Segmental homology and variation in flexor motoneurons of the crayfish abdomen

Motor neurons in the escape response circuit of white shrimp (Litopenaeus setiferus)

Many decapod crustaceans perform escape tailflips with a neural circuit involving giant interneurons, a specialized fast flexor motor giant (MoG) neuron, populations of larger, less specialized fast flexor motor neurons, and fast extensor motor neurons. These escape-related neurons are well described in crayfish (Reptantia), but not in more basal decapod groups. To clarify the evolution of the escape circuit, I examined the fast flexor and fast extensor motor neurons of white shrimp (Litopenaeus setiferus; Dendrobranchiata) using backfilling. In crayfish, the MoGs in each abdominal ganglion are a bilateral pair of separate neurons. In L. setiferus, the MoGs have massive, possibly syncytial, cell bodies and fused axons. The non-MoG fast flexor motor neurons and fast extensor motor neurons are generally found in similar locations to where they are found in crayfish, but the number of motor neurons in both the flexor and extensor pools is smaller than in crayfish. The loss of fusion in the MoGs and increased number of fast motor neurons in reptantian decapods may be correlated with an increased reliance on non-giant mediated tailflipping.

Interactions between stretch and startle reflexes produce task-appropriate rapid postural reactions

Neural pathways underpinning startle reflex and limb stretch reflexes evolved independently and have served vastly different purposes. In their most basic form, the pathways responsible for these reflex responses are relatively simple processing units that produce a motoric response that is proportional to the stimulus received. It is becoming clear however, that rapid responses to external stimuli produced by human and non-human primates are context-dependent in a manner similar to voluntary movements. This mini review discusses the nature of startle and stretch reflex interactions in human and non-human primates and the involvement of the primary motor cortex in their regulation.

Position of larval tapeworms, Polypocephalus sp., in the ganglia of shrimp, Litopenaeus setiferus

Parasites that invade the nervous system of their hosts have perhaps the best potential to manipulate their host's behavior, but how they manipulate the host, if they do at all, could depend on their position within the host's nervous system. We hypothesize that parasites that live in the nervous system of their host will be randomly distributed if they exert their influence through non-specific effects (i.e., general pathology), but that their position in the nervous system will be non-random if they exert their influence by targeting specific neural circuits. We recorded the position of larval tapeworms, Polypocephalus sp., in the abdominal ganglia of white shrimp, Litopenaeus setiferus. Tapeworms are more common within ganglia than in the section of the nerve cord between ganglia, even though the nerve cord has a greater volume than the ganglia. The tapeworms are also more abundant in the periphery of the ganglia. Because most synaptic connections are within the central region of the ganglion, such positioning may represent a trade-off between controlling the nervous system and damaging it.

Functional motifs composed of morphologically homologous neurons repeated in the hindbrain segments

Segmental organization along the neuraxis is a prominent feature of the CNS in vertebrates. In a wide range of fishes, hindbrain segments contain orderly arranged reticulospinal neurons (RSNs). Individual RSNs in goldfish and zebrafish hindbrain are morphologically identified. RSNs sharing similar morphological features are called segmental homologs and repeated in adjacent segments. However, little is known about functional relationships among segmental homologs. Here we investigated the electrophysiological connectivity between the Mauthner cell (M-cell), a pair of giant RSNs in segment 4 (r4) that are known to trigger fast escape behavior, and different series of homologous RSNs in r4-r6. Paired intracellular recordings in adult goldfish revealed unidirectional connections from the M-cell to RSNs. The connectivity was similar in morphological homologs. A single M-cell spike produced IPSPs in dorsally located RSNs (MiD cells) on the ipsilateral side and excitatory postsynaptic depolarization on the contralateral side, except for MiD2cm cells. The inhibitory or excitatory potentials effectively suppressed or enhanced target RSNs spiking, respectively. In contrast to the lateralized effects on MiD cells, single M-cell spiking elicited equally strong depolarizations on bilateral RSNs located ventrally (MiV cells), and the depolarization was high enough for MiV cells to burst. Therefore, the morphological homology of repeated RSNs in r4-r6 and their functional M-cell connectivity were closely correlated, suggesting that each functional connection works as a functional motif during the M-cell-initiated escape.

Loss of escape-related giant neurons in a spiny lobster, Panulirus argus

When attacked, many decapod crustaceans perform tailflips, which are triggered by a neural circuit that includes lateral giant interneurons, medial giant interneurons, and fast flexor motor giant neurons (MoGs). Slipper lobsters (Scyllaridae) lack these giant neurons, and it has been hypothesized that behavioral (e.g., digging) and morphological (e.g., flattening and armor) specializations in this group caused the loss of escape-related giant neurons. To test this hypothesis, we examined a species of spiny lobster, Panulirus argus. Spiny lobsters belong to the sister taxon of the scyllarids, but they have a more crayfish-like morphology than scyllarids and were predicted to have escape-related giant neurons. Ventral nerve cords of P. argus were examined using paraffin-embedded sections and cobalt backfills. We found no escape-related giant neurons and no large axon profiles in the dorsal region of the nerve cord of P. argus. Cobalt backfills showed one fewer fast flexor motor neuron than in species with MoGs and none of the fast flexor motor neurons show any of the anatomical specializations of MoGs. This suggests that all palinuran species lack this giant escape circuit, and that the loss of rapid escape behavior preceded, and may have driven, alternative predator avoidance and anti-predator strategies in palinurans.

