Neuroanatomy of a crayfish thoracic ganglion: sensory and motor roots of the walking-leg nerves and possible homologies with insects

Intrinsic sources of tachykinin-related peptide in the thoracic ganglion mass of the crab, Cancer borealis

Neuropeptides comprise the largest class of neural and neuroendocrine signaling molecules. Vertebrate tachykinins (TKs) and the structurally-related invertebrate tachykinin-related peptides (TRPs) together form the largest neuropeptide superfamily, with a number of conserved neural and neuroendocrine functions across species. Arthropods, including crustaceans, have provided many insights into neuropeptide signaling and function. Crustacean tachykinin-related peptide occurs in endocrine organs and cells and in two of the major crustacean CNS components, the supraoesophageal ganglion ("brain") and the stomatogastric nervous system. However, little is known about TRP sources in the remaining major CNS component, the thoracic ganglion mass (TGM). To gain further insight into the function of this peptide, we aimed to identify intrinsic TRP sources in the TGM of the Jonah crab, Cancer borealis. We first adapted a clearing protocol to improve TRP immunoreactivity specifically in the TGM, which is a dense, fused mass of multiple ganglia in short-bodied crustaceans such as Cancer species of crabs. We verified that the clearing protocol avoided distortion of cell body morphology yet increased visibility of TRP immunoreactivity. Using confocal microscopy, we found TRP-immunoreactive (TRP-IR) axon tracts running the length of the TGM, TRP-IR neuropil in all ganglia, and approximately 110 TRP-IR somata distributed throughout the TGM, within and between ganglia. These somata likely represent both neural and neuroendocrine sources of TRP. Thus, there are many potential intrinsic sources of TRP in the TGM that are positioned to regulate behaviors such as food intake, locomotion, respiration, and reproduction.

Comparative morphology of ultimate and walking legs in the centipede Lithobius forficatus (Myriapoda) with functional implications

BACKGROUND

In the context of evolutionary arthopodial transformations, centipede ultimate legs exhibit a plethora of morphological modifications and behavioral adaptations. Many species possess significantly elongated, thickened, or pincer-like ultimate legs. They are frequently sexually dimorphic, indicating a role in courtship and mating. In addition, glandular pores occur more commonly on ultimate legs than on walking legs, indicating a role in secretion, chemical communication, or predator avoidance. In this framework, this study characterizes the evolutionarily transformed ultimate legs in Lithobius forficatus in comparison with regular walking legs.

RESULTS

A comparative analysis using macro-photography, SEM, μCT, autofluorescence, backfilling, and 3D-reconstruction illustrates that ultimate legs largely resemble walking legs, but also feature a series of distinctions. Substantial differences are found with regard to aspects of the configuration of specific podomeres, musculature, abundance of epidermal glands, typology and distribution of epidermal sensilla, and architecture of associated nervous system structures.

CONCLUSION

In consideration of morphological and behavioral characteristics, ultimate legs in L. forficatus primarily serve a defensive, but also a sensory function. Moreover, morphologically coherent characteristics in the organization of the ultimate leg versus the antenna-associated neuromere point to constructional constraints in the evolution of primary processing neuropils.

X-ray imaging of a water bear offers a new look at tardigrade internal anatomy

BACKGROUND

Tardigrades (water bears) are microscopic invertebrates of which the anatomy has been well studied using traditional techniques, but a comprehensive three-dimensional reconstruction has never been performed. In order to close this gap, we employed X-ray computed tomography (CT), a technique that is becoming increasingly popular in zoology for producing high-resolution, three-dimensional (3D) scans of whole specimens. While CT has long been used to scan larger samples, its use in some microscopic animals can be problematic, as they are often too small for conventional CT yet too large for high-resolution, optics-based soft X-ray microscopy. This size gap continues to be narrowed with advancements in technology, with high-resolution imaging now being possible using both large synchrotron devices and, more recently, laboratory-based instruments.

