published_or_final_version / Anatomy / Master / Master of Science

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No description available.

Interactions between two digenean parasites, Diplostomum spathaceum and Schistosoma mansoni , and their respective intermediate snail hosts, Lymnaea stagnalis and Biomphalaria glabrata , were investigated. D.spathaceum infections in juvenile L.stagnalis became patent 9 to 10 weeks post-infection and inhibited initiation of host oviposition. Adult L.stagnalis resisted infection with D.spathaceum . Both juvenile and adult B.glabrata were susceptible to S.mansoni and the infections become patent 5 weeks after infection. B.glabrata infected as juveniles failed to begin egg-laying whilst those infected as adults showed a dramatic reduction of oviposition 4 weeks post-infection, i.e before the onset of patency. Infection with D.spathaceum , but not S.mansoni , resulted in the appearance of three polypeptides (68.3, 40.8 &'38 34.5 kDa) in snail serum. The additional peptides were not directly related to failure of snail reproduction and are probably of parasite origin. Reproductive failure of infected snails was linked to the later stages of cercarial development by a method developed to manipulate infections using the anthelmintic, praziquantel. D.spathaceum and S.mansoni infections were suppressed in their snail hosts following oral treatment with praziquantel. Praziquantel was selective for cercariae and inhibited cercarial shedding of both digenean species for approximately 5 to 6 weeks before recovery. In mature B.glabrata , harbouring 7 week-old infections of S.mansoni , suppression of cercarial shedding was accompanied by temporary recovery of reproduction. Snail fecundity was halted once more 2 weeks prior to resumption of cercarial shedding. Reproductive recovery depended upon the state of infection and maturity of the host at infection. Failure of D.spathaceum to infect adult L.stagnalis was attributed to the more developed and therefore more competent internal defence system of adult snails. Infection of juvenile, immunologically immature, L.stagnalis with D.spathaceum resulted in several changes in the snail immune system and infection seemed to prevent normal development of several facets of snail immunity.

The aim of this thesis was to identify key neural mechanisms underlying decision making in a model invertebrate system, the pond snail Lymnaea stagnalis. Specifically, this was examined with respect to two decision making processes associated with Lymnaea's feeding behaviour; first a stimulus present/stimulus absent perceptual decision making task for selecting between appetitive and consummatory behaviours, second a behavioural choice between ingestion and egestion. A behavioural paradigm was designed in order to study stimulus present and stimulus absent decision making in Lymnaea. The switch between appetitive and consummatory behaviours was used as a read out of the decision. During stimulus absent decisions, the animal made a judgement about the absence of a sensory stimulus and entered into a period of quiescence. During stimulus present decisions the animal switched from the appetitive behaviour into the consummatory behaviour. The decision about the presence of a stimulus was reliant on the tactile cues from the potential food. Importantly the task was amenable for in vitro preparations and the identification of the decision neurons. A candidate stimulus present decision neuron, the ventral trigger neuron (vTN), was identified in the buccal ganglia. vTN received appropriate sensory input and was able to initiate fictive feeding cycles. vTN was able to initiate fictive feeding cycles via monosynaptic connections with feeding central pattern generator (CPG) interneurons. Development of an in vitro paradigm of the stimulus present and stimulus absent decision provided strong evidence that vTN was a stimulus present decision neuron. A stimulus absent decision neuron was identified as the inhibitory interneuron and member of the feeding CPG, N3t. An in vitro paradigm was used to test interactions of the stimulus present decision neuron with the stimulus absent decision neuron. Goal directed behaviours were also shown to lower the threshold of activity needed for vTN to initiate fictive consummatory behaviours, lowering the sensory threshold needed for stimulus present decisions to be made. The neural mechanism of behavioural choice between ingestion and egestion in Lymnaea was characterised using in vivo and in vitro preparations. A novel motor neuron was identified whose phase of firing activity was selectively shifted between the two behaviours. The interneuronal control of ingestion was identified and consisted of previously identified command-like neurons, whereas the interneuronal control of egestion consisted of novel interneurons located in the buccal ganglia. The two networks showed evidence of both dedicated and multifunctional interneurons. The studies presented in this thesis demonstrate that even within the same relatively simple neural network several distinct mechanisms are utilised for different types of decision. These were identified at the level of individual neurons and their synaptic connectivity. Uniquely in these studies, distinct stimulus absent and stimulus present decision neurons were identified in the perceptual decision task and it was also shown that performing the decision making task facilitates the stimulus present decision. This provides fundamental new insights into the neural mechanisms of decision making. The studies also provide a model system for comparison between a form of decision making studied extensively in mammals (perceptual decision making in a stimulus absent/stimulus present task) and a form studied primarily in invertebrates (behavioural choice between two incompatible behaviours), bridging the gap between invertebrate and vertebrate decision making studies.

Thesis--Hebrew University. / Added title page and abstract in Hebrew. "Reprint from the Bulletin of the Research Council of Israel, Section B: Zoology, vol. 9B, July 1960, no. 2-3."

Thesis (M. Sc.)--University of Hong Kong, 1975. / Typewritten.

Thesis (M.S.)--Missouri State University, 2009. / "May 2009." Includes bibliographical references (leaves 27-34). Also available online.

No description available.

A theoretical treatment has been combined with practical methods to obtain quantitative measurements from the transient responses of electrically coupled cells. Communication via electrical synapses is characterized as a low resistance pathway, whereas membrane response is represented by a resistance in parallel with a capacitance. / The validity and consequences of this model have been investigated using data from identified pairs of electrically coupled neurons in the freshwater snail, Helisoma trivolvis. An automated procedure has been developed to monitor the degree of coupling over extended periods of time and transform these measurements into equivalent electrical units. / The model has been used to: functionally assess the coupled system employing a single micropipette; provide a spatial profile of cell load; monitor induced changes in coupling; examine the role of coupling in controlling the rate of spread of excitation; and evaluate electrical changes which occur within cells during axonal injury and subsequent recovery.