The abdominal muscle receptor organ in Astacus leptodactylus (Crustacea)

The structure of both the slow- and the fast-adapting abdominal muscle receptor organ of Astacus leptodactylus is described with particular reference to differences between the two systems. The receptors are composed of a thin muscle that extends from the front edge of one segment to the front edge of the following and a sensory cell connected with this muscle. In the zone where the sensory cells enter their respective muscle, muscle fibers are reduced (zone of relative muscle exclusion = ZRME) and partly replaced by connective tissue. The occurrence of dendritic processes of both the slow and the fast neurons is confined to this zone. The following differences between the two receptor types are established: (1) The fast receptor muscle reveals a smaller sarcomere length than the slow receptor muscle and a higher myosin/actin filament ratio. (2) Muscle fibers that pass the ZRME are always found at its periphery in the fast system, separated from dendritic processes by layers of connective tissue, while in the slow system muscle fibers frequently are intermingled with the sensory elements. (3) The ZRME of the slow receptor is 20-30% longer than that of the fast receptor. (4) The dendritic varicosities of the slow neuron, on an average, contain many more mitochondria than those of the fast neuron. (5) Dendritic processes (fine twigs as well as varicosities) are juxtaposed to the sarcolemma of the muscle fibers only in the slow system; in the fast system dendrites and muscle are spatially separated by connective tissue. It is assumed that these differences between the two receptor types are at least in part responsible for the different thresholds observed in physiological experiments.     

Muscle receptor organs do not mediate load compensation during body roll and defense response extensions in the crayfish Cherax destructor.

It has been proposed that the abdominal muscle receptor organ (MRO) of decapod crustaceans acts in a sensory feedback loop to compensate for external load. There is not yet unequivocal evidence of MRO activity during slow abdominal extension in intact animals, however. This raises the possibility that MRO involvement in load compensation is context-dependent. We recorded from MRO tonic stretch receptors (SRs) in freely behaving crayfish (Cherax destructor) during abdominal extension occurring during two different behaviors: body roll and the defense response. Abdominal extensions are similar in many respects in both behaviors, although defense response extensions are more rapid. In both situations, SR activity typically ceased when the abdominal extension commenced, even if the joint of the SR being monitored was mechanically prevented from extending by a block. Since extensor motor neuron activity increased when the abdomen was prevented from extending, we concluded that the load compensation occurring in these behaviors was not mediated by the MROs.     

Muscle Receptor Organs in the Crayfish Abdomen: A Student Laboratory Exercise in Proprioception

The primary purpose of this experiment is to demonstrate primary sensory neurons conveying information of joint movements and positions as proprioceptive information for an animal. An additional objective of this experiment is to learn anatomy of the preparation by staining, dissection and viewing of neurons and sensory structures under a dissecting microscope. This is performed by using basic neurophysiological equipment to record the electrical activity from a joint receptor organ and staining techniques. The muscle receptor organ (MRO) system in the crayfish is analogous to the intrafusal muscle spindle in mammals, which aids in serving as a comparative model that is more readily accessible for electrophysiological recordings. In addition, these are identifiable sensory neurons among preparations. The preparation is viable in a minimal saline for hours which is amenable for student laboratory exercises. The MRO is also susceptible to neuromodulation which encourages intriguing questions in the sites of modulatory action and integration of dynamic signals of movements and static position along with a gain that can be changed in the system.     

TTX sensitive plateau potentials in the crayfish slowly adapting stretch receptor neuron

Summary Electrophysiological experiments showed that a tetrodotoxin (TTX) sensitive slowly inactivating Na⁺ current contributed to the excitability of the sensory neuron (SN1) that innervates the slow receptor muscle in the abdominal muscle receptor (MR1) of crayfish, Procambarus clarkii. Following either tetraethylammonium (TEA) blockage of the K⁺ delayed rectifier currents or exposure to high temperature, a depolarizing plateau potential was evoked by the slow Na⁺ current. Ca⁺⁺ substitution by other divalent cations had no effect on the plateau potential, demonstrating that Ca⁺⁺ is not involved in plateau potential genesis. Simultaneous intrasomatic and extraaxonic recordings coupled with 4-aminopyridine (4-AP) exposure indicated that the slowly inactivating Na⁺ current is primarily somatic, and does not contribute significantly to spiking.Abbreviations 4-AP 4-aminopyridine - HAP hyperpolarizing after-potential - MR1 slowly adapting muscle receptor organ - SR1 sensory neuron of MR1 - TEA tetraethylammonium - TTX tetrodotoxin     