RESULTS

Here we use a recently developed prototype lab-based nano-computed tomography device to image a 152 μm-long tardigrade at high resolution (200-270 nm pixel size). The resulting dataset allowed us to visualize the anatomy of the tardigrade in 3D and analyze the spatial relationships of the internal structures. Segmentation of the major structures of the body enabled the direct measurement of their respective volumes. Furthermore, we segmented every storage cell individually and quantified their volume distribution. We compare our measurements to those from published studies in which other techniques were used.

CONCLUSIONS

The data presented herein demonstrate the utility of CT imaging as a powerful supplementary tool for studies of tardigrade anatomy, especially for quantitative volume measurements. This nanoCT study represents the smallest complete animal ever imaged using CT, and offers new 3D insights into the spatial relationships of the internal organs of water bears.

Three-dimensional organization of the brain and distribution of serotonin in the brain and ovary, and its effects on ovarian steroidogenesis in the giant freshwater prawn, Macrobrachium rosenbergii

The giant freshwater prawn, Macrobrachium rosenbergii, is an economically important crustacean species which has also been extensively used as a model in neuroscience research. The crustacean central nervous system is a highly complex structure, especially the brain. However, little information is available on the brain structure, especially the three-dimensional organization. In this study, we demonstrated the three-dimensional structure and histology of the brain of M. rosenbergii together with the distribution of serotonin (5-HT) in the brain and ovary as well as its effects on ovarian steroidogenesis. The brain of M. rosenbergii consists of three parts: protocerebrum, deutocerebrum and tritocerebrum. Histologically, protocerebrum comprises of neuronal clusters 6-8 and prominent anterior and posterior medial protocerebral neuropils (AMPN/PMPN). The protocerebrum is connected posteriorly to the deutocerebrum which consists of neuronal clusters 9-13, medial antenna I neuropil, a paired lateral antenna I neuropils and olfactory neuropils (ON). Tritocerebrum comprises of neuronal clusters 14-17 with prominent pairs of antenna II (AnN), tegumentary and columnar neuropils (CN). All neuronal clusters are paired structures except numbers 7, 13 and 17 which are single clusters located at the median zone. These neuronal clusters and neuropils are clearly shown in three-dimensional structure of the brain. 5-HT immunoreactivity (-ir) was mostly detected in the medium-sized neurons and neuronal fibers of clusters 6/7, 8, 9, 10 and 14/15 and in many neuropils of the brain including anterior/posterior medial protocerebral neuropils (AMPN/PMPN), protocerebral tract, protocerebral bridge, central body, olfactory neuropil (ON), antennal II neuropil (Ann) and columnar neuropil (CN). In the ovary, the 5-HT-ir was light in the oocyte step 1(Oc₁) and very intense in Oc₂-Oc₄. Using an in vitro assay of an explant of mature ovary, it was shown that 5-HT was able to enhance ovarian estradiol-17β (E₂) and progesterone (P₄) secretions. We suggest that 5-HT is specifically localized in specific brain areas and ovary of this prawn and it plays a pivotal role in ovarian maturation via the induction of female sex steroid secretions, in turn these steroids may enhance vitellogenesis resulting in oocyte growth and maturation.

Arthropod neurons and nervous system

Arthropods are very diverse, come in many different forms with diverse adaptations, and through such diversity have populated all environmental niches on the planet. Almost 80% of the animals on planet Earth belong to this phylum. Despite their very diverse phenotypes they share fundamental similarities which were previously used to generate a phylogenetic tree. All arthropods have segmented bodies and possess jointed limbs at all or many of their body segments. An additional common feature of arthropods is their exoskeleton, made mainly of chitin and/or sclerotin.

Localization patterns of dopamine active transporter synthesizing cells during development of brine shrimp

There have been many studies on dopamine active transporter (DAT) in humans and laboratory animals; however, there is a lack of information on DAT in brine shrimp. In this study, we demonstrated the neuronal and nonneuronal characteristics of DAT-synthesizing (DAT⁺ cells) during development of brine shrimp. In neuronal cells, the DAT⁺ neurons in the central body and lobes of a protocerebrum (PC) controlled the deutocerebrum. The sensory cells of nauplius eyes projected their decussated axons to the PC, and the DAT⁺ cells at the posterior region were associated with migration and control of the 10 posterior neurons during the early nauplius stage. In nonneuronal cells, the five types of glands, that is, the salt, antennal, mandible, and accessory glands and posterior gland1 and gland2 synthesized DAT protein. In addition, the gut and rectum dilator muscles and renal cells expressed DAT protein. Thus, DAT protein acts in the development of several types of cells during development of brine shrimp.

