On the anatomy of the central nervous system and the morphological value of the anterior end appendages of Ampharetidae, Pectinariidae and Terebellidae (Polychaeta)

Earlier papers dealing with the anatomy of the central nervous system of ampharetids, pectinariids and terebellids were studied. On this basis a re‐investigation appeared necessary: statements in the literature about the structure of the brain and the innervation of the appendages of the anterior end were incomplete and contradictory. In the present paper, the brain, the circum‐oesophageal connectives and the innervation of, inter alia, the tentacular membrane (including the dorsal ridge), the buccal tentacles, the alimentary canal, the nuchal organs and the branchiae (when present) of Amphicteis gunneri, Anobothrus gracilis, Melinna cf. cristata, Pectinaria auricoma, P. belgica, P. koreni, Petta pusilla, Pista cristata, Eupolymnia nebulosa, Thelepus cincinnatus and Polycirrus medusa are described. The results are summarized in schematic diagrams and compared with each other and with the central nervous system of other polychaetes. It is concluded that the ampharetids, the pectinariids and the terebellids bear no antennae and no palps and that their buccal tentacles belong to the alimentary canal. It is emphasized that all attempts to range their cephalic nervous system into previously proposed common and general schemes of ‘the polychaete nervous system’ seem totally fruitless.     

Morphology of the nervous system of Polychaeta (Annelida)

The article summarizes our up to date knowledge about the morphology of the annelid, especially the polychaete, central and peripheral nervous system. Since the cephalic nervous system was in the focus of controversial discussions for decades, the structure of its neuropile, associated ganglia and nerves is reviewed in detail. The enormous variation of the ventral nerve cord and peripheral nerves is presented as well as a theory how this might have evolved. A ground pattern of the polychaete nervous system is suggested, based on developmental and regeneration studies.     

The nervous system of the tube-worm Chaetopterus variopedatus (Polychaeta)

Silver impregnation of serial histological sections of the tubeworm Chaetopterus variopedatus revealed the presence of a subepidermal nervous system. The anterior nervous system is delimited by the first 11 segments and comprises (1) two dorsolateral cerebral ganglia and lateral instead of ventral nerve cords which are widely separated and thus connected by unusually long commissures, (2) a pharyngeal ganglion in the fourth segment which is connected to the cerebral ganglia by pharyngeal nerves and constitutes along with the pharyngeal plexus a stomatogastric or enteric nervous system, and (3) small, presumably segmental ganglionic swellings along the lateral nerve cords from which emerge commissures and parapodial nerves. No subesophageal ganglion or periesophageal connective could be identified. The lateral nerve cords converge toward the midline in the 12th segment to form the posterior nervous system comprising a pair of ventromedian nerve cords with their repetitive segmental ganglia from which emerge numerous short commissures and three segmental nerves coursing toward the dorsal and ventral regions of parapods and toward the neuropod. Light and electron microscopic investigations of cerebral and segmental ganglia showed an arrangement of inner neuropile and of unipolar neuron somata at the periphery. The neuropile comprises numerous neurites ranging in diameter from 0.5 to 10 μm and making polarized or symmetrical synaptic junctions with each other. The pharyngeal ganglion consists of a similar neuropile and of a large mass of cell bodies which is traversed by an elaborate network of sinuses and harbors three types of neurosecretory cells in addition to the conventional neuron somata. These findings are interpreted in the framework of the highly specialized morphological features and habits of Chaetopterus, and the welldeveloped stomatogastric system is considered to be related to control of the feeding activities.     

Structure of the planarian central nervous system (CNS) revealed by neuronal cell markers

Planarians are considered to be among the most primitive animals which developed the central nervous system (CNS). To understand the origin and evolution of the CNS, we have isolated a neural marker gene from a planarian, Dugesia japonica, and analyzed the structure of the planarian CNS by in situ hybridization. The planarian CNS is located on the ventral side of the body, and composed of a mass of cephalic ganglions in the head region and a pair of ventral nerve cords (VNC). Cephalic ganglions cluster independently from VNC, are more dorsal than VNC, and form an inverted U-shaped brain-like structure with nine branches on each outer side. Two eyes are located on the dorsal side of the 3(rd) branch and visual axons form optic chiasma on the dorsal-inside region of the inverted U-shaped brain. The 6(th)-9(th) branches cluster more closely and form auricles on the surface which may function as the sensory organ of taste. We found that the gross structure of the planarian CNS along the anterior-posterior (A-P) axis is strikingly similar to the distribution pattern of the "primary" neurons of vertebrate embryos which differentiate at the neural plate stage to provide a fundamental nervous system, although the vertebrate CNS is located on the dorsal side. These data suggest that the basic plan for the CNS development along the A-P axis might have been acquired at an early stage of evolution before conversion of the location of the CNS from the ventral to the dorsal side.     

