Bibliothek

Notizen: Wild migrating sockeye salmon Oncorhynchus nerka exposed to 20, 50 and 80 mg 1−1 of clove oil could be handled within 3 min, recovered within 10 min, and survived 15 min exposure trials. Fish tested at 110 mg 1−1 did not recover from 15 min exposure trials. Response curves developed for induction and recovery time considered the following predictors: clove oil concentration, sex, fish length and depth. A significant positive dependence was observed between induction time and fish length for 20, 50 and 80 mg 1−1 test concentrations; no dependence was observed between induction time and length at 110 and 140 mg 1−1. Recovery time differed as a function of clove oil concentration, but not fish size. A concentration of 50 mg 1−1 is recommended for anaesthetizing sockeye salmon ranging in length from 400 to 550 mm at water temperatures averaging 9–10° C.

Notizen: Abstract A simulated neuron was constructed in which effects on spike discharge of altering certain fundamental biophysical parameters could be studied. The simulation was performed by use of a digital computer. The simulation was tested by comparing performance of the simulated neuron with that of actual neurons. Rates and patterns of spike discharge were achieved for the simulated neuron that were comparable to those recorded from units in the motor cortex of awake cats. Altering biophysical parameters such as firing threshold, levels of synaptic input or rates of transverse and longitudinal current spread produced appropriate alterations of discharge rate. On the above basis it was possible to investigate interrelated effects on spike discharge of changing levels of synaptic input and rates of current spread within the simulated neuron. With low rates of longitudinal current spread, graded levels of synaptic input produced correspondingly graded levels of ouput discharge. With high rates of longitudinal current spread, the transfer properties of the neuron were markedly altered. The neuron became a bistable operator where synaptic inputs above a certain level were enhanced and all those below were suppressed. A linear relationship was found to exist between firing threshold and the level of synaptic input required to reach the transition from quiescence to near-tetanic rates of discharge. Alterations in behavior are increasingly thought to be subserved by changes in the efficacy of synaptic transmission or in the post-synaptic intergrative propeties of neurons. The results of our investigations describe the interplay between those two processes.

Notizen: Abstract In a simulated neuron with a dendritic tree, the relative effects of active and passive dendritic membranes on transfer properties were studied. The simulations were performed by means of a digital computer. The computations calculated the changes in transmembrane voltages of many compartments over time as a function of other biophysical variables. These variables were synaptic input intensity, critical firing threshold, rate of leakage of current across the membrane, and rate of longitudinal current spread between compartments. For both passive and active dendrites, the transfer properties of the soma studied for different rates of longitudinal current spread. With low rates of current spread, graded changes in firing threshold produced correspondingly graded changes in output discharge. With high rates of current spread, the neuron became a bistable operator where spiking was enhanced if the threshold was below a certain level and suppressed if the threshold was above that level. Since alterations in firing threshold were shown to have the same effect on firing rate as alterations in synaptic input intensity, the neuron can be said to change from graded to contrast-enhancing in its response to stimuli of different intensities. The presence or absence of dendritic spiking was found to have a significant effect on the integrative properties of the simulated neuron. In particular, contrast enhancement was considerably more pronounced in neurons with passive than with active dendrites in that somatic spike rates reached a higher maximum when dendrites were passive. With active dendrites, a less intense input was needed to initiate somatic spiking than with passive dendrites because a distal dendritic spike could easily propagate by means of longitudinal current spread to the soma. Once somatic spiking was initiated, though, spike rates tended to be lower with active than with passive dendrites because the soma recovered more slowly from its post-spike refractory period if it was also influenced by refractory periods in the dendrites. The experiment of comparing neurons with active and passive dendrites was repeated at a different, higher value of synaptic input. The same differences in transfer properties between the active and passive cases emerged as before. Spiking patterns in neurons with active dendrites were also affected by the time distribution of synaptic inputs. In a previous study, inputs had been random over both space and time, varying about a predetermined mean, whereas in the present study, inputs were random over space but uniform over time. When inputs were made uniform over time, spiking became more difficult to initiate and the transition from graded to bistable response became less sharp.