Stick insects walking along inclined surfaces

In the experiments stick insects walk on an inclined substratesuch that the legs of one side of the body point uphill andthe legs of the other side point downhill. In this situationthe vertical axis of the body is rotated against the inclinationof the substrate as if to compensate for the effect of substrateinclination. A very small effect has been found when the experimentwas performed with animals standing on a tilted platform whichshows that the effect depends on the behavioral context. When,however, animals first walked along the inclined surface andthen, before measurement, stopped walking spontaneously, a rotationof the body has been observed similar to that in walking animals.In a second experiment it was tested whether the observed bodyrotation is caused by the change of direction of gravity vectoror by the fact that on an inclined surface gravity necessarilyhas a component pulling the body sideways. Experiments withanimals standing on horizontal ground and additional weightsapplied pulling the body to the side showed similar body rotationssupporting the latter idea. In a simulation study it could beshown that the combined activity of proportional feedback controllersin the leg joints is sufficient to explain the observed behavior.This is however only possible if the gain factors of coxa-trochanterjoint controller and of femur-tibia joint controller show aratio in the order of 1 : 0.05 to 1 : 1.8. In order to describethe behavior of animals standing on a tilted platform, a ratioof 1 : 1.7 is necessary. In walking animals, this body rotationrequires to change the trajectories of stance and swing movements.The latter have been studied in more detail. During swing, thefemur-tibia joint is more extended in the uphill legs. Conversely,the coxa-trochanter joint appears to be more elevated in thedownhill legs which compensates the smaller lift in the femur-tibiajoint. The results are discussed in the context of differenthypotheses.     

A cell wall protein down-regulated by auxin suppresses cell expansion in Daucus carota (L.)

We investigated the function of the auxin-regulated cell wall gene DC 2.15, a member of a small gene family, present in Daucus carota (L.) and other plants. Cultured cells derived from carrot hypocotyls transformed by the DC 2.15 cDNA in antisense direction were ten-fold longer than wild-type cells, indicating a function of the corresponding protein in suppression of cell expansion. The analysis of carrot plants expressing the DC 2.15 gene in antisense direction showed that the corresponding protein and/or related proteins probably are involved in leaf and vascular bundle development. The antisense plants generally displayed a retarded growth phenotype and delayed greening in comparison to wild-type plants. The asymmetric architecture of the wild-type leaves was degenerated in the DC 2.15 antisense plants and the leaves showed a torsion within and along their major vein. The vascular bundles showed a lowered ratio of the phloem/xylem area in cross sections of the leaf middle vein whereas the bundle sheath and the cambium showed no obvious phenotype. Expression of a promoter-GUS construct was found primarily in vascular bundles of stems, leaves and in the nectar-producing flower discs. The observed pleiotropic antisense phenotype indicates, by loss of function, that one or several related cell wall proteins of this gene family are necessary to realize several complex developmental processes.     

The functional sense of central oscillations in walking

 Rhythmic motor output is generally assumed to be produced by central pattern generators or, more specific, central oscillators, the rhythmic output of which can be entrained and modulated by sensory input and descending control. In the case of locomotor systems, the output of the central system, i.e., the output obtained after deafferentation of sensory feedback, shows many of the temporal characteristics of real movements. Therefore the term fictive locomotion has been coined. This article concentrates on a specific locomotor behavior, namely walking; in particular walking in invertebrates. In contrast to the traditional view, an alternative hypothesis is formulated to interpret the functional sense of these central oscillations which have been found in many cases. It is argued that the basic function of the underlying circuit is to avoid cocontraction of antagonistic muscles. Such a system operates best with an inherent period just above the maximum period observed in real walking. The circuit discussed in this article (Fig. 2) shows several properties in common with results described as “fictive walking”. It furthermore could explain a number of properties observed in animals walking in different situations. According to this hypothesis, the oscillations found after deafferentation are side effects occurring in specific artificial situations. If, however, a parameter called central excitation is large enough, the system can act as a central oscillator that overrides the sensory input completely. Received: 18 May 2001 / Accepted in revised form: 20 November 2001     

