Central gustatory projections and side-specificity of operant antennal muscle conditioning in the honeybee

Gustatory stimuli to the antennae, especially sucrose, are important for bees and are employed in learning paradigms as unconditioned stimulus. The present study identified primary antennal gustatory projections in the bee brain and determined the impact of stimulation of the antennal tip on antennal muscle activity and its plasticity. Central projections of antennal taste hairs contained axons of two morphologies projecting into the dorsal lobe, which is also the antennal motor centre. Putative mechanosensory axons arborised in a dorso-lateral area. Putative gustatory axons projected to a ventro-medial area. Bees scan gustatory and mechanical stimuli with their antennae using variable strategies but sensory input to the motor system has not been investigated in detail. Mechanical, gustatory, and electrical stimulation of the ipsilateral antennal tip were found to evoke short-latency responses in an antennal muscle, the fast flagellum flexor. Contralateral gustatory stimulation induced smaller responses with longer latency. The activity of the fast flagellum flexor was conditioned operantly by pairing high muscle activity with ipsilateral antennal sucrose stimulation. A proboscis reward was unnecessary for learning. With contralateral antennal sucrose stimulation, conditioning was unsuccessful. Thus, muscle activity induced by gustatory stimulation was important for learning success and conditioning was side-specific.     

Improving stable isotopic interpretations made from human hair through reduction of growth cycle error

A recent trend in stable isotopic analysis involves the reconstruction of short-term variations in diet using hair segments. However, bulk hair samples typically contain a growth cycle error, which may conceal or confound the most recently incorporated isotopic information. It is assumed that, at any given time, ～85-90% of scalp hairs are actively growing, while the remaining 10-15% have transitioned into a resting or inactive phase, which lasts up to 4 months before hairs are shed. This study uses growth phase to determine the effects of age, sex, and health status on carbon and nitrogen isotopic ratios of hair analyzed in sequential segments. For this study, we selected archaeological hair samples from 10 individuals from Dakhleh Oasis, Egypt. Isotopic analyses of actively growing hair segments were compared to those for mixed growth phase segments from each individual. These data demonstrate the presence of growth cycle error and show that an understanding of structural-functional relationships is essential for interpreting normal versus pathological changes in hair follicle and fiber production. In situations where diet change and mobility produce variations in an individual's isotopic composition, elimination of positional-temporal error in sequential segment hair analyses can facilitate greater understanding of intraindividual metabolic reactions and changes in hair growth cycles. Phase identification may aid in determining the presence of pathological conditions in individuals, especially in those lacking skeletal indications, and provide a more precise estimation of seasonal dietary patterns, access to changing food resources, and metabolic equilibration to a new locality.     

The innervation and chemical sensitivity of single aesthetasc hairs

Summary Each aesthetasc hair of the lateral antennule of the California spiny lobsterPanulirus interruptus (Randall) is shown by light and scanning electron microscopy to be innervated by a basally situated cluster of sensory neurons encased in a glial sheath which isolates each cluster from those of other hairs (Figs. 1, 3, 4). The dendrites of these neurons penetrate the aesthetasc hairs and their axons extend to the central nervous system. Extracellular recordings with suction electrodes from the axons of single neuronal clusters were used to determine the responsiveness of individual hairs to a spectrum of amino acids, amines, amides, carbohydrates, carboxylic acids, nucleotides, and a tripeptide (Tables 1, 2, Figs. 6, 8). Randomly selected hairs from the antennules of juvenile, and male and female adult lobsters were shown to be broadly sensitive to a variety of stimuli and are homogeneous in their breadth of responsiveness (Figs. 5, 7). Cluster analysis does not reveal distinct chemoreceptive hair types based on their response spectra, suggesting that the receptor populations of single hairs are uniformly competent to respond to diverse chemical stimuli (Figs. 6, 8). Further, the sensitivity profile of aesthetascs to these stimuli correlates well with behavioral responses ofPanulirus interruptus to these same stimuli (Tables 1, 2).Abbreviation ² Chi-squared     

The organization of plurisegmental mechanosensitive interneurons in the central nervous system of the wandering spider Cupiennius salei

