Transplantation of neurons reveals processing areas and rules for synaptic connectivity in the cricket nervous system

In order to assess the nature of spatial cues in determining the characteristic projection sites of sensory neurons in the CNS, we have transplanted sensory neurons of the cricket Acheta domesticus to ectopic locations. Thoracic campaniform sensilla (CS) function as proprioceptors and project to an intermediate layer of neuropil in thoracic ganglia while cercal CS transduce tactile information and project into a ventral layer in the terminal abdominal ganglion (TAG). When transplanted to ectopic locations, these afferents retain their modality-specific projection in the host ganglion and terminate in the layer of neuropil homologous to that of their ganglion of origin. Thus, thoracic CS neurons project to intermediate neuropil when transplanted to the abdomen and cercal CS neurons project to a ventral layer of neuropil when transplanted to the thorax. We conclude that CS can be separated into two classes based on their characteristic axonal projections within each segmental ganglion. We also found that the sensory neurons innervating tactile hairs project to ventral neuropil in any ganglion they encounter after transplantation. Ectopic sensory neurons can form functional synaptic connections with identified interneurons located within the host ganglia. The new contacts formed by these ectopic sensory neurons can be with normal targets, which arborize within the same layer of neuropil in each segmental ganglion, or with novel targets, which lack dendrites in the normal ganglion and are thus normally unavailable for synaptogenesis. These observations suggest that a limited set of molecular markers are utilized for cell–cell recognition in each segmentally homologous ganglion. Regenerating sensory neurons can recognize novel postsynaptic neurons if they have dendrites in the appropriate layer of neuropil. We suggest that spatial constraints produced by the segmentation and the modality-specific layering of the nervous system have a pivotal role in determining synaptic specificity. © 1993 John Wiley & Sons, Inc.     

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.     

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.     

Wind-activated thoracic interneurons of the cockroach: I. Responses to controlled wind stimulation

The cockroach escape response begins with a turn away from a wind puff such as that generated by an approaching predator. The presence and direction of that wind is detected by hairs on the animal's cerci, and this information is conducted to the thoracic ganglia via two populations of giant interneurons. In the thoracic ganglia, the giant interneurons excite a number of interneurons, at least some of which in turn excite motor neurons that control leg movement. In this paper we examine response properties of various thoracic neurons to wind stimuli originating from different directions. Three sets of thoracic neurons were distinguished on the basis of latency. Type A interneurons had short latencies to wind stimuli (1.3-2.25 ms). Type B interneurons had longer latencies (4-6 ms), and motor neurons had the longest latencies (5.6-17.0 ms). Individual type A interneurons either responded equally to wind from all directions or were biased in their response. Directionality was related to the presence of ventral branches near one or both sides of the midline of the ganglion. Cells with ventral median (VM) branches on either side tended to be omnidirectional or front-rear biased, whereas cells with VM branches on only one side were biased to that side. Although several type B interneurons had strong wind responses and were directionally sensitive, they did not have VM branches. We hypothesize that the presence of VM branches in type A interneurons permits connection with ventral giant interneurons, and this connection accounts for their short latency and directional properties. This hypothesis will be tested in the companion paper.     

Sensory Projections from Ectopic Appendages in an insect: Inherent Specificity and Influence of Location

Peripheral and central pathfinding by sensory axons from appendages was investigated in the fly Sarcophaga bullata . (a) Supernumerary appendages (haltere, wing, antenna and leg) were produced by imaginal disc transplantation at various ectopic sites, (b) Leg neuropil was deafferented by leg disc extirpation and in its place another leg disc was implanted. (c) The basal stalk of a leg disc connecting it with the thoracic ganglion was transected. Using cobalt chloride and HRP backfilling methods the pathways taken by the afferents from these experimentally altered appendages was examined. The results indicate that the larval nerves and the imaginal disc stalks act as guides for growing axons to locate their correct entry sites within the ventral ganglion. In the absence of these guides the axons follow any peripheral nerve, such as abdominal nerve, and enter the ganglion at inappropriate sites. However, within the ganglion they take particular routes, almost identical to those taken by axons from in situ appendages suggesting the existance of some kind of a labelled pathway. Deafferentation does not make the leg neuropil more attractive to ingrowing ectopic sensory axons.     

