Bipolar Sensory Cells Research Articles

Fine structure and molting of aesthetasc sense organs on the antennules of the isopod, Asellus aquaticus (Crustacea).

In Asellus aquaticus certain distal antennular segments bear single sensilla referred to as aesthetascs. These show a proximal stem and a distal bulbous region. Depending on its position, each aesthetasc is innervated by either 50-60 or 70-80 bipolar sensory cells, the perikarya of which are situated within the pedunculus. Within the antennular segment the dendrites develop unbranched cilia (9 X 2 + 0 structure). The sensory cells are unusual in that mono- as well as biciliary dendrites are present within a single aesthetasc, the ratio of both types being correlated with the number of sensory cells. Cilia and receptor lymph cavity are enveloped by a set of 3-4 inner and 13-14 outer sheath cells, which terminate at the base of the sensillum, so that the delicate and poreless cuticle of the bulbous region encloses only outer segments within the receptor lymph fluid. A new molting type in arthropods is described in which the outer sheath cells alone build the new cuticle, whereas the inner sheath cells most probably have a protective function. A definition of aesthetascs is proposed based on fine-structural criteria. Functionally the sensilla are considered to be chemoreceptors. This assumption is confirmed by experiments with diluted vital dye as well as lanthanum showing that dissolved substances penetrate the poreless cuticle instantaneously.

GASTROPOD CHEMORECEPTION

Summary(I). Gastropods use chemoreception for a wide variety of behaviours including feeding, homing, escape from predators and a variety of social and reproductive behaviours. Chemoreception is used to locate distant food sources, and to discriminate between potential foods. Responses to chemical food stimuli result from a combination of innate and experiential factors. Gastropods use chemical cues in mucus trails to home. They also home by direct olfactory orientation. Reproductive behaviour in a variety of gastropods appears to involve chemical cues. Evidence exists for pheromones controlling aggregation and mating. Numerous gastropods use chemical cues to avoid or escape from predators.(2). Amino acids appear as likely candidates for attractants and phagostimulants for gastropod feeding. Macromolecules are probably also involved. Amino acids have also been shown to stimulate reproductive behaviours in certain gastropods, thus suggesting a pheromonal function. However, the significance of this finding to the behaviour of the organisms in the field has yet to be evaluated. Saponins have been implicated as the active substances found in sea stars that elicit escape responses of marine gastropods. Choline esters may play a homologous role in gastropod—prey and gastropod‐predator interactions.(3). Gastropods can apparently use a number of different methods to orient to olfactory cues. These include anemotaxis or rheotaxis, klinotaxis and tropotaxis.(4). The major chemosensory organs of gastropods have been identified. They include the anterior and posterior tentacles and lips of terrestrial pulmonates; the cephalic tentacles, the lips and buccal cavity lining, and possibly the osphradium of aquatic pulmonates; the cephalic and mantle tentacles, the anterior margin of the foot, the siphon tip, and the osphradium of prosobranchs; and the rhinophores, tentacles, oral veil and osphradium of opisthobranchs.(5). Many of the organs named above have been examined by both light and electron microscopy. The most common anatomical organization includes bipolar primary sensory cells with cell bodies located subepithelially, and a distal dendrite extending to the free surface. Often a peripheral ganglion is located deep to the sensory epithelium. It is unclear whether axons of the sensory cells project directly to the central ganglion or by way of interneurones located in the peripheral ganglia.(6). The dendritic specializations of the sensory cells vary considerably. Most bear cilia or a combination of cilia and microvilli. The functional significance of the variation in the types of sensory endings is unknown, although the chemosensory epithelia also respond to other sensory modalities, and it is often difficult to ascribe any one cell type to any one modality. Species‐specific variations may also complicate the picture.(7). Prospects for and importance of future studies on gastropod chemoreception are discussed.

The nervous system and ciliary band of Müller's larva.

