Ectopic neurons and the organization of insect sensory systems

Summary The transplantation of appendages from one place to another on the body of crickets (Acheta domesticus) has been used to study the similarities and differences between the sensory systems of various ganglia. Mesothoracic legs have been transplanted to the abdomen in place of a cercus and cerci have been transplanted to thoracic leg stumps. After the ectopic sensory neurons had time to regenerate into the CNS, they were stained and their axonal arborizations examined. The results, which were concerned primarily with bristle receptors, revealed that bristle afferents on ectopic cerci arborized in ventral neuropil (the ventralmost association center) and leg afferents arborized in a ventral anterior region of the terminal abdominal ganglion. The results support the idea that each ganglion contains only a few distinct regions of neuropil (probably three), each receiving separate subsets of the afferent projection.The ectopic cerci were also shown to excite interneurons in the thoracic ganglia whose dendrites were located in the most ventral neuropil. These neurons normally respond to thoracic bristle afferents. Thus, the segregation of afferent axons has a correlate in the interneurons they excite.     

Metamorphic changes in the central projects of hair sensilla inTenebrio molitor L. (Insecta: Coleoptera)

Summary This paper describes the afferent projections of hair sensilla of the pro- and mesothoracic legs and the lateral thoracic sclerites of larval and adultTenebrio molitor and the corresponding set of pupal hair sensilla. The sensory neurons that innervate the hair sensilla of larval or adult insects project somatotopically into the thoracic neuropil. Different types of sensilla on the same region of the body surface project to the same zone of the ipsilateral thoracic ventral neuropil but exhibit different arborization patterns. Although there is a profound reorganization of body surface sensilla, the basic somatotopic layout of the larva is maintained in the adult. The sensory neurons that innervate the pupal hair sensilla possess central projections similar to those of the corresponding adult sensory neurons. The central projections of pupal sensory neurons are somatotopically oriented. Their projection pattern is serially homologous in the thoracic and the abdominal ganglia. The central projection pattern of the described pupal sensory neurons is constant throughout pupation. MAb 22C10 immunoreactivity allows an estimate of the timing of the early differentiation of the imaginal sensory neurons originating during pupation. Ablation experiments indicate that pupal sensory neurons influence the central projection pattern of the differentiating imaginal sensory neurons.     

Properties of descending dorsal unpaired median (DUM) neurons of the locust suboesophageal ganglion

A group of six dorsal unpaired median (DUM) neurons of the suboesophageal ganglion (SOG) of locusts was studied with neuroanatomical and electrophysiological techniques. The neurons are located posteriorly in the SOG and have axons that descend into the ganglia of the ventral nerve cord, some as far as the terminal abdominal ganglion. Within thoracic ganglia the neurons have profuse dendritic ramifications in many neuropiles, including ventral sensory neuropiles. Based on their projection patterns three different morphological types of neurons can be distinguished. These neurons receive excitatory inputs through sensory pathways that ascend from the thoracic ganglia and are activated by limb movements. They may be involved in the modulation of synaptic transmission in thoracic ganglia.     

The terminal abdominal ganglion of the wood cricket Nemobius sylvestris

The abdominal cerci of the wood cricket, Nemobius sylvestris, are covered by a variety of hair‐like sensilla that differ in length, thickness, and articulation. Fillings from the cercal nerves with cobalt chloride and fluorescent dyes revealed the projection of sensory axons into the terminal abdominal ganglion of the ventral nerve chain. Two projection areas on each side of the terminal abdominal ganglion midline could be identified: a posterior cercal glomerulus and an anterior bristle neuropil. Axons from some cercal sensilla ascend through the connectives to reach the metathoracic ganglionic mass. As their axons pass through each segmental abdominal ganglion, they project medial arborization. Cross‐sections of the terminal abdominal ganglion and retrograde fills with cobalt chloride and fluorescent dyes from connectives revealed several small cells and seven pairs of giant ascending interneurons organized symmetrically. Giant somata are located contralateral to their axons (diameters between 20 and 45 μm). The cercal projections overlap extensively with the dendritic fields of the giant interneurons. In the terminal abdominal ganglion, we identified nine longitudinal tracts, two major tracts, and seven smaller ones. The functional implications of the neuranatomical organization of the system are discussed on a comparative basis. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Synapse formation by sensory neurons after cross-species transplantation in crickets: the role of positional information