A recurrent network for landmark-based navigation

A new type of network is proposed that can be applied to landmark navigation. It solves the guidance task, that is, it finds a nonvisually marked location using knowledge concerning its spatial relation to other, visible landmarks. The path to the searched location is not disturbed if a landmark is not visible for some time. The network can also describe findings obtained by experiments with insects and rodents, where the position of the landmarks has been changed after training. In this net, recognition does not occur by searching for a match between a pattern seen and the same pattern being stored but by searching for a match between a pattern seen with a prediction calculated from different data. A simple extension allows a unique match of the landmarks seen with the items stored in memory. With this extension a recognition of the individual landmark is not necessary. A specific output unit of the network can be interpreted in such a way as to show properties of place cells found in vertebrates and the function of the network proposed here as to determine the input to a place cell. The model can explain the observation that a given place cell can also be active when the animal moves in a different environment. An extension is discussed of how the network could be exploited for recognition-triggered response that allows animals to follow fixed routes.     

Local lymph node assay: differentiating allergic and irritant responses using flow cytometry.

The murine local lymph node assay (LLNA) is a method for assessing the contact sensitization potential of chemicals. Based on events that occur during the induction phase of a contact sensitization response, the LLNA measures the in vivo proliferation of cells in the draining lymph nodes (DLNs) of mice following topical exposure to chemicals. In terms of predictive identification of important skin sensitizers, the LLNA has been shown to be at least as sensitive as, and much more reliable than, current guinea pig tests. However, proliferation has also been observed following treatment with some irritants. In an attempt to distinguish allergic from irritant-induced proliferation, flow cytometric techniques have been used to examine the phenotype of lymphocyte subsets in the DLNs as well as markers of T-lymphocyte activation and memory. Mice were treated on the ears for 3 consecutive days with allergens or irritants. The DLNs were harvested 72 h after the final treatment. Single-cell suspensions were prepared, counted, and stained for analysis of the percentages of T cells and B cells and T-cell expression of two adhesion molecules that have been associated with differentiating na?ve and activated/memory T cells, CD62L (L-selectin) and CD44 (H-cam). Increases in lymph node cellularity were observed in both allergen- and irritant-treated mice relative to na?ve and vehicle-treated animals. Mice treated with allergens showed a preferential increase in the percentage of B220(+) B cells compared with irritant-treated mice. Treatment with allergens, but not irritants, resulted in a selective increase in the percentages of CD4(+) and CD8(+) cells expressing the T-cell activation/memory phenotype CD62L(lo)CD44(hi). Taken together, flow cytometric analysis of cell phenotype and expression of T-cell activation/memory markers may provide important information for differentiating allergen- and irritant-induced proliferative responses in the DLNs of chemically treated mice.     

Sequence-dependent conformation of an A-DNA double helix. The crystal structure of the octamer d(G-G-T-A-T-A-C-C)

The crystal structures of the synthetic self-complementary octamer d(G-G-T-A-T-A-C-C) and its 5-bromouracil-containing analogue have been refined to R values of 20% and 14% at resolutions of 1.8 and 2.25 A, respectively. The molecules adopt and A-DNA type double-helical conformation, which is minimally affected by crystal forces. A detailed analysis of the structure shows a considerable influence of the nucleotide sequence on the base-pair stacking patterns. In particular, the electrostatic stacking interactions between adjacent guanine and thymine bases produce symmetric bending of the double helix and a major-groove widening. The sugar-phosphate backbone appears to be only slightly affected by the base sequence. The local variations in the base-pair orientation are brought about by correlated adjustments in the backbone torsion angles and the glycosidic orientation. Sequence-dependent conformational variations of the type observed here may contribute to the specificity of certain protein-DNA interactions.     