The newly hatched Müller’s larva of the polyclad Pseudoceros canadensis is described at the electron microscopical level with attention to the arrangement and innervation of the ciliary band and the organization of the larval nervous system. Distinctive ultrastructural features allow the trochal cells of the band to be distinguished from general epithelium of the body surface and the specialized oral field epithelium. The band comprises a ventral and a dorsal marginal loop that run along the margins of the six projecting lobes of the larva, and a suboral plate that forms a bridge between these behind the mouth. These three components are joined asymmetrically to form a single, but discontinuous, band: on the left the ventral loop and suboral plate are joined, but these fail to connect with the dorsal loop; on the right it is the ventral and dorsal loops that join, but there is no connection between these and the suboral plate. A system of intraepithelial nerves is associated with the ciliary band, the largest nerves being those in the ventrolateral lobes and the intraepithelial commissure that connects these across the oral field. The system is truly peripheral: it lies outside the basement membrane and is separated by it from the central nervous system, which at this stage comprises a brain and four radiating nerve cords. The peripheral and central nervous systems are in direct contact only at two points, located just behind the ventrolateral lobes on either side of the larva, where a few neurites pass through the basement membrane from one system to the other. The neurites of the peripheral system arise mainly from bipolar sensory cells located in the ciliary band. These are concentrated along the ventrolateral lobes, and their projecting cilia face incoming water currents. Observations on larval swimming behaviour do not, however, suggest any obvious function for these cells. The ciliary band is thus organized as a self-contained unit supplying its own innervation. Other primitive invertebrate larvae have ciliary bands that are similar to some extent in their organization and ultra­structure. This, added to what is already known about Müller’s larva, supports the idea that it is primitive and is closely related to at least several other larval types, but it is not clear how the overall arrangement of the band in Müller’s larva as described here relates to what is seen in other larvae. Several ways in which the pattern might have originated from simpler patterns in hypothetical ancestral forms are, however, discussed.

Single-pore sensilla of damselfly-larvae: representatives of phylogenetically old contact chemoreceptors?

The four single-pore sensilla on the maxillary palp of Agrion and Ischnura possess two bipolar sensory cells for A-sensilla and five for B-sensilla. Distally, their dendrites are enclosed by a dendritic sheath and a pore tube. The dendritic tips have access to the outside via the opening of the pore tube. The inner dendritic segments join the receptor cell bodies laterally. The receptor cells of each sensillum are enveloped by three sheath cells and three receptor lymph cavities. Receptor lymph cavity 1 and 3 are connected with the outside by the pore tube and a minute canal system in the cuticle, respectively. All three sheath cells show microvilluslike processes of the apical membrane. Moreover, all sheath cells contain two centrioles in tandem arrangement. The structure of the single-pore sensilla is compared with that of similar sensilla of other arthropods. Functional and phylogenetic aspects are discussed.

Die digitiformen Sensillen auf dem Maxillarpalpus von Coleoptera

The digitiform sensilla on the distal segment of the maxillar palps ofAgabus bipustulatus (L.) andHydrobius fuscipes (L.) were investigated by electron microscopic methods. Each sensillum is innervated by a single bipolar sensory cell. The sensilla ofHydrobius are associated with three enveloping cells, which enclose an inner and outer receptor lymph cavity. A single enveloping cell only is found in the completely differentiated sensilla ofAgabus. These sensilla do not form an outer lymph cavity. The area beneath the hair base is filled by the distal process of the enveloping cell and by extensions of epidermal cells. Only one extra-cellular space exists, which seems to be homologous to an inner receptor lymph cavity.