Serotonin-immunoreactivity in the ventral nerve cord of Pycnogonida--support for individually identifiable neurons as ancestral feature of the arthropod nervous system

BACKGROUND

The arthropod ventral nerve cord features a comparably low number of serotonin-immunoreactive neurons, occurring in segmentally repeated arrays. In different crustaceans and hexapods, these neurons have been individually identified and even inter-specifically homologized, based on their soma positions and neurite morphologies. Stereotypic sets of serotonin-immunoreactive neurons are also present in myriapods, whereas in the investigated chelicerates segmental neuron clusters with higher and variable cell numbers have been reported. This led to the suggestion that individually identifiable serotonin-immunoreactive neurons are an apomorphic feature of the Mandibulata. To test the validity of this neurophylogenetic hypothesis, we studied serotonin-immunoreactivity in three species of Pycnogonida (sea spiders). This group of marine arthropods is nowadays most plausibly resolved as sister group to all other extant chelicerates, rendering its investigation crucial for a reliable reconstruction of arthropod nervous system evolution.

RESULTS

In all three investigated pycnogonids, the ventral walking leg ganglia contain different types of serotonin-immunoreactive neurons, the somata of which occurring mostly singly or in pairs within the ganglionic cortex. Several of these neurons are readily and consistently identifiable due to their stereotypic soma position and characteristic neurite morphology. They can be clearly homologized across different ganglia and different specimens as well as across the three species. Based on these homologous neurons, we reconstruct for their last common ancestor (presumably the pycnogonid stem species) a minimal repertoire of at least seven identified serotonin-immunoreactive neurons per hemiganglion. Beyond that, each studied species features specific pattern variations, which include also some neurons that were not reliably labeled in all specimens.

CONCLUSIONS

Our results unequivocally demonstrate the presence of individually identifiable serotonin-immunoreactive neurons in the pycnogonid ventral nerve cord. Accordingly, the validity of this neuroanatomical feature as apomorphy of Mandibulata is questioned and we suggest it to be ancestral for arthropods instead. The pronounced disparities between the segmental pattern in pycnogonids and the one of studied euchelicerates call for denser sampling within the latter taxon. By contrast, overall similarities between the pycnogonid and myriapod patterns may be indicative of single cell homologies in these two taxa. This notion awaits further substantiation from future studies.

A network model comprising 4 segmental, interconnected ganglia, and its application to simulate multi-legged locomotion in crustaceans

Inter-segmental coordination is crucial for the locomotion of animals. Arthropods show high variability of leg numbers, from 6 in insects up to 750 legs in millipedes. Despite this fact, the anatomical and functional organization of their nervous systems show basic similarities. The main similarities are the segmental organization, and the way the function of the segmental units is coordinated. We set out to construct a model that could describe locomotion (walking) in animals with more than 6 legs, as well as in 6-legged animals (insects). To this end, we extended a network model by Daun-Gruhn and Tóth (Journal of Computational Neuroscience, doi: 10.1007/s10827-010-0300-1 , 2011). This model describes inter-segmental coordination of the ipsilateral legs in the stick insect during walking. Including an additional segment (local network) into the original model, we could simulate coordination patterns that occur in animals walking on eight legs (e.g., crayfish). We could improve the model by modifying its original cyclic connection topology. In all model variants, the phase relations between the afferent segmental excitatory sensory signals and the oscillatory activity of the segmental networks played a crucial role. Our results stress the importance of this sensory input on the generation of different stable coordination patterns. The simulations confirmed that using the modified connection topology, the flexibility of the model behaviour increased, meaning that changing a single phase parameter, i.e., gating properties of just one afferent sensory signal was sufficient to reproduce all coordination patterns seen in the experiments.