Ultrastructure of a muscle spindle-analogous receptor organ in the mandible of Oncopeltus fasciatus (Insecta,Heteroptera) with remarks on the homology of the mandibular muscles

Summary A non-ciliary muscle receptor organ in the first mandibular retractor muscle of Oncopeltus fasciatus is described. The organ consists of two specialized muscle fibres of the first retractor, which are embedded in a thickened layer of connective tissue. The sensory innervation is supplied by three multiterminal sense cells sending several dendrites to the receptor muscle fibres. Naked dendritic terminals are attached to the muscle surface or connective tissue fibrils. The far-reaching analogy of the receptor to the intrafusal chain-fibres of vertebrate muscle spindles is remarkable. The existence of a muscle receptor organ in the first mandibular retractor may serve as an argument in favor of the homology of this muscle with the musculus tentorio-mandibularis of orthopteroid insects.Supported by a grant from the Deutsche Forschungsgemeinschaft     

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)     

The activity of abdominal stretch receptors during non-giant swimming in the crayfish<Emphasis Type="Italic"> Cherax destructor</Emphasis> and their role in hydrodynamic efficiency

Recordings were made from the nerve innervating the stretch receptors of the abdominal muscle receptor organs and slow extensor muscles of tethered crayfish, Cherax destructor, during so-called non-giant swimming. The stretch receptors were active during the flexor phase of swimming but the duration and pattern of activity varied from cycle to cycle. Their pattern of firing was modified by the activity of the large accessory neurons which make direct inhibitory synapses upon them. Neither the stretch receptors nor the accessory neurons were active during the extensor phase of the cycle. The timing and extent of tailfan movements during the period of stretch receptor activity were measured from video records before and after the stretch receptor nerves were cut in the second to fifth segments. The promotion of the tailfan during flexion was significantly delayed and the minimum angle to which the uropods were remoted at the end of flexion significantly larger in denervated animals. We propose that afferent information from the stretch receptors coordinates the timing and extent of tailfan movements according to variations in the positioning and movement of the abdominal segments such that the hydrodynamic efficiency of the tailfan is enhanced on a cycle by cycle basis during non-giant swimming.Abbreviations A# abdominal segment number - Acc accessory neuron - LUU large unidentified unit - MRO muscle receptor organ - NGS non-giant swimming - SEMN slow extensor motor neuron - SR stretch receptor neuron     

Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Fast and slow receptor muscles

Receptor muscles of the abdominal muscle receptor organs of the crayfish, Procambarus clarkii, were examined by electron microscopy. Both the fast and the slow receptor strand comprises a single muscle fibre which is divided by invagination of the cell membrane into numerous cytoplasmic processes in its intermediate region (the so-called intercalated tendon). Most of these myofibrillar processes insert in this region, but some of them pass through the intermediate region without interruption and join the other portion of the fibre. Thus the receptor muscles, whilst maintaining cytoplasmic continuity throughout their whole length, are modified in their intermediate regions, becoming fasciculated and providing spaces which are occupied by the connective tissue and the dendrites of the sensory neurone. Clear-cut differences in fine structure are shown between the muscle of the two types of receptor unit. The fast receptor muscle shows the typical features of arthropod fast muscles, including short sarcomere length (on average 3.3 μm), cylindrical myofibrils, well-developed sarcoplasmic reticulum, and regular hexagonal array of the myofilaments. By contrast, the slow receptor muscle fibre is characterized by long sarcomeres (average 6.5 μm) and unique organization of the myofilaments, with very thick ‘thick’ filaments having diameters in the range of 25–36 nm surrounded by about 12 thin filaments.     

