Neuronal co-processing of course deviation and head movements in locusts

Summary Descending deviation detector neurons (DDNs) of Locusta migratoria are characterized physiologically by their responses to light on/off stimuli, simulated course deviation (rotation of an artificial horizon), passive rotation of the head, frontal wind, and flight activity. The investigation emphasises on the co-processing of exteroceptive input signalling course deviation (mainly movement of the retinal image, but also wind), and proprioceptive input signalling head movement and position. Stimuli were presented in combinations as expected during natural behavior. Eight DDNs are described for the first time, and 3 previously described DDNs are characterized further. Responses to horizon rotation and imposed head movements are assigned to one of 4 response types: (1) the horizon-only type codes retinal slip and/or the position of the horizon in the visual field but ignores cervical proprioception; (2) the head-only type ignores visually simulated course deviation but codes for movement or position of the head; (3) in the compensating type, head rolling causes visual input and cervical proprioceptive input of opposite signs, so that head movements themselves are ignored, whereas course deviations are recognized; (4) in the amplifying type, head rolling causes visual input and cervical proprioceptive input of the same sign, i.e. one input amplifies the other. This classification does not take the various responses to wind into account. In several DDNs, responses to phasic and tonic stimuli of the same modality, and/or responses to deviations about different axes could be assigned to different response types. Activity in DDNs has been shown previously to result in steering responses of wings, legs, abdomen and/or the head. It is proposed that different kinds of flight steering (e.g. corrective course control, intentional steering, orientation towards or away from a target) may be controlled by selective enhancement or suppression of responses or motor effects of DDN-subpopulations.Abbreviations AP action potential - DDN descending deviation detector neuron - DNI, DNC, DNM descending deviation detector neurons receiving major input from the ipsilateral, contralateral, and median ocellus respectively - PDDSMD protocerebral, descending direction-selective motion-detecting neuron - PI(2)5 descending deviation detector neuron with the cell body in the pars intercerebralis medialis - TCG tritocerebral commissure giant neuron     

Visual receptive field properties of feature detecting neurons in the dragonfly

The dragonfly, (Aeshna, Anax) which feeds on small flying insects, requires a visual system capable of signaling the movements of airborne prey. A group of 8 descending feature detectors in the dragonfly are tuned exclusively to moving contrasting objects. These target-selective descending neurons project from the brain to the thoracic ganglia. Their activity drives steering movement of the wings.In this study, we recorded target-selective descending neuron activity intracellularly.To define their receptive fields, we recorded responses to the movement of black square targets projected onto a screen in front of the animal. Each neuron was identified by dye injection.Target-selective descending neurons exhibit several receptive field properties. Our results show that they are strongly directionally selective. Two TSDNs, exclusively tuned to small targets, have receptive fields restricted to visual midline. Others, which are not selective for target size, have asymmetric receptive fields centered laterally.We suggest that the behavioral function of these specialized feature detectors is to steer the dragonfly during prey-tracking so as to fix the position of the prey image on the retina. If the dragonfly maintains a constant visual bearing to its prey over time it will intercept its prey.Abbreviations TSDN target-selective descending neuron - DCMD descending contralateral movement detector - MDT median dorsal tract - DIT dorsal intermediate tract - VNC ventral nerve cord     

Differential regulation of a homologous peptidergic neuron in grasshoppers: evolution of a neural circuit

The vasopressin-like immunoreactive (VPLI) neurons of grasshoppers have paired cell bodies in the suboesophageal ganglion and both anterior and posterior running axons. In non-oedipodine grasshopper species (e.g. Schistocerca gregaria), most of their arborisations are distributed in dorsal and lateral neuropil, while in oedipodine species (e.g. Locusta migratoria), the neurons have additional extensive axonal projections in both the optic lobes and proximal portions of the ganglionic peripheral nerves. This study demonstrates that these morphological differences correlate with their physiology. In L. migratoria, VPLI neuron activity is regulated primarily via a spontaneously active interneuron which descends from the brain. This descending interneuron is inhibited by a light-activated brain extraocular photoreceptor. Regulation of VPLI neuron activity by an extraocular photoreceptor is also seen in the other oedipodine grasshopper investigated. In the four non-oedipodines examined (from two subfamilies), we find no extraocular photoreceptor regulation of VPLI neuron activity. Despite this, VPLI neuron in S.␣gregaria does appear to be driven by a descending interneuron homologous to that in L. migratoria. The descending interneuron in both species receives similar mechanosensory input and excites the VPLI neuron via cholinergic synapses. Histamine injection into the medial protocerebrum of both species causes strong inhibition of the descending interneuron. The evolution of the neural circuitry, by which an extraocular photoreceptor comes to regulate the descending interneuron in oedipodine species, is discussed. Accepted: 6 January 1998     