Changes in the nitric oxide system in the shore crab Hemigrapsus sanguineus (Crustacea, decapoda) CNS induced by a nociceptive stimulus

Using NADPH-diaphorase (NADPH-d) histochemistry, inducible nitric oxide synthase (iNOS)-immunohistochemistry and immunoblotting, we characterized the nitric oxide (NO)-producing neurons in the brain and thoracic ganglion of a shore crab subjected to a nociceptive chemical stimulus. Formalin injection into the cheliped evoked specific nociceptive behavior and neurochemical responses in the brain and thoracic ganglion of experimental animals. Within 5–10 min of injury, the NADPH-d activity increased mainly in the neuropils of the olfactory lobes and the lateral antenna I neuropil on the side of injury. Later, the noxious-induced expression of NADPH-d and iNOS was detected in neurons of the brain, as well as in segmental motoneurons and interneurons of the thoracic ganglion. Western blotting analysis showed that an iNOS antiserum recognized a band at 120 kDa, in agreement with the expected molecular mass of the protein. The increase in nitrergic activity induced by nociceptive stimulation suggests that the NO signaling system may modulate nociceptive behavior in crabs.

Recruitment in a heterogeneous population of motor neurons that innervates the depressor muscle of the crayfish walking leg muscle

According to the size principle the fine control of muscle tension depends on the orderly recruitment of motor neurons from a heterogeneous pool. We took advantage of the small number of excitatory motor neurons (about 12) that innervate the depressor muscle of the crayfish walking leg to determine if the size principle applies to this muscle. We found that in accordance with the size principle, when stimulated by proprioceptive input, neurons with small extracellular spikes were recruited before neurons with medium or large spikes. Because only a small fraction of the motor neurons responded strongly enough to sensory input to be recruited in this way, we extended our analysis to all neurons by characterizing properties that have classically been associated with recruitment order such as speed of axonal conduction and extracellular spike amplitude. Through a combination of physiological and anatomical criteria we were able to identify seven classes of excitatory depressor motor neurons. The majority of these classes responded to proprioceptive input with a resistance reflex, while a few responded with an assistance reflex, and yet others did not respond. Our results are in general agreement with the size principle. However, we found qualitative differences between neuronal classes in terms of synaptic input and neuronal structure that would in theory be unnecessary, according to a strict interpretation of the size principle. We speculate that the qualitative heterogeneity observed may be due to the fact that the depressor is a complex muscle, consisting of two muscle bundles that share a single insertion but have multiple origins.

Nitric oxide synthase in crayfish walking leg ganglia: Segmental differences in chemo-tactile centers argue against a generic role in sensory integration

Nitric oxide (NO) is a diffusible signaling molecule with evolutionarily conserved roles in neural plasticity. Prominent expression of NO synthase (NOS) in the primary olfactory centers of mammals and insects lead to the notion of a special role for NO in olfaction. In insects, however, NOS is also strongly expressed in non-olfactory chemo-tactile centers of the thoracic nerve cord. The functional significance of this apparent association with various sensory centers is unclear, as is the extent to which it occurs in other arthropods. We therefore investigated the expression of NOS in the pereopod ganglia of crayfish (Pacifastacus lenisculus and Procambarus clarkii). Conventional NADPH diaphorase (NADPHd) staining after formaldehyde fixation gave poor anatomic detail, whereas fixation in methanol/formalin (MF-NADPHd) resulted in Golgi-like staining, which was supported by immunohistochemistry using NOS antibodies that recognize a 135-kDa protein in crayfish. MF-NADPHd revealed an exceedingly dense innervation of the chemo-tactile centers. As in insects, this innervation was provided by a system of prominent intersegmental neurons. Superimposed on a putatively conserved architecture, however, were pronounced segmental differences. Strong expression occurred only in the anterior three pereopod ganglia, correlating with the presence of claws on pereopods one to three. These clawed pereopods, in addition to their role in locomotion, are crucially involved in feeding, where they serve both sensory and motor functions. Our findings indicate that strong expression of NOS is not a universal feature of primary sensory centers but instead may subserve a specific requirement for sensory plasticity that arises only in particular behavioral contexts.

