Physiologie der Gangliensysteme der Wirbellosen

Innervation of ascidian siphons and their responses to stimulation

The distribution of sensory cells and nerves was studied in the siphons of Corella inflata Huntsman, 1912 and Corella willmeriana Herdman, 1898 by immunohistology and electron microscopy. Each siphon has about 8000 primary sensory neurons. A coronal organ of the compound type is present on the oral tentacles. Convergence in the afferent pathway is estimated at >10:1. A new category of cells associated with the velar sphincter muscle is described at the tentacle bases. Responses to stimulation were recorded using flow meters. Both siphons are sensitive to touch and near-field vibrations. Removal of the oral tentacles did not diminish vibration sensitivity. Gentle stimulation of the oral siphon evokes crossed responses in which the atrial siphon closes and the velar sphincter contracts. Stronger stimulation produces squirts with closure of both siphons and branchial ciliary arrest. Experiments with polystyrene beads show that the oral tentacles are sensitive to contact with inflowing particles. Beads of 500–600 μm diameter evoked rejection responses 88% of the time, 355–425 μm beads 61%, and beads <125 μm less than 8%. These responses, attributed to the coronal organ, were lost after amputation of the tentacles. Electrophysiology confirmed that crossed responses and squirting are centrally mediated reflexes, but local conduction pathways also exist and survive deganglionation.

The nervous system in adult tunicates: current research directions

This review covers 25 years of progress on structural, functional, and developmental neurobiology of adult tunicates. The focus is on ascidians rather than pelagic species. The ascidian brain and peripheral nervous system are considered from the point of view of ultrastructure, neurotransmitters, regulatory peptides, and electrical activity. Sensory reception and effector control are stressed. Discussion of the dorsal strand plexus centres on its relationship with photoreceptors, the presence in it of gonadotropin-releasing hormone and its role in reproductive control. In addition to hydrodynamic sense organs based on primary sensory neurons (cupular organs), ascidians are now known to have coronal sense organs based on axonless hair cells resembling those of the vertebrate acustico-lateralis system. The peripheral nervous system is remarkable in that the motor neuron terminals are apparently interconnected synaptically, providing the equivalent of a nerve net. Development of the neural complex in ascidians is reviewed, highlighting recent embryological and molecular evidence for stomodeal, neurohypophyseal, and atrial placodes. The nervous system forms similarly during embryogenesis in the oozooid and blastogenesis in colonial forms. The regeneration of the brain in Ciona intestinalis (L., 1767) is discussed in relation to normal neurogenesis. Finally, the viviparous development of salps is considered, where recent work traces the early development of the brain, outgrowth of nerve roots, and the targetting of motor nerves to the appropriate muscles.

Conduction and coordination in deganglionated ascidians

The behaviour of Chelyosoma productum and Corella inflata (Ascidiacea) was studied in normal and deganglionated animals. Chelyosoma productum lived for over a year after deganglionation and the ganglion did not regenerate. Electrophysiological recordings were made from semi-intact preparations. Responses to stimulation and spontaneous activity continued to be transmitted through the body wall and branchial sac after deganglionation. Spread was slow, decremental, and facilitative. Treatment with >10 µg·mL-1 d-tubocurarine abolished all responses, indicating that nerves mediate conduction of excitation after deganglionation. Histological study using cholinesterase histochemistry and immunolabelling with antisera against tubulin and gonadotropin-releasing hormone showed no evidence of a peri pheral nerve net in regions showing conduction, contrary to previous claims. The cell bodies of the motor neurones were found to lie entirely within the ganglion or its major roots. Their terminal branches intermingled to form netlike arrays. Sensory neurons were identified with cell bodies in the periphery, in both the body wall and the branchial sac. Their processes also intermingled in netlike arrays before entering nerves going to the ganglion. It is concluded that the "residual" innervation that survives deganglionation is composed of either interconnected motor nerve terminals, interconnected sensory neurites, or some combination of the two. In re-inventing the nerve net, ascidians show convergent evolution with sea anemones, possibly as an adaptation to a sessile existence.