Ultrastructural observations on feeding appendages and gills ofAlvinella pompejana (Annelida,Polychaeta)

The feeding appendages ofAlvinella pompejana obtained from a deep-sea hydrothermal vent environment are described. They are characterized by a ciliated groove, the cells of which have a very distinctive ultrastructure, by groups of bipolar receptor cells and by several kinds of gland cells. Among these, one cell type is in an upside down position suggesting a function completely different from other epidermal secretory cells. The gills differ considerably from the feeding appendages on the basis of their ultrastructure. Their epidermis is very irregular in height; basal infoldings give the blood access to a space coming very near to the external medium. The blood vascular system is open. On the other hand, the gills ofAmphicteis gunneri are not effective sites of gas exchange, since their columnar epithelium is underlain with muscle cells. The cells composing the feeding appendages and gills ofAlvinella pompejana are characterized by ultrastructurally very different mitochondria.     

Monoamines in the nervous system of the tube-wormChaetopterus variopedatus (Polychaeta): Biochemical detection and serotonin immunoreactivity

Summary The presence and distribution of biogenic monoamines in the tube-wormChaetopterus variopedatus were investigated by a radioenzymatic method and HPLC with electrochemical detection, and the cellular localization of serotonin by peroxidase-antiperoxidase (PAP) immunohistochemistry with an antibody against serotonin-formaldehyde-protein conjugate. Dopamine, norepinephrine, epinephrine, serotonin (5-HT) and some of their metabolites were detectable, dopamine and norepinephrine being present in substantially larger amounts than 5-HT and epinephrine. With few exceptions, the largest amounts of amines were localized in the most nerve-rich tissues such as tentacles, and those containing cerebral ganglia and the ventral nerve cord. Serotonin-immunoreactive unipolar neurons were widely distributed in the dorso-lateral cerebral ganglia, the neurosecretory pharyngeal ganglion and the segmental ganglia of the anterior (dorsolateral) and posterior (medioventral) nerve cords. Some nerve-fiber tracts stained in the cerebral ganglia, but the neuropiles of segmental ganglia were the most intensely reactive CNS structures. Numerous reactive fibers were also present in connectives, commissures and segmental nerves. All peripheral sensory structures included serotonin-immunoreactive cells and neurites, especially the parapodial cirri and the bristle receptors of the setae. Trunk and parapodial muscles contained reactive varicose fibers and neuronal somata. These results suggest that monoamines are abundant and widespread in these worms and that 5-HT appears to have a key sensory role.     

Comparative internal structure of dorsal lips and radiolar appendages in sabellidae (Polychaeta) and phylogenetic implications

Fan worms (Sabellidae) possess paired modified prostomial structures at the base of the radiolar crown, dorso‐lateral to the mouth, called dorsal lips. The dorsal lips are involved in the sorting of particles collected by the radiolar crown. The range of variation in the morphology of dorsal lips is extensive, and probably this is not only due to adaptations to different environments and feeding preferences but also due to phylogenetic constraints. In this study, we describe and compare the morphology of dorsal lips in a range of sabellid taxa based on histological cross‐sections of these structures, and compare our data and terminology with those of previous studies. Dorsal lips are maintained erect in most taxa by a modified radiole fused to them known as dorsal radiolar appendage. We suggest that dorsal radiolar appendages with an internal supporting axis (cellular or acellular) and probably also the ventral lips are synapomorphies of the family. J. Morphol., 2011. © 2010 Wiley‐Liss, Inc.     

Structure of the nervous system in Tubiluchus troglodytes (Priapulida)

Abstract. The nervous system of the meiobenthic priapulid species Tubiluchus troglodytes is described by immunohistochemistry and confocal laser scanning microscopy. The brain is circumpharyngeal, consisting of a central ring of neuropil and both anterior and posterior somata. From the brain emerges a ventral nerve cord, which shows ganglion-like swellings in the neck and caudal region. The introvert includes longitudinal neurite bundles running below and between the rows of scalids, with a small cluster of sensory cells under each scalid. In the body wall of the neck and trunk region, longitudinal and circular neurite bundles are present in an orthogonal pattern. The tail is innervated from the caudal swelling of the ventral nerve cord; it also includes longitudinal and circular bundles in an orthogonal pattern. The pharynx has a reticulated system of neurite bundles running between the pharyngeal teeth and fimbrillae. Below each tooth and fimbrilus is a ganglion-like cluster of somata. The intestine is surrounded by a nerve net. The data on the nervous system are compared within other priapulids and with other species of Scalidophora (Kinorhyncha and Loricifera).     

