Fine structural study of the statocysts in the veliger larva of the nudibranch,Rostanga pulchra

Summary The two statocysts of the veliger larva of Rostanga pulchra are positioned within the base of the foot. They are spherical, fluid-filled capsule that contain a large, calcareous statolith and several smaller concretions. The epithelium of the statocyst is composed of 10 ciliated sensory cells (hair cells) and 11 accessory cells. The latter group stains darkly and includes 2 microvillous cells, 7 supporting cells, and 2 glial cells. The hair cells stain lightly and each gives rise to an axon; two types can be distinguished. The first type, in which a minimum of 3 cilia are randomly positioned on the apical cell membrane, is restricted to the upper portion of the statocyst. The second type, in which 9 to 11 cilia are arranged in a slightly curved row, is found exclusively around the base of the statocyst. Each statocyst is connected dorso-laterally to the ipsilateral cerebral ganglion by a short static nerve, formed by axons arising from the hair cells. Ganglionic neurons synapse with these axons as the static nerve enters the cerebral ganglion. The lumen of the statocyst is continuous with a blind constricted canal located beneath the static nerve.A diagram showing the structure of the statocyst and its association with the nervous system is presented. Possible functions of the statocyst in relation to larval behavior are discussed.     

Ultrastructure of the Statocysts in the Apodous Sea Cucumber Leptosynapta inhaerens (Holothuroidea, Echinodermata)

The burrowing sea cucumber Leptosynapta inhaerens possesses five pairs of statocysts, one pair on either side of each radial nerve cord where it arises from the circumoral nerve ring. The nerve cords exhibit only ectoneural components at the level of the statocysts. A sinus-like epineural canal lies superjacent to each cord. This canal is lined by a robust monociliated neuroepithelium which lacks any special support cells. Beneath the neuroepithelium, the somata of the ectoneural neurons form a perikaryal layer whereas the axons are located within the proximal parts of the cords. Glial cells have not been found. Each statocyst is a hollow sense organ. Its central cavity is lined by a monolayer of monociliated parietal cells. Axons of these parietal cells extend towards the statocyst nerve which connects each statocyst with the ectoneural pathways of the cord. A single lithocyte floats within each central statocyst cavity. This unciliated cell contains a voluminous vacuole with the statolith and several smaller vacuoles. It is concluded that statocysts do not belong to the basic organization of the Holothuroidea but have been evolved within this group. The statement, that the statocysts of apodous sea cucumbers and that of the enigmatic Xenoturbella bocki are homologous organs, is rejected.     

Mosaic arrangement of SCPb-, FMRFamide-, and histamine-like immunoreactive sensory hair cells in the statocyst of the gastropod mollusc Pleurobranchaea japonica

A pair of statocysts are located in the periganglionic connective tissue of the pedal ganglia of the opisthobranch mollusc Pleurobranchaea japonica. Light- and electron-microscopic observations show that the sensory epithelium of the statocyst consists of 13 disk-shaped hair cells. Each hair cell sends a single axon to the cerebral ganglion through the static nerve. Neurotransmitters in the hair cells were examined by means of immunocytochemistry. Our results show that the 13 sensory hair cells include two SCPB-, three FMRFamide-, and eight histamine-like immunoreactive cells. One hair cell contains a transmitter substance other than SCPB-, FMRFamide, histamine, serotonin, or GABA. One of the two SCPB-like immunoreactive cells, located in the ventral region of the statocyst, is the largest cell in the statocyst. The other, located in the anterodorsal region, shows co-immunoreactivity to both SCPB and FMRFamide antisera. Among the three FMRFamide-like immunoreactive hair cells, one is located in the posteroventral region, separated from the other two, which are adjacent to each other in the anterodorsal region. All the eight histamine-like immunoreactive hair cells are adjacent to one another, occupying the remainder of a triangular pyramid-shaped region. These immunoreactive cells are symmetrically placed in the right and left statocysts. This mosaic arrangement was identical among specimens. Thus the static nerve may code information about position or movement of the statoliths, with the use of different transmitters in the mosaic arrangement of the hair cells.     