Ultrastructure of the sensory palps ofTetranchyroderma papii (Gastrotricha, Macrodasyida)

The sensory palps of the macrodasyoid gastrotrichTetranchyroderma papii contain processes from two types of cell: 22–23 bipolar primary sensory cells and two to three support cells. In the proximal region of the palp each sensory cell contains a short ciliary segment with a basal body and from this ciliary segment a longer distal segment lacking axonemal microtubules extends through the major part of the length of the palp. Each support cell process bears microvilli and contains a conspicuous bundle of microtubules running the entire length of the process. The cell bodies of both cell types are situated in the epidermis of the head region. The palps are interpreted as having a chemosensory function. They are considered to be homologous to the posterior cephalic sensory organ ofTurbanella cornuta, but not the head tentacles ofChordodasys antennatus or nematode amphids.

Mechanoreceptive bristles on the head of the blowfly: Mechanics and electrophysiology of the macrochaetae

On the heads of the femaleCalliphora studied, in the region between the two compound eyes and at the dorsal back margin, there are about 80 macrochaetae and about 500 smaller mechanoreceptive hairs. Their topography is described, with data on size, angle of inclination and direction of curvature (Figs. 1, 2, 3). When a frontal macrochaeta is rotated about its base in different directions, restoring force is maximal with displacement in the direction of curvature; for the bristles on the back of the head, the force is maximal in the opposite direction. As either type of hair is displaced to progressively greater angles in the direction of greatest restoring force, the bristle joint behaves like a nonlinear spring until the shaft meets the edge of the socket (e.g., 8–10° for the ocellar bristles; Fig. 4). When released the shaft returns to the resting position with a non-oscillatory, strongly damped movement. Each of the bristles and hairs is innervated by one bipolar sensory cell. Nerve impulses are recorded from this cell only when the hair is displaced into the semicircular sector associated with the greatest restoring forces. The threshold angle is about 1°. The angles to which the bristles are moved by air flow (up to 4.5 m/s), airborne sound (50–1000 Hz, 90 dB) and cuticle-conducted vibration (oscillation amplitude in the frequency range of the wingbeat ca. 100 μm) do not exceed this threshold. Stepwise displacement in the direction of curvature elicits phasic excitation that decays approximately exponentially. In the ocellar bristles, for example, the latency to appearance of the first nerve impulse amounts to about 2 ms and the time constant of frequency decay is about 30 ms. With ramp stimuli peak impulse frequency rises in proportion to the logarithm of the rate of displacement, within the range 10–500°/s (Fig. 7). The peak frequency is independent of final angle. The sensitivity of the receptor to movement of the hair shaft within the sector from resting position to socket edge shows a distinct maximum at 1–3° past the threshold angle (Figs. 11, 12). With imposed sinusoidal oscillation the frequency characteristics are approximately linear up to a stimulus frequency of 160 Hz (Fig. 9). But there is considerable adaptation, dependent on stimulus frequency. The macrochaetae are probably pure event detectors, signalling the moment and site at which any suprathreshold contact is made between the body surface and its surroundings. Stimulus-excitation transformation by the ocellar bristles is discussed.

A neuroanatomical study of the cochlea of the little brown bat (Myotis lucifugus).

AbstractThe population and density of the bipolar ganglion cells and sensory hair cells were determined from serial sections and graphic reconstructions of the cochlea and spiral ganglion of four little brown bats. The spiral ganglion lacks a bony canal and is a one and one‐half turn spiral of almost uniform thickness which ends in a broad flat nodule at its apex. The average length of the nerve cell body is 7.4 μ (range: 5–15 μ) and it is 6.4 μ wide (5–10 μ). The average number of ganglion cells enumerated was 63.2 × 103 (range: 60.6 × 103–65.2 × 103) and following correction for “split cell error,” the total number was 55.3 × 103 ganglion cells. The ganglion cell density curve increases gradually to a maximum value of 11.7 × 103 cells/mm2 at 2.4–3.2 mm from the basal end. The two and one‐half turn cochlea is 6.9 mm long (6.8–7.1 mm) with an estimated total of 700 inner and 2,800 outer sensory hair cells. The densities of outer and inner hair cells increase from the base to the apex by 21% and 32%, respectively. However, the density of sensory cells per unit of sensory epithelium decreases from base to apex. The ratio increases to a maximum value of 22:1 at 2.4 mm and then declines toward the apex. The significance of these findings is discussed and compared to similar studies on other high‐frequency hearing mammals.

