction velocities in the range 0.83 to 4.4 m/s. Only one warm unit was identified.Two classes of nociceptors were identified: mechano-thermal (polymodal) nociceptors and high threshold mechanical nociceptors. The discharge characteristics and stimulus-response curves of both types were described. While the mechanothermal nociceptors were exclusively C-fibres (c.v. 0.4 to 1.86 m/s), the high threshold mechanoreceptors contained both C and A delta fibres (c.v. 1 to 5.5 m/s). Thermal response thresholds for the mechano-thermal units ranged from 41 to 50 °C with mechanical thresholds of 2 to over 50 g. Mechanical thresholds for the high threshold units ranged from 5 to over 50 g.The mechanoreceptors were either slowly or rapidly adapting. The pattern of response together with stimulus-response curves were presented for the slowly adapting units. Conduction velocities of the slowly adapting units varied from 0.7 to 20 m/s and mechanical threshold from 0.1 to 2 g. On the basis of their response to a vibrating, and a ramp-and-hold mechanical stimulus, the rapidly adapting units were divided into Herbst and Grandry units with only the Herbst units responding accurately to the vibrating stimulus. Both units had fibres conducting in the 50 m/s range with thresholds in the 0.1 to 10 g range.The results are discussed in relation to the receptors found in other avian species and mammalian peripheral sensory afferents.Abbreviations c.v. conduction velocity - RA rapidly adapting (receptors) - SA slowly adapting (receptors)     

Sensory Innervation of the Raccoon Forepaw: 1. Receptor Types in Glabrous and Hairy Skin and Deep Tissue

Electrophysiological recordings were made from the median, ulnar, radial, and dorsal ulnar nerves to determine the types of mechanosensory receptors serving glabrous and hairy skin surfaces of the raccoon forepaw. In addition to the cutaneous mechanoreceptors, fibers innervating deep tissues were also recorded from each of these nerves. These included sensory fibers innervating muscles, joints, claws, and the subcutaneous pulp.

The array of receptors serving raccoon glabrous skin was the same as found in monkeys and humans: Rapidly adapting (RA), slowly adapting (SA), and Pacinian (Pc) fibers were characterized. Pacinian fibers have been rarely described in previous physiological studies of the raccoon peripheral nerves, but in the present study they composed between 14% and 18% of the glabrous skin mechanoreceptors recorded. A distal-proximal gradient in the density of skin innervation was evident for all three types of receptors.

Receptors characterized in the hairy skin of the dorsal paw were similar to those described in other mammals, and included both down and guard hair afferents, non-hair-associated RA fibers, and SA I and SA II fibers. The relative proportions of these fibers differed from those generally reported for the hairy skin of other mammals. SA hair-associated afferent fibers, which have been reported previously only in primate hairy skin, were also found in large numbers in the raccoon. Similarities and differences in the frequency and types of receptors innervating the raccoon forepaw, the forepaws of other mammals, and the hands of primates (including humans) are discussed.     

Tactile-Evoked Response of Sensory Fibers in Buccal and Submandibular Regions of the Rat

Evoked neural responses to tactile stimulation were recorded electro-physiologically from the mechanoreceptive afferent fibers innervating the buccal and submandibular regions of Wistar rats anesthetized with sodium thiopental. Miniature probes 200 μm in diameter were used, and data analysis was performed on the mechanosensitivity of responses to tactile stimulation in the areas innervated by the mental, mylohyoid, auriculotemporal, and cervical nerves. Mechanosensitivity of each area showed a characteristic distribution of slowly adapting (SA), rapidly adapting (RA), C-fiber (CF), and hair follicle (HF) units in individual receptive fields. The density of the SA units was high in the areas innervated by the mylohyoid and auriculotemporal nerves. The CF units were concentrated in the small dome in the area of the mylohyoid nerve and the auriculotemporal nerve, as shown by a significant response to the dynamic features of stimulation. Estimation of the current needed for tactile acuity suggests an important role of the SA fibers in the areas innervated by the auriculotemporal, mylohyoid, and cervical nerves.     

