Thermomechanical properties of the stimulus transducing cuticle in the infrared organ of Merimna atrata (Coleoptera,Buprestidae)

The pyrophilous Australian “fire‐beetle” Merimna atrata strongly depends on the occurrence and localization of forest fires for its reproduction. As a special adaptation to its unusual biology, elaborate infrared (IR) organs have evolved in this species. The IR‐organs consist of a specialized cuticular portion, the absorbing area, innervated by a sensory complex. The sensory complex contains a thermosensitive multipolar neuron with a specialized dendritic region, the terminal dendritic mass, and a mechanosensitive unit represented by a chordotonal organ (CO). Evidence for the IR‐receptive function so far has only been provided for the multipolar neuron. Based on morphological data, it has been hypothesized that the CO could also be involved in IR‐reception by measuring minute thermal deformations of the absorbing area. To test this hypothesis, we investigated structural features like cuticle thickness, reduced Young's modulus and hardness of the absorbing area. The results were used in finite element simulations to analyze the thermomechanical behavior and performance of the IR‐organ. Our findings indicate that considerable thermal deformation of the absorbing area occurs, supporting the hypothesis that the CO could function as photomechanical IR‐receptor. Interestingly, at the innervation site of the CO the lowest relative displacements of the absorbing area were found. This may indicate that the CO as putative photomechanic IR‐receptor has not been adapted according to the requirements of highest sensitivity. Probable benefits of the bimodal innervation by a thermosensory and a mechanosensory unit and their possible interaction for an improved performance of the IR‐organ are discussed. J. Morphol. 275:991–1003, 2014. © 2014 Wiley Periodicals, Inc.     

Variation in number and differentiation of the abdominal infrared receptors in the Australian 'fire-beetle' Merimna atrata (Coleoptera, Buprestidae)

Most individuals of the Australian ‘fire-beetle’ Merimna atrata have two pairs of IR receptors which are located ventrolaterally on the second and third abdominal sternite. An IR receptor consists of a specialized IR absorbing area, which is innervated by a neural complex. This complex contains one thermoreceptive multipolar neuron with a unique terminal dendritic mass (TDM) and two scolopidia and was termed ‘sensory complex’. However, also individuals with one pair of IR receptors on the second sternite and beetles with three pairs on the second, third, and fourth sternites were found. Additionally, beetles having one or two pairs of IR receptors may have preliminary stages of IR receptors on the third and fourth sternite, respectively. We found two kinds of preliminary stages, both of which are characterized by a much less pronounced absorbing area. In all five abdominal sternites segmental nerves are attached to the cuticle with a neural complex. Investigation of complexes of non-IR sternites suggests that the sensory cells inside the sensory complex of an IR receptor have developed from common internal stretch receptors. From our results it can be hypothesized that the IR sensory system in Merimna atrata has not yet reached a stage, which can be regarded as evolutionary stable.     

Bimodal innervation of the infrared organ of Merimna atrata (Coleoptera,Buprestidae) by thermo- and mechanosensory units

The pyrophilous Australian “fire-beetle” Merimna atrata approaches forest fires and possesses abdominal infrared (IR) organs. Each round IR organ is centrally innervated by a sensory complex showing two different units: one thermoreceptive multipolar neuron and one mechanosensitive chordotonal organ (CO) consisting of two scolopidia. We investigated the CO and found that the scolopidia are mononematic (the scolopale cap remains below the cuticle) and monodynal (one sensory cell per scolopidium). The dendrites of the scolopidia extend anteriorly and are attached by their caps to the cuticle about in the middle of the absorbing area. Structural features at the site of innervation suggest that the CO measures minute thermal deformations caused by IR absorption. Therefore, an additional photomechanic component which has been described for the IR receptors of pyrophilous jewel beetles of the genus Melanophila can be proposed for the IR organ of Merimna. Because scolopidia can measure displacements in the subnanometer range, the CO may enhance the sensitivity of the IR organ. The sensory complex of the Merimna IR organ shows the same units and similar cuticular modifications as the tympanal organs of some noctuid moths. Therefore, a parallel evolution of insect ears and the Merimna IR organ is discussed.     