Sensitivity of an insect mechanoreceptor during moulting

ABSTRACT. The fine structure and the behavioural threshold for vibration sensitivity of the eight thoracic filiform hairs of Barathra brassicae caterpillars were investigated through an intermoult/moult cycle. Associated with each filiform hair is one bipolar sensory cell and three enveloping cells. The outer dendritic segment terminates in an ecdysial canal in the hair base and a tubular body lies at its distal end. Shortly before apolysis the dendrite elongates. By this means the connection between the sensory cell and the old cuticular apparatus is maintained while the epithelium and the old thoracic cuticle are separating. The new cuticular apparatus of the filiform hair is formed in the second half of the larval stage by the three enveloping cells. A second tubular body in the elongated outer dendritic segment is formed at the base of the replacement hair 10 h before next ecdysis, so that the new hair functions as soon as ecdysis is completed, the old cuticular apparatus with the old tubular bodies being torn away with the exuvia during ecdysis. Sensitivity to a 300 Hz tone was tested in the standing wave of a Kundt's tube. Throughout most of the larval instar the threshold was 2.0 ± 0.3 μm particle displacement amplitude until 1–2h before ecdysis when it rose to 6.8 ± 1.3 μm and at 10–30 min before the beginning of ecdysis no reaction to sound could be detected. Once the old cuticle was shed maximum sensitivity returned as soon as the replacement hairs were erect. The sensilla are therefore physiologically functional at all developmental stages except for 30–60 min during actual ecdysis.

The Ultrastructure of the Presumed Chemoreceptor Aesthetasc “Y” of a Cypridid Ostracode

AbstractThe aesthetasc “Y” of cypridid ostracodes is divided into three parts: distal, intermediate, and proximal. Distally, it is equipped with five pores containing electron‐dense plugs. The distal part shows a peculiar striated pattern formed by two types of cuticle. The intermediate and proximal parts are smooth with no pores or other substructures. 14 cilia run in the large receptor‐cavity of the distal part. They originate from 14 bipolar sensory cell bodies within the antenna. From the intermediate part, the cilia are grouped in five cuticular sheaths, surrounded by five sheath cells arranged mesaxonally. At the level of the ciliary junctions, the sheath cells form cavities in which the cilia join the dendrites. Before molting, a new aesthetasc is formed in the antenna by hypodermal cells. The cuticle is secreted in a fold around the sheath cells, which in a way act as moulds. The cilia of the old aesthetasc run through the five pores of the new aesthetasc. The main function of the pores is apparently to enable the animal to obtain information from the surroundings also during the molting period. On morphological grounds, the aesthetasc “Y” is believed to be a chemoreceptor.

Ciliated sensory neurons in the lip of the squid Lolliguncula brevis Blainville.

The lip of Lolliguncula brevis is a muscular fold covered by a simple columnar epithelium and overlayed medially and distally by a non-cellular cuticle. Bipolar sensory cells in the epithelium have a shallow pocket with cilia and microvilli at the free end of their dendrite. The cilia project out onto the surface through pores in the cuticle. Cells with intracellular cilia were observed below the epithelium. These cells send a process toward the surface of the lip. These two receptors are compared to the receptors previously described from the lip of Sepia.

The ultrastructure of chemoreceptor sensilla in Ciniflo (Araneida, Arachnida)

The morphology and ultrastructure of chemoreceptive hairs on the legs or the cribellate spider Ciniflo are described. About 20 bipolar sensory cells innervate each hair. The dendrites of two of these terminate at the hair base and are probably mechanoreceptive in function. The others continue into the hair shaft, where they terminate without branching. The associated sheath and enveloping cells are fully described and their physiological roles discussed. This sensillum is compared with the chemoreceptors of other arthropods.