Loss of escape responses and giant neurons in the tailflipping circuits of slipper lobsters, Ibacus spp. (Decapoda, Palinura, Scyllaridae)

In many decapod crustaceans, escape tailflips are triggered by lateral giant (LG) and medial giant (MG) interneurons, which connect to motor giant (MoG) abdominal flexor neurons. Several decapods have lost some or all of these giant neurons, however. Because escape-related giant neurons have not been documented in palinurans, I examined tailflipping and abdominal nerve cords for giant neurons in two scyllarid lobster species, Ibacus peronii and Ibacus alticrenatus. Unlike decapods with giant neurons, Ibacus do not tailflip in response to sudden taps. Ibacus can perform non-giant tailflipping: the frequency of tailflips during swimming is adjusted by altering the gap between each individual tailflip. Abdominal nerve cord sections show no LG or MG interneurons. Backfilling nerve 3 of abdominal ganglia revealed no MoG neurons, and the fast flexor motor neuron population is otherwise identical to that described for crayfish. The loss of giant neurons in Ibacus represents an independent deletion of these cells compared to other reptantian decapods known to have lost these giant neurons. This loss is correlated with the normal posture in scyllarids, in which the last two abdominal segments are flexed, and an alternative defensive strategy, concealment by digging into sand.

Neurons in the third abdominal ganglion of the early postnatal crayfish: a quantitative and ultrastructural study

Ultrastructural data on the third abdominal ganglion of the crayfish was heretofore only available for adult individuals. The fine structure of neurons in the adult that are involved in the escape response has been described in detail, but no similar data existed for the postnatal individual. An increase in the number of neurons in the third abdominal ganglion during postnatal stages had been reported, which suggested that several changes in the features of neurons may occur. Here we describe the general anatomy and ultrastructure of the early postnatal third abdominal ganglion, with emphasis on neurons, and we compare their characteristics to those of the adult. Abdominal ganglia of 56 crayfish of 0, 8, 10, 18, 25, 50, 110, and 150 postnatal days were processed under cacodylate buffered aldehyde fixatives, osmicated, embedded in plastic, sectioned, and examined by light and electron microscopy. The anatomy of postnatal ganglia is homologous to the anatomy of the adult ganglia except that the perineurium is not developed in postnatals. The area of neurons within the postnatal ganglion shows no stratification, but neurons are grouped in nuclei according to their size. Neurons constitute a homogeneous population in different stages of maturity, as revealed particularly by the ultrastructure of the nucleolus. Postnatal development is evident in the perineurium, which may provide structural support to the ganglion.

Crustacean cardioactive peptide-immunoreactive neurons in the ventral nervous system of crayfish

Crustacean cardioactive peptide-immunoreactive neurons have been mapped in whole-mount preparations and sections of the ventral nervous system of the crayfish Astacus astacus and Orconectes limosus. Based on their morphology, projection patterns, and staining characteristics, two types of contralaterally projecting neurons are individually identifiable. In both species, these neurons occur in all neuromers as apparent serial homologs. In adult specimens, one type of cell has a small, densely stained dorsal lateral perikaryon, and a descending axon, and appears to be an interneuron. Each neuromer contains a single pair of these cells. Only in maxillary ganglia, these cells may have an additional ascending projection. The other type, a neurosecretory cell, has a larger, weakly stained perikaryon and a projection to the segmental third root of the next anterior neuromer. All neuromers contain a single pair of these neurons adjacent to the interneurons except for the abdominal neuromers, which contain two pairs of the neurosecretory cells. Central arborizations and varicose processes toward the surface of the third roots and within the perineural sheath of the ventral nerve cord arise from these neurons. Electron microscopy of granule-containing terminals substantiated that these newly discovered extensive neurohemal areas are release sites for the peptide. In young immature specimens, the perikarya of both neuron types do not differ in size. Additional weakly stained small perikarya occur in all neuromers of Astacus astacus. These two types of crayfish neurons and other comparable aminergic and peptidergic neurons of crayfish and lobster are differentially distributed in the ventral cord. Furthermore, comparison of similar neuron types in crab, locust, meal worm, and moth species indicates intra- and interphyletic structural homologies.