Flight initiations in Drosophila melanogaster are mediated by several distinct motor patterns

We have monitored the patterns of activation of five muscles during flight initiation of Drosophila melanogaster: the tergotrochanteral muscle (a mesothoracic leg extensor), dorsal longitudinal muscles #3, #4 and #6 (wing depressors), and dorsal ventral muscle #Ic (a wing elevator). Stimulation of a pair of large descending interneurons, the giant fibers, activates these muscles in a stereotypic pattern and is thought to evoke escape flight initiation. To investigate the role of the giant fibers in coordinating flight initiation, we have compared the patterns of muscle activation evoked by giant fiber stimulation with those during flight initiations executed voluntarily and evoked by visual and olfactory stimuli. Visually elicited flight initiations exhibit patterns of muscle activation indistinguishable from those evoked by giant fiber stimulation. Olfactory-induced flight initiations exhibit patterns of muscle activation similar to those during voluntary flight initiations. Yet only some benzaldehyde-induced and voluntary flight initiations exhibit patterns of muscle activation similar to those evoked by giant fiber stimulation. These results indicate that visually elicited flight initiations are coordinated by the giant fiber circuit. By contrast, the giant fiber circuit alone cannot account for the patterns of muscle activation observed during the majority of olfactory-induced and voluntary flight initiations.Abbreviations DLM/DLMn dorsal longitudinal muscle/motor neuron - DVM/DVMn dorsal ventral muscle/motor neuron - GF(s) giant fiber interneuron (s) - PSI peripherally synapsing interneuron - TTM/TTMn tergotrochanteral muscle/motor neuron     

New vistas on the initiation and maintenance of insect motor behaviors revealed by specific lesions of the head ganglia

In insects, thoracic pattern generators are modulated by the two head ganglia, the supraesophageal ganglion (brain) and the subesophageal ganglion, which act as higher-order neuronal centers. To explore the contribution of each head ganglion to the initiation and maintenance of specific motor behaviors in cockroaches (Periplaneta americana), we performed specific lesions to remove descending inputs from either the brain or the subesophageal ganglion or both, and quantified the behavioral outcome with a battery of motor tasks. We show that ‘emergency’ behaviors, such as escape, flight, swimming or righting, are initiated at the thoracic level independently of descending inputs from the head ganglia. Yet, the head ganglia play a major role in maintaining these reflexively initiated behaviors. By separately removing each of the two head ganglia, we show that the brain excites flight behavior and inhibits walking-related behaviors, whereas the subesophageal ganglion exerts the opposite effects. Thus, control over specific motor behaviors in cockroaches is anatomically and functionally compartmentalized. We propose a comprehensive model in which the relative permissive versus inhibitory inputs descending from the two head ganglia, combined with thoracic afferent sensory inputs, select a specific thoracic motor pattern while preventing the others.     

Optic lobe projections of marginal ommatidia in Calliphora erythrocephala specialized for detecting polarized light