Summary In spiders the bulk of the central nervous system (CNS) consists of fused segmental ganglia traversed by longitudinal tracts, which have precise relationships with sensory neuropils and which contain the fibers of large plurisegmental interneurons. The responses of these interneurons to various mechanical stimuli were studied electrophysiologically, and their unilateral or bilateral structure was revealed by intracellular staining. Unilateral interneurons visit all the neuromeres on one side of the CNS. They receive mechanosensory input either from a single leg or from all ipsilateral legs via sensory neurons that invade leg neuromeres and project into specific longitudinal tracts. The anatomical organization of unilateral interneurons suggests that their axons impart their information to all ipsilateral leg neuromeres. Bilateral interneurons are of two kinds, symmetric and asymmetric neurons. The latter respond to stimulation of all legs on one side of the body, having their dendrites amongst sensory tracts of the same side of the CNS. Anatomical evidence suggests that their terminals invade all four contralateral leg neuromeres. Bilaterally symmetrical plurisegmental interneurons have dendritic arborizations in both halves of the fused ventral ganglia. They respond to the stimulation of any of the 8 legs. A third class of cells, the ascending neurons have unilateral or bilateral dendritic arborizations in the fused ventral ganglia and show blebbed axons in postero-ventral regions of the brain. Their response characteristics are similar to those of other plurisegmental interneurons. Descending neurons have opposite structural polarity, arising in the brain and terminating in segmental regions of the fused ventral ganglia. Descending neurons show strong responses to visual stimulation. Approximately 50% of all the recorded neurons respond exclusively to stimulation of a single type of mechanoreceptor (either tactile hairs, or trichobothria, or slit sensilla), while the rest respond to stimulation of a variety of sensilla. However, these functional differences are not obviously reflected by the anatomy. The functional significance of plurisegmental interneurons is discussed with respect to sensory convergence and the coordination of motor output to the legs. A comparison between the response properties of certain plurisegmental interneurons and their parent longitudinal tracts suggests that the tracts themselves do not reflect a modality-specific organization.Abbreviations BPI bilateral plurisegmental interneuron - CNS central nervous system - FVG fused ventral ganglia - LT longitudinal tract - PI plurisegmental interneuron - PSTH peristimulus timehistogram - UPI unilateral plurisegmental interneuron     

A computational fluid dynamics model of viscous coupling of hairs

Arrays of arthropod filiform hairs form highly sensitive mechanoreceptor systems capable of detecting minute air disturbances, and it is unclear to what extent individual hairs interact with one another within sensor arrays. We present a computational fluid dynamics model for one or more hairs, coupled to a rigid-body dynamics model, for simulating both biological (e.g., a cricket cercal hair) and artificial MEMS-based systems. The model is used to investigate hair–hair interaction between pairs of hairs and quantify the extent of so-called viscous coupling. The results show that the extent to which hairs are coupled depends on the mounting properties of the hairs and the frequency at which they are driven. In particular, it is shown that for equal length hairs, viscous coupling is suppressed when they are driven near the natural frequency of the undamped system and the damping coefficient at the base is small. Further, for certain configurations, the motion of a hair can be enhanced by the presence of nearby hairs. The usefulness of the model in designing artificial systems is discussed.     

The shape of wind-receptor hairs of cricket and cockroach

We examined the exact shapes of the thread-like wind-receptor hairs in the cricket and cockroach. The diameters of hairs at various distances from the hair tip as measured by scanning electron microscopy revealed unexpected hair shapes. We had expected, a priori, that the shape of the hair would be a slender linearly tapered cone, but the measurements revealed hairs in the form of extremely elongated paraboloids. The diameter of the wind-receptor hairs varies with the square root of the distance from the hair tip, i.e., the diameter rapidly increases with the distance from the tip and is asymptotic to the base diameter. Both the cricket, Gryllus bimaculatus, and the cockroach, Periplaneta americana, showed the same hair shape. In both insects, the formation of the wind-receptor hair during metamorphosis seems to be controlled by a common cytological program. The shape of the hair constrains the mobility of the wind-receptor hair, because both the drag force caused by moving air and the moment of inertia of motion dynamics are functions of shaft diameter. The shape of the hair is a biological trait which affects the sensory information transmitted to the central nervous system. Accepted: 24 February 1998     

