Unusual tension reception in an insect

Muscle tension receptors in animals monitor the tension generated by muscles. This information is important for the initiation and control of movements and for muscle tone in relation to spatial orientation and gravity. Vertebrates have tendon organs located at the musculo-tendinous junction. The number of muscle fibers attached to one receptor is in the range of 3 to 25. In insects by contrast, only a few examples are known where muscle tension is measured by only single receptors embedded in the muscle. All other muscle activity is monitored by a range of other receptors that detect strains on the cuticle or movements of the joints. Here we describe a set of approximately 200 receptor cells located on a single insect muscle. These receptor cells are associated with ovipositor muscle fibers and were preferentially responsive to muscle tension and not muscle length. Although single receptors may respond differently, their summed response to altered muscle tension characterized them as phasic-tonic type receptors. Experimental activation of muscle receptors in animals producing a basic oviposition motor pattern inhibited homonymous muscle activity without resetting the phase of the rhythm. These results suggest a potential role of tension receptors in regulating ovipositor muscle activity and in particular preventing excessive muscle tension during oviposition. The muscle receptors presented here provide the first example of tension measurement in insects by a few hundred receptor cells associated with a single muscle. Their role in motor control and relation to other tension receptors in vertebrates and invertebrates are discussed.     

Tension receptors on the apodemes of muscles in the walking legs of the crab,Cancer magister †

1. Mechanoreceptors monitoring tension in working muscles are described in the Decapoda Crustacea.

2. The receptors are associated with apodemes of muscles in the walking leg and are well‐developed in the extensor and flexor of the meropodite (Figures 1, 2).

3. The unbranched dendrites of the receptor neurones innervate the tissues surrounding the insertions of the muscle fibres (Figures 3, 4, 5(A)).

4. The receptors show spontaneous activity with the M‐C joint at resting position and this activity increases when the muscle is stretched by holding the joint at a different position (Figure 7).

5. Isometric tension increase in the muscle recruits sensory units (Figures 8, 10(A)) and increases the activity of units firing (Figure 9).

6. Apodeme receptors may be an entirely distinct input channel from chordotonal organs (Figure 10(B,C)). Joint movements produced by a standard muscle stimulus against increasing loads reveal very different responses (Figure 11).

7. Attempts to determine whether chordotonal organs (CP1, Figures 5(B), 6) monitor isometric muscle tension (Figure 12) suggest possible complexities in their dynamic responses.

8. Abbreviations used in this paper are FASN flexor apodeme sensory nerve, EASN extensor apodeme sensory nerve, BASN bender apodeme sensory nerve, and OASN opener apodeme sensory nerve.     

Origin of myoblasts involved in the formation of the flight muscles of a butterfly (Pieris brassicae)

The larval muscle precursors of the dorsal longitudinal flight muscles of Pieris brassicae were examined by electron microscopy at five developmental stages of the fifth larval instar. At the beginning of this instar, a large neuromuscular area (NMA) was observed along the larval muscle precursors, on the side of the muscle where the nerve comes into contact with it. This area was delimited by the basement membrane of the muscle and by a thicker external basal lamina and contained nerve branches, tracheae, neuromuscular endings and cells with a cytoplasm rich is free ribosomes. Cells similar to the ribosome rich cells of the NMA were located along the motor nerves supplying the larval muscle precursors in a compartment joined to the NMA. From 78 hr after the fourth moult onwards, the ribosome-rich cells increased in number, accumulated inside the NMA and in the space around the nerves. Then they penetrated the muscle fibre via the channels of the transverse system. These cells were the myoblasts that later help to form the flight muscles. As regards their earlier origin, they do not seem to derive from cells formed from larval fibre nuclei, but might be present together with the muscle precursors, and along the nerves from the beginning of larval development. The differences with the Nematoceran Diptera are discussed.     

Sense organs in the femur of the stick insect and their relevance to the control of position of the femur-tibia-joint

Summary The sensory innervation pattern is described for the femur of the middle and the hind legs ofCarausius morosus. — In one of the nerves (F121) extracellular recordings show a unit which mirrors the tension of the flexor tibiae muscle (tension receptor). The tension receptor increases the firing rate of the slow extensor tibiae motoneuron. It measures the tension of one or more muscle fibres of the anterior side near the distal end of the muscle. The anatomical basis of this receptor is uncertain. — Another receptor was found on the ventral side of the distal end of the apodeme of the extensor tibiae muscle (apodeme receptor). Recordings from this receptor could not be obtained inCarausius. But inExtatosoma tiaratum it responded to stretching of the nerve. In the natural position it shows a minimum of excitation in the 90°-position of the femur-tibia-joint and an increase in firing rate for both flexion and extension. — Tactile hairs react phasically and have no special sensitivity for one direction. Two receptors at the dorsal side of the femur-tibia-joint (RDAL and RDPL), which are situated in the same position as inSchistocerca hind legs, react phasically to extension movements and fire tonically in the most extended position of the joint. — The influence of these receptors on the position of the femur-tibia-joint is only weak.Supported by Deutsche Forschungsgemeinschaft     