The role of positional information in synapse formation was studied in the cricket cercal sensory system by transplanting epidermis from one species of cricket to another. Strips of cercal epidermis containing identified sensory neurons were transplanted from a black donor species to a tan host species; the color difference was used to distinguish between donor and host tissue in adults. Transplanted sensory neurons regenerated axons into the host terminal abdominal ganglion where they formed functional chimeric synapses. These methods were used to test the role of positional information in central synapse formation. Newly generated sensory neurons, formed by the donor tissue at the border between graft and host, were examined to test the idea that their position would determine their structure, function, and projection pattern. These "intercalated" sensory neurons support the positional information hypothesis. First, they had directional sensitivities which were appropriate to their location on the cercus; receptors of this directionality would never be made by the donor tissue if left in its original position. Second, these sensory neurons projected to regions of the CNS known to be appropriate for their directionality. Finally, simultaneous recordings from these ectopic sensory neurons and host interneurons demonstrated the expected synaptic connection, based on the overlap of pre- and postsynaptic cells. Thus three aspects of receptor function, directionality, afferent projection, and choice of synaptic partners, appeared to be controlled by positional information.     

Target neuron specification of short-term synaptic facilitation and depression in the cricket CNS

We investigated the role of retrograde signals in the regulation of short-term synaptic depression and facilitation by characterizing the form of plasticity expressed at novel synapses on four giant interneurons in the cricket cercal sensory system. We induced the formation of novel synapses by transplanting a mesothoracic leg and its associated sensory neurons to the cricket terminal abdominal segment. Axons of ectopic leg sensory neurons regenerated and innervated the host terminal abdominal ganglion forming monosynaptic connections with the medial giant interneuron (MGI), lateral giant interneuron (LGI), and interneurons 7-1a and 9-2a. The plasticity expressed by these synapses was characterized by stimulating a sensory neuron with pairs of stimuli at various frequencies or with trains of 10 stimuli delivered at 100 Hz and measuring the change in excitatory postsynaptic potential amplitude recorded in the postsynaptic neuron. Novel synapses of a leg tactile hair on 7-1a depressed, as did control synapses of cercal sensory neurons on this interneuron. Novel synapses of leg campaniform sensilla (CS) sensory neurons on MGI, like MGI's control synapses, always facilitated. The form of plasticity expressed by novel synapses is thus consistent with that observed at control synapses. Leg CS synapses with 9-2a also facilitated; however, the plasticity expressed by these sensory neurons is dependent on the identity of the postsynaptic cell since the synapses these same sensory neurons formed with LGI always depressed. We conclude that the form of plasticity expressed at these synaptic connections is determined retrogradely by the postsynaptic cell. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 700–714, 1998     

Metamorphosis of wing motor system in the silk moth,Bombyx mori L. (Lepidoptera: Bombycidae): Anatomy of the sensory and motor neurons that innervate larval mesothoracic dorsal musculature,stretch receptors,and epidermis

Anatomy of dorsal mesothoracic structures, such as muscles, sensory organs, and innervation, was studied in the silkworm, Bombyx mori L. (Lepidoptera : Bombycidae), and compared with the adult wing motor system. Musculature and nerve innervation were investigated by dissection and electron micrograph; and central projection of sensory fibers and morphology of somata and dendrites of motor neurons by cobalt back-filling, followed by silver intensification. There are 23 muscle bundles (DLM) and 2 stretch receptors (SR). The DLMs, SRs, and epidermis are innervated by a branch of the dorsal nerve trunk emerging from the mesothoracic ganglion (MSG). The branch bifurcates into a dorsal sensory branch of about 300 sensory fibers and a dorsal motor branch of 14 fibers. The sensory fibers project mainly to a longitudinal portion near the mid line in the ventral neuropil of MSG and the metathoracic ganglion. Several fibers extend into the prothoracic ganglion (PG) and a few into the subesophageal and 1st abdominal ganglia. At least 13 (probably 14) motor neurons send axons to DLMs: 9 (probably 10) in PG, and 4 in MSG. Their dendrites are located mostly on the dorsoipsilateral side of the neuropil, but several branches cross the mid line and give rise to many fine branches on the contralateral side. Comparison between the larval (present study) and adult motor system shows a significant similarity in the musculature, peripheral nerve pattern, and motor neurons with some peculiarities.     