Summary The structure of ommatidia at the dorsal eye margin of the fly, Calliphora erythrocephala is specialized for the detection of the e-vector of polarized light. Marginal zone ommatidia are distinguished by R7/R8 receptor cells with large-diameter, short, untwisted rhabdomeres and long axons to the medulla. The arrangement of the R7 microvillar directions along the marginal zone is fan-shaped. Ommatidia lining the dorsal and frontal edge of the eye lack primary screening pigments and have foreshortened crystalline cones. The marginal ommatidia from each eye view a strip that is 5 °–20 ° contralateral to the fly's longitudinal axis and that coincides with the outer boundaries of the binocular overlap.Cobalt injection into the retina demonstrates that photoreceptor axons arising from marginal ommatidia define a special area of marginal neuropil in the second visual neuropil, the medulla. Small-field neurons arising from the marginal medulla area define, in turn, a special area of marginal neuropil in the two deepest visual neuropils, the lobula and the lobula plate. From these arise local assemblies of columnar neurons that relay the marginal zones of one optic lobe to equivalent areas of the opposite lobe and to midbrain regions from which arise descending neurons destined for the the thoracic ganglia.Optically, the marginal zone of the retina represents the lateral edge of a larger area of ommatidia involved in dorsofrontal binocular overlap. This binocularity area is also represented by special arrangements of columnar neurons, which map the binocularity area of one eye into the lobula beneath the opposite eye. Another type of binocularity neuron terminates in the midbrain.These neuronal arrangements suggest two novel features of the insect optic lobes and brain: (1) Marginal neurons that directly connect the left and right optic lobes imply that each lobe receives a common input from areas of the left and right eye, specialized for detecting the pattern of polarized light. (2) Information about the e-vector pattern of sky-light polarization may be integrated with binocular and monocular pathways at the level of descending neurons leading to thoracic motor neuropil.     

The organization of giant horizontal-motion-sensitive neurons and their synaptic relationships in the lateral deutocerebrum of Calliphora erythrocephala and Musca domestica

Summary Three giant horizontal-motion-sensitive (HS) neurons arise in the lobula plate. Their axons terminate ipsilaterally in the medial deutocerebrum and suboesophageal ganglion. Both Golgi impregnations and cobalt fills demonstrate that endings of the two HS cells, representing the upper and middle third of the retina, differ in shape and location from that of the HS cell subtending the lower third of the eye. This dichotomy is reflected by the terminals of a pair of centrifugal horizontal cells (CH), one of which invades lobula plate neuropil subtending the upper two-thirds of the retina. The other overlaps the dendrites of the HS cell subtending the lower one-third of the retina.The HS cells are cobalt-coupled to a variety of complexly arborizing descending neurons. In Musca domestica, gap-junction-like apposition areas have been observed between HS axon collaterals and descending neuron dendrites. The three HS cells also share conventional chemical synapses with postsynaptic elements, which include the dendritic spines of descending neurons. Unlike the giant vertical-motion-sensitive neurons of the lobula plate, whose relationships with descending neurons appear to be relatively simple, the horizontal cells end on a large number of descending neurons where they comprise one of several different populations of terminals. These descending neurons terminate within various centres of the thoracic ganglia, including neuropil supplying leg, neck, and flight muscle.     

Coordination of flipflopping neural signals and head turning during pheromone-mediated walking in a male silkworm moth Bombyx mori

Male silkworm moths, Bombyx mori, move their heads side-to-side during zigzag walking toward a source of sex pheromone. High-speed video analysis revealed that changes in walking direction were synchronized with this head turning. Thus the direction of the walking is indicated by the direction of the head turning. Head turning was regulated by neck motor neurons which innervate the cervical ventral muscles and the ventral muscles through the second cervical nerve. To determine the role of the `flipflop' state transition in spike activity carried by descending interneurons from the brain to the thoracic ganglion, we recorded pheromonal responses simultaneously from flipflop descending interneurons and a single cervical ventral 1 neck motor neuron. The activity of the cervical ventral 1 neck motor neuron was synchronized to that of the flipflop descending interneurons. The cervical ventral 1 neck motor neuron was morphologically identified using confocal imaging. Our results demonstrate that the flipflop signals play an important role in instructing turning signals during the pheromone-mediated behavior in a male B. mori. Accepted: 11 June 1998     

The whole-body shortening reflex of the medicinal leech: motor pattern,sensory basis,and interneuronal pathways

The leech whole-body shortening reflex consists of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations — semi-intact, body wall, and isolated nerve cord — to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a fast conducting pathway in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.Abbreviations conn connective - DE dorsal excitor motor neuron - DI dorsal inhibitor motor neuron - DP dorsal posterior nerve - DP:B1 dorsal posterior nerve branch 1 - DP:B2 dorsal posterior nerve branch 2 - MG midbody ganglion - VE ventral excitor motor neuron - VI ventral inhibitor motor neuron     