Dynamic responses of series force receptors innervating the opener muscle apodeme in the blue crab, Callinectes sapidus

Receptors monitoring muscle force innervate the opener muscle apodeme in the walking legs of the blue crab, Callinectes sapidus. Biocytin backfills reveal 9–15 bipolar neurons with somata as large as 60 μm positioned at the distal end of the apodeme. Sensory endings insert into the apodeme and are in series with the opener muscle. The axons of these neurons form the opener apodeme sensory nerve that merges with the most distal branch of the opener motor nerve. Recordings reveal that the receptors are not spontaneously active nor do they respond to passive muscle stretch. Isometric muscle contraction evoked by stimulating the opener excitor motor neuron is the adequate stimulus for receptor firing. Most significant is the finding that during contraction, over a wide range of forces, the firing rate of individual receptors closely parallels the rate of change of isometric force. The peak instantaneous frequency typically occurs at the force derivative maximum, but not at maximum force development. Thus, receptors of the opener apodeme sensory nerve more closely monitor changes in isometric force rather than the total force achieved. Accepted: 20 September 1996     

Adult-specific neurons in the nervous system of the moth, Manduca sexta: selective chemical ablation using hydroxyurea

The segmental ganglia of adults of the moth, Manduca sexta, are constructed both from remodeled larval neurons and from adult-specific cells. The latter are produced by identified stem cells (neuroblasts) during larval life and then differentiate to form functional neurons during metamorphosis. The mitotic activity of the larval neuroblasts could be irreversibly blocked by the DNA-synthesis inhibitor hydroxyurea (HU). Treatment on day 1 of the third larval stage resulted in 80-90% of the neuroblasts being blocked before they produced any progeny while leaving the functional larval neurons unaffected. Treated larvae finished growth, underwent metamorphosis, and produced an adult CNS that contained the normal set of remodeled larval neurons but lacked most of the new adult-specific cells. When HU treatment was delayed until the start of the fourth or fifth larval stage, the neuroblasts produced the early portions of their respective lineages before they were blocked. The immature neurons that were generated prior to treatment survived to contribute adult-specific neurons to the moth CNS, but the remainder of each lineage was missing. This technique therefore enables one to produce adult nervous systems containing the basic set of remodeled larval cells plus defined sets of adult-specific neurons.     

Superextension and supercontraction in locust ovipositor muscles

A ten times elongation of certain abdominal intersegmental muscles occurs in female locusts during digging prior to oviposition. During and after oviposition the muscles contract, shortening by up to 90% or more, restoring the resting positions of the abdominal segments.Discontinuous Z-discs permit supercontraction at the resting length and then fragment into Z-bodies when the muscle is stretched, so enabling it to superextend without loss of the contractile property. In this superextended state the fibres resemble smooth muscles. After oviposition, the muscle fibres contract but the sarcomeres are not restored completely, some of the Z-bodies being unevenly distributed in the recontracted fibres. Locust ovipositor muscle has the most extreme example of Z-disc disagregation known from the insects and is the insect muscle which approaches most closely the smooth muscle condition.Two types of motor nerve innervate this muscle, one is ordinary and the other, containing granules, resembles an octopaminergic fibre possibly involved in regulating a catch mechanism in the muscle.The physiological requirements for egg-laying with an extensible ovipositor, which is also part of the normally functioning abdomen, are well met by the ultrastructural specializations of locust ovipositor muscles.     

The morphology and ultrastructure of tension receptors in the walking legs of the crab,Carcinus maenas

Summary The tension receptor system of the crab merus consists of two size classes of receptor cell body distributed along one face of the flexor muscle apodeme. The receptors show the general arthropod mechanoreceptor structure of cell body, connecting cilium, and sheathed sensory processes, but there are several differences. Many processes show convolutions, and the distal portion of the sensory process is embedded in the apodeme cuticle. The terminations of the sensory processes lack the usual structural specialisations for mechanotransduction. Tension transduction appears to occur by flexion of the cuticle-embedded sensory process.Work supported by an ARGC grant to Dr. D.L. MacmillanAuthor supported by Australian Commonwealth Postgraduate Research Award     

In vitro regulation of the innervation pattern of quail muscle fibres by quail and mouse neurons