Surgical generation of supernumerary appendages for studying neuronal specificity in Drosophila melanogaster

The ability of sensory neurons to establish specific synaptic contacts in the central nervous system (CNS) can be studied by changing the spatial relationship between the periphery and the CNS. In contrast to the genetic displacement of appendages by homoeotic mutations, the surgical approach used in this study allows one to place homologous as well as heterologous appendages to the same site on the body surface. Using an improved technique of “surface transplantation,” we generated supernumerary appendages of any desired type in a particular abdominal position. The sensory axons originating from these grafts enter the CNS through the main abdominal nerve and arborize in the fused abdominal ganglia; many fibers extend also into thoracic centers. In the abdominal ganglia, terminals from dorsal transplants (wings and halteres) stay on the ipsilateral side, whereas terminals from ventral transplants (legs and antennae) distribute ipsi- and contralaterally. The same preference holds true for dorsal and ventral abdominal bristles, respectively, whose projection patterns served as a reference. In thoracic ganglia, axons from dorsal and ventral grafts yield completely different terminal patterns. Dorsal grafts project into the ipsilateral wing center, even in the mutant wingless, in which normal wing afferents are suppressed. In contrast, fibers from ventral grafts often extend along the thoracic midline. These data indicate that sensory axons of homologous appendages on the one hand, and their central targets on the other, share serially repeated surface markers. This may enable sensory fibers to recognize centers of homologous appendages.     

Parallel motor pathways from thoracic interneurons of the ventral giant interneuron system of the cockroach, Periplaneta americana

The data described here complete the principal components of the cockroach wind-mediated escape circuit from cercal afferents to leg motor neurons. It was previously known that the cercal afferents excite ventral giant interneurons which then conduct information on wind stimuli to thoracic ganglia. The ventral giant interneurons connect to a large population of interneurons in the thoracic ganglia which, in turn, are capable of exciting motor neurons that control leg movements. Thoracic interneurons that receive constant short latency inputs from ventral giant interneurons have been referred to as type A thoracic interneurons (TIAs). In this paper, we demonstrate that the motor response of TIAs occurs in adjacent ganglia as well as in the ganglion of origin for the TIA. We then describe the pathway from TIAs to motor neurons in both ganglia. Our observations reveal complex interactions between thoracic interneurons and leg motor neurons. Two parallel pathways exist. TIAs excite leg motor neurons directly and via local interneurons. Latency and amplitude of post-synaptic potentials (PSPs) in motor neurons and local interneurons either in the ganglion of origin or in adjacent ganglia are all similar. However, the sign of the responses recorded in local interneurons (LI) and motor neurons varies according to the TIA subpopulation based on the location of their cell bodies. One group, the dorsal posterior group, (DPGs) has dorsal cell bodies, whereas the other group, the ventral median cells, (VMC) has ventral cell bodies. All DPG interneurons either excited postsynaptic cells or failed to show any connection at all. In contrast, all VMC interneurons either inhibited postsynaptic cells or failed to show any connection. It appears that the TIAs utilize directional wind information from the ventral giant interneurons to make a decision on the optimal direction of escape. The output connections, which project not only to cells within the ganglion of origin but also to adjacent ganglia and perhaps beyond, could allow this decision to be made throughout the thoracic ganglia as a single unit. However, nothing in these connections indicates a mechanism for making appropriate coordinated leg movements. Because each pair of legs plays a unique role in the turn, this coordination should be controlled by circuits dedicated to each leg. We suggest that this is accomplished by local interneurons between TIAs and leg motor neurons.     