Dual pathways for tactile sensory information to thoracic interneurons in the cockroach

The escape system of the American cockroach is both fast and directional. In response to wind stimulation both of these characteristics are largely due to the properties of the ventral giant interneurons (vGIs), which conduct sensory information from the cerci on the rear of the animal to type A thoracic interneurons (TI_As) in the thoracic ganglia. The cockroach also escapes from tactile stimuli, and although vGIs are not involved in tactile-mediated escapes, the same thoracic interneurons process tactile sensory information. The response of TI_As to tactile information is typically biphasic. A rapid initial depolarization is followed by a longer latency depolarization that encodes most if not all of the directional information in the tactile stimulus. We report here that the biphasic response of TI_As to tactile stimulation is caused by two separate conducting pathways from the point of stimulation to the thoracic ganglia. Phase 1 is generated by mechanical conduction along the animal's body cuticle or other physical structures. It cannot be eliminated by complete lesion of the nerve cord, and it is not evoked in response to electrical stimulation of abdominal nerves that contain the axons of sensory receptors in abdominal segments. However, it can be eliminated by lesioning the abdominal nerve cord and nerve 7 of the metathoracic ganglion together, suggesting that the relevant sensory structures send axons in nerve 7 and abdominal nerves of anterior abdominal ganglia. Phase 2 of the TI_As tactile response is generated by a typical neural pathway that includes mechanoreceptors in each abdominal segment, which project to interneurons with axons in either abdominal connective. Those interneurons with inputs from receptors that are ipsilateral to their axon have a greater influence on TI_As than those that receive inputs from the contralateral side. The phase 1 response has an important role in reducing initiation time for the escape response. Animals in which the phase 2 pathway has been eliminated by lesion of the abdominal nerve cord are still capable of generating a partial startle response with a typically short latency even when stimulated posterior to the lesion. © 1995 John Wiley & Sons, Inc.     

The innervation of the wind-sensitive head hairs of the locust, Schistocerca gregaria

ABSTRACT. The innervation of the locust head hairs was investigated by filling the sub-cuticular neurones with cobalt and by recording antidromic spikes at individual hairs when the circumoesophageal and cervical connectives were stimulated. The central projections from most head hairs ( c. 80%) terminate in the suboesophageal ganglion, whereas about 20% extend into the thoracic ganglia. Sensilla with projections to the thoracic ganglia are structurally no different from those whose fibres terminate in the suboesophageal ganglion and they are scattered throughout the hair fields with no consistent positions in different individuals. We have re-investigated the 'accessory response' (Camhi, 1969) and conclude that it is not indicative of a separate afferent or efferent system but is simply cross-talk from adjacent axons. We relate our findings to the fibre composition of the dorsal tegumentary nerve.     

Central projections of campaniform sensilla on the cerci of crickets and cockroaches

Summary Campaniform sensilla associated with filiform hairs comprise an important receptor type of the multimodal sensory system of the cerci of crickets and cockroaches. Their axon projections were investigated using iontophoretic cobalt injection into single sensilla.In crickets (Gryllus bimaculatus, Acheta domestica), six different types of cereal campaniform sensilla projections can be distinguished on the basis of their axonal arborizations and terminations. Typically, a proportion of cereal campaniform sensilla, associated with long filiform hairs, give rise to axons that ascend as through fibres from the terminal ganglion to reach the sixth abdominal ganglion. Cereal campaniform sensilla associated with clavate hairs have projections restricted to the terminal ganglion alone.Whereas in crickets axons of cercal campaniform sensilla invade only certain segmental neuropils in the terminal ganglion, in cockroaches (Periplaneta americana) axons from cercal campaniform sensilla branch in every segmental neuropil. A proportion of cereal campaniform sensilla in this species also gives rise to through fibres to the fifth abdominal ganglion.We discuss morphological and functional interpretations of differences between crickets and cockroaches and consider the significance of this type of receptor in the context of previous studies of the cercal system.     