The coxo-trochanteral muscle receptor organ of locusts

Summary The coxo-trochanteral muscle receptor organ of the hind leg of the locust Locusta migratoria migratorioides (R.&F.) has been investigated by use of scanning and transmission electron microscopy with special emphasis on its distal attachment site. The overall morphology of the receptor muscle, the sensory neuron and its dendrites was found to share many common features with other arthropod sense organs of that type with two important differences: (1) the connective tissue segment (= intercalated tendon) is extremely short compared to that of other muscle receptor organs; (2) the naked dendritic terminals of the non-ciliated, multipolar sensory neuron of the organ contain clusters of microtubules, interconnected by an amorphous matrix, that resemble the tubular bodies of ciliated, epithelial receptor cells.Abbreviation MRO muscle receptor organ Supported by the Deutsche Forschungsgemeinschaft (Br 882 and Hu 223)     

Fine structure of the galeal styloconic sensilla of larval Lymantria dispar (Lepidoptera: Lymantriidae)

Lepidopteran larvae possess two pairs of styloconic sensilla located on the maxillary galea. These sensilla, namely the lateral and medial styloconic sensilla, are each comprised of a smaller cone, which is inserted into a style. They are thought to play an important role in host-plant selection and are the main organs involved in feeding. Ultrastructural examination of these sensilla of fifth instar Lymantria dispar (L.) larvae reveal that they are each approximately 70 um in length and 30 um in width. Each sensillum consists of a single sensory peg inserted into the socket of a large style. Each peg bears a slightly subapical terminal pore averaging 317 nm in lateral and 179 nm in medial sensilla. Each sensillum houses five bipolar neurons. The proximal dendritic segment of each neuron gives rise to an unbranched distal dendritic segment. Four of these dendrites terminate near the tip of the sensillum below the pore and bear ultrastructural features consistent with contact chemosensilla. The fifth distal dendrite terminates near the base of the peg and bears ultrastructural features consistent with mechanosensilla. Thus, these sensilla each bear a bimodal chemo-mechanosensory function. The distal dendrites lie within the dendritic channel and are enclosed by a dendritic sheath. The intermediate and outer sheath cells enclose a large sensillar sinus, whereas the smaller ciliary sinus is enclosed by the inner cell. The neurons are ensheathed successively by the inner, intermediate, and outer sheath cells.     

Control of dendritic branching and tiling by the Tricornered-kinase/Furry signaling pathway in Drosophila sensory neurons

To cover the receptive field completely but without redundancy, neurons of certain functional groups exhibit tiling of their dendrites via dendritic repulsion. Here we show that two evolutionarily conserved proteins, the Tricornered (Trc) kinase and Furry (Fry), are essential for tiling and branching control of Drosophila sensory neuron dendrites. Dendrites of fry and trc mutants display excessive terminal branching and fail to avoid homologous dendritic branches, resulting in significant overlap of the dendritic fields. Trc control of dendritic branching involves regulation of RacGTPase, a pathway distinct from the action of Trc in tiling. Timelapse analysis further reveals a specific loss of the ability of growing dendrites to turn away from nearby dendritic branches in fry mutants, suggestive of a defect in like-repels-like avoidance. Thus, the Trc/Fry signaling pathway plays a key role in patterning dendritic fields by promoting avoidance between homologous dendrites as well as by limiting dendritic branching.     