Immunocytochemical mapping and quantification of expression of a putative type 1 serotonin receptor in the crayfish nervous system

Serotonin is an important neurotransmitter that is involved in modulation of sensory, motor, and higher functions in many species. In the crayfish, which has been developed as a model for nervous system function for over a century, serotonin modulates several identified circuits. Although the cellular and circuit effects of serotonin have been extensively studied, little is known about the receptors that mediate these signals. Physiological data indicate that identified crustacean cells and circuits are modulated via several different serotonin receptors. We describe the detailed immunocytochemical localization of the crustacean type 1 serotonin receptor, 5-HT1crust, throughout the crayfish nerve cord and on abdominal superficial flexor muscles. 5-HT1crust is widely distributed in somata, including those of several identified neurons, and neuropil, suggesting both synaptic and neurohormonal roles. Individual animals show very different levels of 5-HT1crust immunoreactivity (5-HT(1crust)ir) ranging from preparations with hundreds of labeled cells per ganglion to some containing only a handful of 5-HT(1crust)ir cells in the entire nerve cord. The interanimal variability in 5-HT(1crust)ir is great, but individual nerve cords show a consistent level of labeling between ganglia. Quantitative RT-PCR shows that 5-HT1crust mRNA levels between animals are also variable but do not directly correlate with 5-HT(1crust)ir levels. Although there is no correlation of 5-HT1crust expression with gender, social status, molting or feeding, dominant animals show significantly greater variability than subordinates. Functional analysis of 5-HT1crust in combination with this immunocytochemical map will aid further understanding of this receptor's role in the actions of serotonin on identified circuits and cells.

A crustacean serotonin receptor: Cloning and distribution in the thoracic ganglia of crayfish and freshwater prawn

Serotonin (5-HT) is involved in regulating important aspects of behavior and a variety of systemic physiological functions in both vertebrates and invertebrates. These functions are mediated through binding to 5-HT receptors, of which approximately 13 have been characterized in mammals. In crustaceans, important model systems for the study of the neural basis of behaviors, 5-HT is also linked with higher-order behaviors, associated with different 5-HT receptors that have been identified at the physiological and pharmacological levels. However, no crustacean 5-HT receptors have been identified at the molecular level. We have cloned a putative 5-HT(1) receptor (5-HT(1crust)) from crayfish, prawn, and spiny lobster and have raised antibodies that recognize this protein in all three organisms. 5-HT(1crust) immunoreactivity (5-HT(1crust)ir) was observed surrounding the somata of specific groups of neurons and as punctate staining within the neuropil in all thoracic ganglia of crayfish and prawn. In the crayfish, 5-HT(1crust)ir was also found in boutons surrounding the first and second nerves of each ganglion and on the 5-HT cells of T1-4. In the prawn, 5-HT(1crust)ir was also found in axons that project across the ganglia and along the connectives. We found examples of colocalization of 5-HT(1crust) with 5-HT, consistent with the short-term modulatory role of 5-HT, as well as cases of serotonergic staining in the absence of a 5-HT(1crust) signal, which might imply that other 5-HT receptors are found at these locations. We also observed receptors that did not possess counterpart 5-HT staining, suggesting that these may also mediate long-term neurohormonal functions of serotonin.

Ontogeny of the ventral nerve cord in malacostracan crustaceans: a common plan for neuronal development in Crustacea, Hexapoda and other Arthropoda?

This review sets out to summarize our current knowledge on the structural layout of the embryonic ventral nerve cord in decapod crustaceans and its development from stem cell to the mature structure. In Decapoda, neuronal stem cells, the neuroblasts, mostly originate from ectodermal stem cells, the ectoteloblast, via a defined lineage. The neuroblasts undergo repeated asymmetric division and generate ganglion mother cells. The ganglion mother cells later divide again to give birth to ganglion cells (neurons) and there is increasing evidence now that ganglion mother cells divide again not only once but repeatedly. Various other aspects of neuroblast proliferation such as their temporal patterns of mitotic activity and spatial arrangement as well as the relation of neurogenesis to the development of the segmental appendages and maturation of motor behaviors are described. The link between cell lineage and cell differentiation in Decapoda so far has only been established for the midline neuroblast. However, there are several other identified early differentiating neurons, the outgrowing neurites of which pioneer the axonal scaffold within the neuromeres of the ventral nerve cord. The maturation of identified neurons as examined by immunohistochemistry against their neurotransmitters or engrailed, is briefly described. These processes are compared to other Arthropoda (including Onychophora, Chelicerata, Diplopoda and Hexapoda) in order to shed light on variations and conserved motifs of the theme 'neurogenesis'. The question of a 'common plan for neuronal development' in the ventral nerve cords of Hexapoda and Crustacea is critically evaluated and the possibility of homologous neurons arising through divergent developmental pathways is discussed.