Lobula plate and ocellar interneurons converge onto a cluster of descending neurons leading to neck and leg motor neuropil in Calliphora erythrocephala

Summary In the fly, Calliphora erythrocephala, a cluster of three Y-shaped descending neurons (DNOVS 1–3) receives ocellar interneuron and vertical cell (VS4–9) terminals. Synaptic connections to one of them (DNOVS 1) are described. In addition, three types of small lobula plate vertical cell (sVS) and one type of contralateral horizontal neuron (Hc) terminate at DNOVS 1, as do two forms of ascending neurons derived from thoracic ganglia. A contralateral neuron, with terminals in the opposite lobula plate, arises at the DNOVS cluster and is thought to provide heterolateral interaction between the VS4–9 output of one side to the VS4–9 dendrites of the other. DNOVS 2 and 3 extend through pro-, meso-, and metathoracic ganglia, branching ipsilaterally within their tract and into the inner margin of leg motor neuropil of each ganglion. DNOVS 1 terminates as a stubby ending in the dorsal prothoracic ganglion onto the main dendritic trunks of neck muscle motor neurons. Convergence of VS and ocellar interneurons to DNOVS 1 comprises a second pathway from the visual system to the neck motor, the other being carried by motor neurons arising in the brain. Their significance for saccadic head movement and the stabilization of the retinal image is discussed.     

Effects of temperature on properties of flight neurons in the locust

High ambient temperatures increase the wing-beat frequency in flying locusts, Locusta migratoria. We investigated parameters of circuit and cellular properties of flight motoneurons at temperatures permissive for flight (20–40 °C). As the thoracic temperature increased motoneuronal conduction velocity increased from an average of 4.40 m/s at 25 °C to 6.73 m/s at 35 °C, and the membrane time constant decreased from 11.45 ms to 7.52 ms. These property changes may increase locust wing-beat frequency by affecting the temporal summation of inputs to flight neurons in the central circuitry. Increases in thoracic temperature from 25–35 °C also resulted in a hyperpolarization of the resting membrane potentials of flight motoneurons from an average of-41.1 mV to -47.5 mV, and a decrease of input resistances from an average of 3.45 M to 2.00 M. Temperature affected the measured input resistance both by affecting membrane properties, and by altering synaptic input. We suggest that the increase in conduction velocity Q₁₀=1.53) and the decrease of membrane time constant (Q₁₀=0.62) would more than account for the wing-beat frequency increase (Q₁₀=1.15). Hyperpolarization of the resting membrane potential (Q₁₀=1.18) and reduction in input resistance (Q₁₀=0.54) may be involved in automatic compensation of temperature effects.Abbreviations ANOVA analysis of variance - CPG central pattern generator - DL dorsal longitudinal muscles - EMG electromyographic - MN motoneuron - PSP post synaptic potential - Q₁₀ temperature coefficient - RMP resting membrane potential - S.D. standard deviation - SR stretch receptor     

Conditional inactivation of TGF-β type II receptor in smooth muscle cells and epicardium causes lethal aortic and cardiac defects

To understand the role of TGF-β signaling in cardiovascular development, we generated mice with conditional deletion of the TGF-β type II receptor (TβRII) gene (Tgfbr2) in cells expressing the smooth muscle cell-specific protein SM22α. The SM22α promoter was active in tissues involved in cardiovascular development: vascular smooth muscle cells (VSMCs), epicardium and myocardium. All SM22-Cre^+/−/Tgfbr2 ^flox/flox embryos died during the last third of gestation. About half the mutant embryos exhibited heart defects (ventricular myocardium hypoplasia and septal defects). All mutant embryos displayed profound vascular abnormalities in the descending thoracic aorta (irregular outline and thickness, occasional aneurysms and elastic fiber disarray). Restriction of these defects to the descending thoracic aorta occurred despite similar levels of Tgfbr2 invalidation in the other portions of the aorta, the ductus arteriosus and the pulmonary trunk. Immunocytochemistry identified impairment of VSMC differentiation in the coronary vessels and the descending thoracic aorta as crucial for the defects. Ventricular myocardial hypoplasia, when present, was associated to impaired α-SMA differentiation of the epicardium-derived coronary VSMCs. Tgfbr2 deletion in the VSMCs of the descending thoracic aorta diminished the number of α-SMA-positive VSMC progenitors in the media at E11.5 and drastically decreased tropoelastin (from E11.5) and fibulin-5 (from E.12.5) synthesis and/or deposition. Defective elastogenesis observed in all mutant embryos and the resulting dilatation and probable rupture of the descending thoracic aorta might explain the late embryonic lethality. To conclude, during mouse development, TGF-β plays an irreplaceable role on the differentiation of the VSMCs in the coronary vessels and the descending thoracic aorta.     