Über Bau und Funktion der Statocyste bei der Schnecke Aplysia limacina

Zusammenfassung Die Statocyste von Aplysia limacina zeigt in ihrem Bau keine wesentliche Abweichung vom durchschnittlichen Gastropoden-Typ. Besondere statolithfreie Räume oder Sinneshaare, wie sie von Tieren mit echtem Rotationssinn bekannt sind, wurden nicht angetroffen.Frei schwimmende, aus ihrer Normallage gebrachte Aplysien zeigen Lagekorrekturbewegungen, bei denen der Kopfteil führt. Auch fixierte und im Wasser hochgehobene Aplysien zeigen nach Drehung um horizontale Achsen kompensatorische Kopfstellreflexe. Auf Drehung um die Vertikalachse wird nicht reagiert. Einseitige Entstatung (Durchschneidung des N. staticus) ruft keinen, beiderseitige Entstatung einen vollständigen Ausfall der statischen Lagekorrektur- und Reflex-bewegungen hervor; die schwimmende Aplysia vollführt dann Purzelbäume. Taktile Reize vom Untergrund unterstützen die Lageorientierung. Ein orientierender Lichteinfluß machte sich nicht geltend.Nach einseitiger Durchschneidung des Cerebro-Pedal-Konnektivs reagiert eine fixierte Aplysia nur mehr in ipsilateraler Seitenlage mit der kompensatorischen Kopfdrehung zur intakten Seite hin; in kontralateraler Seitenlage wird nicht mehr reagiert. Das Ergebnis der Ausschaltversnche (Tabelle, S. 49) führt zu Schluß-folgerungen über den Verlauf der statischen Reflexbahnen, die in einem Diagramm (Abb. 7, S. 53) zusammengefaßt sind.Diese und andere Befunde werden in Zusammenhang mit den Ergebnissen früherer Autoren diskutiert. Bezüglich des Reizvorganges wird angenommen, daß auch in der Schneckenstatocyste Scherung der Cilien den effektiven, physiologisch adäquaten Reiz darstellt.

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Structure and functioning of the statocyst in the gastropod Aplysia limacina

Summary The statocyst of Aplysia limacina is a rounded vesicle with a diameter of 200–250 . Its wall is composed of two kinds of cells. The outer supporting cells are separate cells in fresh tissue; only under the influence of pressure or fixing agents their walls burst and artificial syncytia are created. The inner sense or giant cells are on their inner surface covered with motile cilia. Each statocyst of Aplysia contains 13 sense cells; their nervous offshoots constitute the statocyst nerve which runs towards the cerebral ganglion. The statolith is a cluster of about 1000 loosely aggregated chalk particles (statoconia). It fills the greater part of the statocyst lumen and is lightly moved by the cilia. Special statolith-free cavities or sense hairs, such as are known from animals with a true rotation sense, were not found in the statocyst of Aplysia.Freely swimming Aplysiae perform correction movements with their head leading, when they are brought out of their normal position in space. Likewise, fixed Aplysiae, when lifted up in the water and rolled or tilted about horizontal axes, show compensatory static head reflexes. Rotation around a vertical axis causes no response. Unilateral section of the statocyst nerve causes neither a loss of the position reflexes nor any asymmetry of posture or movement. Bilateral section of this nerve, however, abolishes all correction movements and compensatory reflexes; swimming animals perform somersaults. Tactile stimuli from the underground support the animal's spatial orientation. An orienting influence of light was not observed.After unilateral section of the cerebro-pedal connective a fixed Aplysia only responds when rolled the towards ipsilateral side (with a compensatory turn of the head towards the contralateral side); when rolled 90° towards the controlateral side no reaction occurs. The results of the elimination experiments (Table, p. 49) lead to the following conclusions: 1) from each statocyst two reflex pathways originate, one of which is activated after a roll around the long axis to the left side and causes a head turn to the right, whereas the other one comes into action after a roll to the right side and causes a head turn to the left; 2) the pathways of both statocysts which turn the head to the left run from the cerebral ganglion through the left cerebro-pedal connective towards the left pedal ganglion; both pathways which turn the head to the right run through the right cerebropedal connective towards the right pedal ganglion (diagram, Fig. 7, p. 53).These and other results are discussed in relation to data of earlier investigations. The course of the static nerve as shown morphologically to occur in other gastropods resembles closely the pathways postulated for Aplysia on physiological grounds. With regard to the process involved in stimulation it is assumed that in the statocyst of gastropods, like in other static organs, a shearing force exerted on the cilia represents the effective, physiologically adequate stimulus. Recent findings about the submicroscopical structure of the cilia in the statocyst of gastropods as well as about the mechanical sensitivity of motile cilia give this assumption strong support.