Structure of allotransplanted ganglia and regenerated neuromuscular connections in crayfish

In adult crayfish, Procambarus clarkii, motoneurons to a denervated abdominal superficial flexor muscle regenerate long-lasting and highly specific synaptic connections as seen from recordings of excitatory postsynaptic potentials, even when they arise from the ganglion of another crayfish. To confirm the morphological origins of these physiological connections we examined the fine structure of the allotransplanted tissue that consisted of the third abdominal ganglion and the nerve to the superficial flexor muscle (the fourth ganglion and the connecting ventral nerve cord were also included). Although there is considerable degeneration, the allotransplanted ganglia display intact areas of axon tracts, neuropil, and somata. Thus in both short (6–8 weeks) and long (24–30 weeks) term transplants approximately 20 healthy somata are present and this is more than the five axons regenerated to the host muscle. The principal neurite and dendrites of these somata receive both excitatory and inhibitory synaptic inputs, and these types of synaptic contacts also occur among the dendritic profiles of the neuropil. Axon tracts in the allotransplanted ganglia and ventral nerve cord consist largely of small diameter axons; most of the large axons including the medial and lateral giant axons are lost. The transplanted ganglia have many blood vessels and blood lacunae ensuring long-term survival. The transplanted superficial flexor nerve regenerates from the ventral to the dorsal surface of the muscle where it has five axons, each consisting of many profiles rather than a single profile. This indicates sprouting of the individual axons and accounts for the enlarged size of the regenerated nerve. The regenerated axons give rise to normal-looking synaptic terminals with well-defined synaptic contacts and presynaptic dense bars or active zones. Some of these synaptic terminals lie in close proximity to degenerating terminals, suggesting that they may inhabit old sites and in this way ensure target specificity. The presence of intact somata, neuropil, and axon tracts are factors that would contribute to the spontaneous firing of the transplanted motoneurons. © 1996 John Wiley & Sons, Inc.     

Co-factors and co-repressors of Engrailed: expression in the central nervous system and cerci of the cockroach, <Emphasis Type="Italic">Periplaneta americana</Emphasis>

In the larval cockroach (Periplaneta americana), knockout of Engrailed (En) in the medial sensory neurons of the cercal sensory system changes their axonal arborization and synaptic specificity. Immunocytochemistry has been used to investigate whether the co-repressor Groucho (Gro; vertebrate homolog: TLE) and the co-factor Extradenticle (Exd; vertebrate homolog: Pbx) are expressed in the cercal system. Gro/TLE is expressed ubiquitously in cell nuclei in the embryo, except for the distal pleuropodia. Gro is expressed in all nuclei of the thoracic and abdominal central nervous system (CNS) of first instar larva, although some neurons express less Gro than others. Cercal sensory neurons express Gro protein, which might therefore act as a co-repressor with En. Exd/Pbx is expressed in the proximal portion of all segmental appendages in the embryo, with the exception of the cerci. In the first instar CNS, Exd protein is expressed in subsets of neurons (including dorsal unpaired medial neurons) in the thoracic ganglia, in the first two abdominal ganglia, and in neuromeres A8–A11 of the terminal ganglion. Exd is absent from the cerci. Because Ultrabithorax/Abdominal-A (Ubx/Abd-A) can substitute for Exd as En co-factors in Drosophila, Ubx/Abd-A immunoreactivity has also been investigated. Ubx/Abd-A immunostaining is present in abdominal segments of the embryo and first instar CNS as far caudal as A7 and faintly in the T3 segment. However, Ubx/Abd-A is absent in the cerci and their neurons. Thus, in contrast to its role in Drosophila segmentation, En does not require the co-factors Exd or Ubx/Abd-A in order to control the synaptic specificity of cockroach sensory neurons.I acknowledge the support of NIH R01 NS45547, NIH-SCORE S06 GM0088224, and RCMI G12 RR03051.     

The organization of plurisegmental mechanosensitive interneurons in the central nervous system of the wandering spider Cupiennius salei

