A new kind of auxiliary heart in insects: functional morphology and neuronal control of the accessory pulsatile organs of the cricket ovipositor.

INTRODUCTION: In insects, the pumping of the dorsal heart causes circulation of hemolymph throughout the central body cavity, but not within the interior of long body appendages. Hemolymph exchange in these dead-end structures is accomplished by special flow-guiding structures and/or autonomous pulsatile organs ("auxiliary hearts"). In this paper accessory pulsatile organs for an insect ovipositor are described for the first time. We studied these organs in females of the cricket Acheta domesticus by analyzing their functional morphology, neuroanatomy and physiological control. RESULTS: The lumen of the four long ovipositor valves is subdivided by longitudinal septa of connective tissue into efferent and afferent hemolymph sinuses which are confluent distally. The countercurrent flow in these sinuses is effected by pulsatile organs which are located at the bases of the ovipositor valves. Each of the four organs consists of a pumping chamber which is compressed by rhythmically contracting muscles. The morphology of the paired organs is laterally mirrored, and there are differences in some details between the dorsal and ventral organs. The compression of the pumping chambers of each valve pair occurs with a left-right alternating rhythm with a frequency of 0.2 to 0.5 Hz and is synchronized between the dorsal and ventral organs. The more anteriorly located genital chamber shows rhythmical lateral movements simultaneous to those of the ovipositor pulsatile organs and probably supports the hemolymph exchange in the abdominal apex region. The left-right alternating rhythm is produced by a central pattern generator located in the terminal ganglion. It requires no sensory feedback for its output since it persists in the completely isolated ganglion. Rhythm-modulating and rhythm-resetting interneurons are identified in the terminal ganglion. CONCLUSION: The circulatory organs of the cricket ovipositor have a unique functional morphology. The pumping apparatus at the base of each ovipositor valve operates like a bellow. It forces hemolymph via sinuses delimited by thin septa of connective tissue in a countercurrent flow through the valve lumen. The pumping activity is based on neurogenic control by a central pattern generator in the terminal ganglion.

Variations in interpulse interval of double action potentials during propagation in single neurons.

In this work, we analyzed the interpulse interval (IPI) of doublets and triplets in single neurons of three biological models. Pulse trains with two or three spikes originate from the process of sensory mechanotransduction in neurons of the locust femoral nerve, as well as through spontaneous activity both in the abdominal motor neurons and the caudal photoreceptor of the crayfish. We show that the IPI for successive low-frequency single action potentials, as recorded with two electrodes at two different points along a nerve axon, remains constant. On the other hand, IPI in doublets either remains constant, increases or decreases by up to about 3 ms as the pair propagates. When IPI increases, the succeeding pulse travels at a slower speed than the preceding one. When IPI is reduced, the succeeding pulse travels faster than the preceding one and may exceed the normal value for the specific neuron. In both cases, IPI increase and reduction, the speed of the preceding pulse differs slightly from the normal value, therefore the two pulses travel at different speeds in the same nerve axon. On the basis of our results, we may state that the effect of attraction or repulsion in doublets suggests a tendency of the spikes to reach a stable configuration. We strongly suggest that the change in IPI during spike propagation of doublets opens up a whole new realm of possibilities for neural coding and may have major implications for understanding information processing in nervous systems.

Giant and dwarf axons in a miniature insect, Encarsia formosa, (Hymenoptera, Calcididae).

Miniaturization effects in the central nervous system (CNS) of a very small calchicid wasp, Encarsia formosa (0.6 mm long), are obvious for the overall morphology and at the level of axon sizes. Parasagittal sections show that most ganglia are fused and leave connectives only in the neck and the petiole. The thoracic complex is partly squeezed between muscles, enwraps cuticular apodemes and protrudes laterally into the coxae of legs. Somata of neurons are similar in size and form a multiple layer around large neuropile regions of the CNS. In TEM sections of connectives the range of axon diameters lies between 0.045 and 3.8 µm. Extremely small axon diameters below 0.1 µm are supposed to present spatial restrictions for ion channels and internal organelles. In theory, that can cause frequent spontaneous releases of action potentials (AP) which impede regular information transfer by normal APs. Therefore, axon sizes were studied in connectives between ganglia where longer distance information transfer requires action potentials even in the smallest axons. The diameters of many interganglionic axons below 0.08 µm contradict the theory. The luxury of large axon diameters exceeding 2-3 µm is reserved for several "giant" interneurons in the thoracic and in the abdominal ganglion complex. They should belong to rapid sensory alerting systems. The largest, a bilateral pair in the abdominal CNS, could integrate afferents from long wind sensitive hairs on the abdomen.

Periodic solutions and refractory periods in the soliton theory for nerves and the locust femoral nerve.

Close to melting transitions it is possible to propagate solitary electromechanical pulses which reflect many of the experimental features of the nerve pulse including mechanical dislocations and reversible heat production. Here we show that one also obtains the possibility of periodic pulse generation when the constraint for the nerve is the conservation of the overall length of the nerve. This condition generates an undershoot beneath the baseline ('hyperpolarization') and a 'refractory period', i.e., a minimum distance between pulses. In this paper, we outline the theory for periodic solutions to the wave equation and compare these results to action potentials from the femoral nerve of the locust (Locusta migratoria). In particular, we describe the frequently occurring minimum-distance doublet pulses seen in these neurons and compare them to the periodic pulse solutions.

Evolution of a new sense for wind in flying phasmids? Afferents and interneurons.

The evolution of winged stick insects (phasmids) from secondarily wingless ancestors was proposed in recent studies. We explored the cuticle of flying phasmids for wind sensors that could be involved in their flight control, comparable to those known for locusts. Surprisingly, wind-sensitive hairs (wsH) occur on the palps of mouthparts and on the antennae of the winged phasmid Sipyloidea sipylus which can fly in tethered position only when air currents blow over the mouthparts. The present study describes the morphology and major functional properties of these "new" wsH with soft and bulging hair bases which are different from the beaker-like hair bases of the wsH on the cerci of phasmids and the wsH described in other insects. The most sensitive wsH of antennae and palps respond with phasic-tonic afferents to air currents exceeding 0.2 ms(-1). The fields of wsH on one side of the animal respond mainly to ventral, lateral, and frontal wind on the ipsilateral side of the head. Afferent inputs from the wsH converge but also diverge to a group of specific interneurons at their branches in the suboesophageal ganglion and can send their integrated input from wsH fields of the palps and antennae to the thoracic central nervous system. Response types of individual wsH-interneurons are either phasic or phasic-tonic to air puffs or constant air currents and also, the receptive fields of individual interneurons differ. We conclude that the "new" wsH system and its interneurons mainly serve to maintain flight activity in airborne phasmids and also, the "new" wsH must have emerged together with the integrating interneurons during the evolution from wingless to the recent winged forms of phasmids.

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.

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

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.