     

A crustacean statocyst with only three hairs: Light and scanning electron microscopy

Standard histological and SEM techniques have been used to examine the pair of statocyst organs located in the telson of the isopod, Cyathara polita. Each organ is formed as an invagination of the dorsal cuticle of the telson. The invagination narrows to form a stalk between the statocyst and dorsal surface. A canal courses longitudinally through this stalk and forms a continuous channel between the lumen of the cyst and the external environment. On the luminal floor of each statocyst, there are three pits; each correlates with a nodule protruding from the ventro-medial wall. From each pit, a single, bifurcating hair projects dorsally to contact the single concretion within the statocyst lumen. No other static organs have been found in this animal. Thus, maintenance of equilibrium in this species appears to be under the control of but six hairs, three in each statocyst. Innervation of each statocyst is provided by a branch of a nerve which connects anteriorly with the last abdominal ganglion.     

Interaction between hair cells in the statocyst ofHelix lucorum

An electrophysiological study of interactions between hair cells within the statocyst ofHelix lucorum was undertaken by intracellular and extracellular recording. Analysis of the results led to the following conclusions. First, some hair cells, subtending on angle on the arc of the statocyst sphere of not more than 90°, were electrically connected; electrical synapses, moreover, possessed polar properties; the coefficient of coupling in one direction was about 10 times greater than the other. Second, some connections between hair cells which subtended an angle of not more than 90° were mixed electrochemical in character. The excitatory chemical component in this case was directed in a direction opposite to effective electrical conduction. Third, inhibitory connections were observed between statocyst receptors: monosynaptic chemical (subtending an angle of about 180°, evidently, between the hair cells) and polysynaptic weak inhibitory interactions (subtending an angle in this case of not less than 90–100° between the test neurons). Fourth, all types of connection between hair cells were observed in CNS preparations with the vestibular nerve divided close to the cerebral ganglion. This means that zones of synaptic contacts between these receptors are located not in the CNS, but close to the statocyst.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 17, No. 2, pp. 230–239, March–April, 1985.     

Evolution of cerebral vesicles and their sensory organs in an ascidian larva

Sorrentino M., Manni L., Lane N. J. and Burighel P. 2000. Evolution of cerebral vesicles and their sensory organs in an ascidian larva. —Acta Zoologica (Stockholm) 81 : 243–258 The ascidian larval nervous system consists of the brain (comprising the visceral ganglion and the sensory vesicle), and, continuous with it, a caudal nerve cord. In most species two organs, a statocyst and an ocellus with ciliary photoreceptors, are contained in the sensory vesicle. A third presumptive sensory organ was sometimes found in an ‘auxiliary’ ganglionic vesicle. The development and morphology of the sensory and auxiliary ganglionic vesicles in Botryllus schlosseri and their associated organs was studied. The sensory vesicle contains a unique organ, the photolith, responding to both gravity and light. It consists of a unicellular statocyst, in the form of an expanded pigment cup receiving six photoreceptor cell extensions. Presumptive mechano‐receptor cells (S1 cells), send ciliary and microvillar protrusions to contact the pigment cup. A second group of distinctive cells (S2), slightly dorsal to the S1 cells, have characteristic microvillar extensions, resembling photoreceptor. We concur with the idea that the photolith is new and derived from a primitive statocyst and the S2 cells are the remnant of a primitive ocellus. In the ganglionic vesicle some cells contain modified cilia and microvillar extensions, which resemble the photoreceptor endings of the photolith. Our results are discussed in the light of two possible scenarios regarding the evolution of the nervous system of protochordates.     