Giant and dwarf axons in a miniature insect,Encarsia formosa, (Hymenoptera,Calcididae)

Miniaturization effects in the central nervous system (CNS) of a very small calchicid wasp, Encarsia formosa (0.6 mm long), are obvious for the overall morphology and at the level of axon sizes. Parasagittal sections show that most ganglia are fused and leave connectives only in the neck and the petiole. The thoracic complex is partly squeezed between muscles, enwraps cuticular apodemes and protrudes laterally into the coxae of legs. Somata of neurons are similar in size and form a multiple layer around large neuropile regions of the CNS. In TEM sections of connectives the range of axon diameters lies between 0.045 and 3.8 μm. Extremely small axon diameters below 0.1 μm are supposed to present spatial restrictions for ion channels and internal organelles. In theory, that can cause frequent spontaneous releases of action potentials (AP) which impede regular information transfer by normal APs. Therefore, axon sizes were studied in connectives between ganglia where longer distance information transfer requires action potentials even in the smallest axons. The diameters of many interganglionic axons below 0.08 μm contradict the theory. The luxury of large axon diameters exceeding 2–3 μm is reserved for several “giant” interneurons in the thoracic and in the abdominal ganglion complex. They should belong to rapid sensory alerting systems. The largest, a bilateral pair in the abdominal CNS, could integrate afferents from long wind sensitive hairs on the abdomen.     

Behavioral responses of Phormia regina (Meigen) to labellar stimulation with amino acids

Behavioral responses to labellar stimulation with 19 L-amino acids were predicted on the basis of electrophysiological responses of largest labellar hairs. With the exceptions alanine, aspartic and glutamic acids, and valine tests of these predictions confirmed that Phormia can discriminate amino acids, and that these acids may be grouped according to their effects. Electrophysiological investigation of the four exceptions was repeated and results were consistent with the behavioral data. In particular, these acids elicited previously unreported responses from the salt receptor. The discrepancies between this and earlier studies may be explainable, in part, on methodological grounds. There was evidence for response differences among hairs of different sizes and among the largest labellar hairs themselves. The significance of amino acid discrimination for the problem of protein recognition can only be speculated upon until more complete electrophysiological and nutritional information is available.     

Fine-Structural Characterization of the Somatic Innervation of the Tympanic Membrane in Normal,Sympathectomized, and Neurotoxin-Denervated Rats

The pars tensa of the rat tympanic membrane (TM) consists largely of a lamina propria of specialized unbanded collagen bounded on the outer surface by an unusually thin epidermal layer and on the inner surface by a flat, single-cell mucosal layer. The mucosal layer is innervated solely by unmyelinated (C) axons, whereas the cutaneous layer is supplied by both myelinated and C axons. The outer surface differs from general body skin, lacking dermal papillae, hairs, sweat glands, and distinctive dermal corpuscular structures.

Epidermal innervation includes distinctive terminals in the basal layer, unassociated with Merkel cells, and deeper intraepidermal smaller endings containing accumulations of mitochondria and vesicles. The sensory nature of these endings can be inferred by their extensive, but not total, elimination following neonatal capsaicin treatment (a potent neurotoxin for thin sensory fibers) and their preservation following surgical or neurotoxin sympathectomy.

The thin mucosal epithelium displays capillaries and beaded axons close to the free surface of the middle ear. The unmyelinated terminals contain predominantly large, dense-core vesicles (LDCVs). Capsaicin treatment results in extensive elimination of terminals containing LDCVs in surface epithelia. A possible small trophic influence of sensory thin-fiber supply was noted on the development of the epidermal layers. The sensory modalities elicited by natural stimulation of the TM is considered in relation to the pattern of innervation.     

Silencing normal input permits regenerating foreign afferents to innervate an identified crayfish sensory interneuron

Interneuron A, an identified first-order sensory interneuron that is innervated by mechanoreceptors on one side of the crayfish tailfan, normally resists extra innervation by regenerating contralateral mechanoreceptor axons. However, if its normal innervation is silenced by covering mechanosensory hairs with a surgical glue, it accepts contralateral innervation. This finding on an arthropod provides evidence for the generality and antiquity of the principle, critical in development of the vertebrate nervous system, that activity of one set of afferents can control whether other afferents form synapses on a target.     