Sensory Innervation of the Raccoon Forepaw: 2. Response Properties and Classification of Slowly Adapting Fibers

Physiological recordings were made from 136 slowly adapting (SA) fibers in the median and ulnar nerves that innervate the glabrous skin of the raccoon. It was found that wetting the skin produced large increases in fiber responsiveness and decreases in threshold. Their responses decreased rapidly with slight displacements of the stimulus away from the center of the receptive field. Responses also decreased with increases in the diameter of the tip of the stimulus probe. The length of time that an SA fiber responded to a prolonged indentation was related to the magnitude of the indentation, and was greater after wetting of the skin. The absence of any clear and consistent grouping of fibers into moderately SA (MSA) and very SA (VSA) units argues against the existence of two types of SA receptors differing in this property. However, the distinction between SA I and SA II fibers that has been made in other species was confirmed in the raccoon.     

Slowly Adapting Cutaneous Mechanoreceptor Afferent Units Associated with Merkel Cells in Frogs and Effects of Direct Currents

In the bullfrog, two types of slowly adapting (SA) cutaneous mechanoreceptor afferent units have been identified physiologically: irregularly discharging frog type I (Ft I) units in both warty and nonwarty skin, and regularly discharging frog type II (Ft II) units in the nonwarty skin. In the present study, mechanosensitive spots of Ft I units were located around the skin warts in the warty skin. The quinacrine technique (Crowe and Whitear, 1978) revealed that quinacrine-accu-mulating Merkel cells were present around the skin warts and near the orifice of skin glands that also surrounded the skin warts. Thus, a significant correlation was found between the location of Merkel cells and the receptive fields (RFs) of Ft I units in the warty skin.

Direct current (DC) stimulation was applied for 1 sec to the skin inside and outside the mechanical RFs of the two types of SA units. RFs for DC stimulation were located on those for mechanical stimulation in both types of SA units. The current threshold required to produce a single spike was lower in cathodal than in anodal pulses in both types of SA units. Greater current intensity elicited an increased number of spikes, but the effective polarity of currents was anodal for Ft I units and cathodal for Ft II units. The optimal current intensity for producing prolonged discharges ranged from +60 to +100 μA in Ft I units and from -50 to -80 μA in Ft II units. The sequence of impulses evoked was irregular in Ft I units and regular in Ft II units, as seen in mechanical responses. When current of the effective polarity for each type of unit was superimposed on the mechanical indentations, it facilitated the mechanical response. Currents of opposite polarity were not effective without mechanical indentation, but when used together, they depressed the mechanical response in both the Ft I units and the Ft II units. Thus, different polarities of DC could selectively activate two different types of SA units in bullfrogs. We consider these findings in connection with a presumed receptor structure for each type of unit; it is likely that the prolonged discharges in the Ft I unit are produced by active involvement of Merkel cells, whereas those in Ft II units are the result of a direct activation of afferent nerve terminals.     

Tactile neural mechanisms in monotremes

Monotremes, perhaps more than any other order of mammals, display an enormous behavioural reliance upon the tactile senses. In the platypus, Ornithorhynchus anatinus, this is manifest most strikingly in the special importance of the bill as a peripheral sensory organ, an importance confirmed by electrophysiological mapping that reveals a vast area of the cerebral cortex allocated to the processing of tactile inputs from the bill. Although behavioural evidence in the echidna, Tachyglossus aculeatus, suggests a similar prominence for tactile inputs from the snout, there is also a great reliance upon the distal limbs for digging and burrowing activity, pointing to the importance of tactile information from these regions for the echidna. In recent studies, we have investigated the peripheral tactile neural mechanisms in the forepaw of the echidna to establish the extent of correspondence or divergence that has emerged over the widely different evolutionary paths taken by monotreme and placental mammals. Electrophysiological recordings were made from single tactile sensory nerve fibres isolated in fine strands of the median or ulnar nerves of the forearm. Controlled tactile stimuli applied to the forepaw glabrous skin permitted an initial classification of tactile sensory fibres into two broad divisions, according to their responses to static skin displacement. One displayed slowly adapting (SA) response properties, while the other showed a selective sensitivity to the dynamic components of the skin displacement. These purely dynamically-sensitive tactile fibres could be subdivided according to vibrotactile sensitivity and receptive field characteristics into a rapidly adapting (RA) class, sensitive to low frequency (相似文献   

Slowly adapting cutaneous mechanoreceptor afferent units associated with Merkel cells in frogs and effects of direct currents.