Organization of statocysts in the Otoplanidae (Plathelminthes): an ultrastructural analysis with implications for the phylogeny of the Proseriata

Summary The statocyst of otoplanids is enveloped by a bipartite capsule which consists of two different extracellular matrices. This capsule encircles three different types of aciliary cells: several peripherally located flattened parietal cells, one central statolith forming cell (lithocyte) and two clusters of accessory cells. Intracapsular lumina exist which are different from extracapsular intercellular spaces. The accessory cells most probably represent those structures that are mainly involved in nervous conduction. These cells extend cytoplasmatic processes towards different peripheral regions of the statocyst where processes of outer nerve cells penetrate the capsule. The statocyst does not seem to represent a more evolved equilibrium receptor system but may function as a relatively simple aciliary sense organ suitable for positive geotactic behaviour. The otoplanid statocyst corresponds to statocysts in other lithophorous proseriates but not to statocysts in other taxa of the free-living Plathelminthes. The monophyly of a taxon Lithophora within the Proseriata is corroborated by this autapomorphic characteristic.Abbreviations ac accessory cell(s) - c capsule of the statocyst - ce cerebrum - ci cephalic intestine - co capsule opening - cp cell process(es) of accessory cell(s) and cell(s) containing filaments - ecm extracellular matrix - fc cell(s) containing filaments - ic intercellular spaces within the capsule - mc muscle cell(s) - n lobed nucleus of the lithocyte - nac nucleus (nuclei) of accessory cell(s) - nc nerve cell(s) - npc nucleus (nuclei) of parietal cell(s) - pc parietal cell(s) - s statolith - sc statolith cell (lithocyte)     

Efferent activity in theOctopus statocyst nerves

Summary Single unit electrophysiological recordings were obtained from efferent fibres in the statocyst nerves ofOctopus vulgaris. A preparation comprising the CNS and a single statocyst was employed. 42% of the efferents displayed a level of resting activity; transient changes in this activity occurred at irregular intervals.The responses of the efferent units were examined during sinusoidal oscillations of the statocyst at stimulus frequencies between 0.01–1 Hz, and amplitudes up to 35°. 84% of the units showed activity synchronised with the imposed oscillations; the time taken to establish this response varied for different units (Fig. 1).The lowest stimulus frequency at which a unit could be entrained varied for different units, with those units with a resting level of activity having the lowest thresholds. The peak firing frequency of the efferents was found to increase with increasing stimulus frequency or amplitude (Fig. 3). However, the change in firing frequency was much smaller than that reported for the statocyst afferents to similar stimuli.The efferent units of the posterior crista nerve were found to respond to clockwise or anticlockwise rotations (Fig. 4), with the individual units having unipolar responses. The phase response of the units changed little with increasing stimulus amplitude but an increase in phase lag occurred with an increase in the stimulus frequency (Fig. 5). The form of this relationship (Fig. 6) was similar to that reported for the statocyst crista afferents.The principal source of the input to the efferents in these experiments was shown to be afferents from the contralateral statocyst. These results are discussed and compared with data from the vertebrate semicircular canal system.     

Neuronal mechanisms of defense reaction to statocyst receptor stimulation in the freshwater snail Planorbarius corneus

Tilting of the freshwater snailPlanorbarius corneus triggering dynamic statocyst receptor response resulted in defense reaction attended by rapid lowering of the shell over the head, foreshortening of the foot, and inhibited locomotion and buccal apparatus operation. Large numbers of neurons from different ganglia were found to take part in this reaction in isolated nervous system preparations. The response usually followed an "all or none" pattern and did not depend on which statocyst receptors had been stimulated. Each successive response arose no sooner than 10–20 sec after the previous reaction to tilting the preparation. It is deduced that defense reaction to statocyst receptor stimulation takes the form of a "fixed action" governed by a special central mechanism. It was found during the process of investigating interaction between response to statocyte receptor and cutaneous nerve stimulation that the same central mechanism serves to produce defense reactions evoked by presentation of different stimuli.     

