Extrasynaptic Communication.

Streams of action potentials or long depolarizations evoke a massive exocytosis of transmitters and peptides from the surface of dendrites, axons and cell bodies of different neuron types. Such mode of exocytosis is known as extrasynaptic for occurring without utilization of synaptic structures. Most transmitters and all peptides can be released extrasynaptically. Neurons may discharge their contents with relative independence from the axon, soma and dendrites. Extrasynaptic exocytosis takes fractions of a second in varicosities or minutes in the soma or dendrites, but its effects last from seconds to hours. Unlike synaptic exocytosis, which is well localized, extrasynaptic exocytosis is diffuse and affects neuronal circuits, glia and blood vessels. Molecules that are liberated may reach extrasynaptic receptors microns away. The coupling between excitation and exocytosis follows a multistep mechanism, different from that at synapses, but similar to that for the release of hormones. The steps from excitation to exocytosis have been studied step by step for the vital transmitter serotonin in leech Retzius neurons. The events leading to serotonin exocytosis occur similarly for the release of other transmitters and peptides in central and peripheral neurons. Extrasynaptic exocytosis occurs commonly onto glial cells, which react by releasing the same or other transmitters. In the last section, we discuss how illumination of the retina evokes extrasynaptic release of dopamine and ATP. Dopamine contributes to light-adaptation; ATP activates glia, which mediates an increase in blood flow and oxygenation. A proper understanding of the workings of the nervous system requires the understanding of extrasynaptic communication.

Fluorescence of serotonin in the visible spectrum upon multiphotonic photoconversion.

The vital molecule serotonin modulates the functioning of the nervous system. The chemical characteristics of serotonin provide multiple advantages for its study in living or fixed tissue. Serotonin has the capacity to emit fluorescence directly and indirectly through chemical intermediates in response to mono- and multiphoton excitation. However, the fluorescent emissions are multifactorial and their dependence on the concentration, excitation wavelength and laser intensity still need a comprehensive study. Here we studied the fluorescence of serotonin excited multiphotonically with near-infrared light. Experiments were conducted in a custom-made multiphoton microscope coupled to a monochromator and a photomultiplier that collected the emissions. We show that the responses of serotonin to multiphoton stimulation are highly non-linear. The well-known violet emission having a 340 nm peak was accompanied by two other emissions in the visible spectrum. The best excitor wavelength to produce both emissions was 700 nm. A green emission with a ∼ 500 nm peak was similar to a previously described fluorescence in response to longer excitation wavelengths. A new blue emission with a ∼ 405 nm peak was originated from the photoconversion of serotonin to a relatively stable product. Such a reaction could be reproduced by irradiation of serotonin with high laser power for 30 minutes. The absorbance of the new compound expanded from ∼ 315 to ∼ 360 nm. Excitation of the irradiated solution monophotonically with 350 nm or biphotonically with 700 nm similarly generated the 405 nm blue emission. Our data are presented quantitatively through the design of a single geometric chart that combines the intensity of each emission in response to the serotonin concentration, excitation wavelengths and laser intensity. The autofluorescence of serotonin in addition to the formation of the two compounds emitting in the visible spectrum provides diverse possibilities for the quantitative study of the dynamics of serotonin in living tissue.

Mathematical and diffractive modeling of self-healing.

To analyze the self-healing of a partially obstructed optical beam, we represent it by two orthogonal field components. The first component is an exact copy of the unobstructed beam, attenuated by a factor that is computed by a simple formula. The second component represents a pure distortion field, due to its orthogonality respect to the first one. This approach provides a natural measure of the beam damage, due to the obstruction, and the degree of self-healing, during propagation of the obstructed beam. As interesting results, derived in our approach, we obtain that the self-healing reaches a limit degree at the far field propagation domain, and that certain relatively small phase obstructions may produce a total damage on the beam. The theory is illustrated considering a Gaussian beam, distorted by different amplitude and phase obstructions. In the case of a soft Gaussian obstruction we obtain simple formulas for the far field limit values of the beam damage and the self-healing degree.

Efficient generation of Hermite-Gauss and Ince-Gauss beams through kinoform phase elements.

We discuss the generation of Hermite-Gauss and Ince-Gauss beams employing phase elements whose transmittances coincide with the phase modulations of such beams. A scaled version of the desired field appears, distorted by marginal optical noise, at the element's Fourier domain. The motivation to perform this study is that, in the context of the proposed approach, the desired beams are generated with the maximum possible efficiency. A disadvantage of the method is the distortion of the desired beams by the influence of several nondesired beam modes generated by the phase elements. We evaluate such distortion employing the root mean square deviation as a figure of merit.

Comparing efficiency and accuracy of the kinoform and the helical axicon as Bessel-Gauss beam generators.

We compare two phase optical elements that are employed to generate approximate Bessel-Gauss beams of arbitrary order. These elements are the helical axicon (HA) and the kinoform of the desired Bessel-Gauss beam. The HA generates a Bessel beam (BB) by free propagation, and the kinoform is employed in a Fourier spatial filtering optical setup. As the main result, it is obtained that the error in the BBs generated with the kinoform is smaller than the error in the beams obtained with the HA. On the other hand, it is obtained that the efficiencies of the methods are approximately 1.0 (HA) and 0.7 (kinoform).