In the bullfrog, two types of slowly adapting (SA) cutaneous mechanoreceptor afferent units have been identified physiologically: irregularly discharging frog type I (Ft I) units in both warty and nonwarty skin, and regularly discharging frog type II (Ft II) units in the nonwarty skin. In the present study, mechanosensitive spots of Ft I units were located around the skin warts in the warty skin. The quinacrine technique (Crowe and Whitear, 1978) revealed that quinacrine-accumulating Merkel cells were present around the skin warts and near the orifice of skin glands that also surrounded the skin warts. Thus, a significant correlation was found between the location of Merkel cells and the receptive fields (RFs) of Ft I units in the warty skin. Direct current (DC) stimulation was applied for 1 sec to the skin inside and outside the mechanical RFs of the two types of SA units. RFs for DC stimulation were located on those for mechanical stimulation in both types of SA units. The current threshold required to produce a single spike was lower in cathodal than in anodal pulses in both types of SA units. Greater current intensity elicited an increased number of spikes, but the effective polarity of currents was anodal for Ft I units and cathodal for Ft II units. The optimal current intensity for producing prolonged discharges ranged from +60 to +100 microA in Ft I units and - from -50 to -80 microA in Ft II units. The sequence of impulses evoked was irregular in Ft I units and regular in Ft II units, as seen in mechanical responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional Diversity and Evolution of Bitter Taste Receptors in Egg-Laying Mammals

Egg-laying mammals (monotremes) are a sister clade of therians (placental mammals and marsupials) and a key clade to understand mammalian evolution. They are classified into platypus and echidna, which exhibit distinct ecological features such as habitats and diet. Chemosensory genes, which encode sensory receptors for taste and smell, are believed to adapt to the individual habitats and diet of each mammal. In this study, we focused on the molecular evolution of bitter taste receptors (TAS2Rs) in monotremes. The sense of bitter taste is important to detect potentially harmful substances. We comprehensively surveyed agonists of all TAS2Rs in platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus) and compared their functions with orthologous TAS2Rs of marsupial and placental mammals (i.e., therians). As results, the agonist screening revealed that the deorphanized monotreme receptors were functionally diversified. Platypus TAS2Rs had broader receptive ranges of agonists than those of echidna TAS2Rs. While platypus consumes a variety of aquatic invertebrates, echidna mainly consumes subterranean social insects (ants and termites) as well as other invertebrates. This result indicates that receptive ranges of TAS2Rs could be associated with feeding habits in monotremes. Furthermore, some orthologous receptors in monotremes and therians responded to β-glucosides, which are feeding deterrents in plants and insects. These results suggest that the ability to detect β-glucosides and other substances might be shared and ancestral among mammals.     

Meeting Announcement: Barrels XII-1999

(1)Microelectrodes were used to record the extracellular activity of 80 single neurons of the main cuneate nucleus (MCN) of raccoons anesthetized with either methoxyflurane or pentobarbital sodium. All 80 MCN neurons had peripheral receptive fields (RFs) that lay entirely on the glabrous surfaces of the forepaw and were responsive to light mechanical stimulation. Neurons were characterized according to the nature of their response to mechanical stimulation of their RFs, as well as to their response to electrical stimulation of the contralateral thalamic ventrobasal complex (VB). (2) All antidromically activated neurons (64% of sample) were histologically verified as falling within the clusters region of the MCN, while synaptically activated neurons (19% of sample), as well as neurons not responsive to VB stimulation (17% of sample), were located in both the clusters and the polymorphic regions. (3) Antidromically activated neurons typically responded with a single fixed-latency spike, although a few responded with a burst of 3 or more spikes. Others responded with a single antidromic spike followed by a train of synaptically activated spikes. In these latter neurons, it was often possible to block the synaptic spikes selectively. (4) MCN neurons were classed according to their response to controlled mechanical stimuli as rapidly adapting (RA), slowly adapting (SA), or Pacinian (Pc). The proportions of neurons falling into these categories did not vary significantly with the type of response to thalamic stimulation, and the overall percentages were 56% RA, 24% SA, and 20% Pc. These figures are very similar to those previously obtained in a sample of primary afferent fibers of the raccoon cervical cuneate fasciculus (L. M. Pubols and Pubols, 1973). (5) Absolute displacement, displacement velocity, and force thresholds, which ranged between 4 and 326 μm, 0.01 and 16.3 μm/msec, and 120 and 3600 mg, respectively, are comparable to those previously found for primary afferents supplying mechanoreceptors of the glabrous surfaces of the raccoon's forepaw. Neither displacement nor force thresholds differed for RA versus SA neurons; however, displacement velocity thresholds were significantly lower for SA than for RA neurons.     

