Reinhold Hustert

Segmental and interganglionic projections from primary fibres of insect mechanoreceptors

SummaryProjections of primary insect mechanoreceptor fibres in Locusta migratoria and Acheta domesticus are visualised with the cobalt intensification technique after axonal or dendritic filling with cobaltous chloride. Chordotonal proprioceptors of the locust mesothoracic thoracocoxal joint have typical segmental projections and more or less widespread interganglionic processes into the metathoracic, prothoracic and even suboesophageal ganglia. Similar observations can be made on locust abdominal chordotonal organs, from some of which all primary axons project into five preceding neuromeres or ganglia. From locust abdominal stretch receptors and cricket mechanosensitive sternal hairs single afferent axons extend through several ganglia. Locust campaniform sensilla and clavate hair projections of crickets terminate locally. It is concluded that many interganglionic primary afferents contribute to intersegmental control of motor coordination in insects.

Rapid mechano-sensory pathways code leg impact and elicit very rapid reflexes in insects

SUMMARY The temporal sequence of mechanoreceptor input arriving at the motoneuron level in the central nervous system (CNS) after distal mechanical contact was studied for the locust middle leg. Different types of afferent information from potential contact areas after selective stimulation showed propagation times of no less than 8 ms from mechanosensory hairs, campaniform sensilla (CS) and spurs of the distal leg segments. Impact of the same mechanical stimuli, even if very delicate, elicits strain that is transferred in less than 1 ms via the cuticle and stimulates proximal CS on the trochanter and femur. These propagate the afferents that code distal leg contact in about 1 ms to the CNS, where they connect mono- and polysynaptically to motoneurons of the depressor trochanteris system. The elicited excitatory postsynaptic potentials (EPSPs) contribute to rapid efferent commands, since single EPSPs already rise near firing threshold of the motoneurons. The short delays in this mechano-neuronal-muscular pathway from the tip of a leg to the neuromuscular synapses (5–7 ms) can very rapidly raise muscle tension in the trochanteral depressors at new leg contacts during locust landing and locomotion. At substrate contact, proximal leg CS contribute to very rapid motor responses supporting the body.

Serotonin-immunoreactive and dopamine-immunoreactive neurones in the terminal ganglion of the cricket, Acheta domestica: Light- and electron-microscopic immunocytochemistry

SummaryThe distribution and ultrastructure of serotonin- and dopamine-immunoreactive (5-HTi and DAi) neurones have been investigated in the terminal ganglion of the cricket, Acheta domestica, using a pre-embedding chopper technique. Special attention has been paid to the immunoreactive structures in the neuropil. 5-HTi structures are extensively distributed and densely packed throughout the 5 neuromeres of the terminal ganglion and originate from several interneurones and efferent neurones. In contrast, DAi fibres are distributed sparsely although they extend to all neuromeres of the ganglion and originate from 6 interneurons only. For both 5-HTi and DAi neurones characteristic axonal projections and branching patterns can be distinguished. The 5-HTi axons exhibit rich varicose arborizations, whereas DAi neurones possess fewer varicosities in the neuropil. Electron microscopy shows that 5-HTi varicosities contain small (∼ 60 nm) and large (∼ 100 nm) agranular vesicles, and large (∼ 100 nm) granular vesicles, whereas in DAi varicosities small (∼ 60 nm) agranular and large (∼ 100 nm) granular vesicles are seen. Both 5-HTi and DAi varicosities form synaptic contacts. We conclude that both serotonin and dopamine may be used as neurotransmitters in the terminal ganglion of the cricket.

Location and major postembryonic changes of identified 5-HT-immunoreactive neurones in the terminal ganglion of a cricket (Acheta domesticus)

SummaryIn the terminal ganglion of the cricket, Acheta domesticus, the somata of certain interneurones and efferent neurones consistently react to 5-HT immunohistochemistry. There are serially homologous pairs of bilateral interneurones seen in the neuromeres of the 7th to the 10th segment and hindgut neurones with their somata located at the posterior median end of the ganglion. In adult crickets, pairs of large efferent neurones with lateral somata supply specific genital muscles in the 8th and the 9th segment of females. In males, only one pair of these efferent neurones supplies genital muscles of the 9th segment only. These identified 5-HT-immunoreactive neurones are not detected in larval crickets before development of the genital apparatus.

Central projections from contact chemoreceptors of the locust ovipositor and adjacent cuticle

Abstract. Contact chemoreceptors (basiconic sensilla) located on the ovipositor and genital segments of the locust serve to control the chemical features of the substrate before and during oviposition. They occur dispersed and also crowded in fields between mechanosensory exteroceptors sensitive to touch or wind (trichoid and filiform sensilla). The central nervous projections of the four chemosensory and one mechanosensory neurons from single basiconic sensilla were stained selectively, focusing on receptors on the ovipositor valves, which usually contact the substrate during the pre-oviposition probing movements. All axons and neurites from one contact chemoreceptor usually stay close together in most of their projections. Segregation occurs mainly when single axons terminate in one neuromere while the others proceed to a different neuromere or ganglion. For projections from one chemoreceptor, there is evidence neither for functional segregation of mechanosensory from chemosensory afferent terminals nor for specific segregation between different chemosensory afferents. The projections from sensilla of dorsal cuticle tend to project rather uniformly along the midline of the terminal ganglion. Comparative staining of touch- and wind-sensitive hair receptor neurons shows mostly central projections, similar to those of neighbouring contact chemoreceptors. From the typical intersegmental projections of most primary afferents and from the lack of segregation into glomerular structures, we conclude that integration of chemosensory information from the genital segments is distributed in the terminal and the 7th abdominal ganglion.