The sensory organs on the glossa, the epipharynx and the hypopharynx of the worker of the honeybee Apis mellifica L. (Insecta, Hymenoptera)

In the mouth of the honey bee several types of sensory organs are found: Sensillae chaeticae on the tongue, sensillae trichodeae on the epipharynx and sensillae basiconicae on the hypopharynx. These organs usually are built up by a trichogen and a tormogen cell and they possess primary bipolar sensory cells in a varying number.

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.

The anatomy and adequate stimulation of a chordotonal organ in the antennae of a hermit crab

1. 1. A chordotonal organ in the basal segment of the antennal flagellum of the hermit crab, Petrochirus californiensis, is described. 2. 2. Adequate stimulation is flexion or intrascolopidial compression of the dendrites. Stretch of the dendrites inhibits responses from the bipolar sensory cells. 3. 3. A theory for the adequate stimulation of the flagellar chordotonal organ is applied to periopod elastic strand stretch receptors. A mechanism that results in flexion of the dendrites in response to strand stretch or relaxation is proposed to account for the responses obtained from the other chordotonal organs. 4. 4. It is suggested that movement of responses, most position responses and responses that are intermediate could arise from a single type of cell, and would depend only on the physical arrangement of the dendrites within the strand.

Histochemical localization of monoamines in the nervous system of the polychaete Nephtys.

A specific fluorescence method has been used for the cellular demonstration of biogenic monoamines in the nervous system ofNephtys. The central nervous system contains both green fluorescent (adrenergic) and yellow fluorescent (5-hydroxytryptaminergic) fibres and neurons, the neurons showing a repetitive pattern in successive segments in the nerve cord. The proportion of yellow neurons is much higher in the ventral nerve cord, while the green neurons predominate in the supra-oesophageal ganglion. Many of the fluorescent fibres in the neuropile are beaded. Three of the four segmental nerves carry fluorescent fibres peripherally. Except for yellow neurons in the wall of the intestine, all peripheral fluorescent cells are green. Of these, a few in each segment are located in peripheral ganglia, and the rest resemble the bi-polar epidermal sensory cells found in other polychaetes. The only yellow fibres visible peripherally are the multiple varicosities which occur at points of muscle insertions on basement membranes. It is possible that these are the end plates of a set of motor fibres. On the basis of the distribution and morphology of structures exhibiting the two types of fluorescence it is postulated that the green system may have a sensory and the yellow system a motor function.

Chemoreceptor sensillum structure in Limulus.

AbstractThe chemoreceptors of Limulus polyphemus (L.) are polyneuronal sensilla found in the spines of the coxal gnathobases of each walking leg, the spines of the chilarial appendages, and the chelae of all the limbs. Each sensillum contains 6–15 bipolar sensory cells that share a single pore in the cuticle. The dendrites of the sensory cells of each sensillum course to the cuticle together. These attenuate sharply and enter a canal in the cuticle as a very narrow terminal thread. The dendrites retain their identity in the thread, but with the light microscope, they are usually not visible individually. Each thread, consisting of 6–15 dendrites, is accompanied to the cuticular surface by a cuticular tubule found within the canal. The chemoreceptor sensilla of the gnathobase, chilarium, and chela, the temperature organs of Patten, and the flabellar receptor organs all have the same basic organization. In general this is the same structural plan shown by chemoreceptors of other arthropods. Several different mechanisms of peripheral physiological interaction among receptor cells are possible with a sensillum organization like that described here for Limulus.