Autoradiographic detection of indolamine and catecholamine neurons in the nervous system of Owenia fusiformis (Polychaeta,Annelida)

Summary Following immobilization stress the supraoptic nucleus exhibits an increased number of coarse, heavily immunostained fibers in the basal glia labyrinth. Ultrastructural immunocytochemistry demonstrates a labeling of the rough endoplasmic reticulum in the neurosecretory perikarya and granule-free, immunoreactive material in the axons adjacent to the basal glia labyrinth. Furthermore, a labeling of the intercellular clefts of the neuropil is demonstrable in the supraoptic nucleus. These results lead to the hypothesis that 1) vasopressin is synthesized and released in two forms, in a granule-bound form and in a granule-free, probably more soluble form, and that 2) the latter might be released already in the nuclear area into the intercellular clefts from where it may reach its target cells via the cerebrospinal fluid of the subarachnoid space.The excellent technical assistance of Mrs. Helga Prien is thankfully acknowledgedSupported by the Deutsche Forschungsgemeinschaft (Grant Nr. Kr 569/2) and the Stiftung Volkswagenwerk     

Immunohistochemical analysis of nervous system regeneration in chimeric individuals of Dorvillea bermudensis (Polychaeta, Dorvilleidae)

In regeneration experiments, 0.5% of the two- or five-segmented fragments of the polychaete Dorvillea bermudensis were found unexpectedly transplanted: two fragments of each that were lying close together during the initial period, fused and regenerated a chimeric individual. Of the three theoretical possibilities (i.e. fusion of (i). two posterior ends; (ii). one anterior and one posterior end; (iii). or two anterior ends) only the last two were realized. The similarly oriented fragments regenerated a normal animal while anterior-anterior fused ones produced two heads or a double head. Whether the ventral cords of the fragments are located vis-à-vis or adjacent, influences the course of regeneration as well. Immunohistochemical methods (anti-acetylated alpha-tubulin) in conjunction with confocal laser scanning microscopy were used to investigate the wiring pattern of the nervous systems of the grafts. In all cases, at least two supraesophageal ganglia were formed and palps, antennae and nuchal organs were innervated by the correct nerves but, in special cases, were innervated vice versa from the other brain. From these results it can be concluded that fusion of a regenerating connective with another connective results in formation of a new brain, irrespective of whether it belongs to the same nerve cord or not.     

Emergence in the central nervous system

“Emergence” is an idea that has received much attention in consciousness literature, but it is difficult to find characterizations of that concept which are both specific and useful. I will precisely define and characterize a type of epistemic (“weak”) emergence and show that it is a property of some neural circuits throughout the CNS, on micro-, meso- and macroscopic levels. I will argue that possession of this property can result in profoundly altered neural dynamics on multiple levels in cortex and other systems. I will first describe emergent neural entities (ENEs) abstractly. I will then show how ENEs function specifically and concretely, and demonstrate some implications of this type of emergence for the CNS.     

Endocannabinoids in the central nervous system

The major psychoactive component of cannabis derivatives, delta9-THC, activates two G-protein coupled receptors: CB1 and CB2. Soon after the discovery of these receptors, their endogenous ligands were identified: lipid metabolites of arachidonic acid, named endocannabinoids. The two major main and most studied endocannabinoids are anandamide and 2-arachidonyl-glycerol. The CB1 receptor is massively expressed through-out the central nervous system whereas CB2 expression seems restricted to immune cells. Following endocannabinoid binding, CB1 receptors modulate second messenger cascades (inhibition of adenylate cyclase, activation of mitogen-activated protein kinases and of focal-adhesion kinases) as well as ionic conductances (inhibition of voltage-dependent calcium channels, activation of several potassium channels). Endocannabinoids transiently silence synapses by decreasing neurotransmitter release, play major parts in various forms of synaptic plasticity because of their ability to behave as retrograde messengers and activate non-cannabinoid receptors (such as vanilloid receptor type-1), illustrating the complexity of the endocannabinoid system. The diverse cellular targets of endocannabinoids are at the origin of the promising therapeutic potentials of the endocannabinoid system.     

Insulin and the central nervous system

Data from literature concerning the neurobiological, electrical and metabolic effects of insulin are reviewed. Emphasis is laid on insulin distribution in the CNS, on distribution and localization of the insulin brain receptors, on insulin transport through the hemato-encephalic barrier. Data concerning insulin effect on the electrical activity of various CNS neurons, particularly, on those of the feeding and satiety centres. The effects of insulin on the brain metabolism are discussed. Insulin shares many properties with the nerve growth factor and may be considered as specific neurotransmitter and neuromodulator.