Sensory activity of the mandibular muscle receptor organ of Homarus gammarus (L)

The mandibular muscle receptor organ (Mand. M.R.O.) of Homarus gammarus (L) exhibits increased activity to receptor muscle (R.M.) contraction (decrease in length) and to stretch (increase in length).

The sensory units of this receptor differ in their threshold to R.M. tension and in the frequency change they exhibit for a given increment in tension. The units fire tonically during maintained tension once their threshold has been reached.

Both the number of active units and their frequency increase with R.M. tension. The Mand. M.R.O. is velocity sensitive and exhibits a higher degree of activity during rapid stretch. This dynamic response increases with rate of R.M. stretch.

The activity of the Mand. M.R.O. sensory neurones is compared with that of other M.R.O.’s and the authenticity of some aberrant units is discussed.     

In search of differences between the two types of sensory cells innervating spider slit sensilla (<Emphasis Type="Italic">Cupiennius salei</Emphasis> Keys.)

The metatarsal lyriform organ of the spider Cupiennius salei is a vibration detector consisting of 21 cuticular slits supplied by two sensory cells each, one ending in the outer and the other at the inner slit membrane. In search of functional differences between the two cell types due to differences in stimulus transmission, we analyzed (1) the adaptation of responses to electrical stimulation, (2) the thresholds for mechanical stimulation and (3) the representation of male courtship vibrations using intracellular recording and staining techniques. Single- and multi-spiking receptor neurons were found among both cell types, which showed high-pass filter characteristics. Below 100-Hz threshold, tarsal deflections were between 1° and 10°. At higher frequencies, they decreased down to values as small as 0.05°, corresponding to 4.5-nm tarsal deflection in the most sensitive cases. Different slits in the organ and receptor cells with slow or fast adaptation did not differ in this regard. When stimulated with male courtship vibrations, both types of receptor cells again did not differ significantly regarding number of action potentials, latency and synchronization coefficients. Surprisingly, the differences in dendrite coupling were not reflected by the physiological responses of the two cell types innervating the slits.     

On the cavity receptor organ (X-organ or organ of Bellonci) of Artemia salina (Crustacea: Anostraca)

Summary The cavity receptor organ (previously X-organ or organ of Bellonci) of Artemia salina consists of ciliated neurons whose cilia protrude into a cavity beneath the cuticle. The neuronal dendrites penetrate a giant accompanying cell and epidermal cells before entering the cavity. The cavity beneath the cuticle, the ciliated neurons and the connexion with the medulla terminalis justifies a homologization with the frontal filament organ of cirripeds and the third unit of copepods. The term cavity receptor is suggested for this organ. It is hardly homologous with the second unit of copepods and the organs described for many malacostracans under the names of sensory pore X-organ or organ of Bellonci. The latter organs are very similar to the cavity receptor but have an internal cavity formed by glial cells.The cavity receptor organ was previously considered neurosecretory but in the light of the present knowledge it is rather sensory although a double function cannot be denied.This investigation was supported by grants (to R. E.) 2760-3 and 2760-4 from the Swedish Natural Science Research Council. One of us (P. S. L.) was on sabbatical leave from the University of Tasmania.     

Effects of Cannabinoids on Caffeine Contractures in Slow and Fast Skeletal Muscle Fibers of the Frog

The effect of cannabinoids on caffeine contractures was investigated in slow and fast skeletal muscle fibers using isometric tension recording. In slow muscle fibers, WIN 55,212-2 (10 and 5 μM) caused a decrease in tension. These doses reduced maximum tension to 67.43 ± 8.07% (P = 0.02, n = 5) and 79.4 ± 14.11% (P = 0.007, n = 5) compared to control, respectively. Tension-time integral was reduced to 58.37 ± 7.17% and 75.10 ± 3.60% (P = 0.002, n = 5), respectively. Using the CB₁ cannabinoid receptor agonist ACPA (1 μM) reduced the maximum tension of caffeine contractures by 68.70 ± 11.63% (P = 0.01, n = 5); tension-time integral was reduced by 66.82 ± 6.89% (P = 0.02, n = 5) compared to controls. When the CB₁ receptor antagonist AM281 was coapplied with ACPA, it reversed the effect of ACPA on caffeine-evoked tension. In slow and fast muscle fibers incubated with the pertussis toxin, ACPA had no effect on tension evoked by caffeine. In fast muscle fibers, ACPA (1 μM) also decreased tension; the maximum tension was reduced by 56.48 ± 3.4% (P = 0.001, n = 4), and tension-time integral was reduced by 57.81 ± 2.6% (P = 0.006, n = 4). This ACPA effect was not statistically significant with respect to the reduction in tension in slow muscle fibers. Moreover, we detected the presence of mRNA for the cannabinoid CB₁ receptor on fast and slow skeletal muscle fibers, which was significantly higher in fast compared to slow muscle fiber expression. In conclusion, our results suggest that in the slow and fast muscle fibers of the frog cannabinoids diminish caffeine-evoked tension through a receptor-mediated mechanism.     