The fine structure of crustacean proprioceptors. II. The thoracico-coxal organs in Carcinus, Pagurus and Astacus

There is a complex of proprioceptor organs spanning the thoracico-coxal joints of the walking legs of decapod Crustacea. The muscular receptor is a specialized portion of the promotor muscle of the leg, associated with sensory fibres which are the dendrites of neuron-like sensory cells. Similar sensory fibres enter strands of connective tissue which are attached distally with the levator and depressor muscles of the basipodite (innervated elastic strands). In macrurans, but not in crabs, there is in addition a thoracico-coxal chordotonal organ. The fine structure of these proprioceptors is described, in Carcinus , Pagurus and Astacus . In Carcinus and in Pagurus the dendrites of all the sensory fibres are associated with specialized strings of connective tissue, within larger connective tissue strands. Fine processes of the dendrites (dendrite fingers) penetrate these connective tissue strings (vacuolated strings) and run up and down them for a certain distance, within the extracellular connective tissue substance, which is amorphous connective tissue containing bundles of collagen fibres. Elsewhere the dendrites are surrounded by sheath cells. The connective tissue of the strands consists mostly of collagen, except proximally in the muscular receptor, where amorphous connective tissue is more plentiful. Blood channels penetrate the strands in places. Appearances which seem to be connected with the formation of the amorphous connective tissue are described. In Carcinus and in Pagurus three sensory fibres are associated with the muscular receptor; these are referred to as T, S and P . The dendrites of T are associated with vacuolated connective tissue strings which attach directly to the proximal ends of the fibres of the proprioceptor muscle. The S dendrites pass into strands of connective tissue flanking the muscle; dendrite fingers of P , which is a smaller nerve fibre, enter some of the vacuolated strings which are associated with the S dendrites. The elastic strands are innervated each by one or more sensory fibres, with dendrites again associated with vacuolated connective tissue strings. In Astacus two sensory fibres send irregularly branched dendrites into a strand of connective tissue proximal to the proprioceptor muscle. No specialized connective tissue strings can be distinguished, but there are unsheathed dendrite fingers penetrating the connective tissue in places. The chordotonal organ contains isodynal scolopidia, each with two bipolar sensory cells of similar fine structure. A rough calculation of the surface area of the dendrite fingers of the S neuron of Carcinus has been made.

IMPULSE ORIGIN AND PROPAGATION IN A BIPOLAR SENSORY NEURON.

Intracellular recording techniques were used to study electrical activity in bipolar sensory cells associated with crayfish tactile receptors. Several lines of evidence indicate that spikes evoked by natural stimulation of the receptor originate at a dendritic locus. Although overshooting spikes are recorded in the soma in response to both natural and antidromic stimulation receptor potentials are observed only rarely, and, when present, their amplitude is less than 5 mv. Impulses propagating centrifugally into the soma following antidromic stimulation always exhibit an inflection in the rising phase of the spike; however, orthodromic spikes are usually uninflected. Occasionally, orthodromic responses (in the soma) exhibit rather unusual wave forms. Such spikes evoked by natural stimuli are indistinguishable from those elicited electrically in the dendrite, but they do not resemble antidromic impulses. Because the axonal and dendritic boundaries of the soma have a low safety factor for spike transmission, at high frequencies invasion of the soma by dendritic spikes is impeded and often blocked. The soma region can thus act as a low-pass filter. The significance of this self-limiting mechanism for the behavior of the animal is not known; it is suggested, however, that this impediment is a potentially critical one, and may, in other situations, have encouraged the evolution of alternative arrangements.

THE FLY PTILINUM: TACTILE RECEPTORS AND THEIR FUNCTION AT EMERGENCE.

Although the musculature of the ptilinum has been studied extensively, no nervous components have been previously demonstrated. The present paper describes a series of minute hairs, interspersed amongst the ‘spinules’ that cover the organ, each of which is innervated by a bipolar sensory nerve cell. The axon arising from the sensory cell is joined by those from other similar cells, and they pass posteriorly to enter a ganglion; from the ganglion a broad nerve leaves to join the antennal nerve to the brain. The general morphology suggests that the hairs are tactile; this is substantiated by the ‘avoiding reactions’ displayed by the fly when the ptilinum is subjected to tactile stimulation, both when held experimentally, and in nature when by-passing soil particles.