Characteristics of dorsal horn neurons expressing subliminal responses to sural nerve stimulation

The present study was designed (1) to characterize the subliminal responses of dorsal horn neurons to stimulation of the sural nerve, and (2) to correlate the type of response to this stimulus with the responses to natural mechanical stimulation of the skin. To accomplish this, intracellular and extracellular recordings were carried out in L6 and L7 dorsal horn neurons in the cat. The excitatory responses of each cell to electrical stimulation of the sural nerve and to mechanical stimulation of the skin were noted. Of 35 dorsal horn cells recorded intracellularly, 11 responded with impulses to sural nerve stimulation, 9 responded with excitatory postsynaptic potentials (EPSPs) but not impulses, and 15 had no excitatory responses to this stimulus. The type of response to sural nerve stimulation was strongly correlated with receptive field modality. Most cells receiving an input from high-threshold cutaneous mechanoreceptors responded with impulses or gave no excitatory response to sural nerve stimulation, whereas most cells that had only low-threshold mechanoreceptor input responded with EPSPs only or gave no response. In cells with only low-threshold (LT) mechanoreceptive input, response to sural nerve stimulation was highly correlated with receptive field locus. Those LT cells with no excitatory responses to sural nerve stimulation had receptive fields confined to the foot and/or toes, whereas those that gave EPSPs had more proximal receptive fields. The possible significance of these data with reference to changes observed after lesions, such as increased response to sural nerve stimulation, increased receptive field size, and somatotopic reorganization, is discussed.     

15th Annual spring brain conference,march 10–13, 2004,summary report

The present study was designed (1) to characterize the subliminal responses of dorsal horn neurons to stimulation of the sural nerve, and (2) to correlate the type of response to this stimulus with the responses to natural mechanical stimulation of the skin. To accomplish this, intracellular and extracellular recordings were carried out in L⁶ and L⁷ dorsal horn neurons in the cat. The excitatory responses of each cell to electrical stimulation of the sural nerve and to mechanical stimulation of the skin were noted.

Of 35 dorsal horn cells recorded intracellularly, 11 responded with impulses to sural nerve stimulation, 9 responded with excitatory postsynaptic potentials (EPSPs) but not impulses, and 15 had no excitatory responses to this stimulus. The type of response to sural nerve stimulation was strongly correlated with receptive field modality. Most cells receiving an input from high-threshold cutaneous mechanoreceptors responded with impulses or gave no excitatory response to sural nerve stimulation, whereas most cells that had only low-threshold mechanoreceptor input responded with EPSPs only or gave no response. In cells with only low-threshold (LT) mechanoreceptive input, response to sural nerve stimulation was highly correlated with receptive field locus. Those LT cells with no excitatory responses to sural nerve stimulation had receptive fields confined to the foot and/or toes, whereas those that gave EPSPs had more proximal receptive fields. The possible significance of these data with reference to changes observed after lesions, such as increased response to sural nerve stimulation, increased receptive field size, and somatotopic reorganization, is discussed.     