Fine structure of the stretch receptor in the bursa copulatrix of the butterfly,Pieris rapae crucivora

Summary A pair of multipolar stretch-receptive neurons were found in the bursa copulatrix of the female cabbage white butterfly, Pieris rapae crucivora. The cell body of each neuron, about 10 m in diameter, lies on the edge of the muscular region in the antero-lateral wall of the corpus bursae. No special accessory structure, such as a receptor muscle, is associated with the neuron. The several dendrites extend radially into the muscle layer. The dendrites are ensheathed except for their terminal tips, and, on their course, they anchor repeatedly on the epithelial cells or the muscle fibers in such a manner that their basement membranes fuse together. While the ensheathed dendrite is usually 0.1–0.2 m in diameter, it often forms 1–2 m varicosities especially at anchor sites, so that it looks like a varicose, or beaded, chain. The varicosities contain a number of mitochondria, but only microtubules are found in the fine interconnecting parts of the dendrite. The naked dendritic tips terminate in the basement membrane of the epithelial cell. The varicosities, as well as naked tips, seem to be important for stimulus transduction in the sensory cell of this type.     

Dendritic reorganization of an identified neuron during metamorphosis of the moth Manduca sexta: The influence of interactions with the periphery

During metamorphosis of the moth, Manduca sexta, an identified leg motor neuron, the femoral extensor motor neuron (FeExt MN) undergoes dramatic reorganization. Larval dendrites occupy two distinct regions of neuropil, one in the lateral leg neuropil and a second in dorsomedial neuropil. Adult dendrites occupy a greater volume of lateral leg neuropil but do not extend to the dorsomedial region of the ganglion. The adult dendritic morphology is acquired by extreme dendritic regression followed by extensive dendritic growth. Towards the end of larval life, MN dendrites begin to regress, but the most dramatic loss of dendrites occurs in the 3 days following pupation, such that only a few sparse dendrites are retained in the lateral region of leg neuropil. Extensive dendritic growth occurs over the subsequent days such that the MN acquires an adult-like morphology between 12 and 14 days after pupation. This basic process of dendritic remodeling is not dependent upon the presence of the adult leg, suggesting that neither contact with the new target muscle nor inputs from new leg sensory neurons are necessary for triggering dendritic changes. The final distribution of MN dendrites in the adult, however, is altered when the adult leg is absent, suggesting that cues from the adult leg are involved in directing or shaping the growth of MN dendrites to specific regions of neuropil. © 1993 John Wiley & Sons, Inc.     

Dorsal longitudinal stretch receptor of Drosophila melanogaster larva - fine structure and maturation

Insects possess two types of sensory neurons: ciliated type I sensory neurons that innervate external sensory organs and chordotonal organs, and type II sensory neurons that form a subepidermal plexus or innervate stretch receptors. Among stretch receptors, a dorsel longitudinal stretch receptor is highly conserved in insects, being found in all insect orders investigated. Here we describe the topology and anatomical structure of this receptor in the fruit fly embryo and larva using transmission electron microscopy and single cell staining for fluorescence microscopy. The receptor is composed of the dorsal bipolar dendrite neuron, which arises from an archetypal cell lineage, its sister glial cell and the peripheral glial cell accompanying the nerve. The neuron is situated among the muscles in the dorsal body wall on the intersegmental nerve. Its two dendrites stretch the length of the segment to the segmental folds. The neuron is wrapped by both glial cells and surrounded by a common basal lamina, which fans out at the dendritic tips to attach them to the epidermal cells at the segmental borders.     

The ultrastructure of the sensory plaque organs of the antennae the chinese lantern fly, Pyrops Candelaria L., (Homoptera, Fulgoridae)

The plaque organs of Pyrops consist of elaborately folded, finely perforated cuticular areas, each associated with numerous bipolar sensory cells organised in groups. The proximal, mitochondrial region of each dendrite narrows to reveal a ciliary ultrastructure. The ciliary fibrils pass into a highly vesiculated region and beyond this are succeeded by a dense array of neurotubules. Peripherally the dendrites proliferate numerous fine branches from which finer filaments extend into the cuticular pores. The plaques are considered to be complex olfactory organs evolved from groups of sensilla basiconica.     

Ultrastructure of the peg and hair sensilla on the antenna of larval Aedes aegypti (L.)

