Regulation of light flux by pupil mechanisms in the UV-sensitive superposition eye of owl-fly Ascalaphus macaronius (Neuroptera) was studied with a fast reflection microspectrophotometric technique. ...The spectral sensitivity of pupil reaction, which was calculated on the basis of changes of transient amplitude reflection, was almost identical with the one of Deilephila eye. This indicates that in spite of different life styles and spectral sensitivities of photoreceptors, pupil closing is triggered by the same photosensitive structure in both eyes. By measuring the spectra of reflected light from the Ascalaphus eye between 400 and 700 nm after different dark periods following light stimulation, it was established that the restoration of reflection was much faster in the red than in the blue spectral range. Based on this, we propose that two different pupil mechanisms with different spectral absorption characteristics are involved in light-flux regulation. Fast-reacting pupil is probably represented by screening pigment migration in the secondary pigment cells and a slow blue-absorbing system by the activity in primary pigment cells. The importance of two different pupils for the photoregeneration of visual pigment is discussed.
The process of light adaptation in blowfly photoreceptors was analyzed using intracellular recording techniques and double and triple flash stimuli. Adapting flashes of increasing intensity caused a ...progressive reduction in the excitability of the photoreceptors, which became temporarily suppressed when 3 x 10(6) quanta were absorbed by the cell. This suppression was confirmed by subsequently applying an intense test flash that photoactivated a considerable fraction of the 10(8) visual pigment molecules in the cell. The period of temporary desensitization is referred to as the refractory period. The stimulus intensity to render the receptor cell refractory was found to be independent of the extracellular calcium concentration over a range of 10(-4) and 10(-2) M. During the refractory period (30-40 ms after the adapting flash) the cell appears to be "protected" against further light adaptation since light absorption during this period did not affect the recovery of the cell's excitability. Calculations showed that the number of quantum absorptions necessary to induce receptor refractoriness is just sufficient to photoactivate every microvillus of the rhabdomere. This coincidence led to the hypothesis that the refractoriness of the receptor cells is due to the refractoriness of the individual microvilli. The sensitivity of the receptor cells after relatively weak adapting flashes was reduced considerably more than could be accounted for by the microvilli becoming refractory. A quantitative analysis of these results suggests that a photoactivated microvillus induces a local adaptation over a relatively small area of the rhabdomere around it, which includes several tens of microvilli. After light adaptation with an intense flash, photoactivation of every microvillus by the absorption of a few quanta produced only a small receptor response whereas photoactivation of every rhodopsin molecule in every microvillus produced the maximum response. The excitatory efficiency of the microvilli therefore increases with the number of quanta that are absorbed simultaneously.
In photoreceptors, a latency of many milliseconds elapses between the absorption of a light quantum and the occurrence of the late receptor potential, even for strong light stimuli. Surprisingly, ...this is much longer than the time necessary for conductance changes such as occur in membranes of neurones or muscles, mediated by chemical transmitters. There are several possible explanations for the long photoreceptor latency. (1) It may be due to properties of the visual pigment molecules. For instance, the temporal coincidence of the occurrence of metarhodospin II with the receptor signal indicates that the meta I-meta II transition might be the trigger for the electrical response in vertebrate photoreception. (2) It may be explained by properties of transport processes. Such a time consuming process could be the diffusion of an internal 'transmitter substance', which diffuses to a 'pore' in the receptor membrane. (3) A third possibility is the time needed to produce and accumulate chemical substances. The light-induced change of the visual pigment molecule might trigger a chemical reaction chain, in which the product of an earlier step triggers the next one. The experiments described here show that a considerable part of the long latency in photoreception is due to processes that are localised at the level of the visual pigment molecule.
This study examined the impact of 4 months of leadership training seminars and participation in a youth advisory council on the civic awareness and leadership development of members of the Saint ...Croix Foundation Youth Advisory Council (SCFYAC). The 16 members of the SCFYAC, between 14 and 21 years of age, participated in this study, which took place in Saint Croix, U.S. Virgin Islands. The instruments used in the study included the Civic Measurement Models: Tapping Adolescents' Civic Awareness and individual interviews; both were administered at the beginning and end of the study. Results from the survey did not indicate significant growth for members; however, analysis of the in-depth interviews demonstrated that participants had a greater understanding of leadership and civic engagement at the end of the study. The theme that suggested this understanding included knowledge of skills that leaders exhibit. Data also indicated an increase in SCFYAC members' perception that civic engagement by citizens improves the community. Another theme was the pivotal experiences of SCFYAC members throughout the study, which included learning leadership skills through experiential activities, being encouraged to engage in civic activity, being supported by peer SCFYAC members in their development as leaders, and participating in discussions at meetings. SCFYAC members anticipated that they would be engaged in civic activities in the future and they envisioned specific roles in which they planned to serve their community. The results suggested that participation in SCFYAC increased participants' civic awareness and plans for future engagement. Canonical correlations from the survey instrument led to further understanding of what influenced the participants' civic awareness and engagement. Parental involvement in civics and having political conversations at home, coupled with discussions among peers, seemed to encourage greater awareness than did exposure to media or even classroom lessons. It was recommended that facilitators and educators who wish to encourage adolescents in the area of civic engagement integrate experiential learning exercises into their instructional programs and training to help adolescents to develop confidence in their civic engagement. The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.
1. The proposed models of two independent groups, which relate the different states of the visual pigment to the excitation of the membrane in invertebrate photoreceptors (with particular reference ...to the prolonged depolarising afterpotential, the PDA) are compared and evaluated. 2. The validity of the late receptor potential (the "normal" receptor response) as an index of photoreceptor sensitivity, i.e., an index of the number of rhodopsin to metarhodopsin transitions, is verified by concurrent spectrophotometry. 3. Electrophysiological observations alone allow the calculation of 1.3 x 10(8) photopigment molecules in the rhabdom of an R1-6 photoreceptor of a vitamin A-bred Calliphora. 4. The PDA is shown to be quantifiable in terms of the number of rhodopsin to metarhodopsin conversions by the absorption of single light quanta. 5. The comparison of discrete membrane fluctuations (quantum bumps) during the PDA and during exposure to sustained light stimuli that mimic the PDA suggest that, the PDA, similar to the late receptor potential, may be due to the summation of quantum bumps.
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