Self base pairing in a complementary deoxydinucleoside monophosphate duplex: crystal and molecular structure of deoxycytidylyl-(3'-5')-deoxyguanosine

The molecular structure of ammonium deoxycytidylyl-(3'-5')-deoxyguanosine, crystallized from aqueous acetone near pH 4, was determined for X-ray diffraction data. The crystals were tetragonal, space group P43212 with a = b = 11.078 (1) A and c = 45.826 (4) A. The structure was solved by tangent expansion of phases based on a derived phosphorus position and refined to R = 0.060 by full matrix least squares. Molecules related by a 2-fold symmetry axis are connected by hydrogen bonds between the bases and form parallel right-handed duplexes. Pairs of cytosines share a proton at N(3) and are joined by three hydrogen bonds: N(4)-H...O(2)...H-N(4), and N(3)-H...N(3). Guanines are joined by two hydrogen bonds: N(2)-H...N(3) and N(3)...H-N(2). Base-stacking interactions within the duplex are weak with the cytosine and guanine ring planes inclined at 24 degrees to each other in each monomer. Despite the unusual arrangement of the molecules, the sugar phosphate backbone has the g-g- conformation normally associated with right-handed double helical structures. Conformational parameters of the nucleosides are also typical with both sugars C(2')-endo and glycosidic torsion angles 55 degrees for cytidine and 94 degrees for guanosine. The bonding geometry of the bases is influenced by hydrogen bonding and charge-transfer networks in the crystal lattice. The solvent molecules interact with the dimer in three fused circular hydrogen bonding domains with a single disordered ammonium cation per d(CpG) dimer. Parallels with the formation of self base pairs and their implications in molecular biology are discussed.     

The coordination of force oscillations and of leg movement in a walking insect (Carausius morosus)

As in the preceding paper stick insects walk on a treadwheel and different legs are put on platforms fixed relative to the insect's body. The movement of the walking legs is recorded in addition to the force oscillations of the standing legs. The coordination between the different legs depends upon the number and arrangement of the walking legs and the legs standing on platforms. In most experimental situations one finds a coordination which is different from that of a normal walking animal.Supported by DFG (Cr 58/1)     

Der Regelkreis des Kniesehnenreflexes bei der Stabheuschrecke Carausius morosus

In Part B the tendon of the femoral chordotonal organ (receptor tendon) of a fixed leg is sinusoidally moved with different amplitudes and frequencies. This causes movements of the tibia. Figures 1–3 show the amplitudes of the tibia movements and the phase-shifts between tibia-movement and stimulus. As it is known, that a tibia-movement of about 13° corresponds to a movement of the receptor-tendon of 100 m, a bode-plot can be constructed. Figure 4 is the first part of a three-dimensional bode-plot (amplitude ratio) which additionally shows the values of amplitudes and frequencies, at which a phase shift of 180° can be observed. The system is stable, if the gain of the system is smaller than 1 at these values. A gain equal or larger than 1 causes instability. As it can be seen in Fig. 4, the system is stable, but it is not very far from instability. In Part C an inert mass is coupled to the tibia in order to enlarge the phase-shift. After a disturbance, which causes a higher gain of the system, intact legs often show long lasting oscillations of small amplitude (Fig. 6a, b). During these oscillations the other legs are not moved. Sometimes active movements of all legs occur. Active movements of the tested legs have larger amplitudes and are always followed by small-amplitude-oscillations. Legs with cut receptor tendons and intact legs of decerebrated animals never show small-amplitude-oscillations but only active movements. Therefore it is probable that the small-amplitude-oscillations are oscillations of the feedback-system. In Part C 4 another possible explanation for these oscillations is discussed: The forces, produced by the muscles, might be represented by a noise of broad bandwidth from which the mechanical system selects only a small band given by its resonance frequency. In order to test this hypothesis, electrophysiological experiments are done (C5): During slow-amplitude-oscillations of legs with an inert mass added a spike-burst can be observed in the flexor tibiae during extension and in the extensor tibiae during flexion of the femur-tibia-joint. Sometimes no activity in the extensor can be observed. This means, that the activity in the muscles has a phase-shift of about 180° relative to the movement of the tibia: These supports the hypothesis, that the small-amplitude-oscillations are oscillations of the control system of the Kniesehnenreflex. In Part D it is discussed, whether the rocking-movements of the whole animal could be explained by oscillations of control systems. It is, deduced, that if this hypothesis is true, the control system in the coxa-trochanter-joint must be as near to instability as the control system of the Kniesehnenreflex.