The antenna of fourth instar larvae of Aedes aegypti has one peg organ of a basiconic type innervated by four neurons. The dendrites are ensheathed to near their terminations at the peg tip by an electron-dense dendritic sheath and by a cuticular sheath. They have easy communication by diffusion with the external environment only at the tip through a peripheral ensheathing membrane and six slit-channels. One of the dendrites resembles a tubular body proximally and may be mechanoreceptive. The peg generally appears to be a contact chemoreceptor. There are three antennal hairs of a typical sensillum trichodeum type innervated at the base by one neuron each. An intricate terminal mechanism at the insertion of the dendrite in the hair is described. These are believed to be tactile hairs. There are also three antennal hairs each innervated by two neurons. The dendrite from one terminates at the base similar to that of a tactile hair, and is believed to function in a similar mechanoreceptive manner. The dendrite from the second neuron extends naked along the length of the hair lumen. It is believed to be primarily chemoreceptive, in a slow-acting general sensory function. In all the sensilla there appear to be secretions produced in the junction body regions of the dendrites, and there is evidence for accumulation of secretory materials in the dendritic tips in some of the sensilla.     

Morphology and ultrastructure of the sensory spine,a presumed mechanoreceptor of Sphaeroma hookeri (Crustacea,Isopoda), and remarks on similar spines in other peracarids

The isopod Sphaeroma hookeri and many other isopods and peracarids have a sensory spine with laterally inserting sensory hair, positioned in the apical region of the propodal palm of pereopod 1. This spine is innervated by five to eight sensory cells (each giving rise to one cilium) the dendrites of which can be divided into an inner and outer dendritic segment. The cilia are surrounded by an extracellular, electron-dense dendritic sheath. Thirteen enveloping cells are present. The outer dendritic segment (structure beyond the basal bodies) contains two receptor lymph cavities; the inner one lying within the dendritic sheath is homologous with the inner receptor lymph cavity of insects. Scolopales, or tubular bodies, are lacking; their function is probably accomplished by the dendritic sheath. Apically the sensory hair does not have a pore, and the spine is heavily sclerotized. The inner dendritic segment begins with a basal body from which rootlets of different length and thickness extend into the dendrite. In the latter is an accumulation of vesicles. The dendrites keep close contact with other dendrites and the enveloping cells by desmosomal membrane structures. The possible importance of the sensory spine for phylogenetic studies is discussed.     

Ultrastructure of a possible new type of crustacean cuticular strain receptor in Carcinus maenas (crustacea,decapoda)

A hitherto unknown sensillum type, the “intracuticular sensillum” was identified on the dactyls of the walking legs of the shore crab, Carcinus maenas. Each sensillum is innervated by two sensory cells with dendrites of “scolopidial” (type I) organization. The ciliary segment of the dendrite is 5–6 μm long and contains A-tubules with an electron-dense core and dynein arm-like protuberances; the terminal segment is characterized by densely packed microtubules. The outer dendritic segments pass through the endo- and exocuticle enclosed in a dendritic sheath and a cuticulax tube (canal), which is suspended inside a slit-shaped cavity by cuticular lamellae. The dendrites and the cavity terminate in a cupola-shaped invagination of the epicuticle. External cuticular structures are lacking. Three inner and four to six outer enveloping cells are associated with each intracuticular sensillum. The innermost enveloping cell contains a large scolopale that is connected to the ciliary rootlets inside the inner dendritic segments by desmosomes. Scolopale rods are present in enveloping cell 2. Since type I dendrites and a scolopale are regarded as modality-specific structures of mechanoreceptors, and since no supracuticular endorgan is present, the intracuticular sensilla likely are sensitive to cuticular strains. The intracuticular sensilla should be regarded as analogous to insect campaniform sensilla and arachnid slit sense organs.     