Comparative morphology of statocysts in the Plathelminthes and the Xenoturbellida

The general fine-structural organization of statocysts in Catenulida, Nemertodermatida, Acoela, Proseriata, Lurus (Dalyellioida), and Xenoturbella are summarized. In lithophorous (statocyst-bearing) members of the Catenulida, the statocysts exhibit a few parietal cells and one or several movable statoliths within a spacious intracapsular cavity. Statocysts in the Nemertodermatida have several parietal cells and two lithocytes, each equipped with one statolith, whereas those of the other acoelomorphan taxon, the Acoela, always have two parietal cells and one movable lithocyte. The statocysts of lithophorous members of the Proseriata represent more sophisticated systems: each has two clusters of accessory cells in addition to several parietal cells and a voluminous lithocyte in which the statolith is movable. In catenulids and proseriates, processes of outer neurons penetrate the capsule of the statocyst, whereas such innervations have not been found in the Nemertodermatida and Acoela. I conclude that the different types of statocysts have evolved independently within the Plathelminthes. Xenoturbella displays an intraepidermal statocyst with many monociliary parietal cells and several mobile cells (lithocytes) within the central cavity of the statocyst. Each of these mobile cells carries a statolith-like structure and one prominent cilium. The statocyst of Xenoturbella does not correspond to any type of plathelminth statocyst.     

The statocyst of Vampyroteuthis infernalis (Mollusca: Cephalopoda)

The statocyst shows a remarkable combination of features of decapods and octopods confirming that Vampyroteuthis is a relic somewhere near the ancestor of both groups. The lining of the statocyst separates from the outer wall, forming an inner sac, filled with endolymph, surrounded by perilymph. This is the condition found in octopods, never in decapods. The macula is partly divided into a macula princeps and macula neglecta, as in decapods but never in octopods. There are numerous statoconia, but no large statolith has been seen. The crista has four parts as in decapods, but they are not sharply separated. There are numerous small anticristae, with the general form found in decapods, differentiated into pegs and hooks.
The wall of the inner sac contains numerous hair cells. These hairs protrude between the epithelial cells. The bases of the cells are drawn out into fine processes, presumably some dendritic and some axonal. There is thus a plexus of nerve fibres all over the wall, communicating with the crista nerve.
There is a very large posterior sac of unknown function, lying behind the crista. It contains only one large anticrista and the opening of Kölliker's canal, which is very large.     

Commissural ring nerve: A female-specific neurosecretory tract supplied by bifurcating median neurons in the cockroach Periplaneta americana (L.) and the cricket Teleogryllus commodus (Walker)

Summary In the American cockroach, Periplaneta americana, and the Australian field cricket, Teleogryllus commodus, the two nerves supplying the bases of the cerci are joined by a branch that crosses behind the last abdominal ganglion. This commissural ring nerve is restricted to females, and it contains many axons filled with granular and agranular vesicles. The axons stem from somata located within the ganglion. There are one (Periplaneta) or two (Teleogryllus) groups of median neurons with bilaterally symmetrical bifurcations, and a group of postero-ventral neurons on each side. In T. commodus, these neurons are distinct from others associated with the cerci. In the two species, the ring nerve neurons contribute to a neuropile near the root of each cereal nerve. The bifurcating median neurons arborize on both sides before entering the ring nerve, while the postero-ventral ones branch more extensively ipsilateral to their somata. The possibilities are discussed that the bifurcating neurons may be homologous to dorsal unpaired median neurons, and that the ring nerve may be a neurohemal area.     

A Fine Structural Analysis of the Statocyst in Turbellaria Acoela

Ferrero, E. (Istituto di Biologia Generale, Universith di Pisa, Pisa, Italy.) A fine structural analysis of the statocyst in Turbellaria Acoela. Zool. Scr. 2 (1): 5–16, 1973.—The fine structure of the statocyst components in the acoelan Convoluta psammophila is described, namely: capsule, parietal cells, lithocyte, and the statolith. The absence of ciliary structures, the highly developed endoplasmic reticulum of the lithocyte and the layered texture of the statolith are remarkable. A functional interpretation of the muscles inserted on the statocyst and of the nerve bundle running nearby is suggested. The morphological similarities and differences between the acoelan statocyst and the statocyst of lower and higher invertebrate phyla are discussed.     