Myoblasts from rudiments of slow and fast muscle, anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) respectively, of 9-day-old quail embryos were cultured in vitro for a period of up to 60 days in order to give rise to well-differentiated muscle fibres. These fibres were innervated by neurons from either quail or mouse embryo spinal cord and their innervation pattern was examined by the visualization of acetylcholine receptors (ACh-R) and of acetylcholinesterase (ACh-E) activity at the neuromuscular contacts. In the culture system used, quail neurons always innervated muscle fibres at several sites and only when a fast-type activity was imposed on these neurons did a reduction in the number of the previously established neuromuscular contacts take place. In contrast, in the muscle fibres innervated by mouse neurons, a spontaneous reduction in the number of the previously established neuromuscular contacts occurred but this spontaneous reduction depended upon the level of differentiation reached by the muscle fibres in vitro. In the cultures of muscle fibres previously innervated by mouse neurons, the addition of quail neurons did not provoke any modification in the initial innervation pattern, and no quail ACh-R cluster was observed. In contrast, in the muscle fibres previously innervated by quail neurons, the mouse neurons contacted these fibres, resulting in a decrease in the number of quail ACh-R clusters. These results emphasize the part played by neurons in the establishment of the innervation pattern when muscle fibres have reached a high level of differentiation. In vitro, the slow and fast characteristics of the muscle fibres do not influence this pattern.     

The fate of specific motoneurons and sensory neurons of the pregenital abdominal segments inTenebrio molitor (Insecta : Coleoptera) during metamorphosis

The anatomy and innervation of the lateral external muscle and sensory cells located in the ventral region of pregenital abdominal segments were examined at the larval and adult stages ofTenebrio molitor (Coleoptera). All seven muscles located in this region degenerate during the pupal stage, whilst only the lateral external median (lem) appears in the adult. Backfillings of the motor nerve innervating this muscle reveal that, at both larval and adult stages, it is innervated by ten neurons. Intracellular records from the muscle fibres show that two neurons are inhibitory, and at least five are excitatory. There are also two unpaired neurons. A variety of sensory organs are located in the ventral region of the larvae, whilst only campaniform sensilla are found in the adult. At both stages, the innervation pattern of the sensory nerve branches is very similar. Also, the central projections of the sensory cells occupy similar neuropilar areas. Finally, prolonged intracellular records from the lem muscle revealed that, at the larval stage, it participates only in segmental or intersegmental reflexes, whilst in the adult it has a primary expiratory role in ventilation. The results show that extensive changes occur in the number of muscles located in the ventral region of the pregenital abdominal segments, as well as in the arrangement and number of sensory neurons, in the structure of the exoskeleton, and even in the central nervous system. In contrast, only minor changes are observed in the sensory and motor nerve branches, in the sensory projections, and in the number and the location of the motoneurons innervating the lateral external median muscle. Correspondence to: G. Theophilidis     

Rescue of the cardiac defect in ErbB2 mutant mice reveals essential roles of ErbB2 in peripheral nervous system development.

ErbB2 receptor tyrosine kinase plays a role in neuregulin signaling and is expressed in the developing nervous system. We genetically rescued the cardiac defect of erbB2 null mutant embryos, which otherwise died at E11. These rescued erbB2 mutant mice die at birth and display a severe loss of both motor and sensory neurons. Motor and sensory axons are severely defasciculated and aberrantly projected within their final target tissues. Schwann cells are completely absent in the peripheral nerves. Schwann cell precursors are present within the DRG and proliferate normally, but their ability to migrate is decreased. Acetylcholine receptors cluster within the central band of the mutant diaphragm muscle. However, these clusters are dispersed and morphologically different from those in control muscle. Our results reveal an important role for erbB2 during normal peripheral nervous system development.     

Similarities in Form and Developmental Sequence for Three Larval Shell Muscles in Nudibranch Gastropods

Shell-anchored muscles that extend into the cephalopodium of five species of planktotrophic nudibranch larvae were studied by ultrastructural examination of sequential larval developmental stages. All species, regardless of larval shell type (inflated or non-inflated), showed a similar basic pattern of shell muscles. The larval retractor muscle (LRM) differentiates prior to hatching and its fibres insert on epithelia of the velum, apical plate, stomodeal region, or mantle fold. Many fibres also connect with subepithelial intrinsic muscles of the cephalopodium. Most but not all LRM fibres Project to left-sided targets and are innervated from the left cerebral ganglion. Two pedal muscles, which are innervated from the pedal ganglia, differentiate during the post-hatching larval stage and both insert primarily on pedal epithelium attached to the operculum. The left pedal muscle is anchored to the shell immediately adjacent to the attachment plaque of the LRM and consists of basal and distal tiers of muscle cells. The right pedal muscle arises on the ventral rim of the shell aperture and consists of a single tier of muscle cells. Ontogenic changes in larval retraction behaviour correlate with developmental change in the muscle effectors. Although some interspecific differences were noted, the presence of a common ground plan for larval shell muscles in these five species contrasts with previous indications of marked variability for nudibranch larval shell muscles.     