Proprioceptive reflex interactions with central motor rhythms in the isolated thoracic ganglion of the shore crab

Summary Experiments were carried out on an isolated central nervous system preparation of the shore crab,Carcinus maenas, comprising the fused thoracic ganglion complex with two proprioceptors of one back leg still attached. These, the thoracic-coxal muscle receptor organ and the coxo-basal chordotonal organ, monitor movement and position of the first and second joints, respectively. Motor activity was recorded extracellularly from the central cut ends of the nerves innervating the promotor and remotor muscles of the thoracic-coxal joint, and the levator and depressor muscles of the coxal-basal joint of the same leg. Simultaneous intracellular recordings were made from central processes of individual motoneurones of each muscle.In the absence of any sensory input, the isolated ganglion exhibited rhythmic bursting in the motor nerve roots, with a slow, usually irregular cycle period of 5–50 s.Both receptor organs had both intra-joint and inter-joint effects on the rhythmically active preparation. In most cases the coxo-basal receptor organ had the greater effect.Resistance reflexes initiated by each of the joint proprioceptors were modulated by the rhythmic activity.It may be concluded that, while the isolated thoracic ganglion of the crab is capable of generating rhythmic motor output, proprioceptive feedback from the two basal joints is important in shaping the motor patterns underlying locomotion. Inappropriate reflexes which would impede active movements about these joints are modulated or reversed so as to permit and even reinforce intended locomotory movements.     

Developmental transition of touch response from slow muscle-mediated coilings to fast muscle-mediated burst swimming in zebrafish

It is well known that slow and fast muscles are used for long-term sustained movement and short bursts of activity, respectively, in adult animal behaviors. However, the contribution of the slow and fast muscles in early animal movement has not been thoroughly explored. In wild-type zebrafish embryos, tactile stimulation induces coilings consisting of 1–3 alternating contractions of the trunk and tail at 24 hours postfertilization (hpf) and burst swimming at 48 hpf. But, embryos defective in flightless I homolog (flii), which encodes for an actin-regulating protein, exhibit normal coilings at 24 hpf that is followed by significantly slower burst swimming at 48 hpf. Interestingly, actin fibers are disorganized in mutant fast muscle but not in mutant slow muscle, suggesting that slower swimming at 48 hpf is attributable to defects of the fast muscle tissue. In fact, perturbation of the fast muscle contractions by eliminating Ca²⁺ release only in fast muscle resulted in normal coilings at 24 hpf and slower burst swimming at 48 hpf, just as flii mutants exhibited. In contrast, specific inactivation of slow muscle by knockdown of the slow muscle myosin genes led to complete loss of coilings at 24 hpf, although normal burst swimming was retained by 48 hpf. These findings indicate that coilings at 24 hpf is mediated by slow muscle only, whereas burst swimming at 48 hpf is executed primarily by fast muscle. It is consistent with the fact that differentiation of fast muscle follows that of slow muscle. This is the first direct demonstration that slow and fast muscles have distinct physiologically relevant contribution in early motor development at different stages.     