Comparative anatomy of serotonin-like immunoreactive neurons in isopods: putative homologues in several species

It is now commonly accepted that the arthropod nervous system has evolved only once, and so homologies between crustacean and insect nervous systems can be meaningfully sought. To do this, we have examined the distribution of serotonin (5-hydroxytryptamine)-like immunoreactive neurons in the central nervous system (CNS) of four common British isopods. Two species of terrestrial woodlouse, Oniscus asellus and Armadillidium vulgare, the littoral sea slater, Ligia oceanica, and the aquatic water hoglouse, Asellus meridianus, all possess approximately 40 pairs of serotonin-like immunoreactive neurons, distributed throughout the CNS in a very similar pattern. Interspecific homology is clearly suggested. Serotonin-like immunoreactive neurons in the first (T1) and fourth (T4) thoracic ganglia are particularly prominent in each of the four species studied. Whole-mount immunohistochemistry shows that the pair of T1 neurons have large dorsolateral cell bodies and prominent neurites that project medially and then anteriorly, whereas the pair of T4 neurons have ventrolateral cell bodies and neurites that bifurcate to form a thin axon projecting anteriorly to terminate in T3 and a thick medial axon that projects posteriorly into the abdominal neuromeres of the terminal ganglion. Intracellular cobalt staining of these neurons reveals more of their arborizations: the T1 neurons send three processes anteriorly, which arborize in the brain and exist from the CNS via peripheral nerves, whereas the T4 neurons contribute considerably to the extensive pattern of serotonin-like immunoreactive fibres in T3-T6 ganglia. The overall pattern of serotonin-like immunoreactive neurons in the isopods is similar to that in decapod crustacea, and a number of putative homologies can be assigned. It is more difficult to homologize the isopod serotonin-like immunoreactive neurons with those in the insect CNS, but some stained brain and thoracic neurons share common cell body positions and axon trajectories in isopods, decapods, and insects and may therefore be homologous.

The inhibitory motoneurons of crayfish thoracic limbs: identification, structures, and homology with insect common inhibitors

The three inhibitory motoneurons supplying crayfish thoracic limbs were identified, stained, and compared structurally. The inhibitors to the walking leg muscles (in Orconectes) were identified anatomically by a combination of immunocytochemical staining for gamma-aminobutyric acid (GABA) or glutamate decarboxylase and differential backfill staining with nickel and cobalt ions. The cheliped inhibitors were identified intracellularly and injected with Lucifer Yellow (Pacifastacus) or cobalt (Procambarus). The common inhibitor (CI) in each thoracic segment has a medial or slightly contralateral soma near the ganglion's posterior boundary, a gently curving primary neurite, an extensive ipsilateral dendritic tree, and an axon emerging through the anterior root. The stretcher-closer inhibitor (SI) has a soma slightly anterior and ipsilteral to the CI's a sharply bent proximal neurite, a smaller dendritic tree, and an axon in the posterior root. The opener inhibitor (OI) lies more laterally and often posterior to the CI; its diagnoally directed neurite enters the posterior root. The inhibitors' structures were related to major neuroanatomical landmarks within the ganglion, to soma positions of excitatory motoneurons revealed by backfilling, and to soma locations of inhibitory interneurons revealed by GABA-like immunoreactivity. In their peripheral distributions to the leg muscles and in their central structures, these crayfish limb inhibitors show striking similarities with those of the locust. Crayfish and locust thoracic ganglia also show more general neuroanatomical similarities. These observations suggest that the crayfish CI, SI, and OI are, respectively, homologous with the locust CI1, CI2, and CI3. The implications of such a homology for arthropod phylogeny are discussed.

Granule containing cells in the crayfish third abdominal ganglion

1. Four of the 850 neuron cell bodies of the crayfish third abdominal ganglion contain large dense secretory granules. 2. The processes of these cells form a neurohemal organ in the dorsal perineurium/neurilemma in the ganglion. 3. None of the immunocytochemically identified peptides accounts for the observed distribution of granules.

Relationships between cell size and nuclear morphology in crayfish neurons

The morphology and ultrastructure of cell nuclei in neurons of the third abdominal ganglion of crayfish were studied from alternating series of ultrathin and semithin sections. The ganglion contains approximately 850 neurons with sizes between 10 and 200 microm. Cell nuclei show a great variability. Their size, the chromatin distribution, the number of nuclear pores, the degree of nucleolar segregation and the size of nucleolus vary in close relationships with the cell size.

GABA-ergic neurons in the crayfish nervous system: an immunocytochemical census of the segmental ganglia and stomatogastric system

We used an antiserum directed against gamma-aminobutyric acid (GABA) fixed with glutaraldehyde (Hoskins et al., Cell Tissue Res. 244:243-252, '86) to label neurons with GABA-like immunoreactivity (GLI) in wholemounts of the stomatogastric ganglion and each segmental ganglion of crayfish, except the brain. Each abdominal ganglion had an average of 63 labeled neurons, or 10% of all their neurons. Each peripheral nerve of each abdominal ganglion except the last contained labeled axons. Within each segment, the first peripheral nerve, N1, had five axons; the second peripheral nerve, N2, had at most four; and the third peripheral nerve, N3, had two. In the last ganglion, N2 had one labeled axon, N3 had two and N6 had two; the other nerves contained no labeled axons. A tabulation of the identified inhibitory neurons in the abdominal ganglia revealed that 40% of these GABA-ergic neurons have been identified. The subesophageal ganglion had many labeled neurons in clusters that formed a repeating pattern; it also had labeled neurons near its dorsal midline. The thoracic ganglia contained more labeled neurons than did the abdominals, but their patterns of labeling were similar. The commissural ganglia contained three clusters of labeled neurons and sent labeled axons to the esophageal ganglion. The esophageal ganglion contained four labeled neurons and many labeled axons. The stomatogastric ganglion contained labeled axon terminals but not labeled neurons.

Unexpected divergence among identified interneurons in different abdominal segments of the crayfish Procambarus clarkii

The command elements that initiate and coordinate the abdominal movements in crayfish show little similarity between the various abdominal segments. Our criteria for similarity among interneurons were based on both cell morphology and electrophysiology. By contrast, previously published evidence shows much greater intersegmental similarity in the skeletal, muscular, motoneuronal, and sensory components of the abdominal system in crayfish, structures that are controlled by or send information to the command elements. Therefore, unlike the command elements, these structures have retained nearly identical form and function in the various segments. We also found in different ganglia examples of interneurons involved with abdominal positioning behavior that have similar morphology but different function and vice versa. Such interneurons could represent divergent pairs of serial homologues. It is unknown why so many of the abdominal positioning interneurons have become different. The various ganglia may perform subtly different functions, requiring differences in the positioning interneurons but not in the motor neurons or muscles. Alternatively, some of the abdominal positioning interneurons underlie more than one behavior; consequently, selection acting on these multiple functions may have changed these interneurons through evolution.

Regeneration studies on a crayfish neuromuscular system. I. Connectivity changes after intersegmental nerve transplants

The superficial flexor muscles of the crayfish are a neuromuscular system of a few muscle cells innervated by six neurons in a precise position-dependent pattern. The neurons are capable of regenerating their normal connectivity patterns within a short span of time when conditions are favorable. The superficial flexor muscles of the second and third segments, despite their similarities in neuronal and muscle cell size and number, have distinctive connectivity patterns; some homologous neurons form similar patterns but other homologous neurons form patterns that are reversed between segments. We transplanted each segment's nerve into each other's muscle in order to observe regeneration of the nerves into a target area that differed in connectivity patterns from their original muscle. During the first weeks of regeneration all neurons formed a connectivity pattern with more connections medially and declining connections laterally, a pattern determined by the medial location of the nerve transplant. This pattern is maintained for most of the neurons, but for some there is an eventual reduction in medial connections as maximum synapse formation shifts to the lateral muscle fibers. Three of the eight neurons studied were able to regenerate connectivity patterns that corresponded to their segment of origin and not to their host muscle. This suggests that intersegmental muscle differences are not influencing the formation of these connectivity patterns, so the neurons will follow their inherent synaptogenesis program.

Extension and retraction of axonal projections by some developing neurons in the leech depends upon the existence of neighboring homologues. I. The HA cells

The role of homologues in the establishment of the pattern of axonal projections of identified segmentally homologous neurons was investigated by means of selective cell ablation and dye injection. The cells studied were the bilateral pairs of heart accessory (HA) neurons found in the fifth and sixth segmental ganglia of the leech ventral nerve cord. Homologues start their morphological differentiation with identical axonal projections, and segmental differences are manifested later, when specific branches stop growing and disappear. The deletion of single HA cells at early stages, however, permits these branches to survive in their ipsilateral homologues and to grow and take over the projections of the deleted neurons. In addition, if both HA homologues on the same side of the nerve cord, or three of the four HA cells, are deleted in an animal, the remaining HA cells often extend novel projections. These observations suggest that either competition for targets, inputs or growth factors, or direct interactions among homologous cells may play a role in the differentiation of segment specific patterns of axonal projections.

Functional organization of crayfish abdominal ganglia: I. The flexor systems

For insect ganglia, Altman (Advances in Physiological Science, Vol. 23. Neurobiology of Invertebrates. New York: Pergamon Press, pp. 537-555, '81) proposed that individual neuropils control different motor activities. A corollary of this hypothesis is that motor neurons involved in many behavioral functions should branch in more neuropils than those active in fewer behaviors. In crayfish, the abdominal fast-flexor muscles are active only during the generation of the powerstroke for tailflips, whereas the slow-flexor muscles are involved in the maintenance of body posture. The slow flexors are thus active in many of the crayfish's behavioral activities. To test the generality of Altman's idea, we filled groups of crayfish fast-flexor and slow-flexor motor neurons with cobalt chloride and described their shapes with respect to the ganglionic structures through which they pass. Individual fast flexors were also filled intracellularly with HRP. Ganglia containing well-filled neurons were osmicated, embedded in plastic, and sectioned. Unstained sections were examined by light microscopy and pertinent sections were photographed. We found that the paths of the larger neurites were invariant, that the dendritic domains of fast and slow motor neurons occupied distinctive sets of neuropils, and that dendrites of slow motor neurons branched in more ganglionic structures than did those of fast motor neurons. These results are consistent with Altman's hypothesis.

Neuronal Circuits: An Evolutionary Perspective

To understand neural circuits completely, it is necessary to know not only how they work, but also why they work that way. Answers to the latter question have been almost teleological in their assumption of optimal design. However, close examination of certain systems has revealed features that apparently lack adaptive value. Their existence can be understood only if the evolution of these circuits is considered and, in particular, how nonadaptive determinants have guided that evoluton.

Morphogenesis of an identified leech neuron: segmental specification of axonal outgrowth

We have investigated the development of segmental diversity in an identified leech neuron, the Retzius cell. Retzius cells in the genital segments differ from those in other segments in lacking central axons and contacting different peripheral targets: the genitalia. These differences are not apparent during initial axon outgrowth, when all Retzius cells follow the same morphogenetic pattern. Rather, they first appear about the time the peripheral axons of the genital segment Retzius cells contact the genital primordia. This suggests that the pattern of central and peripheral axonal outgrowth may be modified by an interaction with peripheral targets.

Mapping of proctolinlike immunoreactivity in the nervous systems of lobster and crayfish

Whole-mount immunocytochemical techniques have been used to map candidate proctolin-containing cells in the central nervous systems of the lobster, Homarus americanus, and the crayfish, Procambarus clarkii. Proctolinlike immunoreactivity was detected in cell bodies and neuropil regions in all central ganglia, and immunoreactive axons were detected in most interganglionic connectives and nerve roots. Cell body staining was confined to fewer than 2% of all cells. Immunoreactive neurons include motoneurons, sensory neurons, neurosecretory cells, and interneurons. Colocalization of the proctolinlike antigen with other neurotransmitters was indicated in a number of cases. Many aspects of the distribution of immunoreactivity were similar in lobster and crayfish; however, staining differences were detected in a number of identified neurons and neural groups, including neurons that innervate the pericardial organs and hindgut motoneurons. Further studies of such neurons might provide interesting clues about the physiological functions of proctolin and the evolution of peptide transmission.

The morphology and ultrastructure of common inhibitory motor neurones in the thorax of the locust

The morphologies of three common inhibitory motor neurones which innervate muscles of a hind leg and the homologous three neurones which innervate muscles in a middle leg are described in relation to known commissures, tracts, and areas of neuropile in their ganglia. The neurones were stained individually by the intracellular injection of cobalt, and the ultrastructure of common inhibitor 1 (CI1) in the metathoracic ganglion was revealed by the intracellular injection of horseradish peroxidase. Homologous inhibitory motor neurones in the meso- and metathoracic ganglia have similar shapes. CI1 has axons in nerves 3, 4, and 5, but common inhibitors 2 and 3 (CI2, CI3) have only a single axon in nerve 5. They nevertheless all share many features in common. All have large (60 70 micron diameter) cell bodies in the ventral cortex near the midline, well separated from those of the excitatory leg motor neurones. Their primary neurites run dorsally and laterally and send many fine branches into the dorsal and lateral neuropile, and some fine branches medially. None enter the ventral neuropile. CI1 and CI2 have a small branch that arises close to the cell body and arborises on either side of the midline. When examined with the electron microscope, CI1 was not found to make any output synapses, even though some of its fine branches are varicose and end in bulbous swellings. These were seen to be packed with mitochondria but not vesicles. Input synapses tend to be grouped together on the secondary neurites and, more especially, on the finer branches and their spines. The majority of processes presynaptic to CI1 contain round agranular vesicles.

Morphological and physiological properties of the giant interneuron of the hermit crab (Pagurus pollicaris)

The physiological and morphological properties of the giant interneurons in the hermit crab Pagurus pollicaris are described. The cell bodies are located anteriorly in the supraesophageal ganglion, close to the mid-line. Each cell sends a neurite posteriorly and then laterally, so that they cross over in the center of the ganglion. Each axon then branches: one branch runs laterally while the other travels posteriorly and leaves the ganglion in the circumesophageal connective on the side contralateral to the cell body. The giant axons travel in the circumesophageal connectives and through the thoracic and abdominal ganglia without branching. Each giant axon makes synaptic contact with its ipsilateral giant abdominal flexor motor neuron and with a second flexor motor neuron that has its axon in the contralateral third root. In the supraesophageal ganglion there is a bidirectional synapse between the two giant interneurons. Intracellular recordings from the giant axons show that there is a delay of 0.5 to 0.75 ms that cannot be accounted for by spike propagation along the axons, and may be accounted for by a chemical synapse between the giant interneurons.

The structure of the fourth abdominal ganglion of the crayfish, Procambarus clarki (Girard). I. Tracts in the ganglionic core

The organization of the fourth abdominal ganglion of the crayfish, Procambarus clarki, was studied with the light microscope in serial sections stained with osmium ethyl gallate. This ganglion is composed of a ventral rind of somata and a core of alternating layers of through-tracts and commissures. The longitudinal tracts of the ganglion are named according to the system in use for the orthopteran insects, because the basic plans of the crustacean and insect ventral ganglia exhibit striking anatomical parallels. The dorsal tracts are the largest and the most regular in their path through the ganglion. In the ventral posterior quadrant of the ganglion the tracts diverge from the basic plan to pass around the major synaptic neuropil and the bases of the peripheral nerves. This paper reports the three-dimensional anatomy of the major longitudinal through-tracts, internal tracts and commissures, and bases of peripheral nerves. Landmark features of the ganglion--including the tracts, the major artery of the vascular system, the shape of the ganglionic core in section, and prominent single cells, all of which make it possible to recognize specific regions of the ganglion--are described.

Heterogeneous properties of segmentally homologous interneurons in the ventral nerve cord of locusts

The G, B1, and B2 neurons are three prominent interneurons located in adjacent segmental ganglia in the central nervous system of locusts. Previous studies on the adult nervous system have shown that each of these cells has its own distinctive morphology and responsiveness to auditory input. Previous studies on the embryonic nervous system have described the lineage and development of one of these cells, the G neuron, in the mesothoracic (T2) segment. In this paper it is shown that the G, B1, and B2 neurons are segmental homologues in that they arise from equivalent lineages during embryogenesis in the T2, T3, and A1 segments, respectively. Each cell arises (along with its identified sibling neuron) from the division of the second ganglion mother cell of neuroblast 7-4. The segment-specific morphology of the G homologues was determined in the T3 and A1 segments between 60-70% of embryonic development, and their identity was established as the adult B1 and B2 neurons by comparing the distinctive cell-specific features of their morphology between embryo and adult. Although all three neurons display striking morphological differences, they all share certain structural features in common, including the location of their primary axons and neurites in specific tracts in the neuropil. By recording intracellularly from the main neurites of the G, B1, and B2 neurons, clear differences were found in the synaptic inputs each of the neurons receives and the synaptic outputs each makes. For example, G and B2, but not B1, receive direct monosynaptic input from the descending contralateral movement detector (DCMD) interneurons and from auditory afferents; B1, but not B2, connects directly to G; and B2, but not B1 or G, connects directly to flight motoneurons. The main conclusion from these observations is that lineally equivalent neurons in different segments can develop similar primary structures but quite different secondary morphologies and synaptic connections. How these segment-specific differences arise during embryogenesis remains unknown.

Plasticity of non-giant flexion circuitry in chronically cut abdominal nerve cords of the crayfish, Procambarus clarkii

We have investigated the pattern of neuronal activity involved in the gradual return of sensory-evoked abdominal flexions in crayfish with chronically transected nerve cords. Recordings were made from eight types of identified neurone that mediate phasic abdominal movements, in a preparation consisting of the isolated abdominal nerve cord and tailfan. Responses of the cells to pinches and dorsiflexions of the tailfan were compared in two groups of animals: animals whose cords had been cut at the thoracic-abdominal junction 4-17 weeks earlier (chronic preparations), and animals whose cords had been cut at the same site either just before the experiment or up to 6 days earlier (acute preparations). Sensory stimuli produced bursts of spikes in 73% of the fast flexor motoneurones impaled in chronic preparations, but never fired these neurones in acute preparations. However, fast flexor motoneurones in both preparations were fired with approximately equal frequency by single impulses in the giant axons, suggesting that the firing thresholds of these motoneurones had not changed. Sensory stimuli also caused spiking in the extensor inhibitor and the flexor inhibitor in chronic preparations; in contrast, responses in the fast extensor motoneurones were always subthreshold and occasionally hyperpolarizing. None of these cells was fired by similar stimuli in acute preparations. Neurones restricted to the giant axon pathways (lateral, medial, segmental and motor giants) were silent during sensory-evoked flexor discharges in chronically transected cords. Flexor discharges were accompanied by intense activity in non-giant axons recorded from the dorsal cord. Two identified, non-giant interneurones with axons in the dorsal cord were substantially depolarized but never fired by sensory input in chronic preparations. Sensory-evoked firing in the fast flexor motoneurones was not abolished by removal of the posterior stump of the nerve cord at the transection site. About 20% of chronic preparations generated cyclic motor output in response to unpatterned sensory stimulation. The pattern of motor activity that develops in chronically transected cords resembles that seen in normal crayfish during non-giant tailflips. Because cord transection permanently isolates the abdomen from rostral neural centres normally required for the generation of such tailflips, the return of co-ordinated motor output in chronically cut cords may result from the sensory activation of non-giant circuitry within the abdominal nervous system.

Distribution and morphology of nociceptive cells in the CNS of three species of leeches

The present study describes the segmental variation in the distribution and morphology of nociceptive neurons (N cells) in the central nervous system of the leech. N cells of midbody ganglia can be segregated into lateral and medial types. We show that monoclonal antibodies specific for N cells can distinguish between the two populations. The monoclonal antibodies were used to map the complete distribution of the cells along the nervous cord. There are two pairs of the medial and lateral nociceptive neurons in the midbody ganglia, one pair of the medial type in the sex ganglia (5 and 6), and a pair of the lateral type in ganglia 20 and 21. The caudal brain is without nociceptive neurons. This distribution was confirmed by electrophysiological means. The morphology of N cells in different parts of the nervous system was investigated by intracellular horseradish peroxidase (HRP) injections. In the terminal segmental ganglia the N cells showed extensive arborizations in the head and tail brains and, contrary to N cells in the midbody ganglia, their arborizations spanned more than three segments. N cells are absent in the tail brain, but the N cells of ganglia 20 and 21 were shown to innervate the entire caudal region. The basic morphology of all N-cell homologues was found to be very similar for three leech species. In the sex ganglia the pair of N-cell homologues were examined in Haemopis, Hirudo, and Macrobdella. The results showed a progressive modification in the three species of the cell's morphology, peripheral projections, and physiological responses, possibly correlated with the evolution and complexity of the sexual organs. HRP injections and monoclonal antibody staining revealed that a common feature of N-cell homologues is the presence of processes that tightly surround the cell soma of other cells. This suggests that N cells may have other functional properties in addition to being primary sensory neurons.

Neural asymmetry in male fiddler crabs

In adult male fiddler crabs, Uca pugnax, there is a marked enlargement of the 1st thoracic ganglion and its nerve root on the side of the major cheliped compared to the side of the minor cheliped. Retrograde uptake of cobalt via the cut ends of the motoneurons revealed a significant hypertrophy of their somata and dendritic fields on the major side of the ganglion compared to the minor side in the male fiddler crabs. (In female fiddler crabs which have two minor chelipeds the motoneurons were similar in size on both sides of the ganglion.) Since the number and distribution of motoneuron somata was relatively constant in the two halves of each ganglion, homologies for individual or groups of neurons could be recognized. The number of axon profiles in a cross-sectional montage of the entire nerve root of the major side in a male fiddler crab was several times greater than that of the minor side in random samples which were appropriately scaled in area. In samples of equal areas the axonal density was similar on the major and minor sides, as was also the range of axon diameters; both signify no difference in size of axons between the contralateral nerve roots. Consequently enlargement of the nerve root on the major side is due to a relative increase in the number of axons. This increase is in sensory fibers since the number of motor fibers are bilaterally constant. Thus neural asymmetry in male fiddler crabs involves hypertrophy of the motoneurons and hyperplasia of the sensory neurons associated with the enlarged condition of the major cheliped.

Fast and slow motoneurons with unique forms and activity patterns in lobster claws

The form of the fast closer excitor (FCE) and the slow closer excitor (SCE) motoneurons to the closer muscle in the claw of the lobster Homarus americanus was determined by injecting cobalt chloride or Lucifer Yellow into their respective somata. Both neurons are monopolar with the single neurite rising vertically to the dorsal surface of the ganglion, then travelling along this surface to where it gives off its dendrites before entering the second nerve root as an axon. The FCE and SCE motoneurons, however, differ in their dendritic form in several respects. First, the FCE completely lacks an anterior dendritic field, which is well elaborated in the SCE. Second, the FCE has fewer large primary dendrites in its posterior field than the SCE. Third, the posterior dendritic field of the FCE is not as extensive as that of the SCE. Fourth, the axon of the FCE originates from one of the posterior primary dendrites while that of the SCE is an axial extension of its neurite. Thus the SCE has a more elaborate dendritic field than the FCE, which may account for its greater excitability. For instance, recordings from intact lobsters show that the SCE has a lower firing threshold and is active for longer periods of time and at higher frequencies than the FCE.

Interneurons in the flight system of the locust: distribution, connections, and resetting properties