Architectonics of crayfish ganglia

The central nervous system of crayfish consists of a chain of segmental ganglia that are linked by cables of intersegmental axons. Each ganglion contains a highly-ordered core of longitudinal tracts, vertical tracts, commissures, and synaptic neuropils. We review from a technical perspective the history of the description of these ganglia, and recognize four episodes of progress. Each major innovation in anatomical methods has led to new insight into the structure and function of this nervous system, and new awareness of the structural patterns that are common to the CNS of all arthropods. Ganglia in different segments of the body differ in size, and appear to differ in anatomy. From a comparison of the structures of the cores of abdominal, thoracic, and subesophageal ganglia, we argue that this apparent difference is illusory. Rather, each of these ganglia is organized on the same plan, a plan also found in insect segmental ganglia. The apparent differences follow from longitudinal compression during development and from allometric growth of particular neuropils associated with innervation of the walking legs. Different authors have described the internal organization of ganglia in different segments, so we provide a cross-reference to the nomenclatures they have introduced. We compare the locations of cell bodies of motor neurons and accessory neurons that innervate different peripheral structures, and demonstrate double-labeling of certain GABAergic peripheral inhibitory neurons. Finally, we describe the construction of digital movies of serial sections of these ganglia, and discuss their utility.

Functional organisation of anterior thoracic stretch receptors in the deep-sea isopod Bathynomus doederleini: behavioural, morphological and physiological studies

The relationship between segmental mobility and the organisation of thoracic stretch receptors was examined in the deep-sea isopod Bathynomus doederleini, which shows a developed adaptive behaviour during digging. The movements of segments during digging were analysed from video recordings, which showed that a large excursion occurred in the anterior thoracic segments. Dye-fills of axons revealed four types of thoracic stretch receptor (TSR): an N-cell type (TSR-1), a differentiated N-cell type (TSR-2), a muscle receptor organ (MRO)-type with a long, single receptor muscle (TSR-3) and an MRO-type with a short, single receptor muscle (TSR-4 to TSR-7). Physiologically, TSR-1 and TSR-2 are tonic-type stretch receptors. TSR-3 to TSR-7 show two kinds of stretch-activated responses, a tonic response and a phasico-tonic response in which responses are maintained as long as the stretch stimulus is delivered. Both TSR-2, with a long muscle strand, and TSR-3, with a single, long receptor muscle, have a wide dynamic range in their stretch-activated response. In addition, TSR-2 is controlled by an intersegmental inhibitory reflex from TSR-3. These results suggest that, although TSR-1 has no receptor muscle and TSR-2 has a less-differentiated receptor-like muscle, they are fully functional position detectors of segmental movements, as are the MRO-type receptors TSR-3 to TSR-7.

Serotonergic and octopaminergic systems in the squat lobster Munida quadrispina (Anomura, Galatheidae)