Mandibular premotor interneurons of larval Manduca sexta

Simultaneous intracellular recordings were made from interneurons and from closer or opener mandibular motor neurons in the isolated suboesophageal ganglion of the larva of Manduca sexta. This article describes various morphologically and physiologically distinguishable premotor spiking interneurons which make direct excitatory connections with the motor neurons. In addition, two presumptive non-spiking interneurons make excitatory and inhibitory connections respectively with opener motor neurons. Both classes of interneurons receive excitatory and inhibitory sensory inputs from the mouthparts. Their circuitry and functions are discussed.Abbreviations A anterior - AP action potential - CEC circumoesophageal connective - Cl-MN closer motor neuron - EPSP excitatory postsynaptic potential - IN interneuron - IPSP inhibitory postsynaptic potential - MdN mandibular nerve - MN motor neuron - MxN maxillary nerve - O-MN opener motor neuron - PSP postsynaptic potential     

The vasopressin-like immunoreactive (VPLI) neurons of the locust,Locusta migratoria. II. Physiology

Summary The two vasopressin-like immunoreactive (VPLI) neurons of the locust, Locusta migratoria, have cell bodies in the suboesophageal ganglion and extensive arborizations throughout the CNS. One of the two peptides responsible for AVP-like immunoreactivity is a vasopressin-related peptide with putative diuretic hormone properties. These neurons also have FLRF-like immunoreactivity, probably due to the FMRF-amide-related peptide, SchistoFLRF-amide, isolated from Schistocerca gregaria. This peptide has cardioinhibitory activity and a dual potentiation/inhibition of slow motoneuron induced muscle-twitch tension. Although haemolymph AVP-like peptide titre fluctuates under various conditions, the mechanism that regulates neurohaemal release of this peptide is not understood. Very little is known of the release of SchistoFLRF-amide. We have used intracellular recording from VPLI neurons in vivo to reveal synaptic inputs that lead to changes in their level of spiking activity, and probably, release of both the AVP-like peptides and SchistoFLRF-amide. This pair of neurosecretory cells has a major, common excitatory input whose sustained rate of activity is inversely related to light intensity; VPLI spiking activity, driven by this input, is greater in the dark than in light. This input is from a pair of descending brain interneurons. Their light-sensitivity persists after ablation of compound eyes, optic lobes and ocelli, showing them to be part of an extra-ocular photoreceptor system. Attempts to record from, and individually stain, the descending neuron have been unsuccessful, although its axon location and diameter in the circumoesophageal connective have been determined. Possible locations for its cell body have been identified; one region, close to the pars intercerebralis, is known to be photosensitive in some insects. Mechanosensory stimuli also lead to brief increases in VPLI spiking activity via the descending interneuron, though this modality rapidly habituates. We detect no changes in VPLI spiking activity that consistently correlate with the osmolality of perfusion salines; such changes might have been expected from their previously proposed role in water homeostasis. Alternative roles for VPLI cells are discussed.Abbreviations AVP arginine-vasopressin - EOP extra-ocular photoreceptor - FLRF Phe-Leu-Arg-Phe - FMRF-amide Phe-Met-Arg-Phe-amide - RH relative humidity - RIA radio-immune assay - SOG suboesophageal ganglion - VPLI vasopressin-like immunoreactive     

