An experimental analysis of neurogenesis requires a detailed understanding of wild-type neural development. Recent DiI cell lineage studies have begun to elucidate the family of neurons and glia produced by each Drosophila embryonic neural precursor (neuroblast). Here we use DiI labeling to extend and clarify previous studies, but our analysis differs from previous studies in four major features: we analyze and compare lineages of every known embryonic neuroblast; we use an in vivo landmark (engrailed-GFP) to increase the accuracy of neuroblast identification; we use confocal fluorescence and Nomarski microscopy to collect three-dimensional data in living embryos simultaneously for each DiI-labeled clone, the engrailed-GFP landmark, and the entire CNS and muscle target field (Nomarski images); and finally, we analyze clones very late in embryonic development, which reveals novel cell types and axon/dendrite complexity. We identify the parental neuroblasts for all the cell types of the embryonic CNS: motoneurons, intersegmental interneurons, local interneurons, glia and neurosecretory cells (whose origins had never been determined). We identify muscle contacts for every thoracic and abdominal motoneuron at stage 17. We define the parental neuroblasts for neurons or glia expressing well-known molecular markers or neurotransmitters. We correlate Drosophila cell lineage data with information derived from other insects. In addition, we make the following novel conclusions: (1) neuroblasts at similar dorsoventral positions, but not anteroposterior positions, often generate similar cell lineages, and (2) neuroblasts at similar dorsoventral positions often produce the same motoneuron subtype: ventral neuroblasts typically generate motoneurons with dorsal muscle targets, while dorsal neuroblasts produce motoneurons with ventral muscle targets. Lineage data and movies can be found at http:// www.biologists.com/Development/movies/dev8623.html/ and http://www.uoneuro.uoregon.edu/doelab/lineages. SUMMARY

Clonal analysis of Drosophila embryonic neuroblasts: neural cell types, axon projections and muscle targets.

A rampwise stretch of the femoral chordotonal organ is known often to elicit a response in the active decerebrate stick insect that is termed an ‘active reaction’, and which can be considered to represent part of the step cycle. During the first part of the response, the flexor motor neurones are excited and the excitatory extensor motor neurones are inhibited, forming a positive feedback loop. When the chordotonal organ reaches a position corresponding to a flexed femur-tibia joint, the flexor motor neurones are inhibited and the extensor motor neurones are excited. In this study, extracellular and intracellular recordings showed that, during an active reaction, the excitation of the retractor unguis motor neurones usually paralleled that of the flexor motor neurones, whereas the protractor coxae motor neurones were less strongly coupled to this system. The first part of the active reaction occurred only at low stimulus velocities. At high stimulus velocities negative feedback was present. The first part therefore represents some kind of velocity-control-system for active flexions. Electrical stimulation of the nerve containing the axons of trochanteral campaniform sensilla and of the hairfield trHP decreased the likelihood that concurrent chordotonal organ stimulation would elicit an active reaction. Furthermore, most of the active reactions that occurred under these stimulus conditions involved only the flexor tibiae muscle. The results indicate that: the walking pattern generator is composed of subunits that are only loosely coupled centrally; it probably does not include a central pattern generator; and generation of an active reaction is a two-step process.

/pdf/functional-principles-of-pattern-generation-for-walking-4sryfjj2t5.pdf

Functional principles of pattern generation for walking movements of stick insect forelegs: the role of the femoral chordotonal organ afferences

The Insects by R. F. Chapman

Stick insect (Carausius morosus) leg muscles contract and relax slowly. Control of stick insect leg posture and movement could therefore differ from that in animals with faster muscles. Consistent with this possibility, stick insect legs maintained constant posture without leg motor nerve activity when the animals were rotated in air. That unloaded leg posture was an intrinsic property of the legs was confirmed by showing that isolated legs had constant, gravity-independent postures. Muscle ablation experiments, experiments showing that leg muscle passive forces were large compared with gravitational forces, and experiments showing that, at the rest postures, agonist and antagonist muscles generated equal forces indicated that these postures depended in part on leg muscles. Leg muscle recordings showed that stick insect swing motor neurons fired throughout the entirety of swing. To test whether these results were specific to stick insect, we repeated some of these experiments in cockroach (Periplaneta americana) and mouse. Isolated cockroach legs also had gravity-independent rest positions and mouse swing motor neurons also fired throughout the entirety of swing. These data differ from those in human and horse but not cat. These size-dependent variations in whether legs have constant, gravity-independent postures, in whether swing motor neurons fire throughout the entirety of swing, and calculations of how quickly passive muscle force would slow limb movement as limb size varies suggest that these differences may be caused by scaling. Limb size may thus be as great a determinant as phylogenetic position of unloaded limb motor control strategy.

https://www.jneurosci.org/content/jneuro/29/13/4109.full.pdf

Neural Control of Unloaded Leg Posture and of Leg Swing in Stick Insect, Cockroach, and Mouse Differs from That in Larger Animals

Publisher Summary This chapter deals with the homologous structures in the nervous systems of Arthropoda. The Arthropoda constitute the phylum with the highest diversity of species in the animal kingdom. Originated in the Precambrium, this monophylum basically consists of two groups, the Chelicerata and the Mandibulata. The common ancestor of the arthropods has to be regarded as an annelid-like species, with an exoskeleton, serial homologous segments bearing a series of repetitively structured appendages, and possessing fused “head”-segments in the anterior region. Recent fossil evidence has debilitated an alternative view, the uniramian concept which regards the Arthropoda as a polyphylum. According to the concept of an arthropod monophylum, compartments of the nervous system reveal some of the synapomorphies of this phylum. The Onychophora represent a taxon which is regarded to have evolved convergently to the arthropod phylum. This chapter explores new evidences that suggest they are a sister-group of the Chelicerata. Peripatus has a brain with several similarities to neuropile regions in the spider brain, for example the “mushroom bodies” and the “central body”.

Homologous Structures in the Nervous Systems of Arthropoda

The innervation pattern of inhibitory motor neurones of the locust has been revealed by intracellular recording from their cell bodies in the meso- and metathoracic ganglion and simultaneous recording from muscle fibres in a middle, or in a hind leg. Three neurones in each ganglion, the common inhibitor (CI = CI1), the anterior inhibitor (AI = CI2), and the posterior inhibitor (PI = CI3) innervate several muscles in one leg and are thus common inhibitory neurones. Metathoracic CI innervates 13 muscles in one hind leg and mesothoracic CI innervates 12 muscles in one middle leg. The muscles are all in the proximal parts of the legs and move the coxa, the trochanter and the tibia. Metathoracic AI and PI innervate four muscles in the more distal parts of one hind leg that move the tibia, the tarsus and the unguis. None of these muscles is innervated by CI. Each inhibitor innervates muscles that have different and often antagonistic actions during movements of a leg. AI and PI receive many synaptic inputs in common and show similar patterns of spikes during imposed movements of a tibia. Tests fail, however, to reveal evidence for any electrical or synaptic coupling between them. A revised scheme of nomenclature for these inhibitory neurones is proposed.

/pdf/innervation-patterns-of-inhibitory-motor-neurones-in-the-mzbsimqt0a.pdf

M. Burrows

Papers

Innervation patterns of inhibitory motor neurones in the thorax of the locust