Electrical activity in the chemoreceptors of the blowfly. I. Responses to chemical and mechanical stimulation

The electrical responses of the neurons associated with the various types of chemosensory hairs of the blowfly, Phormia regina Meigen, following stimulation by chemical and mechanical means have been studied. The singly innervated chemosensory hairs on the ovipositor, maxillary palpi, and antennae respond vigorously to chemical stimulation, but not to mechanical stimulation. The triply innervated chemosensory hairs on the labellum, tarsus, and wing have two neurons which respond only to chemical stimuli. The third neuron responds only to mechanical stimulation. The differential responses of the two chemosensory neurons to various chemical stimuli following the removal of the tip of the hair suggest that the structures responsible for chemoreception are located throughout the distal processes of these neurons. The response of the third neuron to mechanical stimulation is similar to the response recorded from the neuron associated with one type of tactile hair which responds to motion and not to steady deformation. Recordings have been made from the neurons associated with purely tactile hairs using the cut hair as an extension of the micropipette. The mechanosensory neuron of the wing chemosensory hair is capable of responding at the rate of at least 600 impulses per sec. and may serve to indicate changes in air flow over the wing surfaces during flight to enable the fly to correct the wing camber and attack angle.     

Neural correlates of habituation of the proleg withdrawal reflex in larvae of the hawk moth, Manduca sexta

The larval proleg withdrawal reflex of the hawk moth, Manduca sexta, exhibits robust habituation. This reflex is evoked by deflecting one or more mechanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated deflection of a single planta hair caused a significant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Significant response decrement was seen for interstimulus intervals of 10 s, 60 s and 5 min. Response decrement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously following a rest, and electrical stimulation of a body wall nerve facilitated the decremented response (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair deflections. However, when adaptation was eliminated by direct electrical stimulation of sensory neurons, the response in the proleg motor nerve still decreased significantly. Muscle recordings indicated that the response of an identified proleg retractor motor neuron decreased significantly during habituation training. Thus, habituation of the proleg withdrawal reflex includes a central component that is apparent at the level of a single motor neuron. Accepted: 20 December 1996     

Differential effects of nitric oxide on the responsiveness of tactile hairs

The responses of tactile hairs located on legs of the desert locust Schistocerca gregaria (Forskål) are modulated by nitric oxide (NO). There are two types of tactile hair on the tibia of the hind leg of the locust which differ in their thresholds for mechanical stimulation, their location on the leg and in the effect of NO on their responses to deflection. The spike response rates of mechanosensory neurons of low-threshold hairs decreased when exposed to elevated NO levels caused by perfusion of the leg with saline containing the NO donor PAPANONOate. In contrast, in high-threshold hairs, which show low responsiveness under control conditions, an increase in spike rates was observed during PAPANONOate application. These opposing effects of NO reduce the differences in the spike responses of the two types of tactile hairs to mechanical stimulation and are likely to have an impact on behaviours elicited by mechanical stimulation of the legs.     

A model of motor and sensory axon activation in the median nerve using surface electrical stimulation

Surface electrical stimulation has the potential to be a powerful and non-invasive treatment for a variety of medical conditions but currently it is difficult to obtain consistent evoked responses. A viable clinical system must be able to adapt to variations in individuals to produce repeatable results. To more fully study the effect of these variations without performing exhaustive testing on human subjects, a system of computer models was created to predict motor and sensory axon activation in the median nerve due to surface electrical stimulation at the elbow. An anatomically-based finite element model of the arm was built to accurately predict voltages resulting from surface electrical stimulation. In addition, two axon models were developed based on previously published models to incorporate physiological differences between sensory and motor axons. This resulted in axon models that could reproduce experimental results for conduction velocity, strength-duration curves and activation threshold. Differences in experimentally obtained action potential shape between the motor and sensory axons were reflected in the models. The models predicted a lower threshold for sensory axons than motor axons of the same diameter, allowing a range of sensory axons to be activated before any motor axons. This system of models will be a useful tool for development of surface electrical stimulation as a method to target specific neural functions.     

