We examined the possibility that neurons or groups of neurons along the retino-cortical transmission chain have properties of tuned oscillators: To this end, we studied the resonance properties of the retino-thalamo-cortical system of anaesthetized cats by entraining responses with flicker stimuli of variable frequency (2-50 Hz). Responses were assessed from multi-unit activity (MUA) and local field potentials (LFPs) with up to four spatially segregated electrodes placed in areas 17 and 18. MUA and LFP responses were closely related, units discharging with high preference during LFP negativity. About 300 ms after flicker onset, responses stabilized and exhibited a highly regular oscillatory patterning that was surprisingly similar at different recording sites due to precise stimulus locking. Fourier transforms of these steady state oscillations showed maximal power at the inducing frequency and consistently revealed additional peaks at harmonic frequencies. The frequency-dependent amplitude changes of the fundamental and harmonic response components suggest that the retino-cortical system is entrainable into steady state oscillations over a broad frequency range and exhibits preferences for distinct frequencies in the theta- or slow alpha-range, and in the beta- and gamma-band. Concomitant activation of the mesencephalic reticular formation increased the ability of cortical cells to follow high frequency stimulation, and enhanced dramatically the amplitude of first- and second-order harmonics in the gamma-frequency range between 30 and 50 Hz. Cross-correlations computed between responses recorded simultaneously from different sites revealed pronounced synchronicity due to precise stimulus locking. These results suggest that the retino-cortical system contains broadly tuned, strongly damped oscillators which altogether exhibit at least three ranges of preferred frequencies, the relative expression of the preferences depending on the central state. These properties agree with the characteristics of oscillatory responses evoked by non-temporally modulated stimuli, and they indicate that neuronal responses along the retino-cortical transmission chain can become synchronized with precision in the millisecond range not only by intrinsic interactions, but also by temporally structured stimuli.

The invention of new techniques for surgery and interventional radiology demand improved training for ongoing specialists. The Anatomical Institutes in Switzerland support these requirements by establishing hands-on practical training courses by using new procedures for cadaver embalming and model construction. Improvements allow courses to provide students with more realistic simulations of both established and experimental surgical methods. Through these changes, the value of in-depth gross anatomy is enhanced as a topic of fundamental importance for the postgraduate medical and surgical curriculum. The web site www.unifr.ch/sgahe/snga.html contains information on courses using the Thiel embalming solution. Details about training courses in Switzerland using anatomical models are available at www.heartlab.org, www.vascular-international.org, and www.elastrat.com. Anat Rec (New Anat) 265:254 -256, 2001.

The "all cone" retina of the tree shrew (Tupaia belangeri) was examined in the adult and early postnatal stages by light and electron microscopy. Rods are not as rare as previously thought, but make up about 4% of the photoreceptors. They are relatively short and narrow cells, which stain (toluidine blue) more intensively and lie more proximal than cones. Among the cones three morphological varieties could be distinguished. Most cones stain lightly but have a light or a dark giant mitochondrion in their inner segment; a third type stains darker but occurs only rarely. All cones possess extensive radial processes ("lateral fins") around the basal part of their inner segments. Such fins are well known from reptiles and birds, but have only once been described in a mammal (gray squirrel). The maturation of the retina in Tupaia belangeri proceeds centrifugally, i.e., from the vitreal to the scleral side, as in most mammals. A few synapses are already present at birth in the outer and inner plexiform layers, but seem to be more advanced in the latter. Such early synapses are small and have only few synaptic vesicles; they appear almost mature by day 14. The light-sensitive outer segments develop last. The first disks are seen by day 10, but regular membrane stacks are only present by day 18. Thus, it seems that the retina is functional when the young first open their eyes, which occurs around day 18.

Dynamin I, a GTPase involved in the endocytic cycle of synaptic vesicle membranes, is believed to support axonal outgrowth and/or synaptogenesis. To explore the temporal and spatial patterns of dynamin I distribution in neuronal morphogenesis, we compared the developmental expression of dynamin with the expression of presynaptic membrane proteins such as SV2, synaptotagmin, and syntaxin in the chick primary visual pathway. Western blots of retina and tectum revealed a steady increase of synaptotagmin and syntaxin from embryonic Day 7 (E7) to E11, whereas for the same time frame no detectable increase of dynamin was found. Later stages showed increasing amounts of all tested proteins until the first postnatal week. Immunofluorescence revealed that SV2, synaptotagmin, and syntaxin are present in retinal ganglion cell axons from E4 on. In later stages, the staining pattern in the retina and along the visual pathway paralleled the formation and maturation of axons. In contrast, dynamin is not detectable by immunofluorescence in the developing retina and optic tectum before synapse formation. Our data indicate that, in contrast to the early expression of synaptotagmin, SV2, and syntaxin during axonal growth, dynamin is upregulated after synapse formation, suggesting its function predominantly during and after synaptogenesis but not in axonogenesis.(J Histochem Cytochem 47:1297-1306, 1999)

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