Junctional and extrajunctional membrane channels activated by GABA in locust muscle fibres

Iontophoretic application of GABA to voltage-clamped locust muscle fibres has demonstrated the presence of both extrajunctional and junctional GABA receptors. Extrajunctional GABA receptors are distinct from extrajunctional glutamate receptors which also occur in these muscle fibres. Inward GABA currents are nonlinearly dependent on membrane potential. Analysis of membrane current noise produced by iontophoretic GABA application shows that for junctional and extrajunctional GABA receptors the mean channel lifetime is 3-4 ms and the single-channel conductance is approximately 22 pS at - 80 mV (T = 21 degrees C). The mean lifetime as previously demonstrated for glutamate-sensitive excitatory channels in locust muscle fibres.     

Comparative toxicity of some synthetic insecticides,and a growth inhibitor,in the migratory locust,Locusta migratoria

1. The comparative toxicity of four synthetic insecticides (Dieldrin, Methomyl, Fenitrothion and Permethrin) in the different developmental stages of the migratory locust shows that resistance tends to increase with the developmental larval stages except in the adults which show lower resistance than the fifth larval stage.2. The resistance to a growth inhibitor, Teflubenzuron in the different developmental stages shows the reverse phenomenon—the younger larvae were more resistant than the fourth and fifth larval stages while the adults were almost not affected by this product.3. A comparison of the toxicity of the synthetic insecticides in the two forms of the insect, the solitarious and the gregarious, shows that the solitary insects were more resistant than the gregarious ones.     

Developmental expression of tryptophan hydroxylase gene in <Emphasis Type="Italic">Ciona intestinalis</Emphasis>

To describe the serotonergic system in a tunicate larva, we cloned a gene encoding for tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin synthesis, in the ascidian Ciona intestinalis and studied its expression pattern during development. Ci-TPH expression was found from tailbud stage in the precursor cells of the visceral ganglion and in the tail. In the larva, TPH-expressing neurons formed two clusters in the anterior central nervous system at the level of the visceral ganglion. Moreover, we found Ci-TPH expression at the level of the muscle cells of the tail and suggested that this localisation might be at the level of neuro-muscolar junctions. Moreover, we discussed the involvement of serotonin in the control of larval locomotory activity.     

Correlation of electrophysiological,histochemical, and mechanical properties in fibres of the coxa rotator muscle of the locust,Locusta migratoria

Summary The fibre composition of the anterior coxa rotator muscle of the locust middle leg (M92) was examined. The muscle is composed of 90–100 fibres. Muscle fibres were characterized with regard to innervation pattern, electrophysiological properties, and morphological parameters. Activity and isoenzyme composition of myofibrillar ATPase, succinic acid dehydrogenase (SDH) activity and glycogen content were examined employing histochemical techniques. Shortening velocity and the dependence of tension on intracellular Ca²⁺ were determined in skinned fibre experiments. A close match was observed between the innervation pattern of the muscle fibres and their histochemical and physiological properties. The combination of all parameters examined allowed an accurate classification of the muscle fibres into three types. Within a given type, broad variability of some properties was observed (SDH activity, Ca²⁺ sensitivity) while others assumed distinct values (innervation pattern, shortening velocity). The comprehensive characterization of muscle fibre properties permits a functional interpretation of fibre heterogeneity with regard to muscle performance. Fibres with the same innervation pattern may be recruited specifically, according to their electric properties and Ca²⁺ sensitivities. The resulting specific recruitment of fibres with different mechanical responses should allow a subtle control of muscular force, with regard to force amplitude, temporal characteristics of contraction, and metabolic cost.Abbreviations CI₁ common inhibitory neurone one - ejp ijp excitatory, inhibitory junctional potential - EGTA ethylene glycol-bis[-aminoethyl ether] N,N,N,N-tetraacetic acid - mATPase myofibrillar adenosinetriphosphatase - MOPSO 3-[N-morpholino]-2-hydroxypropanesulfonic acid - M92 anterior rotator muscle of the coxa - n Hill coefficient - pCa₅₀ pCa corresponding to half-maximal tension - P₀ maximal isometric tension - SDH succinic acid dehydrogenase - V _max maximal shortening velocity