An acetylcholine receptor lacking both γ and ε subunits mediates transmission in zebrafish slow muscle synapses

Fast and slow skeletal muscle types in larval zebrafish can be distinguished by a fivefold difference in the time course of their synaptic decay. Single-channel recordings indicate that this difference is conferred through kinetically distinct nicotinic acetylcholine receptor (AChR) isoforms. The underlying basis for this distinction was explored by cloning zebrafish muscle AChR subunit cDNAs and expressing them in Xenopus laevis oocytes. Measurements of single-channel conductance and mean open burst duration assigned α(2)βδε to fast muscle synaptic current. Contrary to expectations, receptors composed of only αβδ subunits (presumed to be α(2)βδ(2) receptors) recapitulated the kinetics and conductance of slow muscle single-channel currents. Additional evidence in support of γ/ε-less receptors as mediators of slow muscle synapses was reflected in the inward current rectification of heterologously expressed α(2)βδ(2) receptors, a property normally associated with neuronal-type nicotinic receptors. Similar rectification was reflected in both single-channel and synaptic currents in slow muscle, distinguishing them from fast muscle. The final evidence for α(2)βδ(2) receptors in slow muscle was provided by our ability to convert fast muscle synaptic currents to those of slow muscle by knocking down ε subunit expression in vivo. Thus, for the first time, muscle synaptic function can be ascribed to a receptor isoform that is composed of only three different subunits. The unique functional features offered by the α(2)βδ(2) receptor likely play a central role in mediating the persistent contractions characteristic to this muscle type.     

Sensory Spots of Echinoderes capitatus (Zelinka, 1928) (Kinorhyncha,Cyclorhagida)

The sensory spots of Echinoderes capitatus from the Gulf of Trieste were examined by transmission and scanning electron microscopy. Their arrangement is bilaterally symmetrical and is species-specific. At the cuticle surface the sensory spot appears as a rounded to ovoid area of small cuticular papillae in which two pores open. The sensory organ consists of two different sensory cells, the monociliary receptor and the collar receptor, and one sheath cell. The course of the axons and their connections to the nervous system are described. A survey of collar receptors among invertebrates is given. A comparison of the sensory spots within Kinorhyncha and a comparison with the flosculi of Priapulida and the N-flosculi of Loricifera is made. A possible homology of these three structures is discussed.     

The antennal motor system of crickets: modulation of muscle contractions by a common inhibitor,DUM neurons,and proctolin

In the crickets Gryllus bimaculatus and Gryllus campestris, the two intrinsic antennal muscles in the scape (first antennal segment) control antennal movements in the horizontal plane. Of the 17 excitatory antennal motoneurons, three motoneurons, two fast and one slow, can be stimulated selectively and their effect on muscle contraction, i.e. antennal movement, measured. Simultaneously, either a common inhibitor (CI) neuron or two DUM neurons can be stimulated and the effect on the slow and/or fast muscle contraction measured. The activity of the common inhibitor affected only slow muscle contractions. It decreased contraction rate, increased relaxation rate and suppressed prolonged muscle tension. This effect was blocked by picrotoxin. DUM neuron stimulation affected both slow and fast contractions. It reduced slow and enhanced fast contractions but in only 10% of the experiments could this effect be detected. DUM neuron activity could be mimicked by octopamine application. Proctolin application enhanced both slow and fast contractions but did not increase muscle tension in the absence of motoneuron activity. The results are discussed in relation to the large variability of possible antennal movements during behaviors.Abbreviations CI common inhibitor neuron - DUM dorsal unpaired median neuron     

Zwei Muskelrezeptoren an der Mandibel von Leuctra (Insecta,Plecoptera) — Beispiele für nicht-ciliäre Sinnesorgane mit Tubularkörper-ähnlichen Strukturen

Zusammenfassung Beide Muskelrezeptoren an der Mandibel von Leuctra ziehen vom vorderen Tentorium-Arm zur Mandibel-Basis. Der ventrale Rezeptor besteht aus zwei dünnen Muskelfasern (6 bis 7 m Durchmesser) und mindestens 10 multiterminalen Sinneszellen, deren Dendriten sich im Innern der Fasern verzweigen und an den Z-Scheiben enden. Der dorsale Rezeptor besitzt drei ähnlich dünne Muskelfasern, aber nur eine einzelne multiterminale Sinneszelle. Ihre Dendriten enden im Ansatzgebiet des Muskels, zwischen Muskelfasern und Epidermiszellen.Beide Rezeptoren haben im wesentlichen denselben Feinbau wie bei Coleopteren, jedoch treten — besonders beim dorsalen Rezeptor — in den Dendriten-Enden Strukturen auf, die den Tubularkörpern bei Mechanosensillen ähneln.

