Cellular Neuronal Control of Molluscan Heart

Bivalve and gastropod molluscs undergo large changes in externalenvironmental conditions, as well as in internal state. Cardiacresponses to these changing conditions have been recorded ina variety of species. There is a general tendency for heartrate, and presumably cardiac output, to increase in responseto situations that would increase the load on respiratory andexcretory systems. Changes in molluscan heart function in manycases appear not to be mediated directly by cardiac nerves,but rather by such indirect mechanisms as changes in blood constituentsor mechanical, hemodynamic effects on heart muscle. Three typesof cardiac response in Aplysia have been shown to be mediated,at least in part, by the heart regulator nerves. The neural circuits that regulate heart rate in Aplysia andin Helix have been partially described in cellular detail. InMercenaria, Aplysia and other molluscan species there is evidencethat cyclic adenosine monophosphate has a role in mediatingthe excitatory effects of serotonin on heart muscle. There appearsto be, in fact, a general tendency in the Aplysia nervous systemfor neurons that exert tonic, modulatory effects within neuralnetworks that control a variety of behaviors to use serotoninfor a transmitter. In each case there is evidence to suggestthat changes in cyclic adenosine monophosphate levels may mediatesome of the modulatory effects of serotonin.     

Effects of chronic elevated ozone concentration on antioxidant capacity,photosynthesis and seed yield of 10 soybean cultivars

Crops losses to tropospheric ozone (O₃) in the United States are estimated to cost $1–3 billion annually. This challenge is expected to increase as O₃ concentrations ([O₃]) rise over the next half century. This study tested the hypothesis that there is cultivar variation in the antioxidant, photosynthetic and yield response of soybean to growth at elevated [O₃]. Ten cultivars of soybean were grown at elevated [O₃] from germination through maturity at the Soybean Free Air Concentration Enrichment facility in 2007 and six were grown in 2008. Photosynthetic gas exchange, leaf area index, chlorophyll content, fluorescence and antioxidant capacity were monitored during the growing seasons in order to determine if changes in these parameters could be used to predict the sensitivity of seed yield to elevated [O₃]. Doubling background [O₃] decreased soybean yields by 17%, but the variation in response among cultivars and years ranged from 8 to 37%. Chlorophyll content and photosynthetic parameters were positively correlated with seed yield, while antioxidant capacity was negatively correlated with photosynthesis and seed yield, suggesting a trade‐off between antioxidant metabolism and carbon gain. Exposure response curves indicate that there has not been a significant improvement in soybean tolerance to [O₃] in the past 30 years.     

Functional Organization of the Neural Control of Circulation in Aplysia

The abdominal ganglion of Aplysia provides a useful model forstudying the functional organization of motor systems. Herewe review studies of the neural network controlling circulation,emphasizing the organizational features it may share with othermotor systems controlled by the abdominal ganglion. We identifiedseven motor neurons to the heart and vascular system. Motorneurons having similar motor effects (e.g. the two heart inhibitors,or the three vasoconstrictors), together with cells of unknownmotor function located near them, make up distinct homogeneouscell groups. The members of each group appear to be nearly identicalwith respect to biophysical and neurochemical properties, sizeand effectiveness of synaplie inputs, and firing patterns. Thereare no interconnections between the members of the groups, butfive interneurons innervate the homogeneous groups in variouscombinations, exciting some groups and inhibiting others. Twoof the interneurons, Interneuron I (cell I₁₀) and InterneuronII, are command cells which produce centrally generated motorprograms in the absence of sensory feedback. Eacli command apparentlycodes for a specific homeostatic function, such as increasedcardiac output. Coordination of the two commands is achievedby mutual inhibitory connections between them, ensuring thatthe motor neurons of the system receive only one command ata time. Some synaptic connections made by the command interneuronsappear to be functionally ineffective; the possible significanceof them is discussed. Available evidence suggests that manyfeatures of the network controlling circulation may be characteristicof other visceromotor systems of the abdominal ganglion.