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1) The effects of sine-wave and step inputs of light upon the frog (R. temporaria) electroretinogram were examined at different mean intensities. At all mean intensities tested (3 - 90 lux) the E.R.C. evoked by small amplitude stimuli is approximately linear, although inherent non-linearities are apparent. With larger amplitude stimuli the non-linearities become increasingly evident. At a mean intensity of 3 lux the b-wave evoked by small amplitude stimuli is approximately fitted by the equation for a linear filter with 4 R.C. elements, but the non-linearities have to be accounted for. An increase in mean intensity reduces b-wave sensitivity and shortens the time constants of the response. At a mean intensity of 90 lux the simple filter model does not describe the b-wave response characteristics. However, a reasonable fit to the experimental data may be obtained by closing feedback loop around the transfer function of the simple filter. It is tentatively suggested that a mechanism analogous to feedback may account for the shortening of the b-wave time constants caused by light adaptation. 2) The response characteristics of off-type ganglion cells were studied in response to sine-wave and step inputs of light. Upon the basis of receptive field size, shape and sensitivity distribu tion, three classes of receptive field are identified. Responses evoked by stimulation at different positions within a receptive field are compared and found to differ: peripherally evoked responses having longer time constants than FSP evoked responses. A model, based upon the known anatomy of the retina, is proposed to explain the position dependent changes in ganglion cell response time constants.

The Effect of Small Cations on the Delayed Rectifier and the Resting Potassium Conductance of Frog Sartorius Muscle. By Louise Ann Gay Standard electrophysiological techniques were used to examine the effects of hydrogen ions, thallous and alkali metal ions on the electrical properties of frog sartorius muscle. A reduction in external pH from 9.2 to 5.2 slowed the rising and falling phases of the action potential. In voltage-clamped fibres, a similar pH change shifted the delayed potassium conductance to more positive membrane potentials, but had little or no effect on the maximum delayed conductance. At pH 5-2, the delayed current turned on more slowly than at pH 7.2 and the threshold for the sodium current was shifted to more positive potentials. These results are consistent with the titration of fixed charges at the membrane surface. Reversal potential measurements were used to investigate the selectivity of delayed potassium channels. Calculated mean permeability ratios were PLi/PK = 0.024, PNa/PK = 0.030, PRb/PK = 0.95 and PCs/PK =011. A voltage-clamp technique was used to investigate the effects of caesium and thallium on potassium currents in resting muscle. Caesium blocked the resting potassium conductance in a concentration- and voltage-dependent manner. At a given membrane potential, the blockade was reduced by raising internal potassium, but was largely independent of the external potassium concentration. In fibres hyperpolarized under constant current conditions in the presence of caesium, oscillations in the membrane potential were recorded. In potassium- free solutions, the major effect of thallium was to remove the time- and sodium-dependent permeability change which gives rise to a negative slope in the steady-state current-voltage relations of fibres hyperpolarized in normal Ringer. It is concluded that potassium crosses the resting membrane by way of multi-ion channels containing two or more ion binding sites. A method is described which demonstrates that at least part of the slow conductance decrease which occurs when muscle fibres are hyperpolarized is due to a depletion of potassium from the transverse tubules.

This thesis is divided into two parts. The first part is concerned with the developments and migration of Trichosomoides crassicuada, a common parasite of the urinary bladder of rats. The stages in the development of the egg and the experimental data regarding the hatching in various solutions are given. The morphology of the larvae, their migration in the host and the presebce of a stylet in the adult worms are described. The incidence of Trichosomoides crassicuada and its relationshop to the presence of mucoid alouli in the bladder are discussed. Part II describes an attempt to determine the validity of the theory of retrofection and auto-infection of Enterobius vermicularis. Details of various methods used for observing the development and hatching out of the eggs of Enterobius vermicularis are given. Owing to the difficulties experiences in obtaining an adequate amount of material of E. vermicularis, it was decided to determine whether retrofection and auto-infection occur in a closely allied oxyurid Aspiculuris tetraptera of mice. The results of observations on the development and hatching of eggs in various solutions are given. The bionomics of the larvae are described. It was found that the eggs could hatch out on moist cotton-wool swab in the anal opening of a human volunteer and the anal region of mice. Infective eggs are applied in the anal region of clean mice which were imobilised in specially designed cages to prevent their turning. On examination after a few days, adolescent forms were obtained in the colon in four cases. Experiments are now being continued to find out whether mature forms will develop as a result of retrofection. The theory of internal auto-infection is discussed in the light of our own experimental and the data in the literature.

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