Functional Properties of Slowly Adapting Mechanoreceptors in Cat Footpad Skin

The functional properties of slowly adapting (SA) afferent fibers innervating cat footpad skin were examined. Measurements were taken of receptive field area; spontaneous activity (< 1 impulse/sec); the slope of the stimulus-response curve for steady indentations up to 2 mm in amplitude; variability of the interimpulse intervals, as measured by the coefficient of variation of time interval histograms; decay of the response to steady indentation; and sensitivity to sinusoidal vibration (most sensitive at 5-10 Hz). Where comparable tests were performed on glabrous and hairy skin SA fibers, the functional properties of those in glabrous skin more closely resembled SAI fibers than SAII fibers. Additional results from glabrous skin SA fibers suggest that it is distortion of the nerve endings rather than steady indentation or compression that leads to a brisk response. On the measures described above, there appeared to be only one functional class of SA fiber innervating the cat footpad skin.     

Tactile receptor discharge and mechanical properties of glabrous skin

Current knowledge of the functional properties of mammalian cutaneous mechanoreceptors is reviewed with special reference to receptors associated with the glabrous skin of the raccoon and squirrel monkey hand. Four physiologically defined mechanoreceptor types are recognized: Pacinian afferents, rapidly adapting (RA), and slowly adapting type I (SAI), and slowly adapting type II (SAII). The SAI category is divided into moderately slowly adapting and very slowly adapting (VSA) types in terms of the duration of their response to a prolonged mechanical displacement of skin. Although both RA and SA units are capable of signaling displacement ramp velocity, the pattern of discharge during ramp stimulation may vary widely among units. SAI units also code the depth of skin displacement, but there is no best-fitting function describing the relationship. Static discharge is also markedly influenced by prior ramp velocity. Both raccoon and squirrel monkey VSA units show wide variation in the regularity of their discharge during static displacement. The rate of adaptation of SAI units is less when constant force stimuli are applied to the skin than when constant displacement stimuli are applied. This is partly attributable to mechanical properties of the skin. When either constant force or constant displacement stimuli are spaced too closely in time, there is a progressive (trial-to-trial) decrement in response rate, accounted for in part by failure of the skin to recover to its initial resting level.     

Responses of cutaneous mechanoreceptors within fingerpad to stimulus information for tactile softness sensation of materials

Softness sensation is one of primitive tactile textures. While the psychophysical characteristics of softness sensation have been thoroughly studied, it is lack of a deep understanding of the underlying neuromechanical principles. On the stimulus–response processes of human fingerpad touching fabrics and the physiological properties of slowly adapting type I (SAIs) cutaneous mechanoreceptors within fingerpad, a fabric-skin-receptor coupling model was built and validated. By the fabric-skin-receptor model a series of numerical experiments was conducted, and how the evoked neural responses of cutaneous mechanoreceptors change with the composite compliance of both fingerpad skin and the materials in contact was investigated. The results indicated that the evoked neural responses of populations of cutaneous mechanoreceptors by the physical stimulus from fabrics were nearly proportional to the perceived softness magnitude, and nonlinearly increased and then decreased with the effective elastic modulus of fabrics or the relative elastic modulus of fabrics to soft tissues within fingerpad, where the nonlinear inflection point depended on the touching force level. Therefore, it concluded that the tactile judgment of the physical information for softness sensation of objects was an encoding of neural responses of populations of SAIs cutaneous mechanoreceptors, and the physical information depended on the mechanical interaction of fingerpad and objects in contact.     

Relations between Stimulus Force,Skin Displacement,and Discharge Characteristics of Slowly Adapting Type I Cutaneous Mechanoreceptors in Glabrous Skin of Squirrel Monkey Hand

