The structure and development of a somatotopic map in crickets: The cercal afferent projection

A group of club-shaped sensilla called clavate hairs, located on the cercus of crickets (Acheta domesticus), are part of a specialized sensory system which monitors the orientation of a cricket with respect to the earth's gravitational field. The clavate hairs occur in rows which run proximodistally on the medial aspect of the cercus and each hair can be identified by specifying which row a hair is in and what position it is in within the row. The array of hairs is constant from individual to individual, and thus each hair can be identified in each specimen. The soma of a single bipolar sensory neuron is located in the integument below each hair; its dendrite projects into the hair and its axon projects to a well-defined area of the abdominal ganglion called the cercal glomerulus. All of the neurons within a row project to a particular area of the cercal glomerulus and different rows project to different areas within the glomerulus. Within a row neurons project to slightly different parts of the target area for that row. Thus a highly ordered projection pattern is produced which is tentatively called somatotopic. The development of the first clavate neuron to appear was examined from the first instar to the adult instar. The terminal arborization of this first hair was in no way unusual and its growth paralleled ganglion growth, maintaining a relatively constant position with respect to ganglion coordinates. A second clavate neuron behaved similarly, its arborization was fully formed when the receptor first appeared in the third instar and merely enlarged as the ganglion grew.     

Isolation of Mutations Affecting Neural Circuitry Required for Grooming Behavior in Drosophila Melanogaster

We have developed a screen for the isolation of mutations that produce neural defects in adult Drosophila melanogaster. In this screen, we identify mutants as flies unable to remove a light coating of applied dust in a 2-hr period. We have recovered and characterized six mutations and have found that they produce coordination defects and some have reduced levels of reflex responsiveness to the stimulation of single tactile sensory bristles. The grooming defects produced by all six of the mutations are recessive, and each of the mutations has been genetically mapped. We have also used our assay to test the grooming ability of stocks containing mutations that produce known neural defects.     

Assembly of the cricket cercal sensory system: genetic and epigenetic control

The cercal sensory system of the cricket is being examined using anatomical, physiological, and computer simulation techniques in order to better understand the assembly of sensory systems. This particular sensory system is of interest because it functions like numerically more complex vertebrate sensory systems but offers, to the neuroscientist, the technical advantages of a small number of large identified neurons. Two aspects of sensory processing are being examined in this system; the spatial aspects of a stimulus that tell an animal where a target is in its environment, and the qualities of a stimulus that help the animal to identify the stimulus. The spatial aspects of a stimulus are analyzed by a topographic mapping of the animal's sensory environment. The feature extraction machinery for other aspects of the stimulus lacks any obvious anatomical order and is embedded within the topographic map. We are attempting to tease apart the genetic and the epigenetic components of the assembly process for this sensory system. Here we review our progress with emphasis on the epigenetic aspects of its assembly. We describe previously published work on plasticity as well as new experiments focussed on the role of neuronal activity in the assembly of this neural circuit. Finally, we briefly describe simulation experiments that are helping us understand the role of various forms of synaptic plasticity in the determination of receptive fields.     

The myth of the inflexible invertebrate: competition and synaptic remodelling in the development of invertebrate nervous systems

As this quote and the article it was taken from reflect, it is a commonly held belief that the development of invertebrate nervous systems is so rigidly programmed that it is qualitatively different from that of vertebrates. The facts do not support this assertion, and I will show, using examples from among the arthropods, that appropriate experiments often reveal competition, feedback, and prolonged periods of malleability much as they do for the vertebrates. Indeed, given the well known advantages of studying the invertebrate nervous system, I think it likely that invertebrates can serve as important and useful preparations for studying, at the cellular level, the same biological phenomena that Easter et al. (1985) are trying to understand in vertebrates.     

The effects of transplantation and regeneration of sensory neurons on a somatotopic map in the cricket central nervous system

The restoration of the cercal afferent projection of crickets was examined after severing the cercal nerve or amputating the cercus and reimplanting it. After either maneuver the sensory neurons regenerated arborizations in the central nervous system (CNS) within about 1 month. In order to assess the role of the pathway taken to the CNS in controlling the growth of the terminal arborization, we transplantated left cerci to the right side of the host. The operation mismatched the mediolateral axes of host and graft tissues. In one-third of the neurons examined, the axon trajectories of the regenerated neurons were altered. The terminal arborizations in these cases were unusual; for example, one neuron arborized in an abnormal area as well as in its normal area. In rare instances this neuron arborized only in incorrect areas of the CNS. Thus, it appears that axon pathway can have an effect on the central structure of sensory neurons. However, in most cases after the surgery, the neurons were able to reach their proper target areas even by circuitous routes. The proximodistal coordinate of the map is isomorphic with sensory neuron age, because the most distal receptors are produced early in postembryonic development and new ones are added proximally at each molt. We tested the possibility that the order of differentiation was critical for generating the afferent projection with two experiments. First, the distal cercus including the distal members of the clavate array was amputated. The specimen regenerated an entire distal cercus including distal clavate receptors. When newly generated, distal neurons were stained, the terminal arbors were identical to the amputated neurons they replaced. In this case, both age and order of arrival were reversed from normal yet the topographic projection pattern was not altered. Second, we transplanted young cerci onto older specimens and then examined the regenerated arbors of the transplanted sensory neuron. The immature neuron arborized in the adult nervous system exactly as the mature homolog. Thus the age of a sensory neuron did not appear to be a controlling variable in the elaboration of a terminal arborization. The significance of these results is discussed in the context of two models for development of orderly neuronal connections.