Exploring optimal current stimuli that provide membrane voltage tracking in a neuron model

Studying neurons from an energy efficiency perspective has produced results in the research literature. This paper presents a method that enables computation of low energy input current stimuli that are able to drive a reduced Hodgkin–Huxley neuron model to approximate a prescribed time-varying reference membrane voltage. An optimal control technique is used to discover an input current that optimally minimizes a user selected balance between the square of the input stimulus current (input current ‘energy’) and the difference between the reference voltage and the membrane voltage (tracking error) over a stimulation period. Selecting reference signals to be membrane voltages produced by the neuron model in response to common types of input currents i(t) enables a comparison between i(t) and the determined optimal current stimulus i*(t). The intent is not to modify neuron dynamics, but through comparison of i(t) and i*(t) provide insight into neuron dynamics. Simulation results for four different bifurcation types demonstrate that this method consistently finds lower energy stimulus currents i*(t) that are able to approximate membrane voltages as produced by higher energy input currents i(t) in this neuron model.     

Characterization of cardiac innervation in the nudibranch,Archidoris montereyensis

Summary The heart of the nudibranch mollusc Archidoris montereyensis is regulated by a small number of powerful effector neurons located in the right pleural and visceral ganglia. Two identifiable neurons in the pleural ganglion, a heart excitor (pl_HE) and a heart inhibitor (Pl_HI), are especially important regulators of cardiac function in that low levels of spontaneous activity in either cell significantly alters the amplitude and rate of heart contractions. These neurons have extensive dendritic arbors within the right pleural ganglion and branching axonal processes within the visceral ganglion. The visceral ganglion also contains a heart excitor neuron (V_HE) and at least two heart inhibitor neurons (V_HI cells), but their influence on cardiac activity is weaker than that of the pleural ganglion cells. All of these heart effector cells appear to be motor neurons with axons that terminate predominately in the atrio-ventricular valve region of the heart via the pericardial nerve. The simplicity and strength of these neuronal connections to the heart of Archidoris make this a favorable preparation for studies of cardiac regulation.Abbreviations Pl _HE pleural ganglion heart excitor neuron - Pl _HI pleural heart inhibitor neuron - V _HE visceral ganglion heart excitor neuron - V _HI cells, visceral heart inhibitor neurons - V _K visceral kidney excitor neuron - V _G visceral gill excitor neuron     

The morphology of suboesophageal ganglion cells innervating the nervus corporis cardiaci III of the locust

Summary The nervus corporis cardiaci III (NCC III) of the locust Locust migratoria was investigated with intracellular and extracellular cobalt staining techniques in order to elucidate the morphology of neurons within the suboesophageal ganglion, which send axons into this nerve. Six neurons have many features in common with the dorsal, unpaired, median (DUM) neurons of thoracic and abdominal ganglia. Three other cells have cell bodies contralateral to their axons (contralateral neuron 1–3; CN 1–3). Two of these neurons (CN2 and CN3) appear to degenerate after imaginal ecdysis. CN3 innervates pharyngeal dilator muscles via its anterior axon in the NCC III, and a neck muscle via an additional posterior axon within the intersegmental nerve between the suboesophageal and prothoracic ganglia. A large cell with a ventral posterior cell body is located close to the sagittal plane of the ganglion (ventral, posterior, median neuron; VPMN). Staining of the NCC III towards the periphery reveals that the branching pattern of this nerve is extremely variable. It innervates the retrocerebral glandular complex, the antennal heart and pharyngeal dilator muscles, and has a connection to the frontal ganglion.Abbreviations AH antennal heart - AN antennal nerves - AO aorta - AV antennal vessel - CA corpus allatum - CC corpus cardiacum - CN1, CN2, CN3 contralateral neuron 1–3 - DIT dorsal intermediate tract - DMT dorsal median tract - DUM dorsal, unpaired, median - FC frontal connective - FG frontal ganglion - HG hypocerebral ganglion - LDT lateral dorsal tract - LMN, LSN labral motor and sensory nerves - LN+FC common root of labral nerves and frontal connective - LO lateral ocellus - MDT median dorsal tract - MDVR ventral root of mandibular nerve - MVT median ventral tract - NCA I, II nervus corporis allati I, II - NCC I, II, III nervus corporis cardiaci I, III - NR nervus recurrens - NTD nervus tegumentarius dorsalis - N8 nerve 8 of SOG - OE oesophagus - OEN oesophageal nerve - PH pharynx - SOG suboesophageal ganglion - T tentorium - TVN tritocerebral ventral nerve - VLT ventral lateral tract - VIT ventral intermediate tract - VMT ventral median tract - VPMN ventral, posterior, median neuron - 1–7 peripheral nerves of the SOG - 36, 37, 40–45 pharyngeal dilator muscles     