(1) The contributions of viscoelastic properties of squirrel monkey glabrous skin to slowly adapting Type I (SAI) mechanore-ceptive afferent fiber discharge were examined in the present study. Individual fibers of the median and ulnar nerves were isolated by microdissection in six monkeys anesthetized with pentobarbital sodium. Utilizing mechanical stimulation and data analysis techniques identical to those of a previous study of raccoon glabrous skin and its mechanoreceptors (Pubols, 1982a; Pubols and Maliniak, 1984), we studied and compared responses to punctate mechanical stimuli controlled with respect to force or displacement. (2) Squirrel monkey glabrous skin was found to be more compliant than raccoon glabrous skin, in that a given force applied to either a digital or a palmar skin pad produced a greater displacement of squirrel monkey skin. Skin displacement increased approximately linearly with increasing forces at the beginning of static stimulation, but over time (at least up to 20 sec), the relationship became negatively accelerated. (3) Absolute-force thresholds of individual SAI units were significantly lower in squirrel monkey (mean = 122 mg, range = 48-340 mg) than in raccoon (mean =484 mg, range = 70-1,290 mg). However, absolute-displacement thresholds were insignificantly lower (squirrel monkey: mean = 17.24 μm, range = 5-30μ raccoon: mean = 30 μm, range = 5-185 μm). (4) Application of suprathreshold forces (range = 1-20 g) and displacements (range = 500-1,000 μm) revealed greater interunit variability in response to maintained stimulation than previously found in raccoon. In 8 out of 15 fibers, the rate of adaptation was significantly greater during constant-displacement than during constant-force stimulation; in 4 cases there was no significant difference; and in 3 cases the rate of adaptation was significantly greater during constant-force stimulation. (5) Potential sources of interunit variability include surface topography of the hand, properties of cutaneous and subcutaneous tissues in the vicinity of the receptor, and experimental variables such as stimulus amplitude and rate of stimulus onset. (6) It is suggested that both regional and species differences in functional properties of cutaneous mechanore-ceptors are more likely attributable to differences in mechanical properties of skin and subjacent tissues than to any inherent differences in receptor properties.     

The frequency selectivity of information-processing channels in the tactile sensory system

The frequency selectivity of the P, NP I, and NP II channels of the four-channel model of mechanoreception for glabrous skin was measured psychophysically by an adaptation tuning curve procedure. The results substantially extend the frequency range over which the frequency selectivity of these channels is known and further confirm the hypothesis that the input stage of each of these channels consists of specific sensory nerve fibers and associated receptors. Specifically, the frequency characteristics of Pacinian nerve fibers, rapidly adapting (RA) nerve fibers, and slowly adapting Type II (SA II) nerve fibers were found to be the peripheral neurophysiological correlates of the P, NP I, and NP II channels, respectively. The finding that the tuning characteristic for a test stimulus of 250 Hz delivered through a small (0.008 cm2) contactor depended dramatically on the duration of the test stimulus whereas the detection threshold did not, provides new evidence in support of the hypothesis that separate NP II and P channels exist.     

SA-I mechanoreceptor position in fingertip skin may impact sensitivity to edge stimuli

BACKGROUND: The skin plays a role in conditioning mechanical indentation into distributions of stress/strain that mechanoreceptors convert into neural signals. Solid mechanics methods have modelled the skin to predict the in vivo neural response from mechanoreceptors. Despite their promise, current models cannot explain the role that anatomical positioning and receptor organ morphology play in producing differences in neural response. This work hypothesises that the skin's intermediate ridges may help explain, in part, the sensitivity of slowly adapting type I (SA-I) mechanoreceptors to edge stimuli. METHOD: Two finite-element models of the fingertip were built, validated and used to analyse the functionality of the intermediate ridges. One of the two-dimensional, cross-sectional models included intermediate ridges, while the other did not. The analysis sought to determine if intermediate ridges (1) increase the magnitude of strain energy density (SED) near the SA-I location and (2) help differentiate one 2.0-mm indenter from two 0.5-mm wide indenters with a 1.0-mm gap. RESULTS: Higher concentrations of SED were found near the tips of the intermediate ridges, the anatomical location that coincides with the SA-I receptors. This first result suggested that the location of the SA-Is in the stiffer epidermal tissue helps magnify their response to edge stimuli. The second result was that both models were equally capable of predicting the spatial structure within the in vivo neural responses, and therefore the addition of intermediate ridges did not help in differentiating the indenters. CONCLUSION: The finding, a 15%-35% increase in response when the sampling point lies within the stiffer tissue at the same depth, seeks to inform the positioning of force sensors in robotic skin substrates.     