Different levels of the homeodomain protein cut regulate distinct dendrite branching patterns of Drosophila multidendritic neurons

Functionally similar neurons can share common dendrite morphology, but how different neurons are directed into similar forms is not understood. Here, we show in embryonic and larval development that the level of Cut immunoreactivity in individual dendritic arborization (da) sensory neurons correlates with distinct patterns of terminal dendrites: high Cut in neurons with extensive unbranched terminal protrusions (dendritic spikes), medium levels in neurons with expansive and complex arbors, and low or nondetectable Cut in neurons with simple dendrites. Loss of Cut reduced dendrite growth and class-specific terminal branching, whereas overexpression of Cut or a mammalian homolog in lower-level neurons resulted in transformations toward the branch morphology of high-Cut neurons. Thus, different levels of a homeoprotein can regulate distinct patterns of dendrite branching.     

Comparative ultrastructure of juvenile and adult nuchal organs of an annelid (Polychaeta: Opheliidae)

Opheliid nuchal organs are composed of ciliated cells, retractor muscles, and sensory cells. The perikarya of sensory cells are located in the posterior portion of the brain, and their distal processes extend along the body wall, as the nuchal nerve, and terminate just anterior to the ciliated region. The nuchal nerve of the juvenile is composed of 30–35 dendrites; the adult nuchal nerve has 35–40 dendrites. The ends of the sensory dendrites form sensory bulbs which are clustered around the olfactory chamber, and each bulb bears a modified cilium. Sensory cilia lose their axonemes and extend as microvillous-like structures into the olfactory chamber. Supportive cells delineate approximately the posterior and dorsal portions of the chamber with sensory bulbs forming the remaining ventral and anterior portions. On the lateral aspect of the chamber, cuticular matrix extends into it, and in this area supportive cells bear microvilli which extend into the matrix. The adult nuchal organ is larger than that of the juvenile, and the sensory portion of the olfactory chamber wall is expanded. Expansion of the sensory area is apparently the result of size increase in sensory bulbs and by intrusion of supportive cells between sensory bulbs.     

Ultrastructural investigation of the nuchal organs ofPygospio elegans (Polychaeta). I. Larval nuchal organs

The structural differentiation of the nuchal organs during the post-embryonic development ofPygospio elegans is described. The sensory organs are composed of two cell types: ciliated cells and bipolar primary sensory cells, constituting the nuchal ganglion, which is associated with both the sensory epithelium and the brain. Since the sensory neurons are largely integrated into posterolateral parts of the cerebral ganglion, the nuchal organs are primary presegmental structures. The microvilli of the ciliated cells form a cover over the cuticle with a presumed protective function. An extracellular space extends between cuticle and sensory epithelium. The distal dendrites of the sensory cells terminate in sensory bulbs, bearing one modified sensory cilium each that projects into the olfactory chamber, embedded within the secretion of the ciliated cells. During development, the nuchal organs increase in size. This is accompanied by a shift in position, an expansion of the sensory area, and secretory activity of the ciliated cells. The nuchal ganglion differentiates into three nuchal centres forming three distinct sensory areas around the ciliated region. Each nuchal complex reveals two short nuchal nerves comprising the sensory axons, which enter the posterior circumesophageal connective. The sensory cells lying in the brain exhibit neurosecretory activity; the sensory cilia enlarge their surface area by dilating and branching. Nuchal organs accomplish the basic structural adaptions of chemoreceptors and show structural analogies to arthropod olfactory sensilla; thus, there is every reason to suppose chemoreceptor function.     