Afferent projections of infrared-sensitive sensilla in the beetle Melanophila acuminata (Coleoptera: Buprestidae)

Beetles of the genus Melanophila are able to detect infrared radiation by using specialized sensilla in their metathoracic pit organs. We describe the afferent projections of the infrared-sensitive neurons in the central nervous system. The axons primarily terminate in the central neuropil of the fused second thoracic ganglia where they establish putative contacts with ascending interneurons. Only a few collaterals appear to be involved in local (uniganglionic) circuits. About half of the neurons send their axons further anterior to the prothoracic ganglion. A subset of these ascend to the subesophageal ganglion, and about 10% project to the brain. Anatomical similarities suggest that the infrared-sensitive neurons are derived from neurons supplying mechanosensory sensilla. The arborization pattern of the infrared afferents suggests that infrared information is processed and integrated upstream from the thoracic ganglia.     

Central afferent pathways of long hair sensilla in the ventral nerve cord of the Indian black scorpion,Heterometrus fulvipes Koch

Histological and electrophysiological studies of identified long hair sensilla (LHS) have provided information on primary afferent fibre pathways in the ventral nerve cord of the Indian black scorpion, Heterometrus fulvipes.Cobalt-filling of single LHS on the metasoma showed that sensory axons enter the respective segmental ganglion, ascend ipsilaterally through the next anterior ganglia and terminate in a 4th ganglion. In each ganglion, these plurisegmental fibres give off collateral branches that terminate in the ganglionic neuropil. Fibres entering heterolateral connectives were not found.Recordings from peripheral nerves after deflections of a hair showed single or multiple spike discharges. A single spike could be recorded from ipsilateral anterior connectives of the ventral nerve cord, indicating a through-conductance of the sensory pathways. Strong deflections of a single hair activated several ipsilateral and fewer contralateral ascending interneurons and some segmentai motor neurons. Behavioral studies demonstrate the mechanoreceptive function of the LHS.The present study provides evidence in support of the notion that sensory afferents of the postabdomen in the scorpion bring about rapid, co-ordinated intersegmental movements of the multisegmented tail of the scorpion.Abbreviations CNS central nervous system - LHS long hair sensillum - TR trichobothria     

Distributing coordinated motor outputs to several body segments: escape movements in the cockroach

In the escape behavior of the cockroach, all six legs begin to make directed movements nearly simultaneously. The sensory stimulus that evokes these leg movements is a wind puff. Posterior wind receptors excite giant interneurons that carry a multi-cellular code for stimulus direction — and thus for turn direction-to the three thoracic ganglia, which innervate the three pairs of legs. We have attemptd to discriminate among various possible ways that the directional information in the giant interneurons could be distributed to each leg's motor circuit. Do the giant interneurons, for instance, inform separately each thoracic ganglion of wind direction? Or is there one readout system that conveys this information to all three ganglia, and if so, might the identified thoracic interneurons, which are postsynaptic to the giant interneurons, subserve this function? We made mid-sagittal lesions in one or two thoracic ganglia, thus severing the initial segments of all the known thoracic interneurons in these ganglia, and thus causing their projection axons to the other thoracic ganglia to degenerate. This lesion did not sever the giant interneurons, however (Fig. 5). Following such lesions, the legs innervated by the intact thoracic ganglia made normally directed leg movements (Figs. 4, 6, 7). Thus, the projection axons of the thoracic interneurons are not necessary for normal leg movements. Rather, the giant interneurons appear to specify to each thoracic ganglion in which direction to move the pair of legs it innervates.     