3-D finite-element models of human and monkey fingertips to investigate the mechanics of tactile sense

The biomechanics of skin and underlying tissues plays a fundamental role in the human sense of touch. It governs the mechanics of contact between the skin and an object, the transmission of the mechanical signals through the skin, and their transduction into neural signals by the mechanoreceptors. To better understand the mechanics of touch, it is necessary to establish quantitative relationships between the loads imposed on the skin by an object, the state of stresses/strains at mechanoreceptor locations, and the resulting neural response. Towards this goal, 3-D finite-element models of human and monkey fingertips with realistic external geometries were developed. By computing fingertip model deformations under line loads, it was shown that a multi-layered model was necessary to match previously obtained in vivo data on skin surface displacements. An optimal ratio of elastic moduli of the layers was determined through numerical experiments whose results were matched with empirical data. Numerical values of the elastic moduli of the skin layers were obtained by matching computed results with empirically determined force-displacement relationships for a variety of indentors. Finally, as an example of the relevance of the model to the study of tactile neural response, the multilayered 3-D finite-element model was shown to be able to predict the responses of the slowly adapting type I (SA-I) mechanoreceptors to indentations by complex object shapes.     

Changes in skin levels of two neutotrophins （glial cell line derived neurotrohic factor and neurotrophin-3） cause alterations in cutaneous neuron responses to mechanical stimuli

Neurotrophins are important for the development and maintenance of both high and low threshold mechanoreceptors (HTMRs and LTMRs). In this series of studies, the effects of constitutive overexpression of two different neurotrophins, neurotrophin-3 (NT-3) and glial cell line derived neurotrohic factor (GDNF), were examined. Previous studies indicated that both of them may be implicated in the normal development of mouse dorsal root ganglion (DRG) neurons. Neurons from mice transgenically altered to overexpress NT-3 or GDNF (NT-3-OE or GDNF-OE mice) in the skin were examined using several physiological, immunohistochemi-cal and molecular techniques. Ex vivo skin/nerve/DRG/spinal cord and skin/nerve preparations were used to determine the response characteristics of the cutaneous neurons; immunohistochemistry was used to examine the biochemical phenotype of DRG cells and the skin; RT-PCR was used to examine the levels of candidate ion channels in skin and DRG that may correlate with changes in physiologi-cal responses. In GDNF-OE mice, I-isolectin B4 (IB4)-immunopositive C-HTMRs (nociceptors), a large percentage of which are sensitive to GDNF, had significantly lower mechanical thresholds than wildtype (WT) neurons. Heat thresholds for the same cells were not different. Mechanical sensitivity changes in GDNF-OE mice were correlated with significant increases in acid sensing ion channels 2a (ASIC2a) and 2b (ASIC2b) and transient receptor potential channel AI (TRPAI), all of which are putative mechanosensitive ion channels. Overexpression of NT-3 affected the responses of A-LTMRs and A-HTMRs, hut had no effect on C-HTMRs. Slowly adapting type 1 (SA1) LTMRs and A-HTMRs had increased mechanical sensitivity compared to WT. Mechanical sensitivity was correlated with significant increases in acid-sensing ion channels ASIC1 and ASIC3. This data indicates that both neurotrophins play roles in determining mechanical thresholds of cutaneous HTMRs and LTMRs and that sensitivity changes involve the ASIC family of putative mechanoreceptive ion channels.     

Electroreception in the Guiana dolphin (Sotalia guianensis)

Passive electroreception is a widespread sense in fishes and amphibians, but in mammals this sensory ability has previously only been shown in monotremes. While the electroreceptors in fish and amphibians evolved from mechanosensory lateral line organs, those of monotremes are based on cutaneous glands innervated by trigeminal nerves. Electroreceptors evolved from other structures or in other taxa were unknown to date. Here we show that the hairless vibrissal crypts on the rostrum of the Guiana dolphin (Sotalia guianensis), structures originally associated with the mammalian whiskers, serve as electroreceptors. Histological investigations revealed that the vibrissal crypts possess a well-innervated ampullary structure reminiscent of ampullary electroreceptors in other species. Psychophysical experiments with a male Guiana dolphin determined a sensory detection threshold for weak electric fields of 4.6 μV cm(-1), which is comparable to the sensitivity of electroreceptors in platypuses. Our results show that electroreceptors can evolve from a mechanosensory organ that nearly all mammals possess and suggest the discovery of this kind of electroreception in more species, especially those with an aquatic or semi-aquatic lifestyle.