A Golgi study on the neuronal morphology in the hypothalamus of the Japanese quail (Coturnix coturnix japonica)

Neuronal morphology and dendritic architecture of the tuberal and mammillary regions in the hypothalamus of the quail (Coturnix coturnix japonica) were investigated by means of classical neuroanatomical methods (Bodian silver impregnation, Luxol-fast blue, cresyl violet, toluidine blue, rapid Golgi method). The tuberal region is characterized by isodendritic neurons, in particular: a) pyriform and bipolar neurons, occasionally arranged diagonally to the ventricular surface; b) CSF-contacting neurons, located subependymally or more deeply in the periventricular gray, which are especially abundant in the paraventricular organ and in the proximity of the median eminence; c) numerous multipolar neurons, endowed with stout, almost unbranched dendritic processes, occupying generally the medio-lateral areas of the hypothalamus. Some multipolar neurons display somata, pyramidal or ovoidal in shape, almost imperceptibly tapering into three or more dendritic trunks. These relatively straight and long dendrites are rich in dendritic spines. In the mammillary region, Golgi impregnation shows multipolar neurons of medium size, most likely belonging to the lateral mammillary nucleus.     

Intrinsic organization of the paramedian pontine reticular formation of the cat.

A morphoquantitative analysis was carried out to clarify the cytoarchitectural organization of the paramedian pontine reticular formation (PPRF) which is considered to be an important site in the control of eye movements. The study was carried out on the cat, using the Golgi staining method. The topographic position and detailed structure of the neurons were demonstrated using morphoquantitative methods. On the basis of their neuronal arborization, fusiform neurons and two types of multipolar cells were identified. Fusiform neurons show dendrites which are given off from the two poles of the small- to medium-sized cell body. The arborization generally runs caudorostrally, ending inside the PPRF. These neurons are ubiquitous. Type 1 multipolar neurons, the most frequent elements of the neuronal population (60%), have a small- to large-sized cell body from which 2 or 3 primary spiny dendrites and the axon emerge. Their dendritic field is oval and generally oriented in the vertical plane. These neurons are scattered everywhere in the PPRF. Type 2 multipolar cells are large neurons endowed with numerous primary spiny dendrites constituting a wide round dendritic field and with a thick axon. They are located almost exclusively at the boundaries of the PPRF and preferentially in the caudal region. The characteristics of the neurons suggest that the fusiform cells may play an interneuronal role, while the multipolar neurons could have both a projective function and an important receptive role for the afferent fibers to the PPRF. The lack of homogeneity found among the multipolar neurons is in agreement with the variety of projective elements shown by functional investigations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Signal integration on the dendrites of a pyramidal neuron model

This paper studied the synaptic and dendritic integration with different spatial distributions of synapses on the dendrites of a biophysically-detailed layer 5 pyramidal neuron model. It has been observed that temporally synchronous and spatially clustered synaptic inputs make dendrites perform a highly nonlinear integration. The effect of clustering degree of synaptic distribution on neuronal responsiveness is investigated by changing the number of top apical dendrites where active synapses are allocated. The neuron shows maximum responsiveness to synaptic inputs which have an intermediate clustering degree of spatial distribution, indicating complex interactions among dendrites with the existence of nonlinear synaptic and dendritic integrations.     

Development of the dendritic branching pattern of the Medial Giant Interneuron in the grasshopper embryo

The embryonic development of the grasshopper's Medial Giant Interneuron (MGI) was examined by injecting the cell with the fluorescent dye Lucifer Yellow at a series of stages in its growth. Particular attention was given to the way in which this neuron constructs its stereotyped dendritic branching pattern. The MGI's dendrites originate as secondary processes which sprout at characteristic points along the neurite after the primary growth cone has passed. These processes then arborize to form a miniature version of their adult branching pattern before the end of embryonic life. While growing, the dendritic branches are covered with a radiant profusion of filopodia; however, these filopodia are ephemeral structures and disappear once the cell matures. By contrast there is no significant reduction in either the number or the spatial extent of the actual dendrites at any embryonic stage. This implies that the stereotyped branching pattern of the mature MGI is primarily determined by a precise pattern of initial growth, and that secondary pruning of branches does not play an important role in shaping the final form of this cell. The coordinate ingrowth of the first cercal sensory axons was examined by cobalt filling the embryonic nerve, and the means by which these sensory axons make their initial contacts with the MGI's dendrites is herein discussed. The following paper considers the degree to which this sensory innervation regulates dendritic growth and branching.