The sensory basis for the trap-jaw mechanism in the ant Odontomachus bauri

Ants of the ponerine genus Odontomachus have evolved a mechanism that allows them to instantaneously close their long mandibles to catch prey or defend themselves. This trap-jaw action is triggered by contact of trigger hairs with a potential prey item. Two of these long mechanosensory hair sensilla reside proximally on each mandible and are supplied by giant sensory cells.Extracellular recordings demonstrate that the sensory cells respond to tactile stimulation. Their phasic responses encode amplitude and velocity of hair-deflection away from the midline, but not hair position. The discharge of action potentials follows stimulus frequencies of more than 300 Hz. During sinusoidal stimulation, the cells adapt very little, sustain discharge rates of more than 200 Hz for more than 20 s, and reach peak spike rates of about 450 Hz.The afferent axons of these sensory cells give rise to huge axon terminals within the suboesophageal ganglion. One of the afferents has a prominent contralateral branch, the other is confined to ipsilateral neuropil. Anatomical data indicate that the 4 afferents may be coupled and may serve as the substrate for a very fast reflex.Abbreviations HRP horseradish peroxidase - LGS lateral giant sensillum - MGS median giant sensillum - SEM scanning electron microscopy - SOG suboesophageal ganglion     

The external morphology of contact-chemoreceptive hairs of flies and the motility of the tips of these hairs

The external morphology of contact-chemoreceptive hairs (taste hairs) of six fly species, Calliphora vicina, Lucilia caesar, Musca domestica, Phormia terranovae, Sarcophaga carnaria and Stomoxys calcitrans, is described. The species can be distinguished by the differences between the patterns of taste hairs at the ventral side of their prothoracic tarsi. Taste hairs can be subdivided into morphological types, using the shape of the cuticle around the apical pore as criterion, even though this shape changes slightly on opening and closing of the pore. Light microscopical studies reveal that the nature and osmolarity of stimuli are decisive for the effect stimuli have on the shape of the top of the labellar hairs. The motions of the apical cuticle appear to be reversible. Gentle ultrasonic treatment preserves the shape of the cuticle of the top and the diameter of the pores on fluid stimulation. This technique makes it possible to study the effect of a previous stimulation on both tarsal and labellar hairs with the scanning electron microscope. It is supposed that stimuli can affect cuticular components around the pore, producing volume changes in that cuticle which alter the diameter of the pore.     

Axonal Noise as a Source of Synaptic Variability

Post-synaptic potential (PSP) variability is typically attributed to mechanisms inside synapses, yet recent advances in experimental methods and biophysical understanding have led us to reconsider the role of axons as highly reliable transmission channels. We show that in many thin axons of our brain, the action potential (AP) waveform and thus the Ca⁺⁺ signal controlling vesicle release at synapses will be significantly affected by the inherent variability of ion channel gating. We investigate how and to what extent fluctuations in the AP waveform explain observed PSP variability. Using both biophysical theory and stochastic simulations of central and peripheral nervous system axons from vertebrates and invertebrates, we show that channel noise in thin axons (<1 µm diameter) causes random fluctuations in AP waveforms. AP height and width, both experimentally characterised parameters of post-synaptic response amplitude, vary e.g. by up to 20 mV and 0.5 ms while a single AP propagates in C-fibre axons. We show how AP height and width variabilities increase with a ¾ power-law as diameter decreases and translate these fluctuations into post-synaptic response variability using biophysical data and models of synaptic transmission. We find for example that for mammalian unmyelinated axons with 0.2 µm diameter (matching cerebellar parallel fibres) axonal noise alone can explain half of the PSP variability in cerebellar synapses. We conclude that axonal variability may have considerable impact on synaptic response variability. Thus, in many experimental frameworks investigating synaptic transmission through paired-cell recordings or extracellular stimulation of presynaptic neurons, causes of variability may have been confounded. We thereby show how bottom-up aggregation of molecular noise sources contributes to our understanding of variability observed at higher levels of biological organisation.