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Two muscle receptor organs at the mandible of Leuctra (Insecta, Plecoptera) — examples for non-ciliary sense organs with tubular-body-like structures

Summary The muscle receptor organs of the mandible of Leuctra extend between the anterior tentorial arm and the mandible base. The ventral receptor is composed of two thin muscle fibres (6–7 m in diameter) and at least ten multiterminal sensory cells, the dendrites of which branch in the interior of the fibres and end near the z-bands. The dorsal receptor organ consists of three muscle fibres of similar diameter and only one multiterminal sensory cell. The dendritic ends lie at the distal end of the muscle, where muscle fibres and epidermal cells make contact.Both receptor organs essentially show the same ultrastructural characteristics as in Coleoptera. However, the dorsal receptor organ in particular possesses organelles in its dendritic ends, which look like the tubular bodies in ciliary mechanoreceptors.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft     

Cardiac and skeletal muscle troponin I isoforms are encoded by a dispersed gene family on mouse Chromosomes 1 and 7

We mapped the locations of the genes encoding the slow skeletal muscle, fast skeletal muscle, and cardiac isoforms of troponin I (Tnni) in the mouse genome by interspecific hybrid backcross analysis of species-specific (C57BL/6 vs Mus spretus) restriction fragment length polymorphisms (RFLPs). The slow skeletal muscle troponin I locus (Tnni1) mapped to Chromosome (Chr) 1. The fast skeletal muscle troponin I locus (Tnni2), mapped to Chr 7, approximately 70 cM from the centromere. The cardiac troponin I locus (Tnni3) also mapped to Chr 7, approximately 5–10 cM from the centromere and unlinked to the fast skeletal muscle troponin I locus. Thus, the troponin I gene family is dispersed in the mouse genome. Received: 10 May 1995 / Accepted: 1 September 1995     

Feinstruktur eines maxillaren Muskelrezeptors bei Oncopeltus fasciatus (Insecta,Heteroptera)

Zusammenfassung Im Kopf von Oncopeltus fasciatus wird erstmals für Insekten ein maxillarer nicht-ciliärer Muskelrezeptor in seiner Feinstruktur beschrieben. Er ist zwischen dem Hypopharynx-Flügel und dem Maxillen-Hebel aufgespannt und wird dementsprechend bei der Protraktion der Maxille gedehnt. Der reizübermittelnde Teil ist zusammengesetzt aus zwei caudal gelegenen, sehr dünnen Muskelfasern und einem Bindegewebs-Strang im vorderen Drittel. Die beiden Fasern des Rezeptor-Muskels bilden in ihrem vorderen Bereich eine Röhre. Die motorische Innervierung der Fasern geschieht vom Lumen der Röhre her. Fünf Sinneszellen senden ihre Dendriten zum Bindegewebs-Strang, wo sie zwischen den Fibrillen enden. Der beschriebene Maxillen-Rezeptor ist möglicherweise serial-homolog dem dorsalen Mandibel-Rezeptor anderer Insekten.

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Fine structure of a maxillary muscle receptor in Oncopeltus fasciatus (Insecta, Heteroptera)

Summary The fine structure of a non-ciliary muscle receptor in the maxilla of Oncopeltus fasciatus is described. The receptor extends anteriorly from its origin at the hypopharyngeal wing to its insertion at the maxillary lever. Accordingly it is stretched during protraction of the maxilla. The stimulus-transmitting part consists of two very thin, caudally situated muscle fibres and a connective tissue strand situated in the cranial third. The receptor muscle fibres form a tube in their cranial part. They are innervated by a motor axon lying in the lumen of the tube. Five sensory cells send their dendrites to the connective tissue strand, where they end among the filaments. The maxillary receptor described here is possibly an organ serially homologous to the dorsal mandibular muscle receptor of other insects.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft