Neurobiology of Stomotoca. I. Action systems.

The layout of nerves, muscles, and conducting epithelia is described for the simple hydrozoan medusa Stomotoca. Comparisons are drawn with Sarsia and other recently studied forms. The major action systems are those responsible for swimming, crumpling (protective involution), tentacle posture, pointing (unilateral reciprocal flexions of the manubrium and margin), and visceral movements (barely mentioned). Crumpling is a simple summating response in this species. Crumpling and pointing are considered to use the same effectors but different conduction pathways. New histological results include the demonstration of a nerve plexus running through the endodermal canal system and a nerve plexus in the ectoderm encircling the peduncle. Special attention is given to the distribution of synapses and gap junctions, as possible trasmission pathways in behavioral responses. Some details are included on organization within the marginal nerve rings.     

The organization of the nervous system in Plathelminthes. The neuropeptide F-immunoreactive pattern in Catenulida,Macrostomida, Proseriata

The organization of the nervous system of Archilopsis unipunctata Promonotus schultzei and Paramonotus hamatus (Monocelididae, Proseriata) and Stenostomum leucops (Catenulida) and Microstomum lineare (Macrostomida) was studied by immunocytochemistry, using antibodies to the authentic flatworm neuropeptide F (NPF) (Moniezia expansa). The organization of the nervous system of the Monocelididae was compared to that of the nervous system of Bothriomolus balticus (Otoplanidae), a previously studied species of another family of the Proseriata. The results show that the main nerve cords (MCs), independent of lateral or ventral position in the Monocelididae and the Otoplanidae, correspond to each other. The study also confirms the status of the lateral cords as main cords (MCs) in S. leucops and M. lineare. Common for MCs in the members of the investigated taxa are the following features: MCs consist of many fibres, originate from the brain and are adjoined to 5-HT-positive neurons. In Monocelididae and Otoplanidae, the MCs additionally have the same type of contact to the pharyngeal nervous system. Also common for both proseriate families is the organization of the two lateral nerve cords, with weaker connections to the brain, and the pair of dorsal cords running above the brain. The organization of the minor cords differs. The Monocelididae have a pair of thin ventral cords forming a mirror image of the dorsal pair. Furthermore, an unpaired ventral medial cord connecting medial commissural cells was observed in P. schultzei. Marginal nerve cords, observed in Otoplanidae, are absent in Monocelididae. All minor nerve cords are closely connected to the peripheral nerve plexus. The postulated trends of condensation of plexal fibres to cords and/or the flexibility of the peripheral nerve plexus are discussed. In addition, the immunoreactivity (IR) pattern of NPF was compared to the IR patterns of the neuropeptide RFamide and the indoleamine, 5-HT (serotonin). Significant differences between the distribution of IR to NPF and to 5-HT occur. 5-HT-IR dominates in the submuscular and subepidermal plexuses. In the stomatogastric plexus of M. lineare, only peptidergic IR is observed in the intestinal nerve net. The distribution of NPF-IR in fibres and cells of the intestinal wall in M. lineare indicates a regulatory function for this peptide in the gut, while a relationship with ciliary and muscular locomotion is suggested for the 5-HT-IR occurring in the subepidermal and submuscular nerve, plexuses. In M. lineare, the study revealed an NPF- and RFamide-positive cell pair, marking the finished development of new zooids. This finding indicates that constancy of these cells is maintained in this asexually reproducing and regenerating species.     

Primitive nervous systems: Electrical activity in ventral nerve cords of the flatworm,Notoplana acticola

Electrical activity evoked in the major cords of the ventral submuscular nerve plexus were measured. Recordings and stimulation utilized suction electrodes attached directly to exposed nerve cords. Four categories have been recorded: (a) short latency spikes which have relatively high thresholds and appear to be single units; (b) short duration fast compound spikes that are made up of a large units; (c) long duration compound potentials that are made up from a large number of smaller units, and (d) small amplitude potentials with long latencies and a characteristic shape. These can be conducted diffesely through the nerve plexus. The first two categories of spikes are called “fast” potentials because of their characteristic rise and fall times and the last categories are known as “diffuse” potentials. The spikelike fast potentials were only recorded from the main trunks (nerves VI), while diffuse potentials could also be recorded from side branches of these nerves. The diffuse potential appears to be concluded throughout the plexus but preferential conducting pathways occur lesions. Both diffuse and fast potentials show facilitation of response to repeated stimulation. Facilitation can be demonstrated in the presence of high Mg²⁺ concentrations. Conductance of the diffuse potential also occurs in the presence of high ambient Mg²⁺. In Ca²⁺ free medium containing 1-mM EGTA one can also observe facilitatory events. The possibility of Mg²⁺-insensitive synapses is discussed.     

The catecholaminergic nerve plexus of Holothuroidea

Catecholamines have been extensively reported to be present in most animal groups, including members of Echinodermata. In this study, we investigated the presence and distribution of catecholaminergic nerves in two members of the Holothuroidea, Holothuria glaberrima (Selenka, 1867) (Aspidochirotida, Holothuroidea) and Holothuria mexicana (Ludwig, 1875) (Aspidochirotida, Holothuroidea), by using induced fluorescence for catecholamines on tissue sections and immunohistochemistry with an antibody that recognizes tyrosine hydroxylase. The presence of a catecholaminergic nerve plexus similar in distribution and extension to those previously reported in other members of Echinodermata was observed. This plexus, composed of cells and fibers, is found in the ectoneural component of the echinoderm nervous system and is continuous with the circumoral nerve ring and the radial nerves, tentacular nerves, and esophageal plexus. In addition, fluorescent nerves in the tube feet are continuous with the catecholaminergic components of the radial nerve cords. This is the first comprehensive report on the presence and distribution of catecholamines in the nervous system of Holothuroidea. The continuity and distribution of the catecholaminergic plexus strengthen the notion that the catecholaminergic cells are interneurons, since these do not form part of the known sensory or motor circuits and the fluorescence is confined to organized nervous tissue.     

Early Forms of Evolution of the Basiepidermal Nerve Plexus of Bilateria as a Possible Evidence for Primary Diversity of Its Initial State

To find out whether there is a real parallelism, as it was understood by Academician A.A. Zavarzin, in the structure of the basiepidermal nerve plexus in primitive representatives of such distant groups of animals, as polychaetes and phoronids, the experimental material obtained earlier on neuronal relations in Myriochele oculata (Polychaeta, Oweniidae) and Phoronopsis harmeri (Tentaculata, Phoronoidea) was analyzed. The similarity of the basiepidermal plexus in the representatives of polychaetes and phoronids has been shown to be merely apparent and has a convergent character, i.e., it does not belong to the type of systemic parallelisms. The structural differences revealed between these two considered evolutionary initial types of nerve plexuses are supposed to be a cause of different directions of differentiation of the Bilateria nervous system. The nerve plexus structure close, by its general organization, to that of oweniids can have many common features with the basiepidermal plexus of especially primitive turbellarias and xenoturbellids, whose neuronal relations are so far non-studied. At the same time, a similar nerve plexus could originate formation of the nervous system of some archiannelids (of the nerillids type), typical polychaetes, and primitive oligochaetes. The neuronal relations similar to those of phoronids probably had more chances to progress in evolution to the direction that is principally close to the neuronal relations in pogonophoras and typical Deuterostomata (including the lower chordates).     

Ultrastructure of sea urchin tube feet

Summary An analysis of the ultrastructure of the tube feet of three species of sea urchins (Strongylocentrotus franciscanus, Arbacia lixula and Echinus esculentus) revealed that the smooth muscle, although known to be cholinoceptive, receives no motor innervation.The muscle fibers are attached to a double layer of circular and longitudinal connective tissue which surrounds the muscle layer and contains numerous bundles of collagen fibers. On its outside, the connective tissue cylinder is invested by a basal lamina of the outer epithelium to which numerous nerve terminals are attached. These are part of a nerve plexus which surrounds the connective tissue cylinder. The plexus itself is an extension of a longitudinal nerve that extends the whole length of the tube foot. It is composed of axons, but nerve cell bodies and synapses are conspicuously lacking, suggesting that the axons and terminals derive from cells of the radial nerve. Processes of the epithelial cells penetrate the nerve plexus and attach to the basal lamina. There is no evidence that the epithelial cells function as sensory cells.On the basis of supporting evidence it is suggested that the transmitter released by the nerve terminals diffuses to the muscle cells over a distance of several microns and in doing so affects the mechanical properties of the connective tissue.Supported by the Sonderforschungsbereich 138 of the Deutsche Forschungsgemeinschaft     

Neurobiology of Stomotoca. II. Pacemakers and conduction pathways

Evidence is presented for separate conduction pathways for swimming and for tentacle coordination in the marginal nerves of the jellyfish Stomotoca. The effector muscles are fired through junctions sensitive to excess Mg⁺⁺, probably represented by the neuromuscular synapses observed by electron microscopy. The swimming effector (striated muscle) fires one-to-one with nerve input signals and myoid conduction occurs. Tentacle responses (smooth muscle contractions) involve facilitation, presumably at the neuro-effector junction; responses are graded and nonpropagating. Electrical correlates of two further conducting systems using the marginal nerves have been recorded. Their functions are unknown. One, the bridge system, extends up the four radii and encircles the peduncle; the other (ring system) is confined to the margin. A fifth conducting system is inferred in the case of the pointing response and its distribution is plotted. Signals have not been obtained from it. Pointing is accompanied by a burst of muscle potentials in the radial smooth muscles and is exhibited after a lengthy latency, indicating a local pacemaker. A sixth conducting pathway is the epithelial system, which mediates crumpling, a response involving the radial muscles without pacemaker intervention. Characteristic conduction velocities and wave forms are noted for the first four systems and for epithelial pulses. All systems, except perhaps the pointing conduction system, through-conduct under excess Mg⁺⁺. Spontaneous activity patterns are described for the swimming, tentacle pulse, and ring systems. Abrupt increases in light intensity inhibit spontaneous activity, sudden decreases augmenting it. In the absence of specialized photoreceptors, light is presumed to act directly on central neurons. Epithelial pulses inhibit swimming, apparently by blocking the generation or conduction of the primary nervous events. This observation, taken in conjunction with evidence of feedback inhibition of the primary swimming system by the cells it fires, is discussed in relation to possible mechanisms whereby the output of nerve cells might be altered by activity in the excitable epithelial cells which envelop them.     

Distribution of Nerve Elements Showing FMRFamide-like Immunoreactivity in Hydromedusae

Immunofluorescence tests with antisera to various peptides reveal the presence of a FMRFamide-like peptide in neurons of hydromedusae belonging to three orders. An avian pancreatic polypeptide-like peptide may also be present in certain neurons. Distribution of FMRFamide-like immunoreactivity (Fa-IR) has been studied in Proboscidactyla (O. Limnomedusae), Phialidium and Aequorea (O. Leptomedusae) and Aglantha (O. Trachymedusae). These findings are compared with results obtained with Polyorchis (O. Anthomedusae) by Grimmelikhuijzen and Spencer [J. comp. Neurol. 230 , 361 (1984)]. All species show Fa-IR neurons in the tentacles linked by interconnecting neurites running in the outer marginal nerve ring. A Fa-IR plexus is present in the manubrium and, except in Aglantha, this system merges with a plexus associated with the subumbrellar radial muscles. As in Polyorchis, the swimming motor neurons are unstained. In contrast to this species, only a small percentage of the neurites composing the nerve rings are stained. The giant axons of Aglantha show no Fa-IR. The cnidothylaces of Proboscidactyla contain neurons reacting with antisera to FMRFamide and APP. Present evidence suggests that in hydromedusae Fa-IR is confined to distinct subsets of neurons. These appear to be either sensory units or units supplying smooth muscles, but they are not involved in the innervation of striated muscles.     

The Monoaminergic Paraventricular Organ in the Teleost Ictalurus nebulosus LeSueur,with Special Reference to Its Vascularization

Specific, fluorescent, subependymal perikarya were found in the pars anterior of the paraventricular organ (PVOpa), in the nucleus recessus lateralis (NRL) and in the nucleus recessus posterioris (NRP). No fluorescent perikarya were present in the nucleus lateralis tuberis (NLT). Fluorescent nerve tracts connect the PVOpa and the NRL with the NRP, and interconnect the paired NRP. The nucleus preopticus (NPO) and the NLT receive a large input of aminergic nerve fibers. The monoaminergic nuclei are well vascularized, and their vascular plexes seem to be connected. A capillary plexus is situated dorsal to the NRP and exhibits no contact with the pituitary. It is surrounded by the prominent fluorescent tracts connecting the aminergic nuclei.     

Nerves in the endodermal canals of hydromedusae and their role in swimming inhibition

N eoturris breviconis (Anthomedusae) has a nerve plexus in the walls of its endodermal canals. The plexus is distinct from the ectodermal nerve plexuses supplying the radial and circular muscles in the ectoderm and no connections have been observed between them. Stimulation of the endodermal plexus evokes electrical events recorded extracellularly as “E” potentials. These propagate through all areas where the plexus has been shown by immunohistology to exist and nowhere else. When Neoturris is ingesting food, trains of “E” potentials propagate down the radial canals to the margin and cause inhibition of swimming. This response is distinct from the inhibition of swimming associated with contractions of the radial muscles but both may play a part in feeding and involve chemoreceptors. Preliminary observations suggest that the “E” system occurs in other medusae including Aglantha digitale (Trachymedusae) where the conduction pathway was previously thought to be an excitable epithelium.     

Immunohistochemistry and nerve lesion experiments on the methionine-enkephalin immunopositive neurons in the small intestine of the bullfrog (Rana catesbeiana)

Summary Nerve elements in the small intestine of the bullfrog. Rana catesbeiana, were studied by immunohistochemistry with anti-methionine enkephalin antisera and by nerve lesion experiments, using laser irradiation. Methionine-enkephalin immunopositive nerve fibers occur in the myenteric plexus, circular muscle layer, submucosa, and mucosa. Immunopositive nerve cell bodies in the myenteric plexus have dendrite-like and a long axon-like processes. In the froglet (3 months after metamorphosis), these axon-like processes lead posteriorly in the nerve strand of the myenteric plexus. Some bifurcate, one branch continuing posteriorly, the other doubling back to lead anteriorly; both form terminal varicose fibers in the circular muscle layer. Nerve lesion experiments, in the adult bullfrog, resulted in accumulations of methionine-enkephalin immunoreactivity at the oral and hinder edges of the laser-irradiated necrotic area; there were sprouting and nonsprouting immunopositive stumps. It is suggested that bidirectional flow of methionine-enkephalin in the myenteric plexus is mediated via the anterior and posterior branches of the axon-like process. The difference in sprouting behavior of immunopositive nerve fiber stumps, after nerve lesion, is discussed with reference to regional differences of the axon-like process.     

VIP-, Bombesin- and Neurotensin-Like Immunoreactivity in Neurons of the Gut of the Holostean Fish,Lepisosteus platyrhincus

VIP-like immunoreactivity was found in nerve fibres in all layers of the gut wall in both stomach and intestine, and was abundant in the myenteric and submucous plexuses. A few fibres were associated with blood vessels. Nerve cells showing VIP-like immunoreactivity were found in the myenteric plexus. Neurotensin-like immunoreactivity was found in nerve cells and numerous nerve fibres in the myenteric plexus of both stomach and intestine and in nerve fibres of the circular muscle layer, while bombesin-like immunoreactivity was confined to a low number of nerve fibres in the myenteric plexus of the stomach. The results indicate that a VIP-like, a neurotensin-like and a bombesin-like peptide are present in neurons of the gut of Lepisosteus.     

Postembryonic development of leucokinin I-immunoreactive neurons innervating a neurohemal organ in the turnip moth Agrotis segetum

Summary In the abdominal ganglia of the turnip moth Agrotis segetum, an antibody against the cockroach neuropeptide leucokinin I recognizes neurons with varicose fibers and terminals innervating the perisympathetic neurohemal organs. In the larva, the abdominal perisympathetic organs consist of a segmental series of discrete neurohemal swellings on the dorsal unpaired nerve and the transverse nerves originating at its bifurcation. These neurohemal structures are innervated by varicose terminals of leucokinin I-immunoreactive (LKIR) fibers originating from neuronal cell bodies located in the preceding segment. In the adult, the abdominal segmental neurohemal units are more or less fused into a plexus that extends over almost the whole abdominal nerve cord. The adult plexus consists of peripheral nerve branches and superficial nerve fibers beneath the basal lamina of the neural sheath of the nerve cord. During metamorphosis, the LKIR fibers closely follow the restructuration of the perisympathetic organs. In both larvae and adults the LKIR fibers in the neurohemal structures originate from the same cell bodies, which are distributed as ventrolateral bilateral pairs in all abdominal ganglia. The transformation of the series of separated and relatively simple larval neurohemal organs into the larger, continuous and more complex adult neurohemal areas occurs during the first of the two weeks of pupal life. The efferent abdominal LKIR neurons of the moth Agrotis segetum thus belong to the class of larval neurons which persist into adult life with substantial peripheral reorganization occurring during metamorphosis.     

Neurosecretory system of the American dog tick,Dermacentor variabilis (Acari: Ixodidae). II. Distribution of secretory cell types,axonal pathways and putative neurohemal-neuroendocrine associations; comparative histological and anatomical implications

Histological observations using specialized techniques reveal neurosecretory cells in 18 centers throughout the rind (cortex) of the central nerve mass or synganglion of Dermacentor variabilis. Many cells contribute to complicated networks of neurosecretory pathways and tracts in pre- and post-esophageal portions of the synganglion. The four types of neurohemal-neuroendocrine associations found in Dermacentor resemble structures found in soft ticks (Argasidae) and in other Arachnida, but are more diverse than those described from any other single species. Neurosecretory terminals are distributed diffusely and in two concentrated associations within the perineurium of the synganglion and major peripheral nerves. Terminals are also distributed in the perineurial layers of lateral segmental organs which lie in the general hemocoel at the level of the pedal nerves. A retrocerebral organ complex surrounds the esophagus at its junction with the midgut. The complex includes dorsal and ventro-lateral lobes (containing neurosecretory terminals and intrinsic secretory cells) and the proventricular (neurohemal) plexus. This plexus seems to be a modified (concentrated) cardioglial association. Cardioglial associations are also formed by the neurosecretory innervation of vascular walls of the dorsal aorta and circulatory sinuses which envelope the synganglion and major peripheral nerves. Inferential considerations of neurosecretory and endocrine interactions in the Acari are based on these anatomical and histological data which also provide the basis for evolutionary considerations of anatomical relationships and specializations in the neurosecretory systems of other Arachnida.     

Calbindin neurons of the guinea-pig small intestine: quantitative analysis of their numbers and projections

Summary The distribution of nerve cells with immunoreactivity for the calcium-binding protein, calbindin, has been studied in the small intestine of the guinea-pig, and the projections of these neurons have been analysed by tracing their processes and by examining the consequences of nerve lesions. The immunoreactive neurons were numerous in the myenteric ganglia; there were 3500±100 reactive nerve cells per cm² of undistended intestine, which is 30% of all nerve cells. In contrast, reactive nerve cells were extremely rare in submucous ganglia. The myenteric nerve cells were oval in outline and gave rise to several long processes; this morphology corresponds to Dogiel's type-II classification. Processes from the cell bodies were traced through the circular muscle in perforating nerve fibre bundles. Other processes ran circumferentially in the myenteric plexus. Removal of the myenteric plexus, allowing time for subsequent fibre degeneration, showed that reactive nerve fibres in the submucous ganglia and mucosa came from the myenteric cell bodies. Operations to sever longitudinal or circumferential pathways in the myenteric plexus indicated that most reactive nerve terminals in myenteric ganglia arise from myenteric cell bodies whose processes run circumferentially for 1.5 mm, on average. It is deduced that the calbindin-reactive neurons are multipolar sensory neurons, with the sensitive processes in the mucosa and with other processes innervating neurons of the myenteric plexus.     

Antisera to the sequence Arg-Phe-amide visualize neuronal centralization in hydroid polyps

Summary Antisera to the sequence Arg-Phe-amide (RF-amide) have a high affinity to the nervous system of fixed hydroid polyps. Whole-mount incubations of several Hydra species with RFamide antisera visualize the three-dimensional structure of an ectodermal nervous system in the hypostome, tentacles, gastric region and peduncle. In the hypostome of Hydra attenuata a ganglion-like structure occurs, consisting of numerous sensory cells located in a region around the mouth opening and a dense plexus of processes which project mostly radially towards the bases of the tentacles. In Hydra oligactis an ectodermal nerve ring was observed lying at the border of hypostome and tentacle bases. This nerve ring consists of a few large ganglion cells with thick processes forming a circle around the hypostome. This is the first direct demonstration of a nerve ring in a hydroid polyp.Incubation of Hydractinia echinata gastrozooids with RFamide antisera visualizes an extremly dense plexus of neuronal processes in body and head regions. A ring of sensory cells around the mouth opening is the first group of neurons to show RFamide immunoreactivity during the development of a primary polyp. In gonozooids the oocytes and spermatophores are covered with strongly immunoreactive neurons.All examples of whole-mount incubations with RF-amide antisera clearly show that hydroid polyps have by no means a diffuse nerve net, as is often believed, and that neuronal centralization and plexus formation are common in these animals. The examples also show that treatment of intact fixed animals with RFamide antisera is a useful technique to study the anatomy or development of a principal portion of the hydroid nervous system.     

An ultrastructural study of the polyp and strobila of Atorella japonica (Cnidaria,Coronatae) with respect to muscles and nerves

Atorella japonica were observed by TEM to examine the nerve plexus in the capitulum of the polyp and the cross-striated muscle cells of the strobila. The nerve plexus included a number of neuromuscular junctions and many interneural synapses. Neuromuscular junctions contained two types of synaptic vesicle: clear and small (ca 75 nm diam.), and dense cored and large (ca 120 nm diam.). The first type of vesicle always appeared near the presynaptic membrane and the second type was distributed behind the former. In interneural synapses, two types of vesicle which were similar to neuromuscular synaptic vesicles were recognized. They were distributed in a pattern similar to that of the neuromuscular synaptic vesicles, but these vesicles were found on both sides of the two synaptic membranes.     

The fine structure of the buccal tentacles of Holothuria forskali (Echinodermata,Holothuroidea)

Summary Ultrastructural study of the buccal tentacles of Holothuria forskali revealed that each tentacle bears numerous apical papillae. Each papilla consists of several differentiated sensory buds.The epidermis of the buds is composed of three cell types, i.e. mucus cells, ciliated cells, and glandular vesicular cells (GV cells). The GV cells have apical microvilli; they contain bundles of cross striated fibrillae associated with microtubules. Ciliated cells have a short non-motile cilium. Bud epidermal cells intimately contact an epineural nervous plate which is located slightly above the basement membrane of the epidermis. The epineural plate of each bud connects with the hyponeural nerve plexus of the tentacle. This nerve plexus consists of an axonic meshwork surrounded in places by sheath cells. The buccal tentacles have well-developed mesothelial muscles. Direct innervation of these muscles by the hyponeural nerve plexus was not seen.It is suggested that the buccal tentacles of H. forskali are sensory organs. They would recognize the organically richest areas of the sediment surface through the chemosensitive abilities of their apical buds. Tentacles presumably trap particles by wedging them between their buds and papillae.