Model for the retino-tectal projection

A model for the retino-tectal projection is proposed which assumes that axonal growth proceeds predominantly in the direction of maximal slope of a guiding substance (or, more generally, of a system parameter subsuming the effect of several substances). The spatial distribution of this parameter, in turn, results from the interaction of components of retinal axons (which are graded with respect to position of origin in the retina) and tectal components. One or two gradients in each dimension of retina and tectum suffice. Conditions for the generation of a reliable projection on this basis are relatively simple and consistent with conventional enzyme and receptor kinetics. Adhesive forces could but need not be involved in the guiding mechanism. The slope of guiding substances that interfere with an intracellular pattern-forming mechanism within the growth cone may determine the polarity of activation and thus the direction of growth. Generation of primary projections and some features of regulation such as independence of projections on neural pathways, and observations on the innervation of rotated pieces of tectum, can be explained on the basis of the model. The model can be extended by introducing additional production of guiding substance depending on the density, and duration of presence, of fibre terminals in the course of innervation. This simple mechanism would suffice for observed effects of compression and expansion of the map following ablation of retinal and tectal tissue, respectively. It may but need not be involved in the primary projection, too.     

The Extended Branch-Arrow Model of the formation of retino-tectal connections

This paper presents XBAM (the Extended Branch-Arrow Model), a new model of the development of the retino-tectal topographic mapping as observed in frog, toad, and goldfish visual systems. The updating process employed by XBAM is distributed in nature and depends upon interactions between branches of retinal fibers, the branches and the boundaries of the tectum and grafts, and the branches and the tectal surface. Results of computer simulation of the model are related to experimental data obtained from tectal and retinal graft and lesion studies, and comparisons are also made with other models.     

Restoration of retino-tectal connections in frogs during regeneration of the optic nerve

At various times after unilateral division of the optic nerve in the frogRana temporaria L. evoked potentials in response to electrical stimulation of the optic nerve were investigated in a segment distal to the site of operation, spike activity was recorded from endings of regenerating and intertectal axons when stimuli of different shapes were placed in the field of vision, and the distribution of axonal bulbs of growth by depth in the tectum mesencephal was studied electron-microscopically. During regeneration of the axons the responses of the retinal ganglionic cells to visual stimuli retained most of their individual features. Myelinated axons of the retinal ganglionic cells regenerate first (starting on the 21st day after operation). Myelination of these fibers lags significantly behind their growth and is complete more than 100 days after the operation. Unmyelinated axons of the retinal ganglionic cells grow up toward the tectum mesencephali after myelinated axons (80 or more days after the operation). Axonal bulbs of growth in the initial periods after the operation are located close to the pial surface and the level of spread of the myelinated axons of the retinal ganglionic cells differs significantly from their normal level of localization. Intertectal connections persist after division of the nerve and are activated by visual stimuli during regeneration of the axons of the retinal ganglionic cells. Connections were found mainly between intertectal fibers terminating superficially and retinal ganglionic cells belonging to class 1 and 2 detectors. Axons of the retinal ganglionic cells grow up toward the caudal region of the tectum mesencephali later than toward the rostral region.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 5, No. 6, pp. 611–620, November–December, 1973.     

First neurophysiological approach to the retino-tectal projections inDiscoglossus pictus (Anura)

Summary The topography and receptive field properties of the retino-tectal projections ofDiscoglossus pictus were studied electrophysiologically for the first time. The spatial extent and tectal topography of the visual field resemble closelyRana. However, despite the presence of event retinal cells, only one class of sustained retinal cell was found. Nevertheless, receptive field properties and their velocity function suggest thatDiscoglossus retinal cells may be correlated with the classical R3 and the recently described mixed-sustained types inRana (Gaillard and Garcia 1986).     

Development of neuronal connections in chick embryonic retino-tectal system: An overview

Synaptogenesis has been examined in the retino-tectal system of the chick embryo both biochemically and morphologically. We have evaluated the amount and the rate of transport of glycoproteins synthetized in retinal ganglion cells and axonally transported to retinal nerve endings. The distribution of glycoproteins inside the tectum shows that they are part of the synaptosomal membrane either because they are incorporated in it en route or because they flow through an axolemmal flow. The role they might play in the building up of the synaptic membrane is discussed in relation to the maturation of synaptic contacts, observed either with E-PTA staining method or with routine fixation methods. The distribution of a carbohydrate binding protein was followed with histochemical techniques. This protein is synthesized by the matching tectal neurons, and its synthesis is developmentally regulated. Its distribution is affected by innervation. All these findings are discussed in relation to the hypothesis that three different steps are involved in embryonic synaptogenesis. These three steps are: 1) interneuronal recognition; 2) formation of initial contact; and 3) formation of synapses. The possibility that different kinds of cell surface macromolecules might play a different role is also discussed.     

L.D.H. and L.D.H. isoenzymes of the intra-uterine secretions of the cow during the estrous cycle

Uterine secretions were collected from 20 mature cows during estrus (day 0), metestrus (day 5), diestrus (day 10) and proestrus (day-1). Lactate dehydrogenase (LDH) and LDH isoenzymes activity were evaluated. No significant cyclic variations of LDH activity was found in the uterine secretions while the mean of the enzyme activity was higher during the estrogenic period of the cycle. The relative activity of LDH-1, LDH-2 and LDH-3 isoenzymes were higher during proestrus and estrus whereas LDH-5 activity was more important during metestrus. The LDH-3 seems to have the higher relative activity in uterine secretions of the cow.