Different effects of the biogenic amines dopamine,serotonin and octopamine on the thoracic and abdominal portions of the escape circuit in the cockroach

1. The escape behavior of the cockroach, Periplaneta americana, is known to be modulated under various behavioral conditions (Camhi and Volman 1978; Camhi and Nolen 1981; Camhi 1988). Some of these modulatory effects occur in the last abdominal ganglion (Daley and Delcomyn 1981a, b; Libersat et al. 1989) and others in the thoracic ganglia (Camhi 1988). Neuromodulator substances are known to underlie behavioral modulation in various animals. Therefore, we have sought to determine whether topical application of putative neuromodulators of the escape circuit enhance or depress this circuit, and whether these effects differ in the last abdominal vs. the thoracic ganglia. 2. Topical application of the biogenic amines serotonin and dopamine to the metathoracic ganglion modulates the escape circuitry within this ganglion; serotonin decreases and dopamine enhances the response of leg motoneurons to activation of interneurons in the abdominal nerve cord by electrical or wind stimulation. 3. The neuropil of the thoracic ganglia contains many catecholamine-histofluorescent processes bearing varicosities, providing a possible anatomical substrate for dopamine release sites. 4. Topical application of octopamine to the terminal abdominal ganglion enhances the response of abdominal interneurons to wind stimulation of the cerci. In contrast, serotonin and dopamine have no effect at this site. 5. It is proposed that release of these biogenic amines may contribute to the known modulation of the cockroach escape response.     

Lack of evidence for cell death among avian spinal cord interneurons during normal development and following removal of targets and afferents.

Chick embryos and posthatched chicks were examined at several ages for the presence of pyknotic interneurons in the lumbar spinal cord. Because no pyknotic interneurons were found, direct cell counts of healthy interneurons were carried out and a comparison made between early- and late-stage embryos and hatchlings. There was no decrease in the number of interneurons in the ventral intermediate gray matter of the spinal cord between embryonic day (E) 8 and 2 weeks posthatching (PH) or in the dorsal horn between E10 and 2 weeks PH. To study whether interneuron survival is regulated by targets or afferents, a situation known to exist in other developing neural populations, early embryos were subjected to (1) removal of one limb, resulting in the loss of lateral motor column motoneurons and dorsal root ganglion sensory afferents; (2) transection of the thoracic spinal cord, thereby removing both descending afferents and rostral targets of spinal interneurons, or (3) a combination of the two operations. No reductions in interneuron numbers were found as a result of these operations. Furthermore, morphometric analysis also revealed no change in neuronal size following these experimental manipulations. By contrast, there was a slight decrease in the total area of spinal gray matter that was most prominent in the dorsal region following limb bud removal. Our results indicate (1) that spinal interneurons fail to exhibit the massive naturally occurring death of postmitotic neurons that has been observed for several other populations of spinal neurons, and (2) spinal interneurons appear to be relatively resistant to induced cell death following the removal of substantial numbers of afferent inputs and targets.     

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.     

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.     

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.     

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.     

Differing afferent connections of spiking and nonspiking wind-sensitive local interneurons in the terminal abdominal ganglion of the cricket Gryllus bimaculatus

Fifteen local spiking interneurons (LSIs) and twentyone local non-spiking interneurons (LNIs) were identified in the terminal abdominal ganglion (TAG) of the cricket Gryllus bimaculatus on the basis of intracellular recording and staining (Figs. 1, 5, 6). Although the majority of LNIs showed sharp directionalities (Fig. 7) the LSIs did not (Fig. 3). The directionality of LNIs varied with the recording sites within a single cell (Fig. 8). Electrical stimulations of the cereal sensory nerve suggested that the LNIs are connected monosynaptically with the sensory afferents of both the cerci, and that LSIs may possess a variety of bilateral combinations of polysynaptic connections with the sensory afferents. We found that the spiking and the non-spiking local interneurons in the cereal sensory system differ not only in their membrane properties, but also in their afferent connections, and concluded that their differing connectivity to the sensory afferents will associate them with different roles in signal processing.Abbreviations TAG terminal abdominal ganglion - LSI local spiking interneuron - LNI local non-spiking interneurons - CNS central nervous system - PSP post synaptic potential - GI giant interneuron     