The efferent innervation of the genital chamber by an identified serotonergic neuron in the female cricket Acheta domestica

SummaryThe serotonergic innervation of the genital chamber of the female cricket, Acheta domestica, has been investigated applying anti-serotonin (5-HT) immunocyto-chemistry at both light- and electron-microscopic levels as well as using conventional electron microscopy. Whole mount and pre-embedding chopper techniques of immuno-cytochemistry reveal a dense 5-HT-immunoreactive network of varicose fibers in the musculature of the genital chamber. All of these immunoreactive fibers originate from the efferent serotonergic neuron projecting through the nerve 8v to the genital chamber (Hustert and Topel 1986; Elekes et al. 1987). At the electron-microscopic level, 5-HT-immunoreactive nerve terminals, which contain small (50–60 nm) and large (∼ 100 nm) agranular vesicles as well as granular vesicles (∼100nm), contact the muscle fibers or the sarcoplasmic processes without establishing specialized neuromuscular connections. In addition to the 5-HT-immunoreactive axons, two types of immunonegative axons can also be found in the musculature. By use of conventional electron microscopy, three ultrastructurally distinct types of axon processes can be observed, one of which resembles 5-HT-immunoreactive axons. While the majority of the varicosities do not synapse on the muscle fibers, terminals containing small (50–60 nm) agranular vesicles occasionally form specialized neuromuscular contacts. It is suggested that the 5-HTergic innervation plays a non-synaptic modulatory role in the regulation circular musculature in the genital chamber of the cricket, while the musculature as a whole may be influenced by both synaptic and modulatory mechanisms.

Accessory hemolymph pump in the mesothoracic legs of locusts, (Schistocerca gregaria forskal) (Orthoptera, Acrididae)

Abstract An accessory pulsatile organ located in the mesothoracic legs pumps hemolymph towards the tip of the leg ventrally and towards the body near the dorsal side. It consists of a muscle attached to the ventral side of the trochanter and to the central region of a transverse connective tissue diaphragm located at the trochanter-femur border. The diaphragm has a ventral outlet that permits efferent hemolymph flow through a narrow femoral sinus. A second dorsal outlet allows the afferent countercurrent back to the thorax through a separate hemolymph channel. During abdominal ventilation, the pumping rhythm of the leg fn2 heart is neurally synchronized with abdominal ventilation. Expiratory pressure expands tracheal air sacs in the ventral trochanter and helps driving hemolymph out of this space. In idle periods of resting ventilation, an autonomous myogenic rhythm of the leg–heart can maintain hemolymph circulation in the mesothoracic leg without neural control.

Mechanosensory pegs constitute stridulatory files in grasshoppers

Stridulatory files on the inner face of hindleg femora were shown to consist of mechanosensory pegs in males and females of Syrbula montezuma (Saussure) and in males of Chorthippus biguttulus (L.). Females of Chorthippus had stiff protuberances on their stridulatory files, with an innervated tubercle instead of pegs. Pegs and tubercles of adult grasshoppers were shown to develop from innervated tubercular hairs present from the first instar onward in Chorthippus. In adults of Chorthippus, two sensory cells innervated each peg of males and each tubercle of females. Central projections of these afferents from the stridulatory files were very similar to those of the neighboring tactile hairs on the femur. The afferents from pegs in Syrbula responded to deflection and pressure introduced via the widened cuticular cap. In both species, selective stimulation of femoral cuticular receptors elicited antagonistic reflex responses in a coxal retractor muscle: pegs inhibited and neighboring hairs raised the efferent tonic discharges. Apparently, in these two distantly related grasshopper species, stridulatory files function as both sound‐producing and proprioceptive organs. J. Comp. Neurol. 410:444–456, 1999.

The coxo-trochanteral muscle receptor organ of locusts

SummaryThe 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.

External sensilla of the locust abdomen provide the central nervous system with an interganglionic network

External mechanoreceptors and contact chemoreceptors on the cuticle of the sixth abdominal segment of locusts have divergent primary projections of their sensory neurons that form arbours in the segmental and anterior abdominal ganglia. Homologous interganglionic projections from adjacent segments converge in the neuropile of each abdominal ganglion. Of the contributing types of sensilla, three were previously unknown for locust pregenital segments: tactile mechanosensory hairs with dual innervation, external proprioceptors of the hairplate type covered by intersegmental membranes and single campaniform sensilla that monitor cuticular strain in sternites and tergites. In general, interdependence of motor coordination in the abdominal segments is based on a neural network that relies heavily on intersegmental primary afferents that cooperate to identify the location, parameters and strength of external stimuli.