The organization and functional properties of interneurons in the flight system of the locust, Locusta migratoria, were investigated by using intracellular recording and staining techniques. Interneurons were found to be distributed within the three thoracic and the first three abdominal ganglia, and they could be subdivided into three organizational categories: (1) members of one of two serially homologous groups controlling either the forewing or the hindwing, (2) unique individuals with no known homologues in other ganglia, and (3) members of a set of serial homologous in the metathoracic and first three abdominal ganglia. Interneurons in the last two categories influenced both forewing and hindwing motoneurons in a similar manner. Thus interneuronal organization is not characterized by two distinct homologous groups of interneurons for the separate control of forewing and hindwing motor activity. Flight interneurons may also form two separate functional categories: (1) those making short latency connections to motoneurons (premotor interneurons), and (2) those which reset the flight rhythm when depolarized by brief current pulses (pattern generator interneurons). None of the ten premotor interneurons we identified influenced the flight rhythm when depolarized and none of the three groups of pattern generator interneurons were found to form short latency connections with motoneurons. This separation of function may allow phase-shifts in motor output for flight control without changes in wingbeat frequency. Pattern generator interneurons influence motor output to both forewings and hindwings. Thus we conclude that the flight rhythm is generated in a distributed neuronal oscillator driving both the pairs of wings. The organization of flight interneurons is considerably more complex than predicted from existing models of the flight system, or anticipated from the relative simplicity of the motor output. Our finding of homologous sets of interneurons in the abdominal ganglia supports the notion that insect flight evoked from a behavior using appendages distributed along the thorax and the abdomen. Thus the organization of flight interneurons may reflect an interneuronal system which controlled the behavior from which flight evolved.

Modification of motor neuron size and position in the central nervous system of adult snapping shrimps

Cell bodies of claw closer motor neurons in snapping shrimp are dimorphic. Snapper claw motor neurons are larger than corresponding pincer claw motor neurons, but the relative sizes of these cells are reversed during claw transformation. An additional neuronal modification occurs early within this period, in that the pincer claw dorsal inhibitor cell body migrates within the nervous system, from a dorsal to a ventral position. These findings are evidence of rapid, reversible changes in the nervous system following the trigger for the transformation process.

Electrotonic coupling in internally perfused crayfish segmented axons

We have developed a technique for cannulation and internal perfusion of crayfish segmented lateral axons. Experiments on perfused and non-perfused axons lead to the following conclusions: 1. Internally perfused segmented axons behave very similarly to non-perfused axons. 2. The axial electrical resistance of the junctional region is almost as low as a comparable segment of axon. 3. Neither intracellular Ca2+ nor H+ is effective in disrupting the intercellular communication pathway in perfused axons. On the basis of these findings we have formulated a hypothesis for cellular control of intercellular coupling based on the existence of a soluble intermediate for Ca2+ or H+-induced uncoupling. This hypothesis is consistent with data from both internally perfused and non-perfused axons.

Transient, axotomy-induced changes in the membrane properties of crayfish central neurones

1. In crayfish, the normally passive, non-spiking somata of certain unipolar, efferent neurones became spiking within 36 hr of axotomy. 2. The changes persisted for approximately 2 weeks and then waned. The decline in excitability occurred independently of regeneration, and excitability was not restored by recutting the axon stump. 3. The neuropilar processes also became capable of supporting spikes, but synaptic transmission onto the cells and the spike threshold for orthodromic activation were unchanged, as was the gross structure of the neurone. 4. In somata which normally spike, electrogenicity was nevertheless increased, as evidenced by soma spikes that were larger, faster rising, and easier to evoke. 5. We tested for post-axotomy excitability changes in a variety of identified neurones. Every type (n = 5) of phasically active efferent we tested responded as above, as did all three phasic interneurones. One class of spontaneously active interneurones and one spontaneously active efferent did not respond to axotomy. 6. Extensive damage to afferents did not initiate changes in efferents of the same ganglion, nor did it interfere with changes induced by axotomy of the efferents. 7. Transection of the larger of the two main branches of the phasic flexor inhibitor induced soma excitability, but cutting the smaller branch did not. However, after the excitability caused by cutting the larger branch waned, transection of the smaller branch then induced excitability. 8. Neurones with longer axon stumps took longer to develop soma excitability.

Interneurons of the crayfish brain: the relationship between dendrite location and afferent input

This study was undertaken to examine the relationship between the structure and function of the descending interneurons of the crayfish brain. In particular, the dendritic fields were examined to ascertain if the location of an interneuronal dendrite in any of the six cerebral hemineuromeres (which subserve specific sensory modalities) is a necessary or sufficient condition to determine the functional and/or synaptic input to the interneuron. If a neuron projects a dendrite to a hemineuromere of the deutocerebrum or tritocerebrum, the neuron derives sensory input from the corresponding afferent root in 95% of our observations. Most of these inputs (86%) contain at the least a monosynaptic component. Conversely, if a cell derives monosynaptic input from any one of three of the four deutocerebral and tritocerebral roots tested, it has a corresponding dendrite (in 98% of our observations) in the appropriate hemineuromere. Input from the contralateral antennal nerve is an exception to this rule. The presence of a dendrite in the protocerebrum is not sufficient for predicting detectable visual input, but every instance of detectable visual input is associated with a protocerebral dendrite. Polysynaptic inputs are frequently (42%) not associated with corresponding dendrites. In neurons that were repeatedly dye filled in different animals, we observed significant variation only in the number and precise location of the smaller secondary and tertiary neurites. This variation rarely influenced the subset of sensory lobes innervated by the neuron.