The distribution of neurones immunoreactive for β-tyrosine hydroxylase,dopamine and serotonin in the ventral nerve cord of the cricket,Gryllus bimaculatus

The cellular localization of the biogenic amines dopamine and serotonin was investigated in the ventral nerve cord of the cricket, Gryllus bimaculatus, using antisera raised against dopamine, -tyrosine hydroxylase and serotonin. Dopamine-(n<-70) and serotonin-immunoreactive (n<-120) neurones showed a segmental arrangement in the ventral nerve cord. Some neuromeres, however, did not contain dopamine-immunoreactive cell bodies. The small number of stained cells allowed complete identification of brain and thoracic cells, including intersegmentally projecting axons and terminal arborizations. Dopamine-like immunostaining was found primarily in plurisegmental interneurones with axons descending to the soma-ipsilateral hemispheres of the thoracic and abdominal ganglia. In contrast, serotonin-immunostaining occurred predominantly in interneurones projecting via soma-contralaterally ascending axons to the thorax and brain. In addition, serotonin-immunoreactivity was also present in efferent cells and afferent elements. Serotonin-immunoreactive, but no dopamine-immunoreactive, varicose fibres were observed on the surface of some peripheral nerves. Varicose endings of both dopamine-and serotonin-immunoreactive neurones occurred in each neuromere and showed overlapping neuropilar projections in dorsal and medial regions of the thoracic ganglia. Ventral associative neuropiles lacked dopamine-like immunostaining but were innervated by serotonin-immunoreactive elements. A colocalization of the two amines was not observed. The topographic representation of neurone types immunoreactive for serotonin and dopamine is discussed with respect to possible modulatory functions of these biogenic amines in the central nervous system of the cricket.     

Information processing in the femur-tibia control loop of stick insects

The complicated response characteristics of the identified nonspiking interneuron type E4 upon elongation stimuli to the femoral chordotonal organ (fCO) can be obtained by a computer simulation using the neuronal network simulator BioSim, if the following assumptions were introduced: (1) The interneurons receive direct excitatory input from position- and velocity-sensitive fCO afferents but also, in parallel delayed inhibition from the same velocity-sensitive afferents. (2) Position-sensitive afferents in part show adaptation with a rather long time-constant. A subsequent experimental analysis demonstrated that all these assumptions fit the reality: (1) Interneurons of type E4 receive direct excitatory input from fCO afferents. (2) Interneurons of type E4 are affected by velocity dependent delayed inhibitory inputs from the fCO. (3) The fCO does contain adapting position-sensitive sensory neurons, which have not been described before. The described principle of the information processing is also able to generate the response in interneurons of type E6 with less steep amplitude-velocity characteristic due to a different weighting of the direct excitation and delayed inhibition.Abbreviations EPSP excitatory postsynaptic potential - FETi fast extensor tibiae motor neuron - fCO femoral chordotonal organ - FT-control loop femur-tibia control loop - IPSP inhibitory postsynaptic potential - SETi slow extensor tibiae motor neuron     

Activity-dependent induction of facilitation,depression, and post-tetanic potentiation at an insect central synapse

Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations ACh acetylcholine - AChE acetylcholine esterase - CNS central nervous system - EPSP excitatory postsynaptic potential - I _h injected hyperpolarizing current - LTP long-term potentiation - PPR principal planta retractor motoneuron - PTP post-tetanic potentiation - R _in input resistance - V _h hyperpolarized potential - V _m membrane potential - VN ventral nerve - VNA anterior branch of the ventral nerve - V _r resting potential.