Summary In spiders the bulk of the central nervous system (CNS) consists of fused segmental ganglia traversed by longitudinal tracts, which have precise relationships with sensory neuropils and which contain the fibers of large plurisegmental interneurons. The responses of these interneurons to various mechanical stimuli were studied electrophysiologically, and their unilateral or bilateral structure was revealed by intracellular staining. Unilateral interneurons visit all the neuromeres on one side of the CNS. They receive mechanosensory input either from a single leg or from all ipsilateral legs via sensory neurons that invade leg neuromeres and project into specific longitudinal tracts. The anatomical organization of unilateral interneurons suggests that their axons impart their information to all ipsilateral leg neuromeres. Bilateral interneurons are of two kinds, symmetric and asymmetric neurons. The latter respond to stimulation of all legs on one side of the body, having their dendrites amongst sensory tracts of the same side of the CNS. Anatomical evidence suggests that their terminals invade all four contralateral leg neuromeres. Bilaterally symmetrical plurisegmental interneurons have dendritic arborizations in both halves of the fused ventral ganglia. They respond to the stimulation of any of the 8 legs. A third class of cells, the ascending neurons have unilateral or bilateral dendritic arborizations in the fused ventral ganglia and show blebbed axons in postero-ventral regions of the brain. Their response characteristics are similar to those of other plurisegmental interneurons. Descending neurons have opposite structural polarity, arising in the brain and terminating in segmental regions of the fused ventral ganglia. Descending neurons show strong responses to visual stimulation. Approximately 50% of all the recorded neurons respond exclusively to stimulation of a single type of mechanoreceptor (either tactile hairs, or trichobothria, or slit sensilla), while the rest respond to stimulation of a variety of sensilla. However, these functional differences are not obviously reflected by the anatomy. The functional significance of plurisegmental interneurons is discussed with respect to sensory convergence and the coordination of motor output to the legs. A comparison between the response properties of certain plurisegmental interneurons and their parent longitudinal tracts suggests that the tracts themselves do not reflect a modality-specific organization.Abbreviations BPI bilateral plurisegmental interneuron - CNS central nervous system - FVG fused ventral ganglia - LT longitudinal tract - PI plurisegmental interneuron - PSTH peristimulus timehistogram - UPI unilateral plurisegmental interneuron     

Parallel motor pathways from thoracic interneurons of the ventral giant interneurons system of the cockroach,Periplaneta americana

The data described here complete the principal components of the cockroach wind-mediated escape circuit form cercal afferents to leg motor neurons. It was previously known that the cercal afferents excite ventral giant interneurons which then conduct information on wind stimuli to thoracic ganglia. The ventral giant interneurons connect to a large population of interneurons in the thoracic ganglia which, in turn, are capable of exciting motor neurons that control leg movements. Thoracic interneurons that receive constant short latency inputs from ventral giant interneurons have been referred to as type A thoracic interneurons (TI_As). In this paper, we demonstrate that the motor response of TI_As occurs in adjacent ganglia as well as in the ganglion of origin for the TI_A. We then describe the pathway from TI_As to motor neurons in both ganglia. Our observations reveal complex interactions between thoracic interneurons and leg motor neurons. Two parallel pathways exist. TI_As excite leg motor neurons directly and via local interneurons. Latency and amplitude of post-synaptic potentials (PSPs) in motor neurons and local interneurons either in the ganglion of origin or in adjacent ganglia are all similar. However, the sign of the responses recorded in local interneurons (LI) and motor neurons varies according to the TI_A subpopulation based on the location of their cell bodies. One group, the dorsal posterior group, (DPGs) has dorsal cell bodies, whereas the other group, the ventral median cells, (VMC) has ventral cell bodies. All DPG interneurons either excited postsynaptic cells or failed to show any connection at all. In contrast, all VMC interneurons either inhibited postsynaptic cells or failed to show any connection. It appears that the TI_As utilize directional wind information from the ventral giant interneurons to make a decision on the optimal direction of escape. The output connections, which project not only to cells within the ganglion of origin but also to adjacent ganglia and perhaps beyond, could allow this decision to be made throughout the thoracic ganglia as a single unit. However, nothing in these connections indicates a mechanism for making appropriate coordinated leg movements. Because each pair of legs plays a unique role in the turn, this coordination should be controlled by circuits didicated to each leg. We suggest that this is accomplished by local interneurons between TI_As and leg motor neurons.     