An Ultrastructural Account of Otoplanid Turbellaria Neuroanatomy II. The statocyst design: evolutionary and functional implications

The statocyst architecture in the three otoplanid species Notocaryoturbella bigermaria Lanfranchi, 1969, Otoplana truncaspina Lanfranchi, 1969 and Parotoplanella heterorhabditica Lanfranchi, 1969 is compared. Common features are: (a) a fibrillar collagen-like, 0.2 μm thick, investing capsule continuous with the brain capsule; (b) an inner wall made up of six or more flattened and overlapping parietal cells; (c) a statolith forming cell hanging from the dorsal side down in the lumen, with a large statolith containing vacuole; (d) a bilateral pair of spindle shaped accessory cell groups, adjoining the statolith cell and sending projections to the wall—nerve projections run through the capsule; (e) one accessory cell enveloping the other cells of the group has a filament containing cytoplasm, the filaments coverging into a hemidesmosome making contact with a projection coming from a parietal cell; (f) muscles from the longitudinal body musculature inserting onto the capsule externally. The lack of ciliary structures differentiates the turbellarian statocyst from the majority of invertebrate statocysts. The developmental origin, the phylogenetical meaning and the functional and adaptive value of the statocyst in Turbellaria are here commented on.     

Ultrastruktur der Statocyste von Ototyphlonemertes pallida (Keferstein, 1862) (Nemertini)

Zusammenfassung Ototyphlonemertes pallida (Keferstein, 1862) hat zwei Statocysten, die unmittelbar hinter den Dorsalganglien auf den verlängerten Ventralganglien liegen. Jede Statocyste besteht aus einer Statolithenkammerzelle, mehreren Nervenzellen und einer Anzahl Hüllzellen und ist von einer dicken Basalmembran umgeben. Die Statolithenkammerzelle umschließt in der Regel drei Statolithenkammern, die von einer doppelten Membran umgeben sind und untereinander in Verbindung stehen. Sie enthalten je einen frei beweglichen Statolithen. Cilien und Ciliarstrukturen fehlen. Auf der Dorsalseite der Statocyste liegen mehrere stark verästelte Nervenzellen, die einen gemeinsamen Strang bilden. In der Nähe der Statolithenkammerzelle spalten sie sich auf und bilden pro Kammer eine oder mehrere synaptische Platten mit elektrischen Synapsen. Die Statolithenkammerzelle wird von zahlreichen Hüllzellen umgeben, die durch Desmosomen fest verbunden und zusätzlich in der ventralen Hälfte der Statocyste an den Außenseiten stark miteinander verzahnt sind. Die Hüllzellen unterscheiden sich im Aufbau deutlich von den beiden anderen Zelltypen und sind nicht an der Reizperzeption oder Reizleitung beteiligt. Zu den cilienlosen Statocysten bei Coelenteraten, Turbellarien, Holothurien, Xenoturbella und Tunicaten-Larven bestehen keine engeren morphologischen Beziehungen. Die Statocyste von O. pallida stellt eine Bildung sui generis innerhalb der Nemertinen dar.

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Ultrastructure of the Statocyst of Ototyphlonemertes pallida (Keferstein, 1862) (Nemertini)

Summary Ototyphlonemertes pallida (Keferstein, 1862) has two statocysts, which are situated just behind the dorsal ganglions on the elongations of the ventral ganglions. Each statocyst consists of one statolith chamber cell, some nerve cells and a number of covering cells and is surrounded by a thick basement membrane. Usually the statolith chamber cell encloses three statolith chambers, which are intercommunicated and surrounded by a double membrane. Each chamber contains a single mobile statolith. Cilia and ciliary structures are lacking. Within the dorsal part of the statocyst some very ramified nerve cells are situated, which form a nerve fibre. In the vicinity of the statolith chamber cell the nerve cells split up into synaptical plates with electric synapses; there are one or several synaptical plates at the level of each chamber. The statolith chamber cell is surrounded by numerous covering cells, which are connected by desmosomes and additionally linked together at the outside in the ventral part of the statocyst. With regard to their structure the covering cells differ greatly from the other cell types, and they do not participate in impulse perception and impulse conduction. There do not exist any closer morphological relations to the statocysts lacking cilia in Coelenterates, Turbellaria, Holothuria, Xenoturbella and Tunicata. The statocyst of O. pallida represents an indigenous structure within the Nemerteans.