Immunocytochemical mapping of serotonergic and octopaminergic neurons in the central nervous system of the squat lobster Munida quadrispina reveal approximately 120 serotonin-immunoreactive cell bodies (distributed throughout the neuromeres except in abdominal ganglion 5) and 48 octopamine-immunoreactive cell bodies (in brain and thoracic neuromeres but none in the circumesophageal or abdominal ganglia). Immunopositive neuropils for both amines are distributed in multiple areas in each neuromere and overlap extensively. Serotonergic and octopaminergic neurons have extensive bilateral projections in abdominal ganglia, whereas the majority of projections in thoracic and subesophageal ganglia are unilateral (contralateral to soma). This difference correlates with typical differences between abdominal and thoracic motor system coordination. Processes of immunoreactive cells for both amines form extensive, peripheral, neurosecretory-like structures. Serotonin seems to be released peripherally in more segments, and from more nerves per segment, than octopamine. M. quadrispina has fewer serotonergic and octopaminergic immunoreactive cells, in particular, fewer segmentally repeated cells, than other species studied to date. Nevertheless, the general organization of the aminergic systems is similar, and several aminergic cells have locations and morphologies that strongly suggest homology with identified aminergic cells in other crustaceans. Among these are segmentally repeated neurons that, in M. quadrispina, form serotonin-immunopositive tubular structures in the thoracic hemiganglia innervating pereiopods 1-3 that are unlike anything reported previously for any species. Comparisons of immunocytochemical maps within one species and between species exhibiting different behaviors provide insights into possible sites of action, functional differences between, and evolution of biogenic aminergic systems.

Functional organization of crayfish abdominal ganglia. III. Swimmeret motor neurons

Swimmerets are limbs on several segments of the crayfish abdomen that are used for forward swimming and other behaviors. We present evidence that the functional modules demonstrated previously in physiological experiments are reflected in the morphological disposition of swimmeret motor neurons. The single nerve that innervates each swimmeret divides into two branches that separately contain the axons of power-stroke and return-stroke motor neurons. We used Co(++) or biocytin to backfill the entire pool of neurons that innervated a swimmeret, or functional subsets whose axons occurred in particular branches. Each filled cell body extended a single neurite that projected first to the Lateral Neuropil (LN), and there branched to form dendritic structures and its axon. All the motor neurons that innervated one swimmeret had cell bodies located in the ganglion from which their axons emerged, and the cell bodies of all but two of these neurons were located ipsilateral to their swimmeret. Counts of cell bodies filled from selected peripheral branches revealed about 35 power-stroke motor neurons and 35 return-stroke motor neurons. The cell bodies of these two types were segregated into different clusters within the ganglion, but both types sent their neurites into the ipsilateral LN and had their principle branches in this neuropil. We saw no significant differences in the numbers or distributions of these motor neurons in ganglia A2 through A5. These anatomical features are consistent with the physiological evidence that each swimmeret is controlled by its own neural module, which drives the alternating bursts of impulses in power-stroke and return-stroke motor neurons. We propose that the LN is the site of the synaptic circuit that generates this pattern.

Primary afferent depolarizations of sensory origin within contact-sensitive mechanoreceptive afferents of a crayfish leg

Recordings from the central branches of single identified dactyl sensory afferent (DSA) neurons in a crayfish in vitro preparation were performed to study modifications of the sensory message occurring before the first central synapse. These afferents comprised hairs and force-sensitive mechanoreceptors with phasic and phasotonic response characteristics in the terminal segment (dactyl) of the crayfish leg. More than one afferent spike size was often observed in intracellular recordings from these afferents, thus indicating the presence of electrical coupling between the central processes of DSA fibers. Additionally, in identified DSA fibers with large spike sizes, primary afferent depolarizations (PADs) of up to 15 mV were observed, which sometimes triggered antidromic spikes in the afferent. Nevertheless, PADs were clearly inhibitory, because they shunted the afferent spikes. They exhibited the following properties. First, each PAD was preceded by an afferent spike from a neighboring hair, indicating that the PADs had a sensory rather than central origin. Second, PADs could follow high frequencies of afferent discharges without failure, a property suggestive of monosynaptic connections, but because PAD latencies varied by +/-0.5 ms it is more likely that they were mediated by a disynaptic pathway. Third, although PADs were evoked in an extremely reliable manner, their amplitude varied in a quantal manner. Most unitary PADs were the result of the release of < 12 quanta, the mean quantal content lying between 4 and 5; quantal size was large, approximately 1 mV. Fourth, PADs showed facilitation in some fibers, whereas in others they became much smaller when occurring at brief intervals. We suggest that PADs may be an efficient and parsimonious way to limit sensory inflow in space and time, allowing the crayfish to identify precisely both weak and strong mechanical stimuli.