Projection pattern of sensory neurons in the central nervous system of a homeotic mutation of the moth Manduca sexta

Octopod (Octo) is a mutation of the moth Manduca sexta, which transforms the first abdominal segment (A1) in the anterior direction. Mutant animals are characterized by the appearance of homeotic thoracic-like legs on A1. We exploited this mutation to determine what rules might be used in specifying the fates of sensory neurons located on the body surface of larval Manduca. Mechanical stimulation of homeotic leg sensilla did not cause reflexive movements of the homeotic legs, but elicited responses similar to those observed following stimulation of ventral A1 body wall hairs. Intracellular recordings demonstrated that several of the motoneurons in the A1 ganglion received inputs from the homeotic sensory hairs. The responses of these motoneurons to stimulation of homeotic sensilla resembled their responses to stimulation of ventral body wall sensilla. Cobalt fills revealed that the mutation transformed the segmental projection pattern of only the sensory neurons located on the ventral surface of A1, resulting in a greater number with intersegmental projection patterns typical of sensory neurons found on the thoracic body wall. Many of the sensory neurons on the homeotic legs had intersegmental projection patterns typical of abdominal sensory neurons: an anteriorly directed projection terminating in the third thoracic ganglion (T3). Once this projection reached T3, however, it mimicked the projections of the thoracic leg sensory neurons. These results demonstrate that the same rules are not used in the establishment of the intersegmental and leg-specific projection patterns. Segmental identity influences the intersegmental projection pattern of the sensory neurons of Manduca, whereas the leg-specific projections are consistent with a role for positional information in determining their pattern. © 1995 John Wiley & Sons, Inc.     

Ectopic CNS projections guide peripheral neuron axons along novel pathways in leech embryos

Previous studies have indicated that the formation of stereotyped segmental nerves in leech embryos depends on the interactions between CNS projections and ingrowing afferents from peripheral neurons. Especially, CNS-ablation experiments have suggested that CNS-derived guidance cues are required for the correct navigation of several groups of peripheral sensory neurons. In order to directly test this hypothesis we have performed transplantations of CNS ganglia into ectopic sites in segments from which the resident ganglia have been removed. We find that the transplanted ganglia extend numerous axons distributed roughly equally in all directions. When these CNS projections reach and make contact with peripheral sensory axons they are used as guides for peripheral neurons to grow toward and into the ectopic ganglia even when this means following novel pathways that cross the midline and/or segmental boundaries. The peripheral sensory axons turn and grow toward the ectopic ganglia only when in physical contact with CNS axons, suggesting that diffusible chemoattractants are not a factor. These results demonstrate that the guidance cues provided by ectopic CNS projections are both necessary and sufficient to steer peripheral sensory neuron axons into the CNS.     

Convergence of multisensory inputs in Xenopus tadpole tectum

The integration of multisensory information takes place in the optic tectum where visual and auditory/mechanosensory inputs converge and regulate motor outputs. The circuits that integrate multisensory information are poorly understood. In an effort to identify the basic components of a multisensory integrative circuit, we determined the projections of the mechanosensory input from the periphery to the optic tectum and compared their distribution to the retinotectal inputs in Xenopus laevis tadpoles using dye‐labeling methods. The peripheral ganglia of the lateral line system project to the ipsilateral hindbrain and the axons representing mechanosensory inputs along the anterior/posterior body axis are mapped along the ventrodorsal axis in the axon tract in the dorsal column of the hindbrain. Hindbrain neurons project axons to the contralateral optic tectum. The neurons from anterior and posterior hindbrain regions project axons to the dorsal and ventral tectum, respectively. While the retinotectal axons project to a superficial lamina in the tectal neuropil, the hindbrain axons project to a deep neuropil layer. Calcium imaging showed that multimodal inputs converge on tectal neurons. The layer‐specific projections of the hindbrain and retinal axons suggest a functional segregation of sensory inputs to proximal and distal tectal cell dendrites, respectively. © 2009 Wiley Periodicals, Inc. Develop Neurobiol, 2009     

Distribution of Eisenia tetradecapeptide immunoreactive neurons in the nervous system of earthworms