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Abkürzungen bm Basalmembran - d Dendrit - de Desmosom - dg Dorsalganglion - dm doppelte Membran der Statolithenkammer - ds deltaförmige Strukturen - ep Epidermis - es elektrische Synapse - hms Hautmuskelschlauch - hz Hüllzelle - k Zellkern - m Mitochondrium - ma abgewandeltes Mitochondrium - mu Muskulatur - n Nervenstrang - nv Neurosekretvesikel - nz Nervenzelle - rs Rüsselscheide - sc Statocyste - sk Statolithenkammer - skz Statolithenkammerzelle - sp synaptische Platte - st Statolith - v Vakuole - vg Ventralganglion     

Morphology of descending statocyst interneurons in the crayfish Procambarus clarkii Girard

Summary The morphological features of descending interneurons that responded to the artificial bending of statolith hairs were assessed with intracellular recording and staining techniques. Seven statocyst interneurons were identified on the basis of their structure and response characteristics and designated as interneurons S1 to S7. All seven identified interneurons project to the optic lobe, where the optic nerve also projects, and to the dorsal part of the tritocerebrum, where the eyestalk motoneurons originate. All except interneuron S6 also extend their major branches to other neuropilar regions. S2 projects to the dorsal part of the deutocerebrum, where the statocyst nerve terminates, and S3 to the dorsal part of deutocerebrum and the antennal lobe. Four other interneurons (S1, S4, S5, S7) also extend their branches to the parolfactory lobe to which the statocyst nerve projects as well as to the deutocerebrum and antennal lobe. The extensive dendritic projections of S1–S7 suggest that they are complex multimodal interneurons rather than simple relay interneurons, receiving at least visual and statocyst sensory information. The function of the antennal lobe branches, however, has yet to be determined since the functional role of antennal input in equilibrium control is unknown.     

Spontaneous activity of hair cells of the snail statocyst and its changes during intracellular stimulation

The following conclusions were drawn from an electrophysiological study of statocyst hair cell activity inHelix lucorum using intracellular recording. The maximal input resistance of the receptors is observed with hyperpolarizing currents of not more than 0.1 nA, close in magnitude to that arising during inhibitory synaptic transmission. Background noise, a special type of activity of statocyst hair cells, is neither synaptic nor pacemaker in nature, but depends entirely on the degree of contact between the cilia and statoconia. The hair cells possess pacemaker properties which are manifested on depolarization. The zone of action potential generation of the receptors lies in the axon. Inhibitory interactions take place between hair cells, leading to the generation of IPSPs in their spontaneous activity, which do not disappear after division of the vestibular nerve.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 17, No. 2, pp. 222–229, March–April, 1985.     

Some aspects of the fine structure of the statocysts of the molluscsPecten andPterotrachea

Summary The statocyst ofPecten is composed of hair cells and supporting cells. The hair cells bear kinocilia and microvilli at their distal ends and the supporting cells bear microvilli. The cilia have a 9+2 internal filament content, and arise from basal bodies that have roots, basal feet and microtubular connections. Two different ciliary arrangements are described, one with a small number of cilia arranged in a ring, and another with many more cilia arranged in rows. Below the hair cells are probable synapses. A ciliated duct connects to the lumen of the static sac and passes through the centre of the static nerve. The hair cells in the statocyst ofPterotrachea bear kinocilia and microvilli. The possible importance of cilia and microvilli in the transduction process is discussed.We would like to thank ProfessorJ. Z. Young for bringing specimens ofPterotrachea from Naples and also the staff of the Stazione Zoologica for the provision of specimens, Dr.M. Land for providing specimens ofPecten, the Science Research Council (U.K.) for providing the electron microscope used in much of the study and also for a grant to one of us (V.C.B.), and Mrs.J. Parkers and Mr.R. Moss and Mrs.J. Hamilton for much photographic and technical assistance.