Lack of evidence for cell death among avian spinal cord interneurons during normal development and following removal of targets and afferents

Chick embryos and posthatched chicks were examined at several ages for the presence of pyknotic interneurons in the lumbar spinal cord. Because no pyknotic interneurons were found, direct cell counts of healthy interneurons were carried out and a comparison made between early-and late-stage embryos and hatchlings. There was no decrease in the number of interneurons in the ventral intermediate gray matter of the spinal cord between embryonic day (E) 8 and 2 weeks posthatching (PH) or in the dorsal horn between E10 and 2 weeks PH. To study whether interneuron survival is regulated by targets or afferents, a situation known to exist in other developing neural populations, early embryos were subjected to (1) removal of one limb, resulting in the loss of lateral motor column motoneurons and dorsal root ganglion sensory afferents; (2) transection of the thoracic spinal cord, thereby removing both descending afferents and rostral targets of spinal interneurons, or (3) a combination of the two operations. No reductions in interneuron numbers were found as a result of these operations. Furthermore, morphometric analysis also revaled no change in neuronal size following these experimental manipulations. By contrast, there was a slight decrease in the total area of spinal gray matter that was most prominent in the dorsal region following limb bud removal. Our results indicate (1) that spinal interneurons fail to exhibit the massive naturally occurring death of postmitotic neurons that has been observed for several other populations of spinal neurons, and (2) spinal interneurons appear to be relatively resistant to induced cell death following the removal of substantial numbers of afferent inputs and targets.     

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 afferent projection of mesothoracic bristle hairs in the cricket,Acheta domesticus

Summary One of the sensory systems of the thoracic segments of the cricket,Acheta domesticus, was investigated by using cobalt to stain the axonal arborizations of bristles of various types and locations. The results demonstrate that a variety of hair-like receptors project to the ventral areas of thoracic neuropil previously called ventral association center (VAC) and ventralmost ventral association center (vVAC). Bristles located on the dorsal thorax or wings arborize in or near VAC, while those on the ventral thorax and legs arborize in vVAC. Within the area of neuropil occupied by thoracic bristle afferents, the axonal arborizations are distributed according to the position of the parent receptor at the body surface; thus, medial afferents arborize medially and lateral ones more laterally. For leg afferents, the location of the axonal arborization is also correlated with the various leg segments of origin of the parent receptor and its circumferential location on that segment (Fig. 9).Abbreviations vVAC ventralmost ventral association center - VAC ventral association center     

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.     

Anatomical and physiological observations on the organization of mechanoreceptors and local interneurons in the central nervous system of the wandering spider Cupiennius salei

Summary In the wandering spider Cupiennius salei, the functional neuroanatomy of leg mechanosensory receptor neurons and interneurons associated with a single leg neumere was investigated by combined intracellular recording and Lucifer yellow ionophoresis. Trichobothria axons that selectively respond to air currents and to low-frequency airborne vibrations have arborizations restricted to ventral regions of the appropriate leg neuromere. Receptor afferents that respond selectively to substrateborne vibrations are distributed ventrally in the corresponding leg neuromere and extend into certain interganglionic tract neuropiles. Golgi impregnation and intracellular dye filling show that local interneurons originate in ventral sensory neuropiles of leg neuromeres and ascend dorsally to terminate amongst dendrites of motor neurons. Local interneurons generally show higher thresholds for vibration stimuli than do receptors. Local interneurons typically receive inputs from one or several types of receptors. Some respond to stimulation of a single leg, others respond to stimulation of several legs on the same side of the body. The functional morphology of the receptor afferents is correlated with known physiological characteristics of slit sensilla and trichobothria. Structure and activity of the local interneurons are compared with analogous interneurons in other arthropods.