A detailed mapping of Eisenia-tetradecapeptide-immunoreactive neurons in the central and peripheral nervous system combined with quantitative morphological measurements was performed in Eisenia fetida and Lumbricus terrestris. In Eisenia, most labelled neurons were observed in the ganglia of the ventral cord (20.38% of the total cell number of the ganglion) and 15.67% immunoreactive cells occurred in the brain, while 6% of the neurons could be shown in the subesophageal ganglion. In the case of Lumbricus, most immunoreactive cells were found in the subesophageal ganglion (16.17%) and in the ventral ganglia (12.54%). The brain contained 122 ETP-immunoreactive cells (5.6%). The size of the immunoreactive cells varied between 35-75 microm. A small number of Eisenia-tetradecapeptide immunoreactive fibres were seen to leave the ventral ganglia via segmental nerves, and labelled processes could also be observed in the stomatogastric system and the body wall. Labelled axon branches originating from the segmental nerves formed an immunoreactive plexus both between the circular and longitudinal muscle layer and on the inner surface of the longitudinal muscle layer. This inner plexus was especially rich in the setal sac. Among the superficial epithelial cells the body wall contained a significant number of immunoreactive cells. Only a few Eisenia-tetradecapeptide immunoreactive neurons and fibres occurred in the stomatogastric ganglia. In the enteric plexus the number of immunoreactive neurons and fibres decreased along the cranio-caudal axis of the alimentary tract. Eisenia-tetradecapeptide immunoreactive cells were also present among the epithelial cells in the alimentary canal. Some of these cells resembled sensory neurons in the foregut, while others showed typical secretory cell morphology in the midgut and hindgut.     

Central projections of a homoeotic regenerate, antennapedia, in a stick insect, Carausius morosus (Phasmida)

Homoeotic appendages provide a system for the analysis of neural path-finding in which the appendage is mismatched with its segmented ganglion. Central projections of sensory neurons from homoeotic antennapedia regenerates induced by antennal amputation in the stick insect, Carausius morosus, are described. The majority of afferent axons project to the olfactory lobe as in the normal antennal nerve, but they do not give rise to compact glomeruli. Nor does the form of the projection resemble that of leg sensory nerves in thoracic ganglia. The projection of antennapedia regenerate neurons in Carausius resembles the antennapedia mutant of Drosophila except that some primary afferents bypass the olfactory lobe and take several courses through the brain, sometimes reaching distant contralateral areas. It appears that these wandering fibers, having bypassed the olfactory lobe, tend to follow established tracts and to arborize or to deviate at circumscribed synaptic areas. The behavioral evidence for sensory input from antennapedia regenerates is equivocal.     

The cerebral ganglia of Milnesium tardigradum Doyère (Apochela, Tardigrada): Three dimensional reconstruction and notes on their ultrastructure

Differential interference contrast micrographs from stretched animals, serially sectioned semi-thin and ultrathin sections revealed that the cerebral ganglia (supraoesophageal mass) of the eulardigrade Milnesium tardigradum lie above the buccal tube and adjacent tissue like a saddle. It has an anterior indentation which is penetrated by two muscles that arise from the cuticle of the forehead. The cerebral ganglia consist of lateral outer lobes bearing an eye on each side, and two inner lobes which extend caudally. Between the inner lobes a cone-like projection tapers into a nerve bundle. Each outer lobe is joined with the first ventral ganglion. From the outer lobe near the eye the ganglion for a posterolateral sensory field extends to the epidermis. Anterior to the supraoesophageal mass are three dorsal ganglia for the upper three peribuccal papillae. Two additional ganglia attached to the cerebral mass supply the lateral cephalic papillae. The cerebral ganglia are covered by a thin neural lamella. The pericarya which surround the neuropil have large nuclei. Near the axons in the centre of the supraoesophageal mass the cytoplasm is crowded with vesicles of different size and appearance. Some of them resemble synaptic vesicles while others resemble dense core bodies. Structurally different types of synapses and axons can be distinguished within the neuropil.     

The topological organization of primary afferents in the terminal ganglion of crayfish,Procambarus clarkii

Summary The central projections of primary afferents in the terminal ganglion of the crayfish can be seen when an axonal filling with nickel chloride with subsequent silver intensification was used for identification. We describe here the topological relationships of the projections to the landmark structures of the neuropil.The terminal ganglion has five pairs of sensory nerves associated with the mechanosensory hairs and internal proprioceptors. The projection fields of the primary sensory neurons in the nerves Rl and R2 are almost entirely restricted to the ipsilateral half of the ganglion, whereas those of the nerves R3, R4 and R5 cross the midline to form three sensory commissures, A6SCI, A7SCI and A7SCII. The projection fields are segregated from each other, although all are restricted to the ventral neuropil which lies under the ventral intermediate tract (VIT). The intersegmental projections that ascend via the connective ipsilateral to their origins could be observed. This pattern of projection correlates well with the receptive fields exhibited by